When 12:00 PM - 2:00 PM May 08, 2014
Where 1180 Duderstadt Center
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Microstructure Evolution and Tensile Deformation in Mg Alloy AZ61 Through Thixomolding and Thermomechanical Processing


Tracy Berman
Thesis Defense

J. Wayne Jones, advisor


Mg alloy sheets are of considerable interest in automotive, personal electronic, and medical applications because of their low specific density. However, conventional sheet develops a strong basal texture during rolling that persists through further processing, leading to poor room temperature formability. A reduction in basal texture coupled with grain refinement may significantly improve formability.

 

Thixomolding and thermomechanical processing (TTMP) has been shown to pro- duce sheet with a good balance of strength and ductility. The objective of this research has been to identify the deformation and microstructural evolution phe- nomena responsible for these favorable properties.

 

We have demonstrated that TTMP of AZ61 produces a fine grain size stabilized by β-particles and with a weaker deformation texture than commercial Mg sheet, due in part to the presence of the β phase. Texture is further reduced during annealing, leading to decreased planar anisotropy.


Indicators of formability determined from tensile deformation, such as elongation to failure, work hardening coefficient, and r-value, indicate that TTMP AZ61 has the potential for excellent room temperature formability. Yield strength in the annealed sheets varies according to the Hall-Petch relationship; however, texture must also be taken into account. Yield strength and the work hardening coefficient are orientation dependent, and are directly related to the fraction of grains oriented favorably for basal slip in the loading direction.


Damage accumulation in the β-particles during tensile deformation does not prop- agate into the matrix, consistent with the observation that ductility was independent of the β-phase volume fraction. Fractography reveals that failure results from mi- crovoid coalescence in the matrix following the development of shear instability.


This dissertation has identified the microstructural mechanisms responsible for the promising deformation behavior in TTMP AZ61. It also demonstrates the value of considering the addition of secondary phase particles to Mg alloys subjected to rolling. These particles can act to stabilize the grain size and promote a weaker deformation texture, and if kept sufficiently small, will not limit ductility. As the arrangement and distribution of particles evolves little during TTMP, the largest amount of control in the secondary phases will likely be attained during the molding phase of TTMP.