Texture, Second-Phase Particles, and the Anisotropy of Deformation Behavior in TTMP AZ61
- PDF / 4,294,310 Bytes
- 13 Pages / 593.972 x 792 pts Page_size
- 80 Downloads / 127 Views
I.
INTRODUCTION
IT is well known that grain refinement improves both strength and ductility in Mg and its alloys.[1–6] As the grain size is refined strength increases, which can be described by the Hall–Petch relationship. The fracture mechanism also changes with decreasing grain size from intergranular failure to a more ductile rupture by microvoid coalescence in AZ31,[7,8] AZ91,[9] and WE43.[10] As basal slip is the predominant deformation mechanism, the orientation of the basal planes with respect to the loading direction plays a critical role in the deformation of magnesium alloys. Experiments designed to study the influence of starting texture, independent of grain size, by machining samples at different orientations from a textured plate, show that the orientation of the basal planes relative to the loading direction has a strong effect on both the strength and ductility.[11,12] Reduction of the basal texture in sheet material can lead to dramatic improvements in ductility.[13–16] The introduction of secondary phases in magnesium alloy sheet has been reported to have both positive and negative effects on ductility. An increase in the volume fraction of the b-phase is associated with a decrease in ductility. Yet, secondary phases can serve to decrease texture by altering the recrystallization mechanisms and kinetics and by stabilizing a fine grain size.[17,18] Since the contribution of b-phase particles to these processes is still not understood in Mg alloys, the property trade off cannot yet be established. TRACY D. BERMAN, Post-Doctoral Fellow, and J. WAYNE JONES, Professor, are with the Materials Science and Engineering, University of Michigan, 2300 Hayward St, Ann Arbor, MI 48109 Contact-mail: [email protected]. TRESA M. POLLOCK, Professor, is with the Materials Department, University of California, Santa Barbara, CA, 93106-5050 Manuscript submitted November 21, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
Cracking in the b-phase during deformation has been reported in AZ91 by several authors,[9,19–24] as well as in AZ80,[25] AM60,[24] and AM50.[26] In AZ91 alloys with an interconnected network of b-eutectics, cracking in the b-phase can lead to fracture.[21] It has been reasonably hypothesized that even if the bnetwork is not continuous, the growth and coalescence of the particle-initiated cracks are detrimental to ductility.[20,22,23] Yet in AM60 and AM50, with a lower b-phase volume fraction, there is no evidence that particle cracking results in failure. Whether linkage of cracks in the b-phases plays a role in failure of the material depends on the b-phase volume fraction and morphology, and has yet to be thoroughly investigated. Decker and co-workers recently developed a new process designated as thixomolded thermomechanical processing (TTMP), which produces Mg alloy sheet by utilizing untextured thixomolded plates as the warmrolling feedstock.[27,28] Thixomolding is an injection molding process, where the shearing of the liquid metal in the barrel decreases the viscosity, allowing casting at lower t
Data Loading...
