双相区固溶温度对Ti-55531合金组织和力学性能的影响

Solution Temperature in Biphasic Region on Microstructure and Mechanical Properties of Ti-55531 Alloy

通过对Ti-55531合金在双相区不同温度(730~830℃)固溶2 h空冷后,经相同的时效工艺(600℃/6 h/空冷(AC))处理;再结合扫描电子显微镜(SEM)和拉伸试验等分析方法,系统研究了双相区不同固溶温度对该合金组织和力学性能的影响规律。结果表明,随固溶温度的升高,等轴αp含量降低,尺寸减小;后续时效析出的αs含量增多,形态也有显著变化,由全短棒状向短棒状+针状、针状+长片状、全长片状的顺序转变。固溶温度从730℃升高到780℃,塑性较好的αp含量减少导致合金塑性降低,αs含量增加导致合金强度提高;固溶温度从780℃升高到800℃,αs含量继续增多导致合金强度上升,适量的长片状αs促进了合金塑性提高;固溶温度从800℃升高到相变点830℃,过多的长片状αs导致合金强度和塑性都显著下降。合金的强塑性匹配较好时对应的固溶温度为780~800℃。合金的断裂方式都是以微孔聚集型为主、含解理撕裂和沿晶开裂的混合断裂机制,且随固溶温度的增加,合金塑性断裂机制减小,脆性断裂机制增加。

Apart from the hot working processing,the heat treatment was another important influential processing of mechanical properties inβtitanium alloys.It was found that solution treatments could tailor the grain size of originalβgrain or the amount and morphology of primaryα(αp)phase in metastableβtitanium alloys.Subsequently,it would change the characteristics of secondaryα(αs)phase during the aging process,and consequently influence mechanical properties of the alloys.Ti-5 Al-5 Mo-5 V-3 Cr-1 Zr(Ti-55531)alloy was a relatively new metastableβtitanium alloy with high strength based on the BT22(Ti-5 Al-5 Mo-5 V-1 Cr-1 Fe)alloy.The typical application of the alloy was as the replacement of Ti-10V-2 Fe-3Al(Ti-1023)alloy used for thick section forgings in aircraft industry,for instance,landing gears,flap trunks,linkage units between wing and hanging slash.Although there were many studies on same or similar type of alloys as mentioned above but the microstructure-property relationship of the Ti-55531 alloy was still not studied exhaustively.Only limited data on the alloy was available to design.The purpose of this work was to reveal the effect of solution temperature on microstructural characteristics and mechanical properties of the Ti-55531 alloy,and further to understand its relationship between microstructure and tensile property.The experimental material was Ti-55531 alloy forged bar billet with the diameter of 350 mm,supplied by Northwest Institute for Nonferrous Metal Research in China.Samples with the certain dimension(diameter of 11 mm and length of 75 mm)were cut from the bar for subsequent heat treatments.Subsequently,Ti-55531 alloy was treated by the same aging treatment(600℃/6 h/air cooling(AC))after solution at different temperatures(730~830℃)for 2 h in the biphasic temperature region and then air cooling.Furthermore,microstructural features of samples after different heat treatments were characterized by JSM-6700 F scanning electron microscope(SEM).The size and volume fraction of theαphase(including primaryαp particles and as lamellae),and parameters of retainedβmatrix were quantitatively analyzed by the image analysis software according to a large number of microstructural SEM images.Cylindrical tensile specimens with a center diameter of 5 mm and gauge length of 25 mm were machined after above heat treatments according to the Chinese standard GB/T228.1-2010.Tensile tests were carried out on an INSTRON5975 Testing Machine at ambient temperature with an initial strain rate of 1×10^(-3) s^(-1).The reported data of performance were the average of three individual tests to improve accuracy.The fracture surface and profile microstructural features of tensile specimen were observed by JSM-6460 SEM.The effects of solid solution temperatures in the dual-phase zone on microstructure and mechanical properties of the alloy were systematically studied.The results demonstrated that both the volume fraction and size of equiaxedαp particles decreased with increasing of solution temperature.At the same time,the volume fraction of as phase increased and its morphology changed significantly,from short rod to short rod+needle,needle+coarse lamella and full coarse lamella.When the solution temperature rose from 730 to 780℃,the plasticity of the alloy decreased due to the decrease of softerαp particles content,while the strength of the alloy increased with increasing of harder as phase content and the number of theα/βinterphase.With the increase of solution temperature from 780 to 800℃,the strength of the alloy continued to rise because the content ofαs phase andα/βinterphase still increased,and the plasticity of alloys increased slowly with increasing of a suitable amount of coarseαs lamella.With increasing of solution temperature from 800 to 830℃,excessive coarseαs lamella resulted in significantly decreasing of the strength and plasticity of the alloy.The solution temperature range for alloys with good cooperation between strength and plasticity was from 780 to 830℃.In addition,fracture modes of alloys were mixed fracture mechanisms,which were dominated by micro void accumulations and followed by cleavage tearing and intergranular cracking.With the increase of solution temperature,the diameter of dimples and the number of secondary cracks decreased,the volume fraction of intergranular cracks increased,in other words,the plastic fracture mechanism of the alloys decreased and the brittle fracture mechanism increased.It seemed that the strength of the Ti-55531 alloy with bimodal microstructure was primarily dependent on the volume fraction and size of harderαs phase,however,the plasticity of the alloy was mainly related to the volume fraction of softer equiaxedαp particles and the coarseαs lamella.The key effect of different solution temperature on mechanical performance of the Ti-55531 alloy the biphasic temperature region was that the volume fraction of equiaxedαp particles were influenced by the solution temperature,and subsequently the aging process affected the volume fraction and size ofαs lamella.