【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 af。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. Results show that the microstructure of as2cast Ti21523 alloy changes from the features of simplified and larger size of beta grains to finer grain size with increasing solidification cooling rate. After solution treatment and different ageing treatment, alpha phase precipitates in grains interior as well as in grain boundaries. Due to the modification of the precipitate phase, the tensile strength and elongation of the alloy are improved simultaneously. A good bination of the values of of and 4. 5 % of was obtained , which will be satisfied the use of this kind of alloy in critical areas.Key words：cast Ti21523 alloy。（4）總體來說，中等厚度部分的合金的伸長率和延展率均比較厚部分的高，,%時獲得的。同時，相的碎片數(shù)量也隨之增加，二期處理后相變得更加粗糙。同時，合金的延展率升高。4 結(jié)論（1）凝固后的合金的微觀結(jié)構(gòu)是由各方等大的β晶粒和在晶體邊界和內(nèi)部的一些氣泡和熱力孔所組成。當(dāng)考慮到拉伸屈服作用力的時候，我們便可以推斷出多晶材料時：=1/2 σ晶粒邊界應(yīng)力和內(nèi)部應(yīng)力的混合作用關(guān)系式可以表示為：σ s= σ i+ kL b 1/2+ Gb/ π( D l) ln l/ r0(3)增加晶粒的尺寸將會導(dǎo)致屈服強度的下降，但同樣可以導(dǎo)致在晶粒內(nèi)部的析出物的密度變大，從而使析出物之間的距離減小，比較較小尺寸的晶粒而言，較大尺寸的晶粒在晶粒內(nèi)部的析出物在對伸長率的影響與作用上占有優(yōu)勢。晶粒內(nèi)部應(yīng)力可以由Ashby的Orowan公式來描述。這個不期望的后果可以由晶粒從邊界向晶粒內(nèi)部逐漸混合，從而導(dǎo)致了內(nèi)部應(yīng)力起作用而獲得解釋σ s= σ i+ kL d 1/2 (1)其中σ i是斷層混亂運動中磨擦力的反作用力。圖6顯示出了熱處理之后合金中等厚度和較厚部分的機械性能，試樣在800攝氏度下完全處理20分鐘之后在不同的溫度下加熱8小時。雖然破碎是內(nèi)部的微粒，但是相對較小的微粒尺寸也許是高延展性的最好解釋，合金二期加熱后，強度增加而延展性下降，下降到了386兆帕。圖5顯示出了在450℃和650℃下加熱8小時后的合金破碎形態(tài)。隨著加熱時間的不斷增加，合金的伸長率和屈服強度稍有增加而延伸率卻下降了。對比于強度而言，合金延展率的變化有不同的傾向，%（450℃）%（650℃）。在進(jìn)行機械性能測試的過程中，α相最終導(dǎo)致了合金的低強度和高的延展性。導(dǎo)致合金機械性能變化的主要原因是晶粒的大小，數(shù)量以及基體上的α相。 圖4（a）中表示了在不同的加熱溫度下加熱8小時后合金機械性能的變化。圖3（）中展示出了TEM機假想的雙期處理后的合金的對比。隨著加熱時間的增加，α相變得越來越粗糙。在晶粒的邊界上大量的析出α相將導(dǎo)致合金的脆性。圖2（c）中顯示出α相析出于晶粒的邊界上，α相與晶粒邊界所成的角度估計30176。隨著溫度的逐漸升高，針狀的α相變得粗糙。經(jīng)過不同時期和不同方法的處理之后，針狀的α相出現(xiàn)在了晶粒的內(nèi)部以及邊界上，良好的拉伸和延伸率的結(jié)合是可以通過恰當(dāng)?shù)責(zé)崽幚韥磉_(dá)到的。比較較薄的部分而言，中等厚度及較厚區(qū)域在延展性方面并沒有太大的不同之處。隨著冷卻凝固率的增加，合金承受的拉力也隨之增加，與此同時，合金的延伸率也逐漸升高。越靠近模型的內(nèi)表面晶粒的尺寸越小，越小的鑄造尺寸結(jié)果也是如此。帶有黑色的第二幅圖被認(rèn)為是一個不平衡的冷卻凝固結(jié)構(gòu)。它的機械特性是在Instron 1186電子拉伸機上進(jìn)行測試的。合金的顯微結(jié)構(gòu)被放在高倍顯微鏡下和TEM機上進(jìn)行研究。熱處理的試樣在800℃下被加熱20分鐘，然后水冷。為了研究合金不同的冷卻凝固率對于其機械性能和微觀結(jié)構(gòu)的影響，熔化的合金離心后被澆注到一個長235mm，寬100mm，厚度分別為50mm、25mm、10mm的一系列金