Dynamic Properties of Magnesium Alloys

Abstract

With the rapidly expanding use of automobiles globally, the application of lighter structural materials in the form of magnesium alloys has become an urgent necessity to improve fuel efficiency and reduce the environmental impact. Within automobiles, magnesium alloys are used for components such as steering wheel frames and wheel rims, which are subjected to not only quasi-static but also dynamic strains. Indeed, catastrophic failure of a component by dynamic strain effects is a critical consideration, such as that of a wheel rim by braking at a high speed of 320 km/h. A recent consideration of magnesium for crash rail applications was rapidly brought to an end by the poor performance of initial crash test results. Dynamic strain rate testing is also important because of the emergence of magnesium alloys for armor with good ballistic properties. Just as with quasi-static strain, the mechanical behavior at dynamic strain rates depends on the microstructure. Deformation at dynamic strain rates produces more uniform distribution of dislocations and hinders the formation of discrete dislocation cells. Suppression of thermally activated dislocation processes occurs in this regime, which can lead to stresses high enough to activate deformation twins even in face-centered cubic (fcc) alloys with the high-stacking-fault energy. Among the fcc, body-centered cubic (bcc), and hexagonal closepacked (hcp) metals and alloys, high-purity fcc metals exhibit yielding that is nearly independent of the strain rate, while the strain hardening after yielding is strongly dependent on strain rate, which is due to the suppression of dynamic recovery process. In the case of bcc metals and alloys, yielding is strongly dependent on strain rate, while strain hardening is independent of the strain rate. For hcp metals, both the yielding and the strain hardening are dependent on strain rate. In hcp metals and alloys, twins play a very important role in deformation, especially for magnesium alloys in which nonbasal slips are not easily activated. The texture and c/a ratio greatly influence the deformation behavior of hcp metals. The effects of the polarity of twinning are similar to that at quasi-static tests. During in-plane testing of rolled plates with strong basal texture, in which the basal planes are parallel to the loading direction, it is observed that the twinning propensity increases with increasing strain rate. Through-thickness dynamic testing, in which the loading direction is perpendicular to the basal planes, reveals that the main deformation twinning mode, 1012 extension twinning, may not be activated. As a result, the work-hardening response in in-plane loading is more sensitive to strain rate than through-thickness loading. In this issue, we have four contributions on the dynamic behavior of magnesium alloys. The first is titled, ‘‘Dynamic Behavior of a Rare-Earth-Containing Mg Alloy WE43B-T5 Plate with Comparison to Conventional Alloy AM30-F’’ by S.R. Agnew, W. Wittington, A. Oppedal, H. El Kadiri, M. Shaeffer, K.T. Ramesh, J.J. Bhattacharyya, R. DeLorme, and B. Davis, which compares the dynamic behavior of two wrought alloys, a rolled WE43B alloy in peakaged condition, and an extruded AM30 alloy. The WE43B plate exhibited a relative isotropy of yield strength at quasi-static and dynamic strain rates, although it did show tension–compression strength asymmetry and anisotropy in strain hardening, which is consistent with the texture. The extruded AM30 alloy showed stronger anisotropy in strainhardening behavior related to the stronger texture in this material. At larger strain levels, the rate sensitivities along different directions converge in both the alloys. The authors also report that the yield strength is insensitive to strain rates along loading directions in which the rate-controlling process is 1012 type twinning, which is known to be relatively rate insensitive. The second contribution titled, ‘‘Dynamic Behavior of Some Magnesium Alloys’’ by E.S. Prasad, B. Li, N. Dixit, M. Shaefer, S.N. Mathaudhu, and K.T. Ramesh investigates the two important alloys, AZ31B and ZK60, at fine grain and ultrafine grain JOM, Vol. 66, No. 2, 2014