Investigation Of Processing, Microstructure And Properties Of Refined TiAl Alloys

Recently, much attention has been paid toγ-TiAl alloys, which are extensively used in the field of aerospace and rocket engine, gas turbine and vehicle industry due to their low density, high specific strength, high specific stiffness, good oxidation resistance and excellent creep and fractureness resistance. In this paper, refined TiAl alloys have been successfully prepared by a new powder metallurgy technique ( High Energy Milling (HEM) + Spark Plasma Sintering (SPS) or Hot Isostatic Processing (HIP)). And the processing of milling, sintering and desifying and microstructure and properties of as-sintered bulk materials were investigated systemically.The effects of milling temperature and composition on ball milling process and phase evolution of Ti-Al system were investigated. It was shown that the powder size will be decreased prominently when the milling temperature decreased during the milling processing of element powders with the nominal composition of Ti-47Al and Ti-45Al-2Cr-2Nb-1B-0.5Ta at low temperature (-140^-5℃). And the processing of alloying is not occurred in the Ti-47Al system during mechanical milling (MM). In addition, it was found that the alloy elements of Cr,Nb,Ta and B were benefit to weaken the effect of cold wielding during the milling processing of the powders with the nominal composition of Ti-45Al-2Cr-2Nb-1B-0.5Ta, and homogenous Ti/Al composite powder with a nanocrystalline structure and dispersed phases can be prepared.The nanocrystalline structure Ti/Al composite powders were prepared by double mechanical milling (DMM) (first-step milling + heat treatment + second-step milling) and the milling processing routes, powder microstructure, phase constitute and the reaction of powders during the processing of heat treatment was investigated in this paper. The results shown that the processing of multisystem milling was realized by DMM, the partially solid solution was formed, the atomic distance was decreased and the serious deformation occurred after first-step milling after first-step milling, and the TiAl,Al3Ti,Ti₃Al phase formed during the processing of heat treatment and more refined powders were obtaind after second-step milling. Furthermore, the alloy powders with the nominal composition of Ti-47Al(at.%) and Ti-45Al-2Cr-2Nb-1B(at.%) were prepared by DMM,which were mainly composed of nanocrystalline structure TiAl and Ti₃Al phase, and the size could reach 6-14nm.Ti-45Al-2Cr-2Nb-1B-0.5Ta (at.%) alloy with a crystalline structure was successfully prepared by HEM and HIP. It was exhibited that the grain size was increased as the sintering temperature increased, which is, from the size of 50-200nm sintered at 800℃to the size of 100-400nm sintered at 1000℃. However, the phase constitute was not changed and still composed of TiAl and Ti₃Al. After high temperature tensile test, the results shown that the elongation was 63-101% and the strength could reach 80-100MP at 850℃.γ-TiAl bulk materials with refined grain size were prepared by SPS and the effect of sintering temperature and alloy element on microstrcture and grain size ofγ-TiAl was researched. When the sintering temperature was 900 and 1000℃, the Ti-47Al prepared by HEM (mechanical milling and/or DMM) was composed of equixedγ-TiAl, Ti₃Al and Ti₂Al phase, with the sub-micro grain size. As the sintering temperature increased to 1100℃, the lamellar structure of TiAl andTi₃Al occurred, and the grain size increased, too, but it was sub-micro grade. Moreover, the grain size could be refined by adding some alloy element, such as Cr and B. It was shown that the bulk material of Ti-45Al-2Cr-2Nb-1B-0.5Ta alloy prepared by DMM and SPS (900, 1000 and 1100℃) was constituted by homogeneous and equixedγ-TiAl, and a little of Ti₃Al and Ti₂Al, and the grain size could reach sub-micro grade. In addition, it was also found some acicular TiB2 phase to exist in the bulk materials. With the sintering temperature increased, the density of as-sintered bulk materials increased promptly and the grain size was stable.It was shown that the properties of as-sintered bulk materials were highly related to the sintering temperature. With the sintering temperature was 900℃, the bulk material of Ti-47Al prepared by mechanical milling and SPS ( T47A-1) exhibited excellent properties, with the fracture strength of 1963MPa, and the compressive strain was 4.0%, the compressive strength was 1722MPa, the bending strength was 610MPa and the hardness could reach 686Hv. Compared to T47A-1, the compressive strength of the bulk material of Ti-47Al prepared by DMM and SPS (T-47A-2) was 100-200MPa higher, the bending strength was 100-370MPa higher and the compressive strain was 0.3-0.7% lower, respectively.It was found that the alloy elements have important impact on the properties of TiAl alloy. The compressive strain of Ti-45Al-2Cr-2Nb-1B alloy prepared by DMM and SPS was improved evidently due to addition of Cr and B, comparing to Ti-47Al prepared at the same condition. With the sintering temperature was 900℃, the compressive strain of Ti-45Al-2Cr-2Nb-1B alloy was 5.0% , comparing to 3.7% of Ti-47. After adding B, the compressive strength and bending strength was 100-200MPa higher than that of Ti-47Al.