【正文】 程中，α相最終導致了合金的低強度和高的延展性。當加熱溫度為450℃時，,當加熱溫度為650℃時，等于905MPa。對比于強度而言，合金延展率的變化有不同的傾向，%（450℃）%（650℃）。圖4（b）中展示出了在450℃下加熱不同時間后合金機械性能的變化。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。隨著加熱時間的繼續(xù)增加，析出物的距離變得特別小，從而很難再析出，以致于導致了在測試的過程中，金屬的強度升高而延伸率下降。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)。結果顯示出破碎是在內(nèi)部出現(xiàn)的表現(xiàn)出漣漪的特性。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。，二期處理之后，析出物之間較長的距離是導致合金低強度和高延展性的主要原因。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。隨著加熱溫度的逐漸增加，中等厚度和較厚區(qū)域合金的拉伸力下降而延展率升高，總的來說，合金中部的伸長率和延展率都比較厚部分的高，但是當溫度厚度部分高于510攝氏度時加熱8小時后，合金中等厚度部分的伸長率和延展率卻都比較厚部分的低。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，從而導致了內(nèi)部應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。kL是一個常量，d是晶粒直徑。晶粒內(nèi)部應力可以由Ashby的Orowan公式來描述。τ=Gb/2 π( D l) ln l/ r0 (2)其中G是剪切模量，b是伯格斯向量，r0=4b,D是析出物之間的距離，L是析出物的厚度。當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晶粒邊界應力和內(nèi)部應力的混合作用關系式可以表示為：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致屈服強度的下降，但同樣可以導致在晶粒內(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。當在510攝氏度下加熱8小時后，大尺寸晶粒的伸長率在晶體內(nèi)部的析出物中要遠遠超過小尺寸的晶粒。4 結論（1）凝固后的合金的微觀結構是由各方等大的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。隨著冷卻凝固的增加，合金的晶粒尺寸變小，伸長率和屈服強度增加。同時，合金的延展率升高。（2）隨著加熱溫度的升高和加熱時間的增加，針狀的相變得粗糙。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。（3）隨著加熱溫度的升高，伸長率和屈服強度下降而延展率升高，隨著加熱時間的升高，伸長率和屈服強度稍有升高而延展率下降而伸長率下降。（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。它可以滿足臨界領域這種合金的使用要求。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。 solidification cooling rate。 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