Superplastic behaviour and mechanical properties of two phase TiAl alloys

High temperature plastic flow properties of two phase TiAl alloys containing 45 to 49 at.%Al have been investigated in thermo-mechanically grain refined materials in order to clarify the favourable microstructure and chemical composition for TiAl based superplastic materials. Grain sizes of thermomechanical treated materials and their grain size stability during subsequent high temperature deformation strongly depend on chemical composition. It was found that Ti-46at%Al offers the best ady ratio which produces a fine and stable microstructure, whilst exhibiting superior superplasticity at temperatures exceedig 1100°C and a strain rate of around l ~ l o ~ s l (with m-value of 0.44 at 1100°C and 0.64 at 1150°C) as well as preferred mechanical properties at temperature of up to 1000°C. This alloy was proposed as a baseline alloy for superplastic materials to be later modified by third elements effective for the formation of metallic phase. Correspondingly, effects of heat treatment on changes in microstructure and tensile properties have been studied in fine grain TiAl alloys in order to estimate the possibility of improving high temperature strength after superplastic forming. A new kind of microstructure consisting of coarse lamellar colonies and fine colony boundary grains of y and lamellar (to be refereed to as partially-transformed structure) was found to be obtained by heat-treatment of just above the a-transus. It was also found that the partially-transformed structures exhibit a better combination of room temperature ductility and high temperature strength than any other microstructure previously observed. Titanium aluminide alloys based on gamma TiAl have recently received considerable attention because of their potential to replace titanium alloys and nickel-base alloys in aerospace systems, such as advanced turbine engines and hypersonic vehicles, where specific strength and stiffness at high temperature are critical. Many attempts focusing on improving mechanical properties of TiAl alloys have been made and significant improvements on room temperature ductility and fracture toughness have been achieved in two phase TiAl alloys during the last couple of years [I]. In addition to improvements of mechanical properties, it is extremely important to improve plastic formability of these materials to put them into practical use. In particular, rendering superplasticity seems to be indispensable to sheet metal forming application of TiAl alloys, which is the most useful methods of fabricating various kinds of lightweight structures. The possibility of rendering superplasticity to TiAl alloys has been ascertained in several development efforts and N-Mashashi et.al. have shown that formation of P phase at grain boundary is very effective to improve superplasticity of TiAl alloys [2,3]. Superplastic materials generally require a fine and stable grain structure. However, methodical knowledge concerning the possible relationship between chemical composition (Ti/Al ratio) I microstructure (grain size and stability) I plastic flow properties has not yet been established sufficiently to optimise the chemical composition and the manufacturing process of superplastic materials. In this work, the influences of TiJA1 ratio on microstructure and deformation property were closely studied in order to make clear a binary baseline alloy in which a F i e and stable microstructure may be achieved. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:1993761 JOURNAL DE PHYSIQUE IV However, superplastic TiAl alloys might exhibit the undesirable properties of high temperature strength and creep resistance because of their fine grain structure. Therefore, superplastic formed parts should require heat-treatment to improve the mechanical properties especially high temperature strength for structural application in hot sections. In this study the effect of heat-treatment on microstructures and mechanical properties of fine grain TiAl alloys were investigated in order to ascertain the possibility of improving the high temperature strength of superplastic formed parts. EXPERIMENTAL PROCEDURE Two phase binary TiAl alloys having five different chemical composition were prepared in Kobe Steel Ltd. using vacuum induction skull melting, homogenisation and thermo-mechanical treatments. Thermo-mechanical treatment was camed out by extrusion with a reduction of cross section area by a factor of four at 1200"C, followed by annealing. The annealing conditions were optimised respectively for each material within the range of 1000°C to 1200°C and 5H to 10H in order to achieve the finest equiaxed grain microstructure. Aluminium content of thermo-mechanical treated alloys are shown in Table 1. Aluminium content of these materials decreased slightly during thermo-mechanical treatment. Impurity elements are controlled to very low concentrations; the content of oxygen was less than 500ppm, nitrogen and carbon less than 100ppm. The compression test specimens with dimensions of 12mm in both diameter and length as well as tension test specimens having a gage dimensions of 4mm in diameter and 6mm in length were machined from thermo-mechanical treated materials, making length direction parallel to the axis of extrusion. Deformation properties of these materials were measured over the ranges covering temperatures of 1000°C to 1150°C and strain rate of 4x10-5s-1 to 5 ~ 1 0 ~ s l by constant strain rate compression and tension tests within a vacuum. Metallographic observation with optical microscopy was made at various stages of deformation. Heat-treatment tests of these materials were performed at various temperatures between 1100°C and 1425°C for two hours under vacuum conditions, followed by furnace-cool under a flowing argon atmosphere. Systematic metallographic observation was conducted on heat-treated materials. Cylindrical-gauge tensile specimen having gauge dimensions of 4mm in diameter and 15mm in length were machined from both heat-treated and non heat-treated materials having typical microstructures. Tensile tests were camed out at room temperature, 800°C and 1000°C in an argon atmosphere. Table 1 A1 content of TiAl alloys used in this study (at.%Al). as cast after TMT Ti-45at.%Al 45.4 44.9 Ti-46at.%Al 46.1 45.4 Ti-47at.%Al 47.3 46.8 Ti-48at.%A1 47.9 47.4 Ti-49at.%Al 49.0 48.4 RESULTS AND DISCUSSION Effects of Tim Ratio on Microstructure (1) Grain Refinement Fig.1 shows typical microstructures of TiAl in as cast and thermo-mechanical treated conditions. The microstructures in as cast condition are coarse lamellar colony structures consisting of alternate y and azlayers, although small amounts of y grains can be observed in Ti-47Al, Ti-48Al and Ti-49A1 alloys. After thermo-mechanical treatment, the microstructures becomes a mixture of two kinds of structures, one being a dual-phase equiaxed structure (y grain + a2 grain) having a very fine grain size of about 6pm in diameter, and the other a rather coarse equiaxed y grain structure of 20-50pm in diameter. Fig.2 shows the area ratio of a fine dual-phase equiaxed structure as a function of Al content. The area ratio strongly depends on chemical composition and changes drastically around an Al content of 47at.%. This results in Ti-45Al and Ti-46Al consisting dominantly of fine dual-phase equiaxed structure, whilst Ti-484 and Ti49Al consist mainly of equiaxed y grain structure. The area ratio of a f i e dual-phase equiaxed structure may depend in principle on the ratio of y phase and a 2 phase contained in these materials. However, considering that equiaxed y grains should nucleate during annealing at the colony and grain boundaries of lamellar and y grains, the area ratio may more or less be controlled by optimising as cast microstructure and subsequent thermo-mechanical processes. Although fine grain structures could be achievable in Ti-46Al and Ti-45Al to the same extent, Ti-45Al is much more difficult to break up its lamellar structure entirely through thermo-mechanical treatment due to its larger a 2 phase content. Therefore it should be concluded that Ti-46Al offers the best a21y ratio in obtaining a fine grain structure and is therefore most desirable as a superplastic material. Fig.1 Cast and thermo-mechanical treated microstructures of Ti-46at.%Al and Ti-49at.%Al. A 0 before deformation