Mechanical, Creep Properties And Microstruture Character Of Hot Rolled Ti-6Al-4V Alloy

By means of heat treatment, creep properties measurement and microstructureobservation, the influence of preparation technique and heat treatment on microstructure andcreep properties of hot rolled alloy are investigated. By means of SEM/EDS compositionanalysis and XRD analysis, and combined to the contrast analysis of dislocationconfiguration, the influences of heat treatment on the phases constitution and deformationmechanism of the alloy during creep are investigated. Some results are obtained given asfollows:The microstructure of hot rolled alloy consists of and phases, phase in the alloydisplays the strip-like configuration along the rolling direction, and the line-like phase isdistributed along the boundary of the stripe-like phase, but the width of the stripe-like phase is less than that of the forged alloy. In the range of the applied stress(550MPa~600MPa) and temperature (400?~420?), the creep activation energies of theforged and hot rolled alloy are measured to be102kJ/mol and108kJ/mol, respectively. Thedeformation mechanism of the forged alloy during creep is duplex slips of dislocations, the<a+c> dislocations with horizontal direction are activated on the (2110) prismatic planes,and the <a> dislocations with upright feature are activated on the (1103) non-basal planes.The deformation mechanism of the phase in hot rolled alloy during creep is duplexslips of the line-like <a+c> and <a> dislocations on the prismatic and pyramidal planes.Therefore, the lower creep resistance is thought to be the main reason of the alloy having thehigher creep strain rate and short creep life.After solution treatment at the temperature being lower than ? phase transition point,the microstructure of hot rolled alloy consists of bimodal structure, including thatsuper-saturation equiaxed phase and lamellar structure. The quantity of the equiaxedphase decreases as solution temperature enhances, and the needle-like and feather-likemartensite are distributed in the lamellar structure. After solution treatment at thetemperature being higher than ? phase transition point, the microstructure of hot rolled alloyconsists of basketweave structure, the lamellar structure of phases is arranged along thedirections at a certain angle. After aging at480?for150h, phase in the alloy displays theconfiguration with spiral-like feature, the particle-like phase is distributed along theboundary of lamellar phase, and the particle-like ? phase is dispersedly precipitated withinphase along the certain orientation. After aging for150h at480?, the microstructures of the alloy solution treated in ?/? phase and ? phase region still consists of bimodal structureand basketweave structure, respectively, but significant amount of particle-like phase isdispersedly precipitated within the grains and phase.The quantity of the equiaxed phase in the alloy decreases as solution temperatureincreases, which enhances tensile strength of the alloy at room temperature and400?~500?range. Compared to the strength of the alloy solution treated at960?and980?, afteraging at480?for150h, the alloy solution treated at1000?displays a higher tensilestrength at room temperature, but the alloy displays a lower tensile strength at400?~500?ranges.In the applied stress of575MPa~625MPa and temperature of400?~420?ranges, thecreep activation energies of the alloy solution treated at940?,1000?are measured to be185kJ/mol and249.8kJ/mol, respectively. Compared to the alloy with bimodal structure, thealloy with basketweave structure has the better creep resistance under the conditions of theapplied stresses and temperatures. The deformation mechanism of the alloy with bimodalstructure during creep is the wavy-like <a+c> dislocations activated on the pyramidal planesin phase with HCP structure, while deformation mechanism of the alloy with basketweavestructure during creep is (1/2)<111> dislocations activated in slip systems in phase withBCC structure?and it is thought that the V-rich ? phase with high volume fraction isresponsible for improving creep resistance of the alloy.In the range of the applied stress (550MPa~600MPa) and temperature (400?~420?),compared to hot rolled alloy and the alloy solution treated at1000?, after aging for150h at480?, the alloy solution treated at1000?displays the better creep resistance and longercreep lifetime. The dynamic recrystallization of the alloy during creep occurs to form thesubgrains structure, and the size of the subgrains decreases as the creep goes on. Anddislocation slip in the subgrains is thought to be deformation mechanism of the alloy in thelater stage of creep. Moreover, deformation mechanism of the alloy during creep is thehook-like <c> dislocations activated on the prismatic planes in phase with HCP structure.After aging for150h at480?, the alloy solution treated in ?/? phase and ? phase regionstill obtain bimodal structure and basketweave structure, respectively, thereinto, significantamount of particle-like phase precipitating within the grains and phases may hinderdislocation motion, which is thought to be the main reason of the alloy possessing bettercreep resistance and longer creep lifetime.