Characterization of the variant selection occurring during the α→β→α phase transformations of a cold rolled titanium sheet

Abstract

The fact that a cold rolling prior to an α→β→α transformation sequence causes the sharpening of the inherited α textures is analyzed. A restitution method is used to evaluate the intermediate β textures which allowed us to discuss the occurrence of variant selection in the β→α transformation. It appears that cold rolling prior to the α→β transformation makes the high temperature β texture sharper, with a density reinforcement around the {112}/{11-2}<111> components and further the variant selection occurring in the β→α phase transformation becomes stronger. The local texture analysis obtained by EBSD leads to the same results and is used to characterize the variant selection occurring in the β→α transformation.

Introduction

Titanium is characterized by two allotropic modifications: the β body centered cubic phase, stable at high temperature, and the α hexagonal close packed phase stable at room temperature. The phase transformation occurs at about 865°C according to the alloying elements. Both phases are well known to be related by the Burgers orientation relations [1]: {110}_β//{00.2}_α and <111>_β//<1-2.0>_α. These relations are strict in the case of a shear transformation and can be more or less strict when the transformation is diffusive. According to the Burgers relationship and the symmetry of the α and β phases, a total of 12 distinct α orientations may arise from an initial β orientation during the β→α transformation. There is no variant selection in a parent grain when the volume fractions corresponding to each of the 12 inherited orientations are equal. However, at the scale of a grain, this case does not frequently occur, due to many physical reasons. Nevertheless, at the scale of the sample, the transformation can statistically appear without variant selection because the grains, having nearly the same orientation, transform into a limited set of variants, randomly selected. Different studies [2], [3] on the texture change induced by the β→α transformation show that a classical treatment in the β field does not generally induce statistical variant selection during cooling. On the other hand, variant selections were observed when the β phase was strongly deformed just before transformation [4].

Whereas the texture change induced by the β→α transformation at cooling has been widely studied [1], [2], [3], [4], [5], [6] only few works [7], [8], [9] focus on the α→β transformation at heating according to the metallurgical state of the initial α phase. This transformation determines however the high temperature state of the β phase and can therefore have a strong influence on the transformation at cooling. Assuming that the transformation at heating also respects the Burgers orientation relation, an α→β→α transformation sequence would lead to an important multiplication of orientations and to a significant sharpness reduction of the inherited α texture in comparison to the initial one [10]. However, recently published works [5], [6], [11] report a strong density increase of the inherited α phase when the starting α phase was cold deformed. Such a texture change cannot be explained without variant selection when the strict Burgers orientation relation is valid. The comparison between the smooth expected texture and the sharp inherited one demonstrates that variant selection occurred during the α→β→α transformation sequence. Consequently, preferential growth of specific variants to the detriment of others inevitably occurs at the scale of a grain, either during the α→β transformation on heating or (and) during the β→α transformation on cooling.

This contribution reports the influence of cold rolling, imposed prior to an α→β→α transformation sequence, on the inherited α texture in a commercially pure titanium. The study focuses on the texture and the microstructure of the β phase inherited from the α→β transformation and its effects on the β→α transformation at cooling. The textures of the high temperature β phase have been evaluated from the inherited α textures by different restitution methods [12], [13], [14]. These methods include the calculation of indexes sensitive to the variant selection in the β→α transformation. The analysis of the β and α textures allows one to discuss the occurrence of variant selection in the β→α transformation at cooling due to the initial cold rolling. Moreover, local texture analysis on the inherited α phase is effective to characterize the feature of the β→α transformation.