The role of pore wall microstructure and micropores on the mechanicalproperties of Cu–Ni–Mo based steel foams

Nuray Bekoz n, Enver Oktay 1

Metallurgical and Materials Engineering Department, Engineering Faculty, Istanbul University, 34320 Avcılar, Istanbul, Turkey

a r t i c l e i n f o

Article history:Received 25 December 2013Received in revised form3 June 2014Accepted 18 June 2014Available online 27 June 2014

Keywords:Steel foamPore wall microstructureMacroporeMicroporeMechanical property

a b s t r a c t

Mechanical behaviour of metal foams is not only affected by the macrostructure of the foam, i.e. porositycontent, shape and size of the macropores, but also depends on the properties of pore wall. This studyprimarily concerns the role of pore wall microstructure and micropores formed in the pore wall whichinfluences the mechanical behavior of steel foams. Steel foams having rather similar macropore structurebut differences in pore wall microstructure and porosity content were produced by the space holder-water leaching technique in powder metallurgy. Pre-alloyed steel powders with compositions of 1.75 Ni–1.5 Cu–0.5 Mo–0.2 C, 4.0 Ni–1.5 Cu–0.5 Mo–0.2 C and 1.5 Mo–0.2 C as the parent material and carbamideas space holder material were used for the produced steel foams. The structural and mechanical propertyvariations resulting from the use of spacer having different amounts and the various alloy elements inthe chemical composition of powders were investigated. The formation mechanism of macro andmicropores was discussed in terms of mechanical properties of the foams. The results showed that thesize and amount of macropores increased with increasing spacer content, but the size and amount ofmicropores decreased. An increase in content of spacer improved the densification during compaction ofpowder mixtures by the plastic deformation of soft spacer particles. In addition, the copper in alloysreduced micropores in the pore walls. Compression tests were carried out to investigate the deformationmechanism and energy absorbing characteristic of the steel foams. The alloy that contained high nickelresulted in better compressive behavior. The strength and energy absorption capacity of the foamsincreased with a decrease in level of porosity.

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1. Introduction

Steel foams have recently attracted considerable attention inboth industry and academia because of their low cost, low thermalconductivity, high working temperature, high specific mechanicalproperties, high impact energy absorption, high heat resistanceand good weldability, which have high specific properties ascompared to aluminum foams. They can be used as high-functional and lightweight materials in various fields of structuraland functional applications due to their suitable properties [1,2].Mechanical behaviors of metal foams depend on their structures[3–5]. Structure of foams can be characterized in terms ofgeometrical structure and material structure. Geometrical struc-ture includes the shape and size distribution of the pores, and

defects in the pore wall structure. Material structure is simply theinternal and nature microstructure of the pore wall material. Themechanical properties of metal foams are determined by interac-tions between these factors [6]. However, there is still uncertainlyabout the role of these factors in determining the mechanicalproperties, and the use of steel foams in a large scale andsuccessful applications depend on detailed understanding of theeffect of the factors on the mechanical properties.

Among the manufacturing processing of steel foams, the spaceholder-water leaching technique in powder metallurgy has beenstudied by many researches, and is rather cost effective, flexibleand leads to desired properties [7–10]. The foams produced usingthis technique were observed to contain mainly two types ofpores: macropores obtained as a result of spacer, and microporesin cell walls presumably resulting from the compaction andsintering process during the production of the metal foam[11,12]. Markaki and Clyne [13] studied the effect of cell wallmicrostructure on the mechanical behavior of aluminium foams,and found that nature of the microstructure within the cell walls

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Materials Science & Engineering A

http://dx.doi.org/10.1016/j.msea.2014.06.0640921-5093/& 2014 Elsevier B.V. All rights reserved.

n Corresponding author. Tel.: þ90 2124737070x17793; faxþ90 2124737180.E-mail addresses: nbekoz@istanbul.edu.tr (N. Bekoz),

oktay@istanbul.edu.tr (E. Oktay).1 Tel.: þ90 2124737063; fax: þ90 2124737180.

Materials Science & Engineering A 612 (2014) 387–397

can have a significant role on their mechanical behaviors. Theyalso pointed out that optimization of their performance clearlyrequires an understanding of the interplay between processingconditions, cell structure, cell wall microstructure and mechanicalresponse characteristics under applied loads. Park and Nutt [14]reported that the deformation mechanism depends on steel foamsstructure (both micro and macro), which is controlled by theprocess parameters employed. Ahmed et al. [15] investigated themechanical properties and porosity relationship of porous ironcompacts, and found that the stress–strain relationship of thesemetals is largely dependent on the degree of porosity. Bekoz andOktay [8] produced steel foams having different porosity contents,and found that the compressive strength is significantly influencedboth by the amount of total porosity and matrix microstructure.The majority of research on processing of metal foams havingmacro and micropores has the special interest in steel foams withopen or closed pores. In all studies mentioned above, the focuswas given on the effect of macropore structure on the mechanicalproperties of the foams. However, the mechanical behaviour of thefoams is not only affected by the macropore structure of the foam,i.e. porosity content, shape and size of the macropores, but alsodepends on the pore wall structure, i.e. pore wall microstructureand the properties of micropores formed in the pore wall[8,13,16,17]. Although the application areas of the steel foamscontinuously increase, investigations about the effect of pore wallstructure on their mechanical behavior are still lacking.

Up to now, studies on the metal foams have been mainlyfocused on the sintered final parts. There are some issues thatmust be considered on the green strength of powder–spacercompacts because of the influence on the final strength. Laptevet al. [18] investigated the green strength of titanium–ammoniumbicarbonate compacts, and found that the green strength mainlydepends on the compaction pressure and the amount of spacer.Especially the number of contacts between titanium powders andbetween spacer particles as well as between both of them wasfound to be an important parameter for understanding thebehaviour of green compacts under mechanical load. Tunceret al. [19] reported that the volume fraction of soft carbamideparticle and hard titanium powder has an important effect on thecompressibility of powder mixtures. Bouvard [20] observed thenegative influence of hard particles on the densification duringcompaction of powder mixtures. However, no information wasfound about the effect of the different type metal powders formingthe main framework of the foams on the compressibility in theprevious studies. Additionally, volumetric changes are a significantconcern during component fabrication and vary inversely with thegreen density. Three main changes occur during sintering processwhich can be defined as the dimensional change of the pores andpore walls, and shrinkage of the whole specimens [19]. Laptevet al. [21] reported that the shrinkage occurs mainly due toreduction in size of micropores in the pore walls but macroporesusually retain their size or even tend to grow. Manonukul et al.[22] reported that the shrinkage was the shrinkage of the cellularstructure, which was a function of spacer fraction.

The use of pre-alloyed powders has many advantages on themechanical properties of sintered materials. Low alloyed steelpowders commercially known as Distaloy AB, Distaloy AE andAstaloy Mo are a partially pre-alloyed iron powder containingdifferent copper, nickel and molybdenum contents [23]. Distaloypowders are a partially pre-alloyed iron powder containing cop-per, nickel and molybdenum and consist of very pure iron powdersto which finely divided alloying elements have been diffusionbonded. In this way the high compressibility of the iron powder ismaintained and the risk of segregation can be minimized bybenefitting from fine alloying additives. Also, Distaloy powdershave a good green strength and extremely dimensionally stable.

Astaloy Mo is a water-atomized steel powder which is pre-alloyedwith 1.5% molybdenum and exhibits a homogenous microstruc-ture [23,24]. The nickel in the alloy decreases the elongation andmolybdenum is added for high compressibility. Both alloy ele-ments used to enhance hardenability of steels. The microporositycould be partly eliminated by liquid phase sintering. Copper formsa liquid phase during sintering and thus increases the strength[24]. Many researchers manufactured high dense specimens fromthe above mentioned powders by powder metallurgy technique,and investigated the mechanical and microstructural properties[25–27]. Although a few studies [7,8,12] have been published onthe processing and properties of Distaloy AB and Astaloy Mo steelfoam, there is no study on Distaloy AE steel foams in the literature.Also, there is no study on the energy absorbing properties of theabove mentioned steel foams in the literature.

In this study, steel foams having different Cu–Ni–Mo contents andporosity levels were produced by the space holder-water leachingtechnique in powder metallurgy. The structural and mechanicalproperty variations resulting from the use of steel powders havingdifferent contents of alloy elements were investigated in detail. Effectof processing parameters, such as densification of green specimens,volumetric shrinkages, and size distribution of macro and microporesof the foams, was determined as function of the spacer content. Theenergy absorption properties and deformation behavior of steelfoams was also characterized.

2. Experimental procedure

Low alloy steel foams were produced using pre-alloyed steelpowders commercially known as Distaloy AB, Distaloy AE andAstaloy Mo, which is a registered trademark of Höganäs Company.Three types of steel powders were selected with different contentsof alloy elements such as Cu–Ni–Mo, to obtain the foams withdifferent pore wall microstructures. The powder premixes con-sisted of 0.8 wt% zinc stearate as lubricant, and 0.2 wt% carbon wasadded as fine graphite (UF4). The chemical compositions of thesteel powders mixes prepared are given in Table 1. Distaloy AB,Distaloy AE and Astaloy Mo steel powders had a size distributionbetween 45 mm and 150 mm with an average particle size of112 mm, 90 mm and 109 mm, respectively. As a space holdermaterial, carbamide was used for its very high solubility in water.Spacer particles were crushed and sieved to obtain the fraction of710–1000 μm with irregular shape. The binder for green strengthwas polyvinyl alcohol (PVA), supplied by Merck, Germany. Fig. 1shows scanning electron microscope (SEM), Jeol JSM 5600, imagesof the steel powers used as the parent material and irregularshaped carbamide particles used as space holder material. Thegreen specimens were immersed in distilled water at roomtemperature to leach the carbamide. The PVA in the green speci-mens was thermally removed as part of sintering cycle. Thespecimens were sintered at 1200 1C for 60 min under high purityhydrogen in a horizontal tube furnace (Lenton, UK).

The detailed procedure for the production of different steelfoams in this study was described in Refs. [7,8]. The densities ofthe green specimens were calculated from measurements of thespecimens' weights and dimensions. The density and the fraction

Table 1The chemical composition of the powder mixes (wt%).

Base powder Cu Ni Mo C Lubricant Fe

Distaloy AB 1.5 1.75 0.5 0.2 0.80 Bal.Distaloy AE 1.5 4.0 0.5 0.2 0.80 Bal.Astaloy Mo – – 1.5 0.2 0.80 Bal.

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of open and closed porosity content of the sintered foams weredetermined by Archimedes' principle in boiling paraffin at 150 1Cfor 1 h for impregnation of open pores using a Sartorius precisionbalance equipped with a density-determination kit. The relativedensity was calculated thought the ratio of foam density to densemetal density. The pore morphology and pore wall microstructurewere examined using SEM and optical microscopy. The specimens'pores were filled with a cold-hardening epoxy resin (Epofix,supplied by Struers) then etched in 2% Nital solution for opticalexamination. Macro and microporosity content, shape and sizedistributions of the pores were determined using Clemex Vision PEcommercial image-analyzer software. Mechanical properties of thespecimens were studied by the compression test performed on aZwick–Roell Z050 materials testing machine. Compression testswere conducted at a crosshead speed of 0.5 mm min�1 with theload applied parallel to the foam growth direction. Testing wasautomatically halted when the preset load limit of 50 kN wasreached. This load level ensured full compression and hence force–displacement data beyond foam densification. Prior to compres-sion testing, specimen dimensions were measured with digitalcalipers to an accuracy of 0.01 mm. The stress was calculated usingthe apparent cross-sectional area of the respective specimen, afterwhich Young's modulus for each specimen was determined fromthe slope of the corresponding stress–strain graph. Energy absorp-tion capacities of the foams were also evaluated. At least threespecimens were tested under the same conditions to guaranteethe reliability of the results. During the course of this study,sintered foams produced using Distaloy AB, Distaloy AE andAstaloy Mo steel powders were referred to as AB, AE and Mofoams, respectively.

3. Results and discussion

3.1. Effect of processing parameters on the green density

Compacting pressure, properties of powders (hard and soft)and their fractions in the powder mixture can have a determininginfluence on compressibility of the mixtures which is an importantvariable effecting pore wall structure of highly porous materials.

A series of compacting experiments were carried out by changingcompaction pressure from 100 MPa to 500 MPa in order todetermine the effect of compaction pressure on the green densityof compacts as a function of carbamide fraction, and the results aregiven in Fig. 2 for the specimens produced from Distaloy AB steelpowder and carbamide particle mixtures. The results obtained forpure steel powder and pure carbamide particles are also plotted inFig. 2 for comparison. The increases in the densities of the DistaloyAB, Distaloy AE and Astaloy Mo green compacts having 50–80 vol%carbamide particles as a result of compaction pressure increasefrom 100 MPa to 500 MPa are shown in Table 2.

The results showed that the green density of compacts isaffected by the compaction pressure, the volume fraction ratiobetween carbamide particles and steel powders. The green den-sities of the specimens increased with decreasing carbamidefraction. The density of the green compacts increased from3.62 g cm�3 to 4.62 g cm�3 for the AE foams, from 3.53 g cm�3

to 4.43 g cm�3 for the AB foams; and from 3.48 g cm�3 to4.28 g cm�3 for the Mo foams as the volume fraction of carbamidedecreased from 80% to 50%. Densification of the compacts havinghigh carbamide content was slightly higher than those of com-pacts having lower carbamide content. The increase in the densityof the Distaloy AE green compacts is slightly higher than that ofthe Distaloy AB and Astaloy Mo green compacts. The similardensifications within error limits in the obtained results areprobably due to the inhomogeneous density distribution in thegreen compacts. Distaloy AE powders containing higher alloyingelement have a lower average particle size than that of the othertwo steel powders and this may be the reason for the increase indensity. Laptev et al. [18] found that compactibility of titanium–

ammonium bicarbonate mixtures depended on the volume frac-tion of ammonium bicarbonate, and densification of mixturesincreased with an increasing amount of space holder. Also, Laptevet al. [21] claimed that the existence of spacer particles deterioratepressure transfer between titanium particles during compaction.This leads to a decrease in titanium framework density withincreasing amount of the spacers. On the other hand, the densi-fication of the compacts increases by the plastic deformationof soft spacer particles. Tuncer et al. [19] reported that thespacer–spacer and titanium–spacer contacts are easier to distort

Fig. 1. SEM images of raw materials: (a) Distaloy AB, (b) Distaloy AE, (c) Astaloy Mo steel powders, and (d) irregular carbamide particles.

N. Bekoz, E. Oktay / Materials Science & Engineering A 612 (2014) 387–397 389

compared to titanium–titanium contacts. Similar effects wereobtained in this study. Fig. 2 shows that the green densityvariation for 100% carbamide is less dependent on the compactionpressure at the range studied, while that of pure steel powderslightly increases with pressure. Measurement showed thegreen density of carbamide was about to the theoretical densityof 1.34 g cm�3. The green density of the compacts having differentvolume fractions of carbamide showed a behavior similar to that ofpure carbamide powder, and their densities increase as compac-tion pressure increases. The results indicated that the densificationof the pure spacer was expressly enhanced compared to thedensification of steel powder at the same pressure. Soft carbamideparticles increased compaction efficiency of the powder mixturesince they have a greater compactibility and reduces frictionbetween steel particles during compaction.

In addition, the compacting pressures greater than 200 MPabroke the carbamide particles thereby changed the final pore sizeand morphology in the foam structure. These specimens could notbe leached out rapidly. On the other hand, specimens compactedat pressures less than 200 MPa could not hold their shapes afterremoval of the spacer in water. In view of these facts, the optimumcompacting pressures was determined to be 200 MPa, as thecompacts conserve its original shapes with sharp edges, withoutany crashing of carbamide particles and has enough strength forleaching treatment.

3.2. Densities and porosity levels of the sintered steel foams

AB, AE and Mo foams with porosities ranging between 48.5%and 70.5%, 48.2% and 70.2% and 47.4% and 71.2%, respectively wereproduced after sintering. Fig. 3 shows photograph of the sinteredsteel foams. Generally, metal foams are characterized structurallyby pore topology (open and closed pores), relative density, poresize and shape [28]; therefore all of these parameters weredetermined in this study. Table 3 presents the densities, totalporosity contents, and amount of open and closed porosities of thesintered foams.

The final porosity content was directly related to the addedfraction of carbamide. As the spacer content increased, thesintered density decreased and the total porosity increased in allcompacts. Open porosity of the specimens also increased when thetotal porosity level of the sintered specimens increased. A remark-able decline in the expected porosity content of the foamsoccurred as a result of sintering. High porosity content causedopen pores in the sintered foam structure which is desired fordamping and sound absorption applications [3,4]. In addition, theopen porosity content of Mo foams at similar porosity levels ishigher than that of the other foams.

3.3. Volumetric shrinkage of the sintered steel foams

The dimensional changes after sintering are strongly related toboth powder characteristic and porosity content of compacts aswell as processing conditions [29]. Volume changes of the sinteredfoams having different porosities are given in Fig. 4. Considerabledensification occurred during sintering process caused 4.41–6.74%,4.58–6.88% and 4.11–6.52% volumetric shrinkages in the AB, AEand Mo foams, respectively.

The extent of volumetric shrinkage after sintering depended onthe volume fraction of spacer and base material forming mainframework, and effect of the spacer fraction is prominent onshrinkage. Densification of the foams increased with increasingspacer content which is in agreement with the report of Laptevet al. [18]. Bekoz and Oktay [7] found that shrinkages of the steelfoams increased as the fraction of open pores increased. Manonukulet al. [22] studied the shrinkages in stainless steel foams, and found

1

2

3

4

5

6

7

50 200 350 500Compaction pressure (MPa)

Gre

en d

ensi

ty (g

/cm

3 )

Fig. 2. Effect of compaction pressure on the green density of Distaloy AB,carbamide having different volume fractions with Distaloy AB, and pure carbamidecompacts.

Table 2The increase in the densities of the green compacts having different carbamidefractions.

Base powder Carbamide fraction (vol%) Density increase (%)

Distaloy AB 50 5.7570.1660 6.2470.1270 6.9570.1180 7.4270.09

Distaloy AE 50 5.9270.1260 6.3870.1570 7.1670.1380 7.9470.11

Astaloy Mo 50 5.6270.1360 6.2270.1470 6.8570.1680 7.3470.12

Fig. 3. Photograph of the sintered steel foams.

N. Bekoz, E. Oktay / Materials Science & Engineering A 612 (2014) 387–397390

that the shrinkages were a function of the spacer volume fraction.Esen and Bor [30] reported that the titanium foam compacts shrinkto some extend during sintering stage due to the combined effect ofconnection of pores and sintering shrinkage in the walls, and alsonoted that the shrinkage was not significant at lower spacercontents. The results in the present investigation showed that theshrinkage of the Mo foams at similar porosity content is lower thanthat of the other foams. The AE foams had a higher volumetricshrinkage since they contained higher percentages of alloyingelements, Cu and Ni. A lower average particle size possessing ofthe Distaloy AE powder may be the reason for the increase involumetric shrinkage. However, a significant effect of particle sizeon the volumetric shrinkage was not detected since the particlesizes of the powders are very close to each other. It was reportedthat the shrinkage of the high dense Distaloy AE and Distaloy ABspecimens produced by powder metallurgy technique increaseddue to the copper and nickel content [31,32].

3.4. Macro and microstructure of the steel foams

SEM images obtained from the surface of sintered AB, AE andMo foams are provided in Fig. 5. Pore walls separating each porefrom its neighbors can clearly be seen. The pores were distributedreasonably uniform in the foam structure, and the morphology ofthe final pores was similar to that of the spacer particles for all thespecimens. This suggests that the pore structures can be designedby using proper size, shape and content of the spacer. No crackswere observed in the porous structures. Fig. 6(a) shows themorphology of the AB foam having 61.5% porosity. Microporesformed in the pore walls and pore walls consisting of partiallysintered powders are also shown in Fig. 6(b) and (c), respectively.A closer appearance of pore walls can be observed in Fig. 7, which

shows micropores clearly formed in the cell walls of sinteredfoams having 50% and 80% expected porosity levels due toincomplete sintering of the steel powders. Although the macro-pores in foams appear to be isolated from each other, they are infact connected owing to the micropores in the cell walls. Theinterconnectivity of the pores increased with porosity content. It isexpected that the micropores in the cell walls may have aconsiderable effect on the mechanical behavior of sintered foams.

It is well known that phases formed in the microstructure of adense metal may also play an important role on the mechanicalproperties. Markaki and Clyne [13] stated that an important pointconcerning the mechanical behavior of metallic foams is that thesignificance of the microstructure of the cell wall material hasbeen largely ignored in most studies. Pang et al. [33] produced thenickel foams with different alloy compositions, and found that themechanical properties of Ni–Cr–Fe alloy foams are stronglydependent of the foam composition. Markaki and Clyne [13], andPark and Nutt [14] compared the metal foams produced usingdifferent methods, and found the mechanical properties aresignificantly affected from changes in microstructure of the cellwalls caused by production methods. Fig. 8 demonstrates opticmicroscope images taken from cell walls of the sintered AB, AE andMo foams. The sintered foam specimens consisted of ferrite,pearlite, austenite and a small amount of bainite phases butmartensite phase was not observed in the microstructure due tothe slow cooling step in the furnace. The microstructural images ofAB and AE foams showed that copper was precipitated in the grainboundaries as a result of sintering. Copper in the alloy hasfavorable effect on cell structure due to liquid phase formationduring sintering process. Darker regions in the micrographspresent the bainite and pearlite phases while brown regions areaustenite and black regions are the micropores between the steelparticles. Distaloy alloys containing Ni had nickel-rich areas in thegrain boundaries due to the heterogeneous distribution of nickel,especially in AE foams. It was reported that the microstructure ofdense Distaloy specimens consisted of pearlite, martensite andnickel-rich phases by several researchers [25,31,32,34].

3.5. Distribution of macropores in the steel foams

Image analyzer software was used to determine the pore size(spherical diameter) and shape (sphericity) from optical imagestaken from polished cross sections of the sintered foams. Fig. 9(a)and (b) shows the morphology of the pores at high magnificationand the image analyzer results of AE foam having 53.8% porosity,respectively. Mean spherical diameter and mean sphericity of themacropores are computed and given in Table 4. The mean valuesof sphericity were between 0.56 and 0.61 for pores of AB foams,between 0.56 and 0.62 for pore of AE foams and between 0.55 and0.60 for pores of Mo foam. Mean pore sizes were between

Table 3Density, total, open and closed porosities of the sintered foams.

Specimens Carbamide fraction (vol%) Density (g cm�3) Total porosity (%) Open porosity (%) Closed porosity (%)

AB foams 50 4.0270.10 48.571.5 32.7 15.860 3.5570.07 54.671.3 42.2 12.470 3.0170.09 61.571.2 50.2 11.380 2.3070.08 70.571.8 62.9 7.6

AE foams 50 4.0570.11 48.271.3 32.1 16.160 3.6170.08 53.871.2 42.1 11.770 3.0370.06 61.271.4 50.4 10.880 2.3370.08 70.271.7 62.1 8.1

Mo foams 50 4.1170.09 47.471.2 32.9 14.560 3.4570.07 55.871.6 43.7 12.170 2.9470.06 62.471.6 52.9 9.580 2.2570.08 71.271.8 65.7 5.5

3

4

5

6

7

8

45 50 55 60 65 70 75Porosity (%)

Vol

umet

ric

shri

nkag

e (%

)

AE foamsAB foamsMo foams

Fig. 4. Volumetric shrinkage of the sintered steel foams having different porositiesas a function of the base material.

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518.5 μm and 828.5 μm for AB foam' pores, between 514.8 μm and814.3 μm for AE foam' pores and between 526.8 μm and 866.2 μmfor Astaloy foam' pores. When the total porosity content of thefoams increased, the mean macropore size increased but meansphericity values decreased due to interconnection of macroporesat high porosity levels. Mean sphericity and mean size of carba-mide particles were initially determined to be 0.66 and 812.6 μm,respectively. The decrease in size and sphericity of the spacer wasattributed to crushing during pressing and moistening prior to themixing. The present results demonstrated that the mean sphericaldiameter and mean sphericity of the macropores changed as afunction of the spacer volume fraction for all powder groups.

However, the use of different steel powders with compositions of1.75 Ni–1.5 Cu–0.5 Mo–0.2 C, 4.0 Ni–1.5 Cu–0.5 Mo–0.2 C and1.5 Mo–0.2 C has only a minor effect on the distribution ofmacropores in the steel foams.

3.6. Distribution of micropores in the pore walls

Quantitative metallographic studies have shown that in allfoams the micropores with non-spherical shape had average sizein the range of 125.6–144.2 μm for AB, 109.6–131.3 μm for AE and142.6–167.8 μm for Mo foams. Relationship between the meanmicropore size and total porosity content of the steel foams isshown in Fig. 10. The mean micropore size values decreased as thetotal porosity content of the foams increased, and this wasattributed to the positive effect of carbamide content on densifica-tion during compaction of steel–carbamide mixture. Mean micro-pore size of the foams in the similar porosity level was alsoinfluenced from the use of steel powders with different Cu–Ni–Mocontents. Mean micropore size of Mo foams is higher than that ofthe other two group foams. The presence of copper in the chemicalcomposition of Distaloy powders reduces micropores in the cellwalls, i.e. micropore size. Also, the lowest mean micropore sizewas determined for AE foams in all groups due to the higher nickelcontent which causes volumetric shrinkage in the foam structure.

Fig. 11 shows the relationship between total, macro andmicroporosity content of AE foams as a function of carbamidevolume fraction. Micropore content decreased as a result ofincreasing amount of spacer. It is reasonable to expect that lowcompaction efficiency leads to an increase in remaining porositybetween the steel powders mostly in the cell walls duringcompaction. The foams having low green densities experiencehigher volumetric shrinkages (i.e. higher densifications) duringsintering and thus contacts between steel particles are facilitated.Macropores are connected to each other through the micropores.Bafti and Habibolahzadeh [35] also reported that low compactionefficiencies obtained when low carbamide volume fractions usedin the mixes and slightly higher microporosity formed in cell wallsas a result of sintering. Tao et al. [36] pointed out that mechanicalproperties of the porous metal depend on the integrity of its porewalls and the volume fraction ratio between the metal and thespacer.

3.7. Compressive properties of the steel foams

Fig. 12 shows the compressive stress–strain curves of the AB, AEand Mo foams having different porosities. The results show thatthe mechanical properties of all the foams depend on both theporosity and the base metal. Compressive yield strength valuesdecreased and length of the plateau region increased with increas-ing porosity content. Long plateau regions are indication ofopen cellular morphology. The compressive yield strength valuesof the AB, AE and Mo foams were 24–109 MPa, 26–116 MPa and22–92 MPa, respectively. The resultant Young's moduli of thementioned steel foams were 0.44–3.08 GPa, 0.48–3.18 GPa and0.42–2.98 GPa, respectively and also decreased with increasingporosity. The interconnections of macropores in the foam structureincreased with increasing porosity, and the structure tends to beweaker under compressive load. On the other hand, the resultsshowed that the foams having higher total porosity contentpossessed lower micropore content, as a result of enhanceddensification of the pore walls during sintering. However, wallthicknesses of the foams decrease as a consequence of increasingtotal porosity content which in turn weakens the foam's mechan-ical behavior. Yi et al. [37] reported that mechanical properties ofmetal foams depend on cell wall material and structure of the cells(open or closed porosity). Ashby et al. [28] found that properties of

Fig. 5. SEM images of (a) AB foam having 70.5% porosity, (b) AE foam having 70.2%porosity and (c) Mo foam having 71.2% porosity.

N. Bekoz, E. Oktay / Materials Science & Engineering A 612 (2014) 387–397392

foam materials depend upon the properties of the base metal andthe relative density. Parvanian et al. [38] reported that the strengthof the metal foams tend to increase with increasing pore wallthickness. A few researchers [39,40] concluded that high porositycontent causes inhomogeneous pore distributions in metal foams,thus the strength becomes lower. Mátyás et al. [26] reported thatthe mechanical properties of sintered steels are dependent on themicrostructure determined by matrix composition. The compres-sive strength values of the AB and AE foams is higher than that ofthe Mo foams in the similar porosity levels. This can be explainedby the presence of copper as an alloying element in the chemical

composition of Distaloy powders. The copper in the alloy reducesmicropores in the cell walls. This has positive effect on thecompressive strength due to the liquid phase absence duringsintering. A similar improvement has been reported by the useof boron as an additive for liquid phase sintering of the stainlesssteel foams [9], magnesium and tin as an additive for liquid phasesintering of aluminum foams [35]. Another explanation for thisincrease is the differences in the production technique of thepowders, i.e. Disyaloy powders are diffusion bonded and Astaloypowder is water atomized. Also, average particle size of DistaloyAE powders lower than that of the other two powders, and AE

Fig. 6. SEM micrographs of the AB foam having 61.5% porosity at different magnifications: (a) structure of the AB foam, (b) micropores formed in pore walls, and (c) porewalls consisting of partially sintered powders.

Fig. 7. Optical microscope images of micropores formed in the pore walls of (a) AB foam, (b) AE foam and (c) Mo foam having theoretical porosity of 50 vol%; and (d) ABfoam, (e) AE foam and (f) Mo foam having theoretical porosity of 80 vol%.

N. Bekoz, E. Oktay / Materials Science & Engineering A 612 (2014) 387–397 393

foams have higher nickel content compared to AB foams, andhigher copper and nickel contents compared to Mo foams. Gethinget al. [34] reported that the mechanical properties improved withincreasing nickel content as a result of increased levels of hardconstitutes within the microstructure. As a result of increasingalloying element content, the amount of pearlite and bainitephases increased in the cell wall structure of the foams, thusenhanced mechanical properties were obtained. Also, microstruc-ture of the foams did not significantly affect the length of theplateau region. Increase in the compressive yield stress values ofthe AE foams compared to the Mo foams at the similar porositylevels was about 26%, 17%, 11% and 18%. The impact of the alloyingelements on the compressive yield stress of foams decreased

as the total porosity of the specimens increased. The mechanicalproperties of the foams are more sensitive to the cell wall thicknessrather than the alloying element effect. Non-homogeneous poredistribution in the highly porous foams may cause scatter in theresults.

Gibson and Ashby [4] recommended a model to explain themechanical properties of materials having porous structure thatthe model undertakes pore walls to be a solid material. It finds thatthe contribution of cell face stretching to the whole stiffness of thefoam and strength depends linearly on relative density (ρ/ρs). All ofthe structural properties are reputed to be ideal in their scalingrelations. Fixed density exponents of 1.5 and 0.5, respectively arepreassumed in empirical Eq. (1) suggested by Gibson and Ashby[4]:

σy ¼ σysCρ3=2rel ð1þρrel

1=2Þ ð1Þ

where σys refers to the bulk material's yield strength, σy to thefoam's compressive yield strength, and ρrel to relative density. 0.23is the value of the C constant for various bulk materials of themodel. The compressive yield strengths of the foams obtainedduring this investigation were compared with the predictions ofthe model in Fig. 13. The density and the compressive yieldstrength of the bulk low alloy steel are 7.81 g cm�3 and675 MPa, respectively [23].

It can be seen that the relative stress depends on the relativedensity. In this porosity range, compressive yield stress values ofall the foams were lower than the predictions of the model,especially at low relative densities. Pore walls are supposed to besolid metal in the model. In the present investigation, the opticalmicroscope studies showed micropores formed in the pore.Besides, weak and anisotropic pore structure, non-uniform poredistribution and large pores in cell walls significantly affect themechanical properties of porous structure.

3.8. Energy absorption capability of the steel foams

The energy absorption capacity is an important property of themetal foams. Park and Nutt [6] reported the energy absorptionbehavior of Fe–2.5% C steel foams synthesized by a powdermetallurgical route, and found that the energy absorption capacityimproved with a decrease in porosity content. Mutlu and Oktay[10] studied the mechanical behavior and energy absorptionproperties of high alloy steel foams produced by powder metal-lurgy technique, and found that the energy absorption capacity ofthe foams increased with decreasing porosity. Yu et al. [41] studiedenergy absorption capacity of aluminum foams and reported thatit increased with an increase in density of the foams. Michailidiset al. [42] investigated the deformation and energy absorbingcharacteristics and mechanisms of the aluminum foams producedby powder metallurgy, and found that substantial improvementsin energy absorption behavior are possible by reducing the relativedensity of the foam. When these studies are evaluated, it isunderstood that steel foams show higher energy absorptionbecause of higher compressive yield stress of the steel.

The most of the absorbed energy is irreversibly converted intoplastic deformation energy. This occurs mainly in the extensiveplateau stress and gives a high energy absorption capacity. Energyis absorbed extremely by the bending and collapse of cell walls inthe foam during the plateau stress. When this mechanism isended, the stress starts to rise, and the foam acts as dense material[6]. Solid parts do not perform well in this respect. Generally,substantial increase in energy absorption is observed as the foamporosity level is varied from low to high. The energy absorptioncapacity is described as the energy required deforming a givenspecimen to specific strains, and is used to compare the foams

Fig. 8. Microstructure images from the cell walls of (a) AB foam, (b) AE foam and(c) Mo foam having theoretical porosity of 60 vol%.

N. Bekoz, E. Oktay / Materials Science & Engineering A 612 (2014) 387–397394

with different porosities. Eq. (2), integration of the area under thestress–strain curve, is used to evaluate energy absorption capacityof the foams [10].

W ¼Z εd

0σðεÞdε ð2Þ

where W is the energy absorption capacity, ε is the compressivestrain, εd is the densification strain, and σ is the compressive stress.

The energy absorption of metal foams has recommended energyabsorption diagrams for the selection of appropriate foams forspecific applications [6]. The compression behavior of Mo foamswith different porosities is shown in Fig. 14(a). The area under theplateau region of the curves is useful energy, W, or energy per unitvolume, W1, W2, W3 and W4 which can be absorbed in thespecified porosity range. The foams with lowest porosity exhibitthe highest stress in the course of energy absorption. Ashby et al.[28] reported that the energy absorption efficiency is described asthe ratio of the deformation energy absorbed by a real material tothe deformation energy absorbed by an ideal energy absorber. InFig. 14(b), the area under the stress–strain curve represents thereal amount of energy. The area of an ideal absorber is defined bythe maximum stress and strain values and shows a rectangularstress–strain curve.

Fig. 15 shows the energy absorption of the steel foams withdifferent porosities calculated according to Eq. (2), from which itcan be seen that the energy absorption capacities of all steel foamsdecreased with increasing porosity content which is in agreementwith the previous studies. In addition, present results show thatthe energy absorption is dependent on the base material. Thisdependency is more pronounced at the lower porosity range. Theenergy absorption capacity values of the AE foams is higher thanthat of the other two foams because the AE foams possess highercompressive yield strength values. Although length of the plateauregion increased with increasing porosity content, the energyabsorption capacity decreased as a result of low compressive yieldstrengths.

Fig. 9. (a) Optical microscope image of the AE foam and (b) image analyzer illustration of macromorphology.

Table 4Mean spherical diameter and mean sphericity of the macropores.

Specimens Fraction ofcarbamide (vol%)

Spherical diameter(μm)

Sphericity

Mean Standarddeviation

Mean Standarddeviation

AB foams 50 518.5 158.6 0.61 0.21160 568.7 164.3 0.59 0.19870 696.4 167.3 0.58 0.21780 828.5 171.9 0.56 0.219

AE foams 50 514.8 165.5 0.62 0.16760 541.5 156.3 0.59 0.17870 682.7 162.4 0.58 0.19280 814.3 154.5 0.56 0.205

Mo foams 50 526.8 147.8 0.60 0.18860 578.6 155.3 0.58 0.21370 719.5 167.8 0.56 0.20580 866.2 168.3 0.55 0.223

100

120

140

160

180

45 50 55 60 65 70 75

Porosity (%)

Mea

n m

icro

pore

siz

e (μ

m) Mo foams

AB foamsAE foams

Fig. 10. The change of the mean micropore size with varied porosities as a functionof the base material.

0

20

40

60

80

40 50 60 70 80 90 100Fraction of carbamide (%)

Poro

sity

(%)

Total porosityMacro porosityMicro porosity

Fig. 11. The change of total, macro and microporosity contents of AE foams withthe amount of spacer added.

N. Bekoz, E. Oktay / Materials Science & Engineering A 612 (2014) 387–397 395

4. Conclusions

The role of pore wall microstructure and micropores formed inthe pore wall on the properties of different steel foams investi-gated. The following conclusions can be drawn:

� Increasing carbamide content in the steel–carbamide mixturesenhances the densification during compaction. The dimen-sional changes after sintering are related to both powdercharacteristic and porosity content of compacts.

� Two types of pores, macropore and micropore, were typicallyobserved in the foams produced using powder metallurgytechnique. The deformation mechanism depends on both theporosity levels and structure of the foam (micro and macro),which is controlled by the process parameters employed.

� The mean size and amount of macropore increased withincreasing spacer content, but the mean size and amount ofmicropores in the pore walls decreased. In addition, thepresence of copper as an alloying element in the chemicalcomposition of Distaloy alloys leads to liquid phase sinteringand decreased content and size of micropore in the pore walls.

� The resultant mechanical properties of the foams are sensitiveto the cell wall structure forming main framework of the foamsas well as the macropore structure. Ferrite, pearlite, austeniteand a small amount of bainite phases formed varying propor-tions in the cell wall structure and had a determining role onthe compressive yield stress of the foam specimens.

� The compressive strength of the AE foams with 4.0 Ni–1.5 Cu–0.5 Mo–0.2 C and AB foams with 1.75 Ni–1.5 Cu–0.5 Mo–0.2 C is

Fig. 12. Compressive stress–strain curves of the foams having different porosities:(a) AB foams, (b) AE foams and (c) Mo foams.

0.00

0.04

0.08

0.12

0.16

0.20

0.24

0.25 0.35 0.45 0.55Relative density (ρ/ρ s )

Rel

ativ

e st

ress

(σy/σ

ys)

AE foamsAB foamsMo foams

Gibson-Ashby Model

Fig. 13. Relationship between relative density and relative stress of the steel foams.

Fig. 14. (a) Compression behavior of Mo foams with different porosities showingthe absorbed energy, W. (b) Comparison of ideal and real energy absorbers of Mofoams with 47.4% porosity.

N. Bekoz, E. Oktay / Materials Science & Engineering A 612 (2014) 387–397396

higher than that of the Mo foams with 1.5 Mo–0.2 C in thesimilar porosity levels. The reason for this is the difference inchemical composition and the production technique of pow-ders. AE foams had higher strength values than the other foamsas a result of formation of nickel-rich phases in the cell-wallmicrostructure.

� Energy absorption capacity is clearly depend on the length ofthe plateau region during compression, which is controlled bythe relative density, pore topology, size distribution of pores,and the cellular morphology.

� As a general conclusion, it is clear that the properties of porewall e.g. pore wall microstructure and micropores formed inthe pore wall can have a significant effect on the mechanicalbehavior of the steel foams.

Acknowledgment

This work was supported by Scientific Research Projects Coor-dination Unit of Istanbul University, Project number BYP–35272.

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0

50

100

150

200

250

300

45 50 55 60 65 70 75Porosity (%)

Ene

rgy

abso

rptio

n (J

/cm

3 ) AE foamsAB foamsMo foams

Fig. 15. Energy absorption capabilities of the steel foams with varied porosities.

N. Bekoz, E. Oktay / Materials Science & Engineering A 612 (2014) 387–397 397