【正文】 mechanical properties1 　INTRODUCTIONTitanium alloys have received appreciated attentions in the fields of aircraft, aerospace, and others owing to their excellent mechanical properties, especially the high specific strength. With regards to lower the mass of aircraft and improving their suitability for transportation , an important class named beta titanium alloys are developed to meet the requirement of the above situations[1 ,2 ] . As the result s of good properties bination of high eremitic strength, elastic modulus and elongation, the alloy Ti215V23Cr23Sn23Al (Ti21523) has bee a potentially selective material to be used among those beta type alloys [3] .From Ref. [4], it is known that the alloy Ti21523 has good workability at room temperature and suitable for cold working. Unfortunately, the high processing cost and drawbacks of low plasticity and high deformation force of the alloy have made it difficult to produce plex and thin walled ponents that are being the keynotes for aero applications [5]. In order to reduce the processing cost and reach the flexibility of shaping Ti21523 alloy, the technique of precision casting has been involved in the field. But due to the large beta grain size and lower mechanical properties under casting condition, the usage of the as2cast Ti21523 alloy is limited. Because of the strengthen effects of heat treatment on the beta type titanium alloys, the Ti21523 alloy can somewhat be strengthened to the extent of high level of the mechanical properties. The investigations on the effect s of heat treatment on titanium alloys have been carried out by America and the former Soviet Union [6, 7]. As it is pointed out that after heat treatment, the matrix precipitates alpha phase in grain interior and at grain boundaries as well. The appearance and distribution of alpha phase improve the mechanical properties of the alloy dramatically [8]. The purpose of this article is to investigate the effect of different solidification cooling rates and heat treatment on the microstructure and mechanical properties of the alloy in order to find an efficient measurement to further improve the mechanical properties of the alloy.2 　EXPERIMENTALThe experimental raw materials came from spongy titanium, vanadium aluminium master alloy, high purity aluminum block , chrome powder and tin block. Then they were melted in an induction skull melting furnace according to the nominal position of the alloy which posed of 15 %V, 3 %Al, 3 %Cr, 3 %Sn, and the balance Ti. The total mass of the charge was 18 kg. The pouring parameters were set as the speed of 200 r/ min for rotating table and the pouring temperature of about 1750 ℃. In order to study the effect of different solidification cooling rates on the solidification microstructure and mechanical properties of the alloy , the molten alloy were centrifugally pouring into a step metal mould with the gauge of 235 mm in length , 100 mm in width , and 50 mm , 25 mm , and 10 mm in thickness respectively. The samples for the analyses of microstructure and mechanical properties of the alloy came from the step specimen. The samples for heat treatment was solute treated at 800 ℃for 20 min and then water cooling as well as the treatment of different ageing temperatures and times with air cooling. The microstructure of the alloy was studied with optical microscope and TEM. The morphology of fractures after tensile test was also investigated by SEM. The mechanical properties were tested in model Instron 1186 electric tensile machine.3 　RESULTS AND DISCUSSION3. 1 　Effect of solidification cooling rate on microstructure of alloy The microstructure of the alloy after solidification is shown in Fig. 1. The equiaxed beta grain is found with a few of gas and shrinking holes in grain interior and at grain boundaries as well. The secondary phases with black color were confirmed as a non equilibrium solidification structure. With increasing solidification cooling rate , the grain size bees smaller. The grain size is small where attached to the inner surface of mould or positions with smaller casting size because of the action of chilling effect of mould inner surface and smaller casting size on the alloy. Compared with the thin section, the middle and thick section have less difference in grain size.3. 2 　Effect of solidification cooling rate on mechanical properties of alloy Table 1 has shown the effect s of different solidification cooling rates on the tensile properties of the alloy. With increasing solidification cooling rate , the tensile strength of the alloy increases. At the same time the elongation of the alloy is improved. The increase of strength and elongation attributes to small grain size [9]. Compared with the thin section, the middle and thick section has less difference in tensile properties.3. 3 　Microstructure of alloy af ter heat treatmentAt the normal condition the single beta microstructure for the alloy can be obtained by air cooling and water cooling. After solution treatment and different ageing treatments, acicular alpha phase is observed at grains interior as well as in grain boundaries. A good bination of strength and elongation can be achieved with proper heat treatment .Figs. 2 (a) and (b) show TEM image of the alloy aged at 450 ℃ and 650 ℃ for 8 h. With increasing ageing temperature, the acicular alpha phases bee coarse. There exist s incoherent corresponding relationship between the alpha phase and matrix [10]. Fig. 2 (c) shows alpha phases precipitate at grain boundaries. The angles between the grain boundaries and alpha phases were estimated about 30176。附錄2Microstructure and mechanical properties of high strength as cast Ti21523 alloyAbstract：The effects of heat treatment and solidification cooling rate on the microstructure and mechanical properties of as cast Ti21523 alloy prepared by induction skull melting method were investigated. R