Texture Memory in Hexagonal Metals and Its Mechanism

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}(beta)//{0001}(alpha-hex) 1 over bar 1 >(beta)//21 over bar 1 over bar 0(alpha-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 & DEG; rotation about 21 over bar 1 over bar 0(alpha-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 alpha-hex(hexagonal)& RARR;beta(bcc) transformation. The model calculations of texture development during two consecutive cycles of alpha-hex & RARR;beta & RARR;alpha-hex transformation in CP-Ti and Zr are also shown.

Automated summary

Texture memory is a phenomenon where initial textures are retained after a complete cycle of transformations, caused by variant selection. The prediction method using harmonic expansion of orientation distribution functions, including the introduction of double Burgers orientation relation (DBOR), proves to be a powerful tool for analyzing texture memory and providing predictive capabilities. The study explores texture memory in hexagonal metals and mechanisms of variant selections.