Materials Science and Engineering A 452–453 (2007) 682–687
Effect of temperature on the tensile properties of an as-castaluminum alloy A319
E. Rincon a, H.F. Lopez b, M.M. Cisneros a,∗, H. Mancha c, M.A. Cisneros a
a Department of Metal-Mecanica, Instituto Tecnologico de Saltillo, Blvd. V. Carranza 2400, 25280 Saltillo, Coahuila, Mexicob Materials Department, University of Wisconsin-Milwaukee, Milwaukee, WI 53201 USA
c Cinvestav-Saltillo, Carr. Saltillo-Mty. Km. 13, Apdo. Postal 663, Saltillo, Coahuila, Mexico
Received 7 September 2006; accepted 3 November 2006
bstract
The tensile properties of an as-cast A319 alloy were investigated as a function of temperature. It was found that the A319-Al alloy remainednherently brittle in the temperature range of −90 ◦C < T < 270 ◦C and the mechanical integrity was not satisfied as defined by the Considereriterion. Apparently, in this temperature range fracturing of brittle intermetallics, including Si particles is dominant. At T > 270 ◦C the mode of
ailure shifts to being essentially ductile by the development of numerous dimples. Under these conditions the development of critical stressest matrix/particle interfaces needed for brittle fracture no longer occurs. Apparently, at these temperatures thermally activated processes lead toignificant relaxation of stress incompatibilities at particle/matrix interfaces and results in appreciable plastic deformation within the matrix. 2006 Elsevier B.V. All rights reserved.
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eywords: A319 aluminum alloys; Tensile properties; Intermetallics; Brittle be
. Introduction
Automotive cast Al-A319 alloys have been increasingly usedn the manufacture of engine blocks due to a combination of gooduidity properties and mechanical strength [1]. The microstruc-
ural constituents present in this alloy are typically complexultiphases comprising eutectic (acicular) Si, as well as numer-
us intermetallic phases. Since engine blocks operate over a wideange of temperatures and stress conditions, alloying elementsuch as Cu and Mg are often added to improve the room and highemperature strength of these alloys [1,2]. Although Cu and Mgignificantly improve the strength of Al-A319 alloys, both, inhe as-cast condition and after heat treating [1–3], the ductility israstically impaired. Al-A319 alloys typically exhibit ductilityn the range of 0–3%. Apparently, the development of inter-
etallic phases including �-(Al2Cu), Mg2Si, �-(Al8Mg3FeSi6),-(Al15(Mn,Fe)3Si2) and �-(Al5FeSi) promote alloy strength-
ning at expenses of ductility. In this sense, iron impurities arehe most detrimental as they lead to the development of relativelyarge �-(Al8Mg3FeSi6), �-(Al5FeSi) and �-Al15(Fe,Mn)3Si2
rittle intermetallics, with � and � being the most harmful phases3,4] in terms of mechanical integrity.
The tensile properties of A319-Al alloys are also affected byhe increasing tendency to develop porosity partly as a resultf Cu [2,3,5] and Sr additions [6]. However, in practice theevel of porosity can be kept down to a minimum by controllinghe alloy chemistry and by reducing the dendrite arm spacing,
through fast cooling [6]. In current casting processing theominant microstructural features responsible for the exhibitedensile properties have been linked to the exhibited volume frac-ion, morphology and size of the intermetallic phases, includinghe Si precipitates [4,6,7–10].
The limited ductility exhibited by these alloys has beenelated to the development of stress incompatibilities at the inter-ace between elastically strained brittle particles and a plasticallyeformed matrix. In turn, these stress incompatibilities promotearticle cracking when a critical stress condition is reached4,11,12]. Accordingly, the overall alloy strain hardening istrongly influenced by particle cracking. As a given inter-etallic precipitate fractures, the surrounding matrix undergoes
tress relaxation resulting in a transfer of load to neighboringtressed particles. As a result, further work hardening in theelaxed matrix leads to cracking of neighboring particles. Conse-uently, successive particle cracking events take place resulting
E. Rincon et al. / Materials Science and Engineering A 452–453 (2007) 682–687 683
Table 1Chemical composition of investigated A319-Al Alloy (wt%)
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In addition, the plastic flow curves (see Fig. 1) are accom-panied by the development of serrations, typical of thePortenvin–Le Chatelier (PLC) effect [13,14], particularly at tem-peratures above 270 ◦C. It is well known that the PLC effect in
n appreciable accumulated internal damage and eventual alloyracture.
Although, the Al-A319 alloy seems to be inherently brittle,he effect of temperature on the exhibited strength and ductil-ty is not known. Most published works on these alloys haveeen focused on the room temperature tensile properties. Yet,he effect of temperature on the mechanical properties has noteen considered, even though this can be a critical design fac-or in alloys exposed to relatively high temperatures such as inylinder heads. In particular, the alloy tensile properties includ-ng work hardening are expected to be significantly influencedy temperature. Hence, this work further explores the tensileesponse of Al–Si–Cu alloys by considering the effect of tem-erature on the exhibited tensile strength and ductility of ans-cast Al-A319 alloy tested at temperatures between −90 and00 ◦C.
. Experimental
The chemical composition of the as-cast alloy investigatedn this work is given in Table 1. The alloy was supplied in theorm of sectioned chilled blocks. In this alloy, Si modificationnd grain refinement were achieved by employing an Al–10%r master alloy, and a commercial Ti–B (5% Ti–1% B) alloy.
From the chilled blocks, tensile specimens were machinedccording to the ASTM standards E21-92 (1998) and B557-02.ensile testing was carried out on an MTS 810 machine at a strainate of 10−4 s−1. The tensile testing machine was instrumentedith an ambient chamber to maintain the testing temperaturesithin ±2 ◦C. Tensile testing was carried out at −90, −60, −30,, 25, 150, 180, 240, 270, 320, 370, and 400 ◦C.
Four samples were tested at each test temperature in ordero obtain reliable tensile results. After tensile testing to frac-ure, specimens were sectioned parallel to the tensile direction,
ounted, and examined by SEM using a Philips Fei-Quantaicroscope operating at 20 kV and equipped with an EDX detec-
or. The fracture surfaces were also examined under the SEM inrder to establish the dominant mode of failure. Moreover, sam-les for transmission electron microscopy (TEM) were sectionedarallel to the applied stress axis and thinned using an electrolyteonsisting of 30 vol.% acetic acid, 20 vol.% orthophosphoriccid, 40 vol.% H2O and 10 vol.% nitric acid. In addition, ionilling was applied when needed. A Philips CM200 TEM oper-
ting at 200 kV was used for thin foil observations.
. Results and discussion
.1. Strength and ductility
In this work, the effect of porosity was not considered, evenhough in some instances it can explain the scattering in the F
Fig. 1. Tensile stress–strain curves at various temperatures for as-cast A319.
easured tensile properties. This was based on the fact that theensile bars were taken from chilled blocks where the exhib-ted λ was relatively small (<25 �m) keeping porosity downo a minimum. Fig. 1 shows the stress–strain curves exhibitedy the as-cast A319-Al alloy at temperatures below and aboveoom temperature. Notice that the yield strength and the strain-ardening behavior (given by the slope of the flow curves), bothecrease with increasing temperatures. However, no appreciablehanges in the plastic flow properties are observed in this alloyetween −90 and 150 ◦C.
Fig. 2 shows the yield and tensile strength, as well as exhibiteductility for this alloy as a function of temperature. Notice thathere is a significant drop in the alloy strength at temperaturesbove 200 ◦C. However, the alloy ductility is not significantlynfluenced at temperatures below 270 ◦C. Yet, the UTS and yieldtrength are slightly improved between 25 and 180 ◦C. Furtherncreases in temperature, lead to alloy elongations of up to 43%hile both, the UTS and the yield strength continuously drop toalues below 50 MPa.
ig. 2. Variation of tensile properties with temperature of as-cast A319-Al alloy.
6 and Engineering A 452–453 (2007) 682–687
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Fig. 4. SEM micrographs of a region adjacent to the fracture surface oft
84 E. Rincon et al. / Materials Science
l alloys can limit or improve the alloy ductility. In the 5000eries of Al–Mg alloys the PLC effect leads to increasing workardening, and enhanced ductility [15]. However, it also leads toreduction in the strain rate sensitivity, which in turn increaseseck growth rates, hence decreasing ductility.
.2. Work hardening
The work hardening properties of these types of alloys haveeen considered using the Voce Equation [16]. Moreover, theondition for tensile plastic instability resulting in necking cane described by the Considere criterion [17]:
dσ
dε= σ (1)
Rearranging terms in Eq. (1) the condition for necking cane described by dσ/σ = dε. Accordingly, in alloys which exhibitower law behavior, the Considere criterion implies that neckingtarts at a critical plastic strain ε* = n, where n is the work hard-ning exponent. Hence, a sudden drop in work hardening raterior to reaching the Considere criterion is indicative of intrinsictructural defects (major discontinuities present in the casting),hich cause early specimen fracture. From the above expression,
lloys that do not reach the onset of necking (tensile plastic insta-ility) given by the Considere criterion (Eq. (1)) possess majortructural discontinuities. In Al-A319 alloys, the Considere cri-erion is not satisfied as cracking of brittle intermetallic particlesincluding Si) always result in significant damage and early frac-ure at low ductility levels (0–3%). The effect of temperature onhe plastic flow behavior for the Al-A319 alloy indicates thathe Considere criterion was not satisfied in this alloy for theemperatures −90 ◦C < T < 270 ◦C.
Fig. 3 shows the work hardening exponent, n determined fromhe power law expression, σ = Kεn for the various temperatures
onsidered in this work. Notice from this figure, that n dropsonotonically with temperature, and it is not until the alloy is
ested at temperatures of or above 320 ◦C that the n exponentatisfies the ε* = n condition indicative of ductile behavior.
ig. 3. Work hardening exponent, n as a function of temperature for an as-castl-A319 alloy.
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ensile specimens showing cracking of (a) �-Al2Cu intermetallics and (b) �-Al15(Mn,Fe)3Si2) intermetallics. The specimens were tested at 180 ◦C.
.3. Particle cracking
Fig. 4(a and b) shows the fracture of brittle intermetallic pre-ipitates (�-Al2Cu, �-Al15(Fe,Mn)3Si2) in a region adjacent tohe fracture surface of an Al-A319 alloy tested in tension at80 ◦C. It was found that fracture of brittle intermetallics was theominant feature in the −90 ◦C < T < 270 ◦C temperature testingange. Limited plasticity in the form of cavities next to bro-en intermetallics or Si particles was found to accompany theracture process (see Fig. 5(a and b)).
The Al-A319 cast alloy was not heat treated and hence, it wasot expected to exhibit work hardening within the matrix duringlastic straining. However, in the specimens tested at T < 270 ◦C,EM observations of the deformed Al-matrix indicated matrixtrain hardening as a result of dislocation interactions with sec-ndary precipitates in these regions prior to fracture as evidencedy Fig. 6(a). The secondary phases were identified as spheri-
al Si-based nano-sized precipitates. Nevertheless, significantislocation activity was only achieved above 270 ◦C with theevelopment of dislocation cell substructures (Fig. 6(b). Hence,
E. Rincon et al. / Materials Science and Engineering A 452–453 (2007) 682–687 685
Fig. 5. SEM fractographs showing (a) cracking of Si particles and incipientcavitation in the surrounding matrix, and (b) fractured �-(Al (Mn,Fe) Si )i1
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ntermetallics on the exhibited fracture surfaces. The specimens were tested at80 ◦C.
l A319 alloys can be considered inherently brittle at tempera-ures below 270 ◦C as the alloy never reaches the expected UTSs defined by the Considere criterion.
In contrast, at temperatures above 270 ◦C, the alloy exhibitedncreasing plasticity and the mode of fracture was essentiallyuctile. Fig. 7(a and b) shows the fracture surfaces exhibited byensile specimens tested at 320 and 400 ◦C. Notice the devel-pment of multiple dimples. In addition, under these conditionshe brittle intermetallic particles were found to shatter into mul-iple pieces as a result of the increasing stress build-ups at the
atrix/particle interfaces (see Fig. 7(b)).The conditions for cracking of Si and intermetallic particles
n Al–Si–Cu–Mg cast alloys have been widely investigated in theiterature [18–20]. Numerous models have been proposed basedn either continuum mechanics or dislocation theory to accountor particle cracking [18–20]. Among the proposed models Cac-
res et al. [18] considered Al–Si–Cu–Mg alloys to behave asarticulate metal matrix composites. Accordingly, in their modeltress incompatibilities are expected to develop between elasti-
s
σ
ig. 6. TEM micrographs showing the deformed matrix of Al-A319, as well ashe presence of spherical, Si-based precipitates of roughly 50 nm in diameter.he specimens were tested at (a) 180 ◦C and (b) 400 ◦C.
ally deformed particles and the plastically deformed Al-matrix.n turn, cracking of intermetallic particles occurs at low strainss plastic relaxation events are significantly delayed in Cu–Mgrecipitation hardened Al–Si alloys. This effect is further accen-uated by the dendritic structure where the Al-matrix is locallyhielded from plastic deformation [18]. Moreover, a critical vol-me fraction of cracked particles is assumed to be needed torigger total fracture.
In the work of Caceres et al. [18] a finite element analysis isnvoked [21] for calculations of the particle cracking stresses.ccordingly, from the finite element analysis the tensile stress
tress–strain behavior can be given by:
p = Kpεnp (2)
686 E. Rincon et al. / Materials Science and E
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ig. 7. SEM fractographs showing the development of numerous dimples typicalf plastic deformation in (a) a specimen tested at 320 ◦C and (b) at 400 ◦C. Noticehe relatively large amounts of fragmented precipitates within cavities.
here Kp and np are polynomial functions of the volume fractionf particles, and of the K and n values for the Al-matrix.
Calculations of critical stresses σ* for particle cracking haveeen made for Al–Si–Cu–Mg casting alloys [18]. Accordingly,n their work, the volume fraction of particles was assumed con-tant at 7% and the calculated σ* values were 2–3 times theield strength of the casting alloys. Moreover, reduced σ* val-es are always possible when cracking of relatively large sizedarticles is considered. The predictions of the model proposedy Caceres et al. [18] could not be corroborated in this work,s no experimental data were available to confirm their modelredictions.
The work of Caceres et al. [18] is able to account for theracture susceptibility of Al–Si casting alloys containing Cu–Mgdditions such as alloy A319 at low applied strains. In contrast,he temperature effect on the tensile properties of these alloysas not been considered in the literature. Fig. 2 shows that as
emperature increases from room temperature to around 270 ◦C,he alloy yield strength and UTS slightly increase or remainsonstant. Yet, the alloy ductility is relatively poor remaining inhe 2–5% range. Under these conditions, it is apparent that the
3
ngineering A 452–453 (2007) 682–687
onstraints that limit plastic relaxation in the matrix surroundinghe reinforcement particles are still dominant.
Assuming that the stress build-up at the particle/matrixnterfaces is due to dislocation pile-ups in deformation bands,hermally activated processes in combination with the actualtate of stresses should promote local dislocation annihilation,limbing and cross-slip. In turn this should be able to promotetress relaxation and progressively be more effective in reducinghe local stress incompatibilities developed at the matrix/particlenterfaces. It is well known [22], that in Al alloys cross-slip ofcrew dislocations is highly effective in by-passing obstacles,hus reducing the level of stress build-ups and leading to workoftening. In turn, this is expected to promote the developmentf cell substructures such as the ones observed in this worksee Fig. 6(b)). Apparently, at temperatures above 270 ◦C stresselaxation mechanisms in the Al-matrix start to become domi-ant, leading to the formation of a subgrain structure within thel-matrix.The experimental outcome of this work indicates that work
oftening mechanisms in the Al cast alloy A319 become increas-ngly effective at temperatures above 270 ◦C. This is manifestedy the development of increasing ductility levels of up to0% at 400 ◦C. Moreover, conditions for the Considere crite-ion become satisfied in specimens tested at or above 320 ◦Ci.e. in this case, the e* = n condition is met). The mode ofracture is essentially ductile and it is dominated by the devel-pment of numerous ductile dimples. Notice in particular thathe reinforcing intermetallic particles fracture into multipleracks (see Fig. 7(b)) in contrast with a single dominant cleav-ge crack observed at low temperatures (Fig. 5). Apparently,eck localization in the form of increasing plastic strainingmposes increasing stress constraints on the reinforcing parti-les. Hence, the critical stress condition for particle cracking ispparently satisfied at diverse interface locations during the finalracture process resulting in particle shattering as observed inig. 7(b).
. Conclusions
The tensile properties of an as-cast Al A319 were investigateds a function of temperature and the following outcome wasound:
. Alloy Al-A319 is inherently brittle as the alloy fractured priorto reaching the maximum defined by the Considere criterion.In particular, the ε* = n condition was not reached and alloybrittleness was found to be dominant in the temperature rangeof −90 ◦C < T < 270 ◦C.
. Microstructural observations of regions in the vicinity of thefracture surfaces, as well as on the fracture surfaces indicatedthat at temperatures below 270 ◦C the dominant mode of fail-ure was controlled by continuous cracking of intermetallicparticles including Si.
. At temperatures above 270 ◦C the mode of failure becomesductile and it manifests by typical dimple fracture. In thiscase, the Considere criterion is satisfied and the ε* = n con-dition is met.
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