Experimental Study of Fracture Behavior of Magnesium

alloy(Mg AZ31)

N.S. Prasada, K. Naveena, R. Narasimhana,∗, S. Suwasb

aDepartment of Mechanical Engineering, Indian Institute of Science,Bangalore 560012, India. Tel: +91-80-22932959

bDepartment of Materials Engineering, Indian Institute of Science,Bangalore 560012, India. Tel: +91-80-22933245

Abstract

Fracture behaviour of magnesium alloy Mg AZ31 is studied by conducting

Mode-I fracture test using compact tension(CT) specimens cut from a rolled

sheet having basal plane normals slightly off from normal direction(ND) of

the plate. There are two sets of specimens such that in one set loading di-

rection(LD) is parallel to rolling direction(LD ‖ RD specimen) and in the

other LD is parallel to transverse direction of the sheet(LD ‖ TD specimen).

High strain hardening in the load-displacement curves is an outcome of pro-

fuse tensile twinning. Fracture toughness values are found to be 41.62 MPa

m1/2 and 44.33 MPa m1/2 for RD and TD specimens respectively. Crack

propagates along the curved path due to shear lips near the free surface and

tunneling at the mid-thickness indicates that the thickness of the specimen

is in the transition zone. Fractographs shows ductile fracture at the mid-

thickness and shear type failure near shear lips. Using electron backscattered

diffraction(EBSD) data, development of tensile twinning at various load lev-

∗Corresponding author.Email address: narasi@mecheng.iisc.ernet.in (R. Narasimhan )

Preprint submitted to Materials Science and Engineering: A June 6, 2014

els before crack initiation is analyzed near the notch and at the far end on the

central line of the notch. Tensile twins(TTs) are observed near the notch root

named as anomalous twins that are formed as opposed to the compressive

strain along c-axis of basal textured grains. As load increases, it is observed

that tendency for plastic deformation by twining increases. Texture changes

near the extended crack tip both at mid-surface and free surface are also an-

alyzed using EBSD data. At the mid-surface, most of the grains retain the

basal orientation and having twins across the contiguous grains. Most of the

grains on the free surface which were having basal orientation are completely

reoriented to prismatic due to the growth of tensile twins which engulf the

entire grain. TEM images that are taken just below the fracture surface show

dislocations in the twinned region. It is found that during plastic deforma-

tion, formation of TTs along with dislocation slip play an important role in

dissipating energy that leads to high toughness.

Keywords: Magnesium alloy; MgAZ31; Fracture behavior; Tensile

twinning; Fracture toughness; EBSD; TEM

1. INTRODUCTION

Transportation industry always prefers to used light weight materials so

as to increase the fuel efficiency. Among all the alloys used for structural

applications, magnesium alloys are the lightest ones. They have good damp-

ing, corrosion resistance properties and are easily machinable. In spite of

these attractive properties, their formability at room temperature is limited

due to hexagonal crystal structure. However as cast magnesium alloys are

promising alternatives to wrought magnesium alloys, they have poor mechan-

2

ical properties like tensile strength, fracture toughness and fatigue strength

compared to wrought alloys. To use the wrought magnesium alloys as safe

structural materials with out any catastrophic failure, one should investigate

the ductility, tensile strength, fracture toughness of these alloys.

In magnesium, deformation along the basal plane is accommodated by

are basal, non-basal slip systems(prismatic and pyramidal 〈a〉). Deformation

along the c-axis is accomadated by pyramidal 〈a + c〉 slip system, tensile

and compression twin systems. The interaction between these different de-

formation mechanisms result in a significant anisotropy in the macroscopic

response for the textured polycrystalline Mg.

There has been extensive study on compression and tension behaviour

of Mg AZ31 alloy in the current literature by Knezevic et al. (2010), Lou

et al. (2007), Barnett (2007), Barnett (2004), Jiang et al. (2007), Koike et al.

(2008). Tensile twins(TTs) are formed when tensile stress is applied along

c-axis or compressive stress is applied perpendicular to c-axis. In most of

the experiments, samples cut from rolled sheets or extruded rods are loaded

either in compression or in tension such that c-axis is perpendicular to the

loading direction. From the stress-strain curves, tension-compression yield

asymmetry was observed and the difference in strain hardening behaviour

was attributed to the deformation twinning. Effect of temperature on ductil-

ity and strain hardening behaviour were also considered. It was found that

as temperature increases there is a transition from the twin to slip domi-

nated deformation. Decreasing the grain size has similar effect to increasing

temperature in that that twinning get suppressed and deformation is by slip

only.

3

In contrast to extensive study on compression and tension behaviour of

wrought Mg AZ31 alloy, there has not been much focus on fracture behaviour

of these alloys. There are some reports in the literature on fracture behaviour

of magnesium and are described as follows.

Fracture toughness of Aluminum alloys has been reported to be higher

than wrought magnesium alloys. Fracture behaviour of magnesium alloys

have been studied by Purazrang et al. (1991), Sasaki et al. (2003), Barba-

gallo et al. (2004), Somekawa et al. (2006), Somekawa et al. (2009), Somekawa

et al. (2010), and Somekawa et al. (2007). The effect of texture on fracture

toughness in a rolled Mg AZ31 alloy plate has been investigated by Somekawa

et al. (2006). The initial texture of rolled sheet was a basal texture with a

slight deviation of basal poles from ND and were distributed along the RD

in the pole figure. Fracture toughness tests were conducted according to the

ASTM-E399 using three point bend(TPB) specimens cut along RD, TD and

45o to RD from the rolled plate having a grain size of 65 µm. Dimple sort

of patterns were observed on the fracture surface and the fracture toughness

was found to be 17.6-20.7 MPa m1/2. It was concluded that the specimen

whose crack front was parallel to RD has less value of toughness as compared

to the other orientations, since dislocations can easily glide on basal plane.

Somekawa et al. (2005) also performed similar experiments using TPB spec-

imens cut from extruded bar having a grain size of 13.5 µm. The fracture

toughness was reported to be 15.9-22.0 MPa m1/2. Based on the fact that

non-basal planes have higher surface energy, it was found that the specimen

whose crack front is perpendicular to basal plane is having higher toughness

compared to the other orientations. Fracture toughness value of 21.5 MPa

4

m1/2 was reported by Somekawa et al. (2009) for an extruded bar with an

average coarse-grain size of 51.6 µm. Formation of deformation twins near

the crack tip was examined at different stages of loading corresponding to the

peak load and half of the peak load. Tensile twins(TTs) were formed near

the crack tip before crack initiation. At peak load, crack propagates along

the boundary between the TTs and the matrix without blunting resulted in

brittle fracture. Since TTs form easily at the crack tip before crack initia-

tion, delaying or suppressing the formation of TTs at the crack tip is one

way to increase the fracture toughness. It was found that as grain size de-

creases, tendency for twinning decreases as explained by Meyers et al. (2001).

Somekawa et al. (2005A) have found that fracture toughness increases from

12.7 MPa m1/2 to 17.8 MPa m1/2 by reducing the grain size from 55 µm to 1

µm in extruded pure magnesium. Fracture mechanism of fine-grained Mg-Zn

binary alloy has been reported by Somekawa et al. (2010). The grain-size

of extruded Mg-Zn alloy was 1-3 µm. Subgrained structures were observed

near crack tip instead of deformation twins which leads to higher fracture

toughness. Due to higher stresses around the voids, nano-order twins were

observed. Crack propagated along twins and connected to the voids. On the

fractured surface dimple pattern was observed which conforms the ductile

fracture opposed to brittle type in the case of coarse-grained MgAZ31 alloy.

Thus the plastic deformation in fine -grain alloy is dominated by dislocation

slip. In addition to grain refinement, Somekawa et al. (2007) has found that

fracture toughness can further increases by adding fine precipitates which ob-

struct the dislocation movements. Fracture toughness of Mg-Zn-Y alloy with

quasicrystalline phase was found to be 32.5 MPa m1/2. Due to strong inter-

5

face between matrix and quasicrystalline phase, void nucleation was delayed

resulted in high toughness. Many dimples were observed on the fracture sur-

face indicates material fails by ductile fracture. More recently, Kaushik et al.

(2014) has performed mode-I fracture tests on magnesium single crystals.

Dependence of fracture resistance on lattice orientation with respect to the

crack plane was studied. In one of the three point bend(TPB) experiments,

notch front is aligned along c-axis which simulates the basal texture in poly-

crystal rolled Mg AZ31 sheet in the present work. From the EBSD maps on

the free surface, it was observed that tensile twins(TTs) nucleate near the

loading edge and as load increases, twins were extending towards notch root

leaving small region around the notch root free of twins . But in the present

work on polycrystal Mg AZ31, TTs were observed near the notch root as

opposed to compressive strain causing dimple on the free surface. It was

concluded that plastic dissipation due to profuse twinning can enhance the

fracture toughness and crack propagates along the prominent twin-matrix

boundary and finally gets deflected at twin-twin intersections.

In this paper, mode-I fracture experiments have been conducted at room

temperature using compact tension(CT) specimens cut from a rolled Mg

AZ31 sheet having nearly basal texture. It was found that the fracture

toughness values are in the range of 41.62 MPa m1/2 - 44.33 MPa m1/2 which

are different from the previously reported values by Somekawa et al. (2006).

Crack propagates along the curved path due to shear lips near the free surface

and tunneling at the mid-thickness indicates that the thickness of the speci-

men is in the transition zone. Fractographs are analyzed at the mid-thickness

and near shear lips to reveal the the fracture mechanism. In order to un-

6

derstand the high strain hardening behaviour observed in load-displacement

curves, specimens are unloaded at different load levels before crack initiation

and microscopic analysis is conducted using optical metallographs, orienta-

tion imaging microscopy(OIM) maps, TEM bright field(BF) images. Tensile

twins(TTs) were observed near the notch root as well as along the central

line of the notch near free edge of the specimen. It is observed that as load

increases, the tendency for plastic deformation by twinning increases.

2. Experimental Procedure

2.1. Specimen preparation

In the present work, material used for the experiments is a hot rolled

magnesium alloy AZ31. The chemical composition of the alloy is presented

in Table 1. Specimens were cut from a rolled sheet having nearly basal texture

as shown in Fig. 2. Grain sizes are inhomogeneous and the mean grain size

was determined as 13.5 µm. From the pole figure Fig. 2(b), it can be seen

that many grains appear to have c-axis slightly off from ND of the sheet.

The Mode -I fracture experiments are conducted using compact tension (CT)

specimens as shown in Fig. 1. Two sets of pre-notched CT specimens are

tested with loading direction(LD) along transverse direction(TD) in one set,

referred to as LD ‖ TD specimen and along rolling direction(RD) of the sheet

in the other, referred to as LD ‖ RD specimen. The specimens are prepared

with dimensions as recommended in ASTM E813 test procedure. A notch

diameter of 90 µm is machined using Electrical Discharge Machining (EDM).

Referring to Fig. 1, the other dimensions of the CT specimen are a = 22 mm,

W = 54 mm, h = 60 mm and thickness B0 = 9.6 mm.

7

2.2. Test Procedure

The experiments are conducted at a constant displacement rate of 0.3

mm/min using Instron 8502 testing machine having load cell capacity of

250 kN. The crack mouth opening displacement (CMOD) gauge is fixed to

the specimen so that CMOD can be measure in addition to the load line

displacement (LLD) and the load P. Speckle pattern is put on one of the

the free surfaces of the specimen by spraying black paint on the background

white paint. Images of the the specimen are taken successively as load level

increases. Vic-2D software is used to get the 2D Lagrangian strain field con-

tours around the notch root using digital image correlation(DIC) technique.

Some of the specimens were loaded to initiate and propagate the crack. The

deformed specimens were cut near the extended crack tip using EDM for

fractographic studies. Texture changes near the extended crack both on

mid-plane and free surface are analyzed from EBSD scans for RD specimen.

To understand the evolution of microstructure with load level, TD specimens

are unloaded at two different load levels before crack initiation. Samples near

the locations A and B in Fig. 1 are cut using EDM for optical and EBSD

analysis.

2.3. Material characterization

In-order to get the high quality electron backscatter diffraction Kikuchi

patterns for EBSD analysis, samples are mechanically polished up to 3000

grit SIC polishing paper followed by cloth polishing using diamond past of 13

grit. Samples are electro-polished using the electrolyte containing 3:5 solution

of H3PO4 and ethanol to get better surface for indexing. EBSD scans are

8

performed with a step size of 0.8 µm in ESEM Quanta 200, FEI machine

and the patterns are indexed using EBSD(TSL) software.

For optical metallography, following the same sample preparation proce-

dure used for EBSD analysis, sample surface is etched for 5 sec in the etchant

solution. The etchant used for etching process is prepared by mixing 5 gm of

picric acid, 10 ml of acetic acid, 90 ml of ethanol and 10 ml of water(Xia et al.

(2009)). Etched samples are observed using Zeiss Axio Vert.A1 microscope

to get the optical metallographs.

For TEM analysis, slices of 300 µm thickness are cut just below the

fracture surface from the deformed LD ‖ RD specimen such that ’basal plane

is edge-on’ using EDM. Mechanical polishing is done in order to reduce the

thickness to 150 µm using 1500 grit paper and deionized water as a coolant.

Circular disc of 3 mm diameter were punched manually in the sheets using

a punching machine. Further mechanical polishing of discs is done using

2500 grit paper to reduce the thickness to 60 µm. Some selected samples are

electro-polished in a twin jet electropolishing machine using electrolyte which

has 99:1 solution of methanol and HClO4. During twin jet polishing, thin

region is obtained for electron transparent around the holes by adjusting

the flow rate of electrolyte. In order to remove the oxide layer formed on

the surface, further thinning of the sample is done by maintaining the the

temperature at −140◦C during ion milling process in Gatan Precision Ion

Polishing system(PIPS) at an angle of 2◦ degrees. Sample are examined

using Tecnai G2 T20 S-Twin TEM at an operating voltage of 200 kV.

9

3. Results and discussion

3.1. Load versus displacement curves

The load (P) versus load line displacement (∆) and load verses CMOD

curves are shown in Fig. 4(a) and in Fig. 4(b) for TD and RD specimens

respectively. It can be seen from these figures that there is not much dif-

ference between the curves pertaining to specimen loading direction either

parallel to RD or TD. There is a deviation from linearity at a load of 5 kN.

Material displays high strain hardening which is evident from the continued

strong increase in load with displacement beyond the 5 kN, which is an out-

come of profuse tensile twinning. During the experiments, the region around

the notch was carefully imaged using a digital camera mounted in front of

the testing machine. Crack initiation is observed on the free surface at the

peak load of 9.1 kN and CMOD of 1.4mm. But crack starts tunneling at

mid-thickness at a load of 8.7 kN and forming shear lips on the free surface

at peak load. Load drop beyond the peak load seems to be an indication of

stable crack growth.

It can be seen from the Fig. 6, Lagrangian strain(E22) contour(green

contour) obtained from 2D-DIC analysis is symmetric about the notch line

at a load of 8.7 kN for the LD ‖ TD specimen. But strain contours above

1% strain, seems to be assymetric due to blunting of notch tip. The radius

of plastic zone (rmaxp green contour) is 9.16 mm is an order of specimen

thickness(9.6 mm) which is an indication that the specimen thickness is in

transition zone. There is a dimple on the free surface near the notch tip due to

relaxation of thickness constraint. This could be the reason for shear lips on

free surface along with tunneled zone due to high stress triaxiality(Anderson

10

(1995)) at mid-surface as seen in the Fig. 7.

3.2. Energy release rate J versus load

The energy release rate J was computed from the load-displacement

curves presented in Fig. 4(a) following the procedure described in ASTM

E813 as explained in the appendix.

Jt = Je + Jp (1)

where Je and Jp are elastic and plastic components of Jt.

The energy release rate J determined by the above procedure is plotted

against load P for TD and RD specimens in Fig. 5 up to crack initiation stage.

Both the curves Je and Jt are coinciding up to the load of p = 5 kN. There

after the Jt curve is deviating from the Je curve which indicates considerable

plastic dissipation before crack initiation. At peak load the values of Jt are

35 N/mm and 40 N/mm for LD ‖ RD specimen and for LD ‖ TD specimen

respectively.. The equivalent KIC values are 41.62 MPa m1/2 and 44.33 MPa

m1/2 for RD and TD specimens respectively. But Somekawa et al. (2006)

has reported the values of KIC are 17.6 - 20.7 MPa m1/2 for a specimen with

fatigue pre-crack, cut from rolled Mg AZ31 alloy sheet having basal texture.

Kamat et al. (1991) have studied the effect of notch root radius on fracture

toughness. It was found that there is a critical notch root radius below which

fracture toughness is constant and is equal to the toughness of the fatigue

pre-cracked specimen. Thus if the notch root radius is below the critical

notch root radius, the obtained value of fracture toughness is considered as a

material property for a given material. In the present case, notch root radius

11

is 45 µm. Thus the high toughness may be due to notch root effect and shear

lip formation.

3.3. Crack path and fractography

It can be seen from the Fig. 7 in both LD ‖ TD and LD ‖ RD speci-

mens, crack propagates along the curved path from the notch root due to

the formation of shear lips. Crack starts tunneling at the mid-thickness first

and then appears on the free surface by creating the shear lips. There is flat

fracture region shown as Q-R in the mid-thickness and shear lips on the free

surfaces near the locations indicated as P, S. The size of shear lip(PQ) is

defined as the distance measured in the thickness direction from free surface

to the location where shear lip ends. Shear lip sizes for RD, TD specimens

are 3.2 mm and 3.15 mm respectively. It can be concluded from these obser-

vations that the specimen thickness is in the transition zone. SEM images of

the fracture surface in the flat region Q-R of the Fig. 7(b) and (c) are shown

in Fig. 8(c) and (a). Dimples indicate ductile fracture in the flat region of

the fracture surface (Somekawa et al. (2006), Gandhi et al. (1979)). There is

a combination of small and big voids in the range of 9-15 µm comparable to

the grain size of the present material. Tear ridges encompasses the set voids.

There are nucleating inclusions of size 1.5 µm can be seen in some voids.

Fractograph near the shear lip P of the Fig. 7(c) is shown in Fig. 8(b). Voids

are oriented along one direction indicates shear type of failure. Inclusions

can be seem in some of the voids. Hydrostatic stress is high at mid-thickness

which responsible for void initiation, growth and coalescence. As load level

increases, following the crack tunneling at mid-thickness, the material adja-

cent to the tunneled zone near the free surface carry more load and leads

12

to failure by shear along maximum shear stress planes. This leads to the

formation of shear lips near free surface (Venkert et al. (2001)).

It can be seen from the macroscopic load-displacement curves and J verses

load curves, there is considerable non-linear behaviour after 5 kN. In order to

answer the questions like why there is a the high strain hardening and con-

siderable plastic dissipation, It is important to conduct microscopic analysis.

Plastic deformation in Mg AZ31 is due to both slip and twinning. Out of

the four different types of slip systems, the only slip system that can accom-

modate plastic deformation along c-axis is the secondary pyramidal 〈a + c〉

slip system. Since the critical resolved shear stress(CRSS) is very high, it is

difficult to activate at room temperature. Therefore the only other mecha-

nism to accommodate the deformation along c-axis at room temperature is

by deformation twinning. Extension along the c-axis of the crystal is caused

by Tensile twins(TTs). TTs can contribute to strain hardening by Hall-Petch

effect, Basinski mechanism and texture hardening. In Hall-petch effect, the

effective slip length decreases due to the formation of TTs in the matrix re-

gion which can cause hardening. According to Basinski mechanism, glissile

dislocation changes to sessile within the twined regions as a result twinned

region is much harder than the matrix region(Salem et al. (2006)). Since

TTs can engulf the grain quickly, the whole grain can re-orient to harder

orientation and it leads to texture hardening(Knezevic et al. (2010)). To un-

derstand the role of tensile twining(TT), LD ‖ TD specimens are unloaded at

two different load levels before crack initiation, one at 8.1 kN and other 8.7

kN. Optical metallographs and EBSD scans are taken near both locations A

and B as shown in Fig. 1 for two load levels. The evolution of twining seems

13

to play an key role in dissipating the energy during plastic deformation which

is analyzed in the following section.

3.4. Development of tensile twinning

In order to observe the evolution of tensile twinning before crack ini-

tiation, LD ‖ TD specimens are unloaded at 8.1 kN and 8.7 kN and the

microstructure near locations A and B are compared.

At location B, it can be seen from the the optical micrograph Fig. 9

and the IPF Fig. 10(a) , most of the twins of {101̄2} are aligning along the

central line of the notch. Some of the grains are having parallel twins. As

load increases from 8.1 kN to 8.7 kN, compare to Fig. 10(a) most of the grains

in Fig. 13(a) have multiple twins and the twin area fraction obtained using

EBSD maps increases from 9% to 24.2%. This indicates that the tendency

for plastic deformation by twinning increases with load level.

Following the fracture study of single crystals by Kaushik et al. (2014),

twin growth can occur by either twin-twin coalescence of same twin variants

or by lengthening and widening of individual twins. In big grains(like E, one

below M) having multiple thick twins, most of the matrix region is already

reoriented by twin-twin coalescence. If the load is increased further, the

whole grain would have been completely reoriented by twinning which could

be the reason for grain reorientation near the extended crack tip as shown in

the Fig. 17(a). In some grains(top right of G) having multiple intersection of

twins, lengthening of one twin is obstructed by another twin variant which is

also observed by Kaushik et al. (2014). If there is no obstruction, the twins

are forming from one end to the other end of the grain(left of F).

EBSD scan near location A in front of notch root is shown in Fig. 10(b).

14

{101̄2} twins are observed near the notch root. Tensile twins are formed

as opposed to the compressive strain along c-axis of basal textured grains

near the notch root. This type of twins are also seen on the surface of the

tension specimen cut from the present rolled plate and loaded in rolling direc-

tion(RD). These twins are named as anomalous tensile twins by Koike et al.

(2008). It was found that these anomalous twins form in grains having large

deviation of c-axis from the ND of the rolled sheet and are surrounded by

grains having c-axis parallel to ND. Differences in strain at grain boundaries

causes the formation of anomalous twins in the grains having greater basal

slip and less prismatic slip compared to the surrounded grains having greater

prismatic slip activity.

It can be seen that there is a combination of big(like E, F) and small

grains( H, K) having c-axis slightly off from ND of the rolled plate. Length-

ening and widening of twin boundaries in big grains is more predominant

as compare to that in small gains. It was found from the literature, Meyers

et al. (2001) have found that decreasing the grain size reduces the tendency

for twinning. It may be due to the fact that big grains are having critical re-

solved shear stress(CRSS) less compared to that of small grains for the given

stress state. Twin boundaries can be seen across the set of contiguous small

grains. As load increases from 8.1 kN to 8.7 kN, compared to Fig. 10(b),

there are multiple parallel twins in some grains(like E, one on top of G, H)

as shown in Fig. 13(b) and are likely to under go twin-twin coalescence to

increase the twin width. Twin-Twin intersection are also observed in some

grains(like F).

At mid surface near location A, Compared to the Fig. 10(b), most of the

15

grains are smaller size in Fig. 11 and their orientation is not perfect basal. It

can be seen in some grains(like E, F), twins are forming across the contiguous

grains. As load level increases from 8.1 kN to 8.7 kN, twin boundaries are

forming circular shaped arcs across the contiguous grains(like E, F, G) can

seen in the Fig. 14. Since the matrix orientations of the grains are different,

the twinned region is reoriented differently along the lengthening direction

of twin in some grains(like E). Few grains(like Q, one below L) are having

multiple parallel twins and intersection of twins can be seen in grain indicated

as N.

The length of the ligament region(AB) in Fig. 1 is 33 mm. From the

optical metallographs, twins are observed on the free surface up 1 mm in

front of the notch at A and 11 mm from the free edge at B for the specimen

unloaded at p = 8.7 kN .

3.5. Texture change and twin patterns around extended crack

Near the extended crack, EBSD scans are performed both at mid-thickness

and on the free-surface. There are tensile twins in most of the grains near

the extended crack tip at mid-thickness as shown in the Fig. 15. Above the

extended crack, some of the grains are completely reoriented from basal to

prismatic. It can be seen from the enlarged view of the region(indicated with

the rectangle(KLMN)) in front of the crack tip, most of the grains are hav-

ing nearly basal orientation(indicated with unit cells) and are having twins

which are forming like circular arcs across the set of contiguous grains. Mis-

orientation angle across the line X-Y is shown in the Fig. 16. There is an 860

rotation of c-axis about 〈112̄0〉 in the twinned region.

Compared to the initial pole figure Fig. 3(b), due tensile twins in most of

16

the grains there is another texture component seen on the circumference of

the basal pole figure Fig. 16(b). Most of the basal poles are shifted towards

ND at an angle of 45◦.

As deformation progress, growth of tensile twins is such that they can

quickly engulf the entire grain as explained by Knezevic et al. (2010). Most of

the grains above and ahead of the extended crack tip which were having basal

orientation, are completely reoriented to prismatic on the free surface shown

in the Fig. 17. Above the crack tip, in the partially reoriented grains(like

E), there are tensile twins with in which the reoriented region is having the

prismatic orientation and the matrix region having basal orientation. Below

the crack tip, there are few grains which are reoriented to prismatic but most

of the grains have the basal orientation and having tensile twins. There is an

asymmetry in the formation of tensile twins about the crack tip which could

be attributed to the fact that the stress state is different above and below

the crack tip due to formation of shear lips.

Due to complete reorientation of large number of grains, most of the basal

poles are on the circumference of the basal pole figure Fig. 17(b). Thus there

is a complete texture change compared to the one in the initial pole figure

Fig. 2(b). Thus texture hardening by tensile twins could be the reason for

the plastic dissipation in this material.

4. Summary and Conclusions

In this work, mode I fracture experiments have been conducted at room

temperature using compact tension(CT) specimens cut from the rolled plate

such that in one set of specimens loading direction is parallel to rolling di-

17

rection(LD ‖ RD sample) and the other set is loaded along transverse di-

rection(LD ‖ TD sample). The main observations / conclusions from the

present work are summarized below.

• There is not much difference between the load displacements curves

pertaining to specimen loaded either parallel to RD or TD. The fracture

toughness values of the present specimens are in the range of 41.62 MPa

m1/2 - 44.33 MPa m1/2 which are comparable to the fracture toughness

values of Aluminum alloys. High toughness in the present work can be

due to notch root effect.

• Crack propagates along a curved path due to the formation of shear

lips. From the DIC analysis, it is found that the specimen thickness is

in the transition region.

• Fractographs shows ductile fracture at the mid-thickness and shear type

failure near shear lip region.

• Before crack initiation at peak load, there is a considerable non-linearity

in load displacement curves. From the optical metallographs and EBSD

analysis, it was found that twin area fraction increases with load level.

Thus the tendency for plastic deformation by twinning increases with

load level.

• As opposed to compressive strain near the notch root, tensile twins(TTs)

are observed during loading and are named as anomalous tensile twins.

• In front of the extended crack tip near mid section, twin boundaries are

forming circular shaped arcs across the contiguous grains. On the free

18

surface, above and ahead of the extended crack tip, most of the grains

are completely reoriented to prismatic due to profuse tensile twinning.

There is an asymmetry in the formation of TTs about the extended

crack tip, attributed to the fact that the stress state is different above

and below the crack tip due to the shear lips.

Thus it can be conclude that tensile twinning is found to be a dominant

energy dissipation mechanism which leads to high hardening in the load

displacement curves.

Acknowledgement

19

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Table 1: Chemical composition (in % wt.) of the hot rolled Mg AZ31 alloy.

Al Zn Mn Fe Cu Ca Mg

3.13 0.87 0.44 0.002 0.1 0.03 95.428

Appendix A. ASTM E813 for computing J

At a point (vi, pi) on the load verses displacement curve

Ji = Jei + Jp

i (A.1)

Jei =

[Ki]2

[1− ν2]E

(A.2)

and Ki is given by

Ki =

[pi

B0(W )12

]f( aW

)(A.3)

where f(

aW

)is given by

f( aW

)=

[2 +

(aW

)] [0.886 + 4.64

(aW

)− 13.32

(aW

)2+ 14.72

(aW

)3 − 5.6(

aW

)4][1−

(aW

)] 32

(A.4)

Jpi =

η Api

B0 b(A.5)

where η is a shape factor, b = W −a is the uncracked ligament length, B0

is the specimen thickness and Api is the area under load verses displacement

curve described in ASTM E813.

24

Figure 1: Schematic of compact tension (CT) specimen with the various dimensions.

25

(a)

(b)

Figure 2: (a) IPF of the hot rolled Mg AZ31 alloy plate on free surface. (b) Pole figure

26

(a)

(b)

Figure 3: (a) IPF of the hot rolled Mg AZ31 alloy plate on mid surface. (b) Pole figure

27

0 1 2 3 4 5

2

4

6

8

10

Displacement (mm)

Load (

kN

)

TD

RD

(a)

0 0.5 1 1.5 2 2.50

2

4

6

8

10

CMOD(mm)

Load(k

N)

TD

RD

(b)

Figure 4: (a) Load-displacement curves for TD and RD specimens. (b) Load-CMOD

curves for TD and RD specimens.

28

0 2 4 6 8 100

10

20

30

40

Load (kN)

J (

N/m

m)

Jt (RD)

Je

Jt (TD)

Figure 5: Energy released rate J verses load curves for TD and RD specimens

Figure 6: Strain contours obtained from DIC on free surface for the TD specimen at a

load of 8.7 kN

29

(a)

(b)

(c)

Figure 7: crack path (a) Front and (b) Side views for TD specimen. (c) Side view for RD

specimen .

30

(a)

(b)

(c)

Figure 8: Fractographs (a) At mid-thickness. (b) Near shear lip regind for RD specimen

(c) At mid-thickness for TD specimen .

31

x1

x2

(a)

Figure 9: Optical metallograph at loaction B on free-surface for TD specimen at a load of

8.1 kN.

32

(a)

(b)

Figure 10: IPFs (a) At location B (b) At location A on the free surface for TD specimen

at a load of 8.1 kN.

33

(a)

Figure 11: IPF at loaction A on mid-thickness for TD specimen at a load of 8.1 kN.

34

x1

x2

(a)

Figure 12: Optical metallograph at loaction B on free-surface for TD specimen at a load

of 8.7 kN.

35

(a)

(b)

Figure 13: IPFs (a) At location B (b) At location A on the free surface for TD specimen

at a load of 8.7 kN.

36

(a)

Figure 14: IPF at loaction A on mid-thickness for TD specimen at a load of 8.7 kN.

37

K

LM

N

X Y

x1

X2

(a)

Figure 15: IPF near the extended crack tip on mid-thickness for RD specimen.

38

Distance (microns)

Mis

ori

en

tatio

nan

gle

(deg

rees)

X Y

Point­to­pointPoint­to­origin

(a)

(b)

Figure 16: (a) Mis-orientation along the line XY in a grain. (b) Pole figure for RD

specimen

39

K

LM

N

x1

x2

E

(a)

(b)

Figure 17: (a) IPF near the extended crack tip on free surface for RD specimen. (b) Pole

figure.

40