An investigation of the γ → α martensitic transformation in uranium alloys

Abstract

A detailed study of the γ → α martensitic transformation in uranium alloys is presented. Five binary uranium-base alloys containing 0.77Ti, 1.2 Mo, 2.2 Mo, 4.3 Mo and 5.0 Mo, respectively, were examined. As quenched, the U-0.77 Ti and U-1.2 Mo alloys consisted of an orthorhombic a/ a , martensite phase with an acicular morphology. The acicular martensite plates contain deformation twins which result from transformation stresses. The U-2.2 Mo and U-4.3 Mo alloys transformed during quenching to orthorhombic α/ b and monoclinic α/ b , martensite phases, respectively. The banded morphology observed in these two alloys consists of long, parallel martensite plates containing fine arrays of transformation twins. The type I transformation twinning modes were identified as {021},{130} and {131}. There was also evidence for a type II 〈512〉 {111} mode. It was found that adjacent bands could contain different kinds of transformation twins. In the U-5.0 Mo alloy, some of the cubic parent phase was retained during water quenching, and the α γ orientation relationship was determined. The γ phase was completely retained in this alloy by slow cooling from the solution treatment temperature of 800°C, and it was found that a martensitic reaction could be induced by deformation. The strain-induced martensite plates contained {021} transformation twins. The α γ orientation relationship was found to be different than the one determined in the quenched condition, and both orientation relationships are irrational. The invariant plane strain theory of martensite crystallography was applied to the twinned martensites, and a number of different parent/product lattice correspondences were considered for the γ → α transformations. It was concluded that more than one correspondence may be operative during these transformations. The single-shear theory was not able to explain the occurrence of all of the observed transformation twinning modes, although the theory was quite successful in predicting the observed crystallography for the strain-induced transformation.