Texture Memory in Hexagonal Metals and Its Mechanism

Toshiro Tomida,Shigeo Sato,Yusuke Onuki,Sven C Vogel

doi:10.3390/met11101653

Abstract

Texture memory is a phenomenon in which retention of initial textures occurs after a complete cycle of forward and backward transformations, and it occurs in various phase-transforming materials including cubic and hexagonal metals such as steels and Ti and Zr alloys. Texture memory is known to be caused by the phenomena called variant selection, in which some of the allowed child orientations in an orientation relationship between the parent and child phases are preferentially selected. Without such variant selection, the phase transformations would randomize preferred orientations. In this article, the methods of prediction of texture memory and mechanisms of variant selections in hexagonal metals are explored. The prediction method using harmonic expansion of orientation distribution functions with the variant selection in which the Burgers orientation relationship, {110}β//{0001}α-hex <11¯1>β//21¯1¯0α-hex, is held with two or more adjacent parent grains at the same time, called “double Burgers orientation relation (DBOR)”, is introduced. This method is shown to be a powerful tool by which to analyze texture memory and ultimately provide predictive capabilities for texture changes during phase transformations. Variation in nucleation and growth rates on special boundaries and an extensive growth of selected variants are also described. Analysis of textures of commercially pure Ti observed in situ by pulsed neutron diffraction reveals that the texture memory in CP-Ti is indeed quite well predicted by consideration of the mechanism of DBOR. The analysis also suggests that the nucleation and growth rates on the special boundary of 90° rotation about 21¯1¯0α-hex should be about three times larger than those of the other special boundaries, and the selected variants should grow extensively into not only one parent grain but also other grains in α-hex(hexagonal)→β(bcc) transformation. The model calculations of texture development during two consecutive cycles of α-hex→β→α-hex transformation in CP-Ti and Zr are also shown.

Highlights

When materials transform from one phase to another and transform back to the initial phase, crystallographic textures in the initial phase are often retained to a great extent [1,2,3,4,5,6,7], and, in some cases, the textures and the grain structures are reconstructed [8,9,10] despite diffusive transformation
5.2.1. α-hex→ β Transformation β textures were computed using the observed α texture before transformation shown in Figures 3a and 4a–c and the mathematical method explained above, and they were compared to the observed β texture
Minimization was performed between the observed and predicted orientation distribution functions (ODF) changing the value of ω as a parameter

Summary

Introduction

When materials transform from one phase to another and transform back to the initial phase, crystallographic textures in the initial phase are often retained to a great extent [1,2,3,4,5,6,7], and, in some cases, the textures and the grain structures are reconstructed [8,9,10] despite diffusive transformation These phenomena are called ‘texture memory’, and the latter are known as ‘austenite memory’ for fcc austenite (γ)→martensite (or bainite)→γ transformation in steel [8,9,10]. There must be some mechanisms that favor some orientation variants over others, i.e., variant selection, which has been thought to be responsible for the texture memory

Methods

Results

Conclusion