Mechanistic Study On Optimizing Microstructure And Mechanical Properties Of TC18 Titanium Alloy Via Rapid Heat Treatment

TC18 titanium alloy（Ti-5Al-5Mo-5V-1Cr-1Fe,wt%）is usually selected to fabricate load-bearing components served at severe environment,such as landing gears,due to its high specific strength,good hardenability and excellent fatigue properties.At present,ingot metallurgy remains the predominant method to fabricate titanium alloys.However,the high content of refractory elements in TC18 titanium alloy can result in element segregation,and the melting loss of volatile elements often occurs during smelting.Modifying smelting process or adjusting the amount of raw materials can avoid the element segregation and make the content of volatile elements in an acceptable range,but the cost of production will increase.Besides,after traditional solution heat treatment and aging,TC18 titanium alloy usually possesses high strength but low ductility.For all the above reasons,the main purpose in this work is to produce cost-effective and high-performance TC18 titanium alloy.To obtain high strength and good ductility,rapid heat treatment（RHT）and aging（RHTA）was introduced as an alternative to solution heat treatment and aging.Firstly,different heat treatments were applied to the as-received TC18 titanium alloy with non-recrystallized microstructure to study the microstructure evolution and its impact on mechanical properties.Subsequently,the influence of non-recrystallized microstructure was eliminated,and the effect of RHT on phase transformation and composition fluctuation of TC18 titanium alloy was studied.The strengthening mechanism of RHT technology on mechanical properties was also revealed,and the tensile properties obtained via RHTA were better than those strengthened by solution heat treatment and aging.To control the composition accurately,avoid element segregation and reduce the cost of production,blended element powder metallurgy（BEPM）was proposed to fabricate TC18 titanium alloy.The as-sintered TC18 titanium alloy was subjected to hot rolling and recrystallization annealing followed by RHTA.The influence of residual pores on tensile and fatigue properties was studied in detail.In this work,it was found that the pre-hydrogenated titanium（Ti）-containing master alloy can reduce the porosity of as-sintered TC18titanium alloy significantly,and the mechanical properties were further improved.The main conclusions of this thesis are as follows:（1）The evolution of non-recrystallized microstructure during different heat treatments and its influence on mechanical properties were revealed.The evolution of non-recrystallized microstructure was the same to that inβgrains during solution heat treatment and aging.Specifically,the primaryα,characterized by different features,transformed into secondaryαwith uniform characteristics upon solution heat treatment and aging.However,during RHTA,theαlamellae perpendicular to grain boundary and distributed along grain boundary in non-recrystallized microstructure transformed into the secondaryαwith the same distribution characteristics.Compared with the content and size of primaryα,the influence of non-recrystallized microstructure one tensile properties evolution of specimens subjected to solution heat treatment and aging was lower.Therefore,with the increase of solution heat treatment temperature,the tensile strength of specimens increased firstly and then decreased.However,for the specimens after RHTA,the tensile strength increased with an increase in the amount of non-recrystallized microstructure,while ductility exhibited an opposite trend.（2）The strengthening mechanism of RHT on the mechanical properties of TC18 titanium alloy was studied.Owing to the short heat treatment time,RHT was found to mitigate the growth ofβgrains in singleβ-phase field and cause more pronounced composition fluctuation.Finerβgrains contributed to good ductility and the more pronounced composition fluctuation was benefit to refine secondaryα.Different from solution heat treatment and aging,RHTA can obtain finerβgrains and achieve the complete transformation from primaryαto secondaryαsimultaneously,which can optimize both strength and ductility.After RHTA,the tensile strength of specimens fabricated via ingot metallurgy increased firstly and then decreased with the increase of RHT temperature.The highest tensile strength can be obtained when TC18 titanium alloy fabricated by ingot metallurgy was rapidly heated to the temperature above phase transition temperature（T_β）.（3）By the combination of RHT and BEPM technologies,the influence of residual pores on mechanical properties was revealed.When the particle size distributions of titanium hydride（Ti H₂）and master alloy were 0-88μm and 0-38.5μm respectively,the porosity of as-sintered TC18titanium alloy was about 5%,and that was about 0.07%after hot rolling with 91%deformation.The residual pores did not affect the tensile properties of specimens after recrystallization annealing.However,for the specimens after RHTA,residual pores can promote the nucleation and propagation of crack at grain boundary which was harmful to the tensile properties.In the low cycle fatigue experiment with stress ratio of 0.1,the residual pores did not affect the low cycle fatigue life of recrystallization annealed samples,but reduced the fatigue life of specimens after RHTA by reducing the initiation time of fatigue cracks.Despite the negative influence of residual pores,excellent ultimate tensile strength（1570 MPa）and elongation（6.1%）still can be obtained via RHTA,and the low cycle fatigue strength of the specimens after RHTA was about 300 MPa higher than that of recrystallization annealed specimens.（4）The mechanism underlying the porosity reduction of pre-hydrogenated Ti-containing master alloy technology has been elucidated.When the particle size distributions of Ti H₂ and master alloy powders were 0-45μm and 0-28μm respectively,the porosity of as-sintered TC18titanium alloy can be reduced to 1.97%.Furthermore,the addition of 13 wt%Ti in master alloy followed by hydrogenation achieved an even lower porosity of 1.27%.This porosity reduction can be attributed to the addition of Ti,which reduced the formation of Kirkendall pores.The pre-hydrogenation of Ti-containing master alloy was benefit to reduce its oxygen content in the subsequent sintering process,and finally the oxygen content of as-sintered TC18 titanium alloy can be controlled below 0.25 wt%.The TC18 titanium alloy with porosity of 1.27%was subjected to hot rolling,recrystallization annealing and RHTA with the same process parameters.Finally,excellent ultimate tensile strength（1601 MPa）and elongation（10.0%）were obtained.Compared with RHTA samples before modifying BEPM process,the elongation increased by 64%.