Texture Memory in Hexagonal Metals and Its Mechanism

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.

Property Improvement of Additively Manufactured Ti64 by Heat Treatment Characterized by In Situ High Temperature EBSD and Neutron Diffraction

“…The texture transformation upon α → β → α phase transformation was modeled based on so-called double Burgers orientation relation (DBOR) as proposed in the literature [35]. In DBOR, the variants that satisfy BOR:{0001} α //{011} β , < 11 20 > α //< 111 > β [36], in which there are 6 and 12 variants in α → β and β → α transformations, respectively, or relationship near to it (up to about 10 degrees of deviation allowed) preferentially nucleate and grow.…”

“…The second mode is more extended in which the DBOR variants grow not only by the migration of the coherent interfaces, but also by semi-coherent or incoherent interfaces into the grains with which they hold only near-BOR or even no orientation relation (EDBOR) as shown in Figure 5b. The DBOR variants can appear on five and four kinds of special boundaries in α and β parent grain structures, respectively, as listed in Table 3 [35]. Therefore, the probability for a particular variant to be chosen according to DBOR, ρ(g), is related to the density of those special boundaries in the parent phase, where g specifies the orientation of a parent grain as well as the variant to be chosen.…”

“…When ω is unity, ρ c (g) is close to zero and the nucleation and growth are all governed by the proposed DBOR mechanism. For the latter growth mode, EDBOR, the second term in (1), ρ c (g) is replaced by a constant δ, which is determined to maintain total volume of material during transformation [35]. In this study, the EDBOR calculation was used.…”

“…The values of ζ k were chosen to be unity for α → β for simplicity. Those for β → α were chosen to be 0, 3, 1 and 1 for the I, II, III and IV types of the special boundaries that were determined for Ti in the previous study [35], respectively. The values of ζ k determined for α → β in the previous study were not used since they were thought to be influenced by macro-zones caused by rolling processes [35].…”

“…Those for β → α were chosen to be 0, 3, 1 and 1 for the I, II, III and IV types of the special boundaries that were determined for Ti in the previous study [35], respectively. The values of ζ k determined for α → β in the previous study were not used since they were thought to be influenced by macro-zones caused by rolling processes [35]. The ρ(g) in Equations ( 1) and ( 2) can be readily expanded by spherical harmonics [35], and the method for transformation texture calculation based on the harmonic expansion of ODF [37] can be used for the texture prediction.…”