C.A. Calhoun

In-Situ Neutron Diffraction Study of Aging in Alloy ZK60A

The effect of c-axis rod-shaped precipitates on the individual deformation mechanisms of extruded magnesium alloy, ZK60A, are examined using in-situ tension and compression testing of solutionized, under-aged, peak-aged, and over-aged material. The results reemphasize that alloy ZK60A exhibits only a modest aging response. The yield stress is slightly increased during compression testing, however, the flow stress plateau associated with deformation twinning is actually enhanced. This suggests that precipitation simultaneously increases the stress required to initiate significant twinning, while retaining the ease with which the twin volume fraction can increase. The previously published load relaxation associated with tensile twinning is confirmed conclusively by this in-situ study.

What is in a Strain Hardening “Plateau”?

A yield plateau occurs in some Mg alloys during compressive deformation that has been ascribed to the localization of twinning. In fine-grained AZ31, this plateau was explained by a region of large twin volume fraction nucleating in a small band and propagating, similar to a “Luders band” like phenomena. Once the band traverses the entire gauge length, the sample begins to strain harden. Similarly, ZK60 samples exhibit the same Luders like phenomena during extrusion direction compression as confirmed by digital image correlation. However, the band is not sufficient to fully explain the plateau in the stress-strain curve. Postmortem electron backscattered diffraction (EBSD) reveals the twin structure evolution through this plateau. It is found that twinning occurs in large elongated grains before spreading to the finer grains.