Variant selection and intervariant crystallographic planes distribution in martensite in a Ti–6Al–4V alloy

Abstract

The transformation texture was studied in a Ti–6Al–4V alloy for two microstructures produced through different phase transformation mechanisms (i.e. diffusional vs. displacive). Both microstructures revealed qualitatively similar crystallographic texture characteristics, having two main texture components with Euler angles of (90°, 90°, 0°) and (90°, 30°, 0°). However, the overall α texture strength was considerably weaker in the martensitic structure (i.e. displacive mechanism) compared with the α + β microstructure produced through slow cooling (i.e. diffusional mechanism). The intervariant boundary distribution in martensite mostly revealed five misorientations associated with the Burgers orientation relationship. The five-parameter boundary analysis also showed a very strong interface plane orientation texture, with interfaces terminated mostly on the prismatic planes {hki0}, when misorientation was ignored. The highest intervariant boundary populations belonged to the 63.26°/[$\overline{10}55\bar{3}$] and 60°/[$11\bar{2}0$] misorientations, with length fractions of 0.38 and 0.3, respectively. The former was terminated on ($4\bar{1}\bar{3}0$), and the latter was a symmetric tilt boundary, terminated on ($\bar{1}011$). The intervariant plane distribution in martensite was determined more by the constraints of the phase transformation than by the relative interface energies.

Introduction

Phase transformations in metals are the most effective way to tailor their microstructure and properties. The transformations often follow particular relationships between lattice orientations in the parent and daughter (transformed) phases. In titanium and zirconium alloys, the most commonly cited relationship is the Burgers orientation relationship, where the phase transformation takes place between a high-temperature body centered cubic (bcc) phase β, and a low-temperature hexagonal close packed phase α during both cooling (β → α) and heating (α → β) [1]. Here, two parallel planes correspond to both close-packed planes, ${(101)}_{\mathrm{\beta }}\Vert {(0001)}_{\mathrm{\alpha }}$, whereas the two parallel directions relate to the nearest-neighbor directions, ${[11\bar{1}]}_{\mathrm{\beta }}\Vert {[11\bar{2}0]}_{\mathrm{\alpha }}$ (Fig. 1) [2]. Therefore, a single β grain can potentially transform to 12 possible orientations/variants during the β → α phase transformation. This should lead to a weak (random) final α texture if all variants are formed with equal statistical probability during the β → α phase transformation.

The texture resulting from diffusional phase transformations in Ti and Zr alloys, however, differs significantly from the theoretical texture expected from the Burgers orientation relationship [3], [4], [5]. In other words, diffusional phase transformations lead to the occurrence of specific orientations/variants through a phenomenon known as variant selection. Four possible mechanisms have been linked to variant selection: (i) the retained stress in the high-temperature phase, due to a volume change on heating [4], [6] and/or externally applied deformation [7], biases the transformation product characteristics on cooling; (ii) the existence of metastable α at high temperature, which acts as nuclei for growth on cooling [3], [8]; (iii) the self-accommodation of the strains produced by the β → α transformation through the formation of specific variant cluster arrangements [9], [10]; and (iv) the presence of specific grain boundary characteristics in the high-temperature β phase, enhancing the nucleation of certain orientations/variants during phase transformation [11], [12], [13], [14], [15]. Among the different hypotheses, the latter was considered the best possible reason for the strong variant selection observed during phase transformations for both Ti and Zr alloys. Bhattacharyya et al. [11] were among the first to demonstrate that high-temperature β grain boundaries in Ti have a tendency to terminate on {1 1 0} planes, resulting in the nucleation of α variants with close alignment of their (0 0 0 1) poles on both sides of the β boundary.

The extent of variant selection (texture development) through the β → α transformation may, however, be influenced by high-temperature β phase characteristics (texture [3], [16] and grain size [15]), as well as heat treatment parameters (annealing temperature and cooling rate [3]). Among the different parameters affecting the variant selection mechanism, the effect of cooling rate on the variant selection during the β → α phase transformation in Ti alloys has received the least attention. For comparison, it is known that the promotion of the displacive phase transformation (increasing cooling rate or undercooling) weakens the variant selection in Zr alloys [3] and steels [17], [18]. Therefore, the first objective of this paper is to describe the influence of cooling rate on the variant selection mechanism during the β → α transformation in a Ti–6Al–4V alloy (i.e. diffusional vs. displacive phase transformation).

Accelerating the cooling rate promotes a displacive phase transformation mechanism, leading to the formation of a martensitic microstructure consisting of laths/plates with a high dislocation density and/or twins, depending on composition [9], [10], [19]. The crystallography of the β → α martensitic transformation has been comprehensively studied over the past decades. The core emphasis was given to morphology [10], orientation relationship [19], habit plane [19], [20], lath/plate dislocation substructure [10], [21] and self-accommodation effects [9], [10]. A limited attempt was also made to characterize the intervariant interfaces formed through the martensitic transformation using transmission electron microscopy (TEM), though it was restricted to only one boundary type [9], [10]. This is not surprising, as the conventional techniques for the characterization of interface/boundary planes, such as TEM and three-dimensional electron backscatter diffraction (EBSD) techniques, are still relatively complex and time consuming.

Recent developments in characterization techniques make it feasible to statistically measure all five independent boundary parameters (i.e. three lattice misorientations and two plane orientations) in polycrystalline materials with different crystal structures [22], [23], [24], [25], [26], [27], [28], [29], [30], [31]. This approach measures all five macroscopic parameters from conventional EBSD maps using automated trace analysis software [32]. Only one five-parameter analysis has been carried out for Ti–6Al–4V alloy so far, and it did not exactly examine the boundaries expected from the Burgers orientation relationship [29]. The second objective of the current work is, therefore, to provide a detailed description of the intervariant interface/boundary character distribution in a martensitic microstructure formed in a Ti–6Al–4V alloy using the five-parameter grain boundary analysis approach. The motivation for this quantitative analysis of the interface distributions is to develop structure–property relationships for this important alloy. Knowledge of the evolution of the grain boundaries and interfaces during phase transformations and their relationship to properties could ultimately be used to control the mechanical properties of the Ti–6Al–4V alloy.