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Journal of Alloys and Compounds 461 (2008) 154–159

The tensile deformation and fracturebehavior of a magnesium alloy

T.S. Srivatsan a,∗, Satish Vasudevan a, M. Petraroli b

a Division of Materials Science and Engineering, Department of Mechanical Engineering,The University of Akron, Akron, OH 44325-3903, USA

b Division of Research, The TIMKEN Company, Canton, OH 44706-0930, USA

Received 15 June 2007; received in revised form 7 July 2007Available online 22 July 2007

bstract

In this paper, is presented and discussed the quasi-static fracture behavior of a rapid solidification processed magnesium alloy. Test specimens of

he magnesium alloy were deformed under quasi-static loading. The resultant tensile properties and fracture behavior are presented and discussedn light of the competing and synergistic influences of nature of loading, intrinsic microstructural effects, matrix deformation characteristics, and

acroscopic fracture.2007 Published by Elsevier B.V.

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eywords: Rapid solidification; Magnesium alloy; Microstructure; Deformatio

. Introduction

The use of magnesium alloys in a variety of technology-elated applications has seen a progressive growth during theast two decades, and both magnesium and its alloy counter-arts continue to make their impact in a spectrum of automotiveroducts. The preferential disposition towards the selection andse of magnesium is attributed primarily to its light weight,.e., 30% lighter than aluminum, 75% lighter than zinc, and0% lighter than steel [1,2]. Besides, magnesium has the high-st strength-to-weight ratio [σ/ρ] of any of the commonly usedon-ferrous and ferrous metallic materials [1]. Other notewor-hy advantages in favor of choosing magnesium include its goodastability, high die casting rates, electromagnetic inference,hielding properties, part consolidation, dimensional accuracy,nd overall excellent machinability, all of which favor its selec-ion and utilization in automobile products [3–5].

The magnesium alloys produced by conventional ingot metal-

urgy (IM) technique exhibit the drawbacks of less than desirabletrength, inferior formability, low thermal stability, inadequatereep resistance, poor oxidation resistance, and inferior corro-

∗ Corresponding author.E-mail address: tsrivatsan@uakron.edu (T.S. Srivatsan).

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925-8388/$ – see front matter © 2007 Published by Elsevier B.V.oi:10.1016/j.jallcom.2007.07.061

cture; Mechanisms

ion resistance. Furthermore, a limited number of slip systemslaces limitations on achieving enhanced strengthening coupledith degradation in ductility or formability of the alloy. Conse-uently, extensive use of the conventional ingot metallurgy (IM)rocessed magnesium-base alloys has been restricted [6,7]. Theddition of alloying elements having limited solubility in mag-esium, such as: molybdenum, titanium and chromium, havingigh melting points that far exceed the boiling point of mag-esium, were found to be beneficial. Consequently, alloyingy traditional ingot metallurgy methods was difficult and hadts limitation [7]. The most commonly used alloying elementsave limited solid solubility in magnesium and tend to formntermetallic compounds as a result of the electropositive naturef magnesium [8]. These limitations were overcome by use ofhe technique of rapid solidification processing [9–11]. In moreecent years, few studies have focused on magnesium-basedanocomposites, which have shown the promise of attractiveombinations of both strength and ductility.

Rapid solidification processing of magnesium-base alloysacilitates a departure from thermodynamic equilibrium and aidsn the preparation of high purity alloys having compositional

exibility [9–11]. The capability for extended solid solubilitiesnd improved chemical homogeneity that is achievable by rapidolidification, enabled in the preparation of alloy compositionshat cannot be easily made using the traditional ingot metallurgy

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T.S. Srivatsan et al. / Journal of Allo

echnique. The improved compositional flexibility facilitated inmproving the corrosion behavior by minimizing the galvanicoupling between the microscopic in-homogeneities. Also, theresence of new non-equilibrium phases facilitates in improvinghe corrosion behavior. A conjoint and mutually interactive influ-nce of these effects results in improved mechanical and physicalroperties coupled with an elimination of redundant metal work-ng and finishing operations. The development and emergence ofigh strength magnesium-base alloys having lightweight serveds an attractive and potentially viable alternative to aluminumlloys with a concomitant savings in weight.

During the last three decades, i.e., since the early 1980s,here has been a preponderance of research activity onapidly solidified magnesium alloys [9–16]. This paper presentsicrostructural influences on the quasi-static deformation and

racture behavior of a rapid solidification processed (RSP)agnesium alloy. The mechanisms governing the quasi-static

racture behavior of the alloy are presented and discussed inight of the conjoint influences of nature of loading, intrinsic

icrostructural effects, matrix deformation characteristics, andacroscopic aspects of fracture.

. Materials and processing

The magnesium-base alloy used in this study was provided by Allied Signalorporation (Morristown, NJ, USA). The chemical composition of the alloy (int.%) is 5.72 aluminum, 2.96 zinc, 6.05 neodymium the remaining magnesium.he material was manufactured using the powder metallurgy (PM)/rapid solidifi-ation (RS) technique. Planar flow casting was used to produce rapidly solidifiedibbons of the magnesium alloy. The processing of ribbons was conducted inn inert vacuum environment with the objective of: (i) preventing oxidation ofhe liquid metal surface, and (ii) preventing the entrapment of air under theiquid film. The ribbons, about 50 mm in width, were then reduced to powdersieve size 500–250 �m) using a series of high speed mechanical communitionrocesses [10,12,13,17]. The final product consisted of irregularly shaped flatlatelets with a thickness equal to the original ribbon thickness. The powder par-icles obtained from the process have a uniform microstructure irrespective ofhe particulate size [14]. To prepare a consolidated body the powders were eitherut-gassed in a can and then sealed under vacuum, or subjected to hot pressing at73–573 K for different lengths of time, ranging from 1 to 24 h, depending uponize of the billet. The cans were cold compacted and subsequently extruded atemperatures 473–573 K to round bars at an extrusion ratio of 18:1. A marginalise in temperature of up to 15 ◦C above room temperature of the billet occurredue to deformation induced by the mechanical working operation, i.e., extrusion.recise details of the processing technique and the precautionary methods usedan be found elsewhere [14].

. Experimental techniques

The initial microstructure of the as-received material, in the extruded con-ition, was characterized by optical microscopy after standard metallographicreparation techniques. The etched specimens were observed in an optical micro-cope and photographed using standard bright field technique.

Tensile specimens were precision machined such that the longitudinal direc-ion, or major stress axis, of each specimen was parallel to the extrusion direction.hus, in each case, the gross fracture plane was essentially perpendicular to thextrusion direction. The cylindrical test specimens, with threaded ends and aage section, which measured 25.4 mm in length and 6.25 mm in diameter, con-

ormed well to the standards specified in ASTM E-8 [18]. To minimize the effectsf surface irregularities and finish the gage section of all test specimens wereechanically ground using 600 grit silicon carbide impregnated emery paper

n order to remove any and all circumferential scratches and surface machiningarks. Uniaxial tensile tests were performed on a fully automated computer con-

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d Compounds 461 (2008) 154–159 155

rolled servo-hydraulic test machine in the room temperature (27 ◦C) laboratoryir (relative humidity of 55%) environment. The specimens of the magnesiumlloy were deformed at a constant strain rate of 10−4 s−1.

Fracture surfaces of the deformed and failed test specimens were comprehen-ively examined in a scanning electron microscope (SEM) to: (a) determine theacroscopic final fracture mode, and (b) characterize the fine-scale topography

nd microscopic mechanisms governing quasi-static fracture. The distinctionetween macroscopic and microscopic fracture mechanism(s) is based entirelyn the magnification level at which the observations are made. The samplesor observation in the SEM were obtained from the failed tensile specimens byectioning parallel to the fracture surface.

. Results and discussion

.1. Initial microstructure

A triplanar optical micrograph illustrating the grain structuref the magnesium alloy is shown in Fig. 1. The microstruc-ure of the Mg–5.72Al–2.96Zn–6.05Nd alloy reveals the powderarticles to be deformed and elongated in the direction of defor-ation, i.e., extrusion direction. The average size of the grain

ig. 1. Triplanar optical micrograph illustrating the morphology of the grains asdirect consequence of the mechanical deformation resulting from extrusion.

156 T.S. Srivatsan et al. / Journal of Alloys an

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ig. 2. Optical micrograph showing the presence of well-defined grains withinach powder particle of the Mg–5.72Al–2.96 Zn–6.05 Nd alloy.

n the alloy matrix (Fig. 3). These particles have been iden-ified and reported as being the dispersoids [14]. This earliertudy used micro-diffraction to identify the dispersoid particlesnd found them to contain a significant portion of aluminumAl) and neodymium (Nd), i.e., the A12Nd [14]. The forma-ion and presence of the A12Nd dispersoids (melting pointemperature of 1733 K) instead of the Mg–RE (rare-earth) dis-ersoids, which have a lower melting point temperature, inhis rapidly solidified Mg–Zn–Al–RE alloy is interesting. Thel Nd dispersoids are thermally stable and help to pin the grain

2oundaries and prevent the coarsening of grains during high tem-erature consolidation and hot extrusion. The microstructure ofhe Mg–5.72Al–2.96Zn–6.05Nd alloy revealed a non-uniform

ig. 3. Optical micrograph showing the distribution of fine second-phase parti-les in the microstructure of the Mg–5.72Al–2.96Zn–6.05Nd alloy.

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d Compounds 461 (2008) 154–159

rain size along the three orthogonal directions of the extrudedlate. The grains were flattened and elongated in the directionf mechanical deformation, i.e., extrusion.

.2. Tensile response

The ambient temperature tensile properties of the rapidlyolidified magnesium alloys, in the as-extruded condition, areummarized in Table 1. The results reported are the mean val-es based on duplicate tests. The yield strength of the alloy is2 Ksi. The high yield strength of this rapid solidification pro-essed alloy is ascribed to the conjoint and mutually interactivenfluences of (a) fine grain size, (b) solid solution strengtheningf the magnesium matrix, and (c) dispersion strengthening aris-ng from the presence of Al2Nd particles. The ultimate tensiletrength of the alloy is 67 Ksi and only marginally higher than theield strength, indicating that the work hardening rate past yield-ng is low. The ductility, measured by elongation over 12.7 mmage length of the specimen, is low and only 5.4%. However, theeduction in cross-sectional area of the test specimen, anothereasure of tensile ductility, was 12% and more noticeable than

ensile elongation. The engineering stress versus engineeringtrain curve is shown in Fig. 4. The strain hardening character-stics of the alloy was evaluated from examining the variationf stress with plastic strain. The variation of monotonic stressith plastic strain obeyed the relationship σ = K(εp)n, where K is

he monotonic strength coefficient and n is the strain hardeningxponent. A low degree of strain hardening can be inferred fromcareful examination of Fig. 4.

.3. Mechanisms governing tensile fracture behavior

The tensile fracture surfaces are helpful in elucidating usefulnformation on the role and/or influence of intrinsic microstruc-ural features on strength, ductility and fracture properties of

he rapid solidified magnesium alloy. For the duplicate samplesested fracture occurred at the gage section and was essentiallyormal to the far-field stress axis. Representative features arehown in Figs. 5 and 6.

ig. 4. Engineering stress vs. engineering strain curve for the magnesium alloy.

T.S. Srivatsan et al. / Journal of Alloys and Compounds 461 (2008) 154–159 157

Table 1Room temperature tensile properties of the rapidly solidified magnesium alloya

E (GPa) 0.2% yield strengthKsi (MPa)

Ultimate tensilestrength Ksi (MPa)

Engineering fracturestress (σf) Ksi (MPa)

Elongation(G.L. = 12.7 mm) (%)

Reduction in arealn(Ao/Af) (%)

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a Results are the mean values based on duplicate tests.

On a macroscopic scale fracture of the test specimen wasrittle in appearance (Fig. 5a) with the presence of an arrayf fine microscopic cracks distributed randomly through theracture surface (Fig. 5b). Examination of the crack path mor-hology at higher magnifications revealed the macroscopicrack to be essentially non-linear as it propagated through thelloy microstructure (Fig. 5c). The macroscopic cracks wereurrounded by a random distribution of very fine microscopicracks. Examination of the fracture surface at higher magnifi-ations revealed a population of dimples of varying size andhape immediately adjacent to the macroscopic and fine micro-copic cracks, features reminiscent of locally ductile and brittleailure mechanisms. The higher magnifications also revealed a

andom distribution of fine microscopic voids (Fig. 6a). Sincerack extension under quasi-static loading occurs at high stressntensities, comparable to the fracture toughness of the mate-

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ig. 5. Scanning electron micrographs of the tensile fracture surface of the magnesistributed through the fracture surface, (c) high magnification of (b) showing non-linend fine microscopic cracks.

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ial, the presence of a population of fine microscopic voidsegrades the actual strain-to-failure associated with ductile frac-ure. Although the exact nucleation of the fine microscopicoids is difficult to pin-point, their near-equiaxed shape sug-ests that they may have nucleated around the Al2Nd dispersoidarticles during the later stages of tensile deformation, with-ut undergoing appreciable growth that would result in theirvalization. The nucleation of a microscopic void at a disper-oid particle and other second-phase particles present in theicrostructure occurs when the elastic energy in the particle

xceeds the surface energy of the newly formed void surfaces.hile this is a necessary condition, it must also be aided bystress at the matrix-second-phase particle interface that is in

xcess of the interfacial strength. When a critical value of thenterface stress is reached void nucleation is favored to occur.he coalescence of the fine microscopic voids is the last stage

ium alloy showing: (a) overall morphology, (b) array of macroscopic cracksar nature of a macroscopic cracks, and (d) shallow dimples, macroscopic crack

158 T.S. Srivatsan et al. / Journal of Alloys an

Fig. 6. Scanning electron micrographs of the fracture surface of the magnesiumabo

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lloy deformed in tension, showing: (a) view of a microscopic crack surroundedy shallow dimples of varying size, and (b) dimples and cracking in the regionf overload.

n the ductile fracture process. The halves of these voids arehe shallow dimples observed on the tensile fracture surfaceFig. 6b).

During tensile deformation the presence of dislocation pileps and grain boundary dislocations aids in nucleating voids athe second-phase particles distributed within the alloy matrixnd also along the grain boundary regions with the initiation oficro-cracking along the grain boundaries (Fig. 6b). The fineicroscopic and macroscopic cracks were observed traversing

he grain boundaries extending in the direction of the tensiletress axis, suggesting the importance of normal stress in enhanc-ng tensile deformation. Microscopically, the magnesium alloy

pecimens revealed features reminiscent of

(i) Locally ductile mechanisms, namely fine microscopic voidsand shallow dimples, and

d Compounds 461 (2008) 154–159

ii) Brittle mechanisms, i.e., an array of fine microscopic andmacroscopic cracks both through the matrix and along thegrain boundary regions.

The very fine microscopic voids coalesce and it is the halvesf these voids that are the isolated pockets of shallow dimplesbserved on the tensile fracture surface. The growth of the micro-copic voids is dictated by localized plastic deformation. Theimited growth of the fine microscopic voids coupled with lackf their coalescence, as a dominant fracture mode, suggests therittle nature of the microstructure that governs the deformationroperties.

. Conclusions

A study aimed at understanding the quasi-static fractureehavior of a rapidly solidified magnesium alloy provides theollowing useful highlights:

. The grains in the alloy were small in size and elongatedin the direction of deformation, i.e., extrusion. Overall, themicrostructure consisted of well-defined powder particles.The dispersoids were found to be distributed through the alloymicrostructure.

. The yield strength of the alloy was high and the tensilestrength was only marginally higher than the yield strengthindicating the tendency for strain hardening beyond yield tobe low.

. Tensile fracture surface morphology revealed an overallmacroscopically brittle appearance and microscopically fea-tures reminiscent of ductile and brittle failure mechanisms.

cknowledgements

The author (TSS) extends most sincere thanks and appre-iation to the unknown reviewer and editor of the journal firheir many useful comments, corrections and suggestions, whichhen incorporated with care have helped strengthen the techni-

al manuscript.

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