Solid Phase Transformation Kinetics And Microstructure Evolution Of Ti5Mo5V3Al-xCr

Ti5Mo5V3Al-xCr series titanium alloys are a series of beta titanium alloys independently researched and developed by the Beijing Nonferrous Metal Research Institute.The strength and toughness of the alloy are mainly influenced by the content and morphology of the precipitated phase in the aging process,while the difference in the content and morphology of the precipitates is related to the isothermal phase transition process of the alloy.Therefore,the research on the phase transformation process of Ti5Mo5V3Al-xCr series titanium alloy has great significance in theory and engineering application.In this experiment,five kinds of alloys with the Cr content of 1.0%,3.0%,5.0%,7.00/%,and 9.0%in Ti5Mo5V3Al-xCr series alloys were selected as the research object.By adjusting the isothermal aging time,the metallographic microscope,Scanning electron microscopy.simultaneous X-ray diffraction,and Rockwell hardness tester were used to analyze the microstructure changes in the isothermal transformations and their effects on mechanical properties.The ?+?/? isothermal kinetics equation of Ti5Mo5V3Al-xCr alloys were calculated in accordance with the JMA equation.The results show that the phase transformation rate of alloy decreases with the increase of Cr content.The nucleation mechanism varies with Cr content,Nucleation takes place at the grain boundaries and both in the grain which is found in 1Cr and 3Cr alloys.Nucleation preferentially occurred in the grain in 5Cr alloy,and there are brand regions with no precipitates.The precipitates are nucleated preferentially at sub-grain in 7Cr alloy.There is a long period before nucleation both inside the grain and at the grain boundaries in 9Cr alloy.The structure after aging are all flake alpha phase distributed fine and dispersed in the matrix of beta phase.Except for 9Cr alloy,the other four alloys barely have an incubation period during phase transformation.With the increase of Cr content,the amount of a phase precipitates decreases and the size increases accordingly.The hardness of the alloy after isothermal aging is directly proportional to the amount of precipitation of the a phase and inversely proportional to the size of the a phase.The hardness value of Ti5Mo5V3Al-xCr alloys after isothermal aging is directly proportional to the precipitation quantity of the alpha phase,but is inversely proportional to the size of the alpha phase.After full aging at 550 ?,the alloy can get the high strength of Rm more than 1200MPa.When the phase transition degree of 1Cr and 3Cr alloys reaches 50%,the best strengthening effect can be obtained,while 5Cr,7Cr and 9Cr alloys get the best strengthening effect when the phase change degree reaches 100%.The strengthening ability of the alloy decreases with the increase of Cr content.