The Second Martensitic Transformation Stress-Induced in Cu–Zn–Al Alloy Single Crystals

Abstract

The martensite to martensite transformation, β1′→α1′, stress-induced secondly in single crystals of Cu–Zn–Al alloys is crystallographically studied in detail. The transformation proceeds by a simple shear on the basal plane of β1′ martensite with the 9R long period stacking structure to form the 3R stacking structure of fct. The crystallographic orientation relations are obtained to be (001)β1′⁄⁄(111)α1′ and [100]β1′⁄⁄[11\bar2]α1′. Two types of stress-strain curve, “two-stage type” and “stress-drop type”, are observed in the tensile test depending upon the tensile direction and testing temperature. In the former case, the α1′ martensites are transformed after completing the first transformation β1→β1′, while in the latter, two variant crystals of β1′ cross each other to produce the α1′ martensite inside. The drop in stress level is well understood by considering the smaller critical resolved shear stress for the second transformation, τα, as compared with that for the first one, τβ.