The massive transformation in titanium aluminides: Initial stages of nucleation and growth

Abstract

Rapid solidification by twin-anvil splat quenching captures the initial nucleation and growth of the ␣ → ␥ m massive transformation in titanium aluminides. Splat quenching Ti 52 Al 48 and Ti 50 Al 48 Cr 2 from the liquid at slightly below the melting point produces an equiaxed ␣ solidification structure. Solid-state cooling rates that approach 10 6 K/s arrest the ␣ → ␥ m massive transformation with 1-to 5-m-sized ␥ m nuclei, especially in the Ti 50 Al 48 Cr 2 alloy. Classical massive-transformation heterogeneous nucleation occurs at ␣:␣ grain boundaries with an orientation relationship of [111] ␥ //[0001] ␣ and {110} ␥ //{1120} ␣ . The ␥ m nucleus then grows into the adjacent ␣ grain without the orientation relationship by forming an incoherent ␣:␥ interface with {111} ␥ facets. Orthogonal variants of the tetragonal c-axis in the ␥ m product suggest that the massive transformation initially produces an fcc structure which subsequently orders into the L1 0 phase. Nucleation of ␥ m is not only observed at ␣:␣ grain boundaries and triple points, but also within the ␣ grains. The intragranular ␥ m nucleation, which is believed to be heterogeneous, occurs with the same orientation relationship as for the intergranular nuclei. However, the intragrain nuclei do not form {111} ␥ facets and retain a curved ␣:␥ m interface. Although analysis of the {111} ␥ faceted growth using weak-beam dark-field (WBDF) imaging shows no evidence for any type of misfit-compensating dislocations, lattice imaging of the {111} ␥ facets with high resolution transmission electron microscopy (HRTEM) reveals that the planar interface exhibits a slight curvature, produced by atomic steps of (111) planes. These experimental data have been used to estimate a ratio of ledge spacing ( ) to ledge height (h) for the {111} ␥ facets as /h ϭ 41, which is similar to calculated values for a ledge growth mechanism of massive transformations in Cu-Zn and Ag-Cd alloys.

Static and in-situ high-resolution transmission electron microscopy investigations of the atomic structure and dynamics of massive transformation interfaces in a Ti-Al alloy

“…Static and in-situ HRTEM experiments provide evidence ing the ␣ → ␥ M transformation in Ti-Al is principally accomfor the existence of structurally incoherent massive transplished by short-range atom jumps more or less continuously formation interfaces, where there may be a small or negliacross structurally incoherent boundaries. Wittig, [50] in his gible barrier to interface motion under reasonably high article in this publication on the same transformation in driving forces for growth. Quasi-periodic patches of NCS rapidly solidified Ti-48Al-(0-2)Cr alloys, made similar atoms may serve as a possible barrier to interface motion.…”

Mechanisms of the massive transformation

“…kinks on the risers of growth ledges, as described in detail Examples of ␣:␥ m interfaces in near-TiAl alloys formed by Burton et al [75] In the case of crystal → crystal transforby a rational orientation relationship which failed to grow mations involving a change in stacking sequence, the probacross their ␣ grain boundary are shown in other papers lem faced by an atom attempting to jump to the product in these proceedings. [25,87] Figure 7 illustrates several such phase is the inverse of that in the crystal → fluid case. If situations during the → m transformation in MnAl(C).…”

Influence of crystallography and bonding on the structure and migration of irrational interphase boundaries

“…[38] The massive transformation of (hcp)␣ to the (initially [34,35] ) (fcc)␥ m in near-TiAl alloys has been the subject of much detailed study excited by the combination of planar facets with largely irrational crystallography (Figure 1). [3,4,8,34,35,117] Here we consider recent work in which the structure of these facets is analyzed by an edge-to-edge model quite different from that of Kelly and Zhang. [1,2] Fundamental aspects of this model will be described in more detail than was feasible in the original brief publication.…”

“…[2] Whereas the ␥ plates are well formed, exhibit the {111} L1o //{0001) ␣ orientation relationship customary during fcc-type 4 hcp precipitation reactions, and have the dislocation structure [118] anticipated from previous research on ␣:AlAg 2 interfaces, [55,56,57] ␥ m is typically blocky or idiomorphic in appearance, with many planar facets (Figure 1). [8] However, linear misfit compensating defects at these facets are largely [8,117] though not entirely [3,36] absent. Both orientation relationships and conjugate habit planes applicable to these facets are usually irrational, even at the atomic level.…”

“…In addition to {111} ZrN boundaries, the presence of a {111} boundary on intermetallic product phase crystals in contact with a high index plane in the solid solution phase has also been noted in two massive transformations: (hcp) ␣ : (fcc) ␥ m in near-TiAl alloys, [8,38,117,147] and (hcp) : (fcc) m in MnAl(ϩ2 at. pct C).…”