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Preface to the viewpoint set on: The current state of magnesium alloy science and technology
Available online at www.sciencedirect.com
Scripta Materialia 63 (2010) 671–673 www.elsevier.com/locate/scriptamat
Viewpoint Paper
Preface to the viewpoint set on: The current state of magnesium alloy science and technology S.R. Agnewa,* and J.F. Nieb a
University of Virginia, Charlottesville, VA 22904-4745, USA b Monash University, Melbourne, Victoria 3800, Australia Available online 20 June 2010
Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Magnesium; Computational; Thermodynamics; Deformation
Magnesium alloy research and development has expanded tremendously during the past decade, after a period of relatively slow expansion since the 1960s. There are now numerous new applications within the automotive and consumer goods (including cases for portable electronics and hand-held power tools) sectors, and there is currently growing interest in developing new applications within the defense [1] and aerospace [2] sectors. While magnesium alloys have recently received considerable attention and have been intensively researched, there are still many fundamentally and/or technologically important questions to be answered before this class of engineering alloys can find much wider applications. For example, what are the key factors in controlling the strength and creep resistance of magnesium casting and wrought alloys? What dictates the nucleation and growth of various deformation twins under different loading conditions? How to achieve random orientation, i.e. greatly weakened texture, in wrought magnesium alloys and therefore improve the formability of these alloys? What are the phase equilibria and microstructural constituents in existing and emerging magnesium alloys of technological importance? Therefore, it is timely to assemble this viewpoint set to examine, in a coordinated way, the current status in the following four major areas of research: (1) use of the computational materials science and engineering approaches in alloy development including thermodynamic and first-principles modeling; (2) mechanistic understanding and development of creep-resistant casting alloys; (3) mechanistic understanding and modeling of deformation, including mechanical twinning and dynamic recrystallization; and
* Corresponding author. Tel.: +1 434 924 0605; fax: +1 434 982 5660; e-mail: [email protected]
(4) texture modification via alloying and processing, especially strip casting of sheet. The goal of this set of viewpoints is provide a perspective which will guide future research aimed at improving the properties and broadening the structural applications of magnesium alloys. A common theme amongst the papers in each of the four major research areas is a focus on alloying with rare earth (RE) elements. The RE elements have been shown to have a wide range of benefits for magnesium alloys, ranging from grain refinement in castings and wrought processed materials to improved high-temperature strength and creep resistance, improved corrosion resistance, and reduced texture strength in wrought processed alloys. The 0.2% proof strength of RE-containing alloys can exceed 600 MPa when they are produced by rapid solidification processing [3,4], and very recently, an impressive value of over 470 MPa has been obtained for the 0.2% proof strength in an Mg–1.8Gd–1.8Y–0.7Zn– 0.2Zr (at.%) alloy produced by conventional hot extrusion [5]. In the present volume, Groebner and SchmidFetzer use the computational phase diagram (CALPHAD) approach to illustrate distinctions between different ternary Mg–RE–RE alloy systems. They show that the simple assumption that all RE elements behave the same is incorrect. Historical RE alloying efforts have relied upon less expensive mixtures of RE metal elements called “mischmetals”. With the demonstration (in many of the papers within this viewpoint set) that different RE elements give rise to different intermetallic phases and impart very different properties, the need to understand the specific impacts of different RE elements (and their combinations) is emphasized. In addition to the use of empirically developed thermodynamic constants, CALPHAD modelers are increasingly turning to first-principles modeling to inform their calculations. Shin and Wolverton describe three examples
1359-6462/$ - see front matter Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2010.06.029
672
S. R. Agnew, J. F. Nie / Scripta Materialia 63 (2010) 671–673
of how electron density functional theory can be used to provide valuable input to the CALPHAD approach, as well as the data necessary for kinetic modeling. Their examples relate to the Mg–Al system, which is the best studied of all magnesium alloy systems and the basis of the most common commercial alloys. However, they illustrate that the approach can be used to rapidly expand the knowledge base in alloy systems for which there is a complete dearth of experimental data. Similarly, Liu et al. demonstrate how first-principles calculations can be used to predict the shear and Young’s moduli of Mg–Al alloys and the effects of Al content and temperature on such mechanical properties. Three papers in the viewpoint set are devoted to creep mechanisms and development of creep-resistant alloys. The views on the dominant creep mechanisms in the stress and temperature regimes of engineering interest have remained controversial, even though attempts to solve this issue were published in an earlier viewpoint set on magnesium alloys [6]. The traditional view is that grain boundary sliding plays a significant role in the creep process of magnesium alloys, and this view has promoted considerable efforts towards to introduction of large volume fractions of second-phase particles in magnesium grain boundaries in the development of creep-resistant alloys. This view seems to stem from observations that the creep resistance of Mg–Al-based alloys is improved when the amount of discontinuous precipitates in grain boundary regions is reduced. As was pointed out the in the 2003 viewpoint set on magnesium alloys, if discontinuous precipitation is the key factor responsible for the limited creep resistance of magnesium alloys, then an elimination of the discontinuous precipitates would lead to substantial improvement in creep resistance. However, the creep resistance of Mg–Zn–Al alloys, in which there is no discontinuous precipitation, is still significantly inferior to those of magnesium alloys based on the Mg–RE systems, even after a large volume fraction of intermetallic particles is introduced into grain boundaries of the Mg–Zn– Al alloys. It was further pointed out [6] that the diffusivity of solute atoms might play an important role in the creep process, and attention was drawn to the fact that the steady-state creep strain rate of Mg–Al solid solution alloys is almost two orders of magnitude higher than that of Mg– Y solid solution alloys when the two alloys contain similar concentrations of solute atoms. The paper by Saddock et al. demonstrates that there is no significant contribution of grain boundary sliding to creep in alloys based on the Mg–Al–Ca system, at least when tested at 175 °C. It is further demonstrated that an inverse dependence of creep rate on grain size is observed, since the lowest creep rates were observed in the fine-grained die-cast materials. It is also demonstrated in the paper by Zhu et al. that, for Mg–RE binary alloys, dislocation creep is the operating deformation mechanism at 177 °C, and that RE alloying elements with different solid solubility in magnesium can have a significant effect on the creep resistance. The systems with increasing equilibrium solid solubility show the best creep resistance (Nd > Ce > La), since more solute is present within the matrix providing solute strengthening and potential precipitation strengthening, rather than partitioning to the interdendritic regions. This reiterates the conclusion of
Grobner and Schmid-Fetzer mentioned above, that the individual RE element may exert distinctly different effects on the phase relations (and the solid solubility), and it is therefore important to distinguish them in magnesium alloys containing multiple RE elements. A comparison of the constitutive response in creep and hot torsion of both cast and wrought magnesium alloys, obtained in the temperature range 100–150 °C, is made in the paper by Spigarelli and El Mehtedi. The stress exponent values extracted from the plots are all larger than 10. Collectively, all the results reported in each of these three papers challenge prior notions of creep-resistant magnesium alloy design, which focused on grain boundary strengthening. As a side note, although Spigarelli and El Mehtedi find that the RE-containing alloy exhibits greater creep strength, they also report that this has a negative consequence of higher working loads and lower hot workability when compared to the traditional alloys. In response to the fact that RE elements are relatively expensive and may be obtained from relatively few sources throughout the world, many researchers are exploring “RE-free” alloys, seeking the same level of property enhancements that RE elements produce. The successful development of wrought alloys inevitably requires in-depth understanding of precipitation, microstructure and texture evolution during sheet production. The potential to develop high-strength low-cost magnesium wrought alloys through precipitation hardening is discussed in the paper by Hono et al., with emphasis on microalloying additions to Mg–Zn and Mg–Sn alloys. Similar to aluminium alloys, many of the magnesium alloys are precipitation hardenable, but the age-hardening response achievable so far is much lower than that obtained in aluminium alloys. Concerted efforts are still need to identify appropriate alloying additions, either individually or collectively, and particularly the guidelines for the selection of microalloying elements that can significantly enhance the age-hardening response. The most intensively studied magnesium sheet alloys are those based on the AZ31 composition. A strong basal texture is invariably associated with the as-deformed sheets of such alloys, and can become even stronger after recrystallization and grain growth, which leads to anisoptropic mechanical properties and limited formability. In recent years, it has been reported that additions of sufficient amounts of RE alloying elements to magnesium can lead to a weakened basal texture in both as-deformed and recrystallized conditions, but the reason and the mechanism responsible for this texture change are not clear. The paper by Kim et al. reiterates that even twinroll cast material is not free from the concerns associated with strong basal texture that have plagued traditional direct-chill cast and hot-rolled materials. Instead, they have turned to alloying with yttrium as a means to control the texture. Their empirical results highlight the role of recrystallization within shear bands as a key mechanism in controlling the texture of magnesium alloys, as do the more sophisticated crystal plasticity modeling results of the paper by Barnett et al. The work of Hantzsche et al. also seeks an explanation for the so-called “RE texture effect” by studying warm-rolled dilute binary alloys of Mg with Ce, Nd and Y. Hantzsche et al. also observe shear banding, but they conclude that RE additions
S. R. Agnew, J. F. Nie / Scripta Materialia 63 (2010) 671–673
promote compression and double-twinning, while Kim et al. observed more double-twinning in a RE-free alloy. Both studies highlight the possible roles of particles and/or solute as inhibitors of recrystallization and/or grain growth, but no clear connection with the texture effect is available. Clearly, there is still controversy regarding the mechanism(s) responsible for the RE texture effect and this remains a fruitful area for research. The papers by Raeisinia and Agnew and Hutchinson and Barnett both grapple with the question of how to incorporate the effects of strengthening mechanisms into crystal plasticity models, which account for the distinct behaviors of the different slip and twinning mechanisms. Both papers illustrate that conventional dislocation theory-based models originally validated for face-centered cubic (fcc) and body-centered cubic (bcc) metals and alloys can be exploited for explaining the behavior of hexagonal close-packed magnesium and its alloys; however, they emphasize that great care must be taken to account for the distinct behaviors of the different deformation mechanisms. Whereas a simple Taylor factor-type approach can be used to relate the polycrystal behavior to the underlying single crystal for many fcc and bcc metals, the yielding, anisotropy and tension–compression asymmetry of magnesium require more sophisticated models. One area that seems ripe for further study (both experimental and theoretical) is that of latent hardening. While there is convergence on the factors determining the critical resolved shear stresses of different mechanisms, the relative impacts of the different slip and twinning modes on each during deformation have not been actively investigated in recent years [7]. Wang et al. provide a review of recent findings concerning the nucleation of mechanical twins and emphasize the preponderance of evidence for grain boundary nucleation mechanisms. In particular, they present new modeling results suggesting that twinning is grain boundary nucleated and that the propensity for twin nucleation is strongly dependent upon grain boundary geometry. These results may explain why models which depend solely upon grain orientation (i.e. through the Schmid law) fail to accurately explain the propensity for twinning. This may be an area where the detailed microstructural descriptions provided by electron backscatter diffraction and related techniques and analyses (such as the twopoint correlation function first introduced by Adams [8]) may be a fruitful area of study. Relative to the heavy emphasis that has been placed upon exploring creep resistance, plastic anisotropy and formability, the fatigue properties of magnesium alloys have not been investigated with nearly the level of sophis-
673
tication. Koike et al. have now carefully examined the impact of deformation twinning on the fatigue behavior of a magnesium alloy and find that tensile twinning (and detwinning) appears to have a rather benign impact, whereas stresses sufficient to activate non-basal slip seem to required to accumulate significant fatigue damage. Similar to previous observations by the same group, they find that compressive twinning can induce surface offsets which eventually lead to cracking. Jordon et al. apply, to an extruded magnesium alloy, the type of detailed structure– property characterization that has been used to great advantage in the context of aluminum alloys and steel. Perhaps not surprisingly, they find that the size and location of crack-initiating intermetallic particles impacts fatigue life significantly and must be taken into account in any accurate fatigue prognosis models or efforts to improve the fatigue resistance of magnesium alloys. They conclude by posing probing questions about environmental effects on fatigue, which have not been explored at all (to the authors’ knowledge) and are bound to be significant in a material as electrochemically active as magnesium. The number of scientific investigations focused on magnesium and its alloys has exploded in the past few years. Therefore, it seems timely to collect, in one place, some concise reports on the status of the field in these four areas along with the author’s perspectives on where the field is going. In such a compact volume, it is impossible to give comprehensive coverage to all the exciting work that is going on in this rapidly growing field, but we hope to provide the reader with a sense of some of the areas in which significant advances are being made, and also some of the critical issues that remain to be addressed. SRA is grateful for the support of a NSF World Materials Network Grant, DMR-0603066. [1] The 47th Sagamore Army Materials Research Conference on Advanced in Lightweight Metals Technology. (27.05.10). [2] B. Gwynne, in: S.R. Agnew et al. (Eds.), Magnesium Technology 2010, TMS, Warrendale, PA, 2010, p. 13. [3] Y. Kawamura, K. Hayashi, A. Inoue, T. Masumoto, Mater. Trans. 42 (2001) 1172. [4] A. Inoue, Y. Kawamura, M. Matsushita, K. Hayashi, J. Koike, J. Mater. Res. 16 (2001) 1894. [5] T. Homma, N. Kunito, S. Kamado, Scripta Mater. 61 (2009) 644. [6] J.F. Nie, Scripta Mater. 48 (2003) 981–984. [7] F.F. Lavrentev, Yu.A. Pokhil, Physica Status Solidi (a) 32 (1975) 227–232. [8] B.L. Adams, Metall. Trans. A 17 (1986) 2199–2207.
Scripta Materialia 63 (2010) 671–673 www.elsevier.com/locate/scriptamat
Viewpoint Paper
Preface to the viewpoint set on: The current state of magnesium alloy science and technology S.R. Agnewa,* and J.F. Nieb a
University of Virginia, Charlottesville, VA 22904-4745, USA b Monash University, Melbourne, Victoria 3800, Australia Available online 20 June 2010
Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Magnesium; Computational; Thermodynamics; Deformation
Magnesium alloy research and development has expanded tremendously during the past decade, after a period of relatively slow expansion since the 1960s. There are now numerous new applications within the automotive and consumer goods (including cases for portable electronics and hand-held power tools) sectors, and there is currently growing interest in developing new applications within the defense [1] and aerospace [2] sectors. While magnesium alloys have recently received considerable attention and have been intensively researched, there are still many fundamentally and/or technologically important questions to be answered before this class of engineering alloys can find much wider applications. For example, what are the key factors in controlling the strength and creep resistance of magnesium casting and wrought alloys? What dictates the nucleation and growth of various deformation twins under different loading conditions? How to achieve random orientation, i.e. greatly weakened texture, in wrought magnesium alloys and therefore improve the formability of these alloys? What are the phase equilibria and microstructural constituents in existing and emerging magnesium alloys of technological importance? Therefore, it is timely to assemble this viewpoint set to examine, in a coordinated way, the current status in the following four major areas of research: (1) use of the computational materials science and engineering approaches in alloy development including thermodynamic and first-principles modeling; (2) mechanistic understanding and development of creep-resistant casting alloys; (3) mechanistic understanding and modeling of deformation, including mechanical twinning and dynamic recrystallization; and
* Corresponding author. Tel.: +1 434 924 0605; fax: +1 434 982 5660; e-mail: [email protected]
(4) texture modification via alloying and processing, especially strip casting of sheet. The goal of this set of viewpoints is provide a perspective which will guide future research aimed at improving the properties and broadening the structural applications of magnesium alloys. A common theme amongst the papers in each of the four major research areas is a focus on alloying with rare earth (RE) elements. The RE elements have been shown to have a wide range of benefits for magnesium alloys, ranging from grain refinement in castings and wrought processed materials to improved high-temperature strength and creep resistance, improved corrosion resistance, and reduced texture strength in wrought processed alloys. The 0.2% proof strength of RE-containing alloys can exceed 600 MPa when they are produced by rapid solidification processing [3,4], and very recently, an impressive value of over 470 MPa has been obtained for the 0.2% proof strength in an Mg–1.8Gd–1.8Y–0.7Zn– 0.2Zr (at.%) alloy produced by conventional hot extrusion [5]. In the present volume, Groebner and SchmidFetzer use the computational phase diagram (CALPHAD) approach to illustrate distinctions between different ternary Mg–RE–RE alloy systems. They show that the simple assumption that all RE elements behave the same is incorrect. Historical RE alloying efforts have relied upon less expensive mixtures of RE metal elements called “mischmetals”. With the demonstration (in many of the papers within this viewpoint set) that different RE elements give rise to different intermetallic phases and impart very different properties, the need to understand the specific impacts of different RE elements (and their combinations) is emphasized. In addition to the use of empirically developed thermodynamic constants, CALPHAD modelers are increasingly turning to first-principles modeling to inform their calculations. Shin and Wolverton describe three examples
1359-6462/$ - see front matter Ó 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2010.06.029
672
S. R. Agnew, J. F. Nie / Scripta Materialia 63 (2010) 671–673
of how electron density functional theory can be used to provide valuable input to the CALPHAD approach, as well as the data necessary for kinetic modeling. Their examples relate to the Mg–Al system, which is the best studied of all magnesium alloy systems and the basis of the most common commercial alloys. However, they illustrate that the approach can be used to rapidly expand the knowledge base in alloy systems for which there is a complete dearth of experimental data. Similarly, Liu et al. demonstrate how first-principles calculations can be used to predict the shear and Young’s moduli of Mg–Al alloys and the effects of Al content and temperature on such mechanical properties. Three papers in the viewpoint set are devoted to creep mechanisms and development of creep-resistant alloys. The views on the dominant creep mechanisms in the stress and temperature regimes of engineering interest have remained controversial, even though attempts to solve this issue were published in an earlier viewpoint set on magnesium alloys [6]. The traditional view is that grain boundary sliding plays a significant role in the creep process of magnesium alloys, and this view has promoted considerable efforts towards to introduction of large volume fractions of second-phase particles in magnesium grain boundaries in the development of creep-resistant alloys. This view seems to stem from observations that the creep resistance of Mg–Al-based alloys is improved when the amount of discontinuous precipitates in grain boundary regions is reduced. As was pointed out the in the 2003 viewpoint set on magnesium alloys, if discontinuous precipitation is the key factor responsible for the limited creep resistance of magnesium alloys, then an elimination of the discontinuous precipitates would lead to substantial improvement in creep resistance. However, the creep resistance of Mg–Zn–Al alloys, in which there is no discontinuous precipitation, is still significantly inferior to those of magnesium alloys based on the Mg–RE systems, even after a large volume fraction of intermetallic particles is introduced into grain boundaries of the Mg–Zn– Al alloys. It was further pointed out [6] that the diffusivity of solute atoms might play an important role in the creep process, and attention was drawn to the fact that the steady-state creep strain rate of Mg–Al solid solution alloys is almost two orders of magnitude higher than that of Mg– Y solid solution alloys when the two alloys contain similar concentrations of solute atoms. The paper by Saddock et al. demonstrates that there is no significant contribution of grain boundary sliding to creep in alloys based on the Mg–Al–Ca system, at least when tested at 175 °C. It is further demonstrated that an inverse dependence of creep rate on grain size is observed, since the lowest creep rates were observed in the fine-grained die-cast materials. It is also demonstrated in the paper by Zhu et al. that, for Mg–RE binary alloys, dislocation creep is the operating deformation mechanism at 177 °C, and that RE alloying elements with different solid solubility in magnesium can have a significant effect on the creep resistance. The systems with increasing equilibrium solid solubility show the best creep resistance (Nd > Ce > La), since more solute is present within the matrix providing solute strengthening and potential precipitation strengthening, rather than partitioning to the interdendritic regions. This reiterates the conclusion of
Grobner and Schmid-Fetzer mentioned above, that the individual RE element may exert distinctly different effects on the phase relations (and the solid solubility), and it is therefore important to distinguish them in magnesium alloys containing multiple RE elements. A comparison of the constitutive response in creep and hot torsion of both cast and wrought magnesium alloys, obtained in the temperature range 100–150 °C, is made in the paper by Spigarelli and El Mehtedi. The stress exponent values extracted from the plots are all larger than 10. Collectively, all the results reported in each of these three papers challenge prior notions of creep-resistant magnesium alloy design, which focused on grain boundary strengthening. As a side note, although Spigarelli and El Mehtedi find that the RE-containing alloy exhibits greater creep strength, they also report that this has a negative consequence of higher working loads and lower hot workability when compared to the traditional alloys. In response to the fact that RE elements are relatively expensive and may be obtained from relatively few sources throughout the world, many researchers are exploring “RE-free” alloys, seeking the same level of property enhancements that RE elements produce. The successful development of wrought alloys inevitably requires in-depth understanding of precipitation, microstructure and texture evolution during sheet production. The potential to develop high-strength low-cost magnesium wrought alloys through precipitation hardening is discussed in the paper by Hono et al., with emphasis on microalloying additions to Mg–Zn and Mg–Sn alloys. Similar to aluminium alloys, many of the magnesium alloys are precipitation hardenable, but the age-hardening response achievable so far is much lower than that obtained in aluminium alloys. Concerted efforts are still need to identify appropriate alloying additions, either individually or collectively, and particularly the guidelines for the selection of microalloying elements that can significantly enhance the age-hardening response. The most intensively studied magnesium sheet alloys are those based on the AZ31 composition. A strong basal texture is invariably associated with the as-deformed sheets of such alloys, and can become even stronger after recrystallization and grain growth, which leads to anisoptropic mechanical properties and limited formability. In recent years, it has been reported that additions of sufficient amounts of RE alloying elements to magnesium can lead to a weakened basal texture in both as-deformed and recrystallized conditions, but the reason and the mechanism responsible for this texture change are not clear. The paper by Kim et al. reiterates that even twinroll cast material is not free from the concerns associated with strong basal texture that have plagued traditional direct-chill cast and hot-rolled materials. Instead, they have turned to alloying with yttrium as a means to control the texture. Their empirical results highlight the role of recrystallization within shear bands as a key mechanism in controlling the texture of magnesium alloys, as do the more sophisticated crystal plasticity modeling results of the paper by Barnett et al. The work of Hantzsche et al. also seeks an explanation for the so-called “RE texture effect” by studying warm-rolled dilute binary alloys of Mg with Ce, Nd and Y. Hantzsche et al. also observe shear banding, but they conclude that RE additions
S. R. Agnew, J. F. Nie / Scripta Materialia 63 (2010) 671–673
promote compression and double-twinning, while Kim et al. observed more double-twinning in a RE-free alloy. Both studies highlight the possible roles of particles and/or solute as inhibitors of recrystallization and/or grain growth, but no clear connection with the texture effect is available. Clearly, there is still controversy regarding the mechanism(s) responsible for the RE texture effect and this remains a fruitful area for research. The papers by Raeisinia and Agnew and Hutchinson and Barnett both grapple with the question of how to incorporate the effects of strengthening mechanisms into crystal plasticity models, which account for the distinct behaviors of the different slip and twinning mechanisms. Both papers illustrate that conventional dislocation theory-based models originally validated for face-centered cubic (fcc) and body-centered cubic (bcc) metals and alloys can be exploited for explaining the behavior of hexagonal close-packed magnesium and its alloys; however, they emphasize that great care must be taken to account for the distinct behaviors of the different deformation mechanisms. Whereas a simple Taylor factor-type approach can be used to relate the polycrystal behavior to the underlying single crystal for many fcc and bcc metals, the yielding, anisotropy and tension–compression asymmetry of magnesium require more sophisticated models. One area that seems ripe for further study (both experimental and theoretical) is that of latent hardening. While there is convergence on the factors determining the critical resolved shear stresses of different mechanisms, the relative impacts of the different slip and twinning modes on each during deformation have not been actively investigated in recent years [7]. Wang et al. provide a review of recent findings concerning the nucleation of mechanical twins and emphasize the preponderance of evidence for grain boundary nucleation mechanisms. In particular, they present new modeling results suggesting that twinning is grain boundary nucleated and that the propensity for twin nucleation is strongly dependent upon grain boundary geometry. These results may explain why models which depend solely upon grain orientation (i.e. through the Schmid law) fail to accurately explain the propensity for twinning. This may be an area where the detailed microstructural descriptions provided by electron backscatter diffraction and related techniques and analyses (such as the twopoint correlation function first introduced by Adams [8]) may be a fruitful area of study. Relative to the heavy emphasis that has been placed upon exploring creep resistance, plastic anisotropy and formability, the fatigue properties of magnesium alloys have not been investigated with nearly the level of sophis-
673
tication. Koike et al. have now carefully examined the impact of deformation twinning on the fatigue behavior of a magnesium alloy and find that tensile twinning (and detwinning) appears to have a rather benign impact, whereas stresses sufficient to activate non-basal slip seem to required to accumulate significant fatigue damage. Similar to previous observations by the same group, they find that compressive twinning can induce surface offsets which eventually lead to cracking. Jordon et al. apply, to an extruded magnesium alloy, the type of detailed structure– property characterization that has been used to great advantage in the context of aluminum alloys and steel. Perhaps not surprisingly, they find that the size and location of crack-initiating intermetallic particles impacts fatigue life significantly and must be taken into account in any accurate fatigue prognosis models or efforts to improve the fatigue resistance of magnesium alloys. They conclude by posing probing questions about environmental effects on fatigue, which have not been explored at all (to the authors’ knowledge) and are bound to be significant in a material as electrochemically active as magnesium. The number of scientific investigations focused on magnesium and its alloys has exploded in the past few years. Therefore, it seems timely to collect, in one place, some concise reports on the status of the field in these four areas along with the author’s perspectives on where the field is going. In such a compact volume, it is impossible to give comprehensive coverage to all the exciting work that is going on in this rapidly growing field, but we hope to provide the reader with a sense of some of the areas in which significant advances are being made, and also some of the critical issues that remain to be addressed. SRA is grateful for the support of a NSF World Materials Network Grant, DMR-0603066. [1] The 47th Sagamore Army Materials Research Conference on Advanced in Lightweight Metals Technology.