Shigehisa Naka

Phase transformation mechanisms involved in two-phase TiAl-based alloys—I. Lambellar structure formation

Abstract The present paper deals with the mechanism operating in one of the three major transformation modes appearing in two-phase TiAl-based alloys: the formation of the so-called γ α 2 lamellar structure. This microstructure, the most typical of the alloys of this category, results from precipitation of γ (Ll0) lamellae in the α (A3) or α2 (DO19) matrix. This reaction is achieved through (i) pre-nucleation stage corresponding to the formation of a local f.c.c. type stacking sequence by the movement of Shockley partials; and (ii) nucleation and growth stages involving both ordering and ledge movement. The presence of numerous order domain boundaries in a given γ lamellar precipitate is explained by the encounter of separately nucleated order domains.

A quantitative TEM analysis of the lamellar microstructure in TiAl based alloys

The microstructure of lamellar TiAl alloys was analyzed by transmission electron microscopy. Two alloys (a PST crystal and a Ti{sub 54}Al{sub 46} alloy) were investigated. The microstructure is formed by {alpha}{sub 2} and {gamma} lamellae which are very long and very thin with respect to their length. The proportion of {gamma} lamellae is higher than that of {alpha}{sub 2} lamellae. The orientation relationships associated with the {gamma}-{gamma} interfaces are examined: they correspond to the order domain, twin and pseudo-twin relationships. Statistical analyses have been performed on the volume fraction of the two phases, on the orientation of the {gamma} lamellae and on the nature of the {gamma}-{gamma} interfaces. These results are used to have a better understanding of the structural transformation (hexagonal phase - > face centered cubic phase) and the ordering process which are active in the process of formation of the microstructure during cooling.

Phase transformation mechanisms involved in two-phase TiAl-based alloys. II: Discontinuous coarsening and massive-type transformation

Abstract This paper is directed towards examination of the mechanisms involved in two of the three major transformation modes appearing in two-phase TiAl-based alloys: discontinuous coarsening and massive-type transformation. The discontinuous coarsening implying grain boundary migrations is essentially promoted by highly stable interfaces in the primary lamellar structure, which render the normal lamellae coarsening of continuous type extremely difficult. During rapid cooling, the massive-type transformation with a direct transition α → γ replaces the primary γ lamellae precipitation. The nucleation takes place most likely on grain boundaries through the formation of either an ordered nucleus or a disordered f.c.c. nucleus followed by ordering, and the nucleus is coherent with one of the two grains. Since such a coherency renders diffusion across the nucleus/matrix interface a difficult process, the growth of the massive phase occurs into the opposite grain through thermally activated short-range jumps of individual atoms across an incoherent α γ interface.

On the existence of transformation-induced antiphase boundaries in the γ-phase of two-phase TiAl-based alloys

Abstract The existence of transformation-induced γ antiphase boundaries (APBs) is discussed in TiAl-based alloys in which two phases, γ and α2, are in equilibrium at low temperatures. While such APBs are frequently observed in the metastable γ-phase after the massive transformation α → γ, they are rarely observed within the γ lamellae of the so-called lamellar structure. In both cases (massively transformed structure and lamellar structure), however, other possible transformation-induced defects, such as twin and pseudo-twin boundaries as well as order-related domain boundaries, are found in abundance in the γ-phase (ordered tetragonal L10). The possibility of coupling of APBs with the other boundaries is examined in order to explain the above-mentioned experimentally observed difference regarding the γ APBs.

High-resolution electron microscopy study of γ-γT twin interfaces in the lamellar structure of TiAl-based alloys

Abstract The so-called TiAl/Ti3Al lamellar structure has been studied in a two-phase TiAl-based alloy by using high-resolution transmission electron microscopy, in order to verify the existence of various mixed boundaries (MBs) in the γ-phase of the lamellar structure. In this investigation, particular attention was paid to the atomic structure of {111} twin boundaries (TBs) γ-γT. Although only a few cases allowed us to examine this kind of interface, the existence of mixed twin boundaries could be confirmed under particular conditions of specimen orientation. This result strongly suggests that the other boundaries, namely order-domain boundaries (ODBs) and pseudo-twin boundaries (PTBs), can also exist in the mixed form. All these MBs can be formed through two distinct mechanisms: firstly the coupling of rather mobile antiphase boundaries with the other less mobile boundaries such as ODBs, TBs and PTBs which are primarily introduced in the lamellar structure and secondly the meeting of two separately nucl...

Variation in the degree of order and its influence on mechanical behavior in complex B2 aluminides of refractory metals

Abstract This paper deals with the variation in the degree of order and its influence on mechanical behaviour in complex B2 aluminides of refractory metals, comprising three groups of metallic elements: X ≡ Ti, Zr, Hf; Y ≡ Nb, Ta, Mo, W, Cr, V; Al. During high-temperature high-rate deformation (extrusion), many of these alloys, which are B2 single phase, seem to experience plastic instabilities because of a local variation in hardness (soft and hard zones). This variation in hardness stems presumably from the variation in the degree of order of the B2 phase associated with a change in the local chemistry due to dendritic segregation. Consequently, the cross-section of the extruded bars exhibits a surprisingly irregular shape. Metallographic examination of the cross-section shows a wavy contrast bearing a resemblance to “van Goghs sky” (VGS) paintings, although the materials are B2 single phase. Furthermore, waves of VGS follow closely the irregular contour of the cross-section. When tensile tests are conducted at room temperature on specimens machined from the extruded products, a very large tensile elongation (10%–28%) can be obtained in many cases. This unusually high elongation at room temperature for intermetallic materials is, in some cases, accompanied by a composite-like deformation behaviour indicating the existence of soft and hard zones even during tensile straining.

Migration of ordered domain boundaries and its effect on the lamellar interfaces in Ti?Al-based alloys

Microstructural investigations have been carried out on a fully lamellar Ti49Al47Cr2Nb2

Early stage of Al3Zr precipitation in a rapidly solidified Al-Cr-Zr alloy

Abstract Early stages of A13Zr precipitation have been studied in detail in the ternary Al-2.4 at.% Cr-1.1 at.% Zr alloy. A supersaturated solid solution of Zr in the asrapidly solidified state decomposes during subsequent heat treatment at 400°C. This decomposition leads to a homogeneous precipitation of very small Al3Zr (LI2) particles in the grain interior, but this homogeneous precipitation is not stable in the vicinity of mobile grain boundaries and is progressively consumed, giving rise to a fan-shaped precipitation of Al3Zr (Ll2), often coexisting with very small interlamellar particles of Al3Zr (LI2). All these precipitation modes are analysed in the light of the current theory of discontinuous reactions.

Influence of Si and C additions on microstructure and mechanical properties of the Ti-43.5Al-1Mo-4Nb-0.1B alloy

Separate effects of 0.3 at% Si and 0.2 at% C additions have been investigated on the Ti–43.5Al–1Mo–4Nb–0.1B alloy. Both elements appear to stabilise the γ phase. Si addition induces an increase in both γ-lamellar and γ-grain volume fractions. A heterogeneous distribution of primary and secondary silicides is observed in Al-rich regions of the microstructure, but silicon appears to be mainly in solid solution in the lamellar structure. The solid solution effect of silicon in the more plastically deformable β phase is probably very limited because of the high volume fraction of α2 in this alloy. This may explain the lack of beneficial effect of 0.3 at% of silicon on both tensile and creep properties. Carbon appears to stabilise the γ phase through an increase in γ-grain volume fraction and is found to be in solid solution in the lamellar structure. Our results suggest a very strong solid solution effect of carbon, since addition as low as 0.16 at% leads to an increase in yield strength at room and high temperature, a reduction in the secondary creep rate and a longer creep life.