| This paper carried out study on heat treatment processings and phasetransformation behaviors of a new type high-strength and high toughness TC21titanium alloy.Firstly, the phase transformation temperature has been determined withmetallography method. Then the phase contents has been analysed after differentheat treatment processing by x-ray and TEM analysis technique. The results showsthat the (α+β)/β transformation temperatures of two different melting furnacesnumber TC21 titanium alloy are 953℃ and 955℃ respectively. The (α+β)/βtransformation temperature is justly dependent on alloy chemical compositioncontents. So durring the enginerring practice, the phase transformation temperaturemust be determined firstly for titanium alloy semi-products.TC21 alloy acquired α+β phases after high temperature β solutioning treatment atdifferent cool rates. The lower of the cool rate, the α phase content in TC21 alloywill be increased and decreased for the β phase content.Secondly, the relationship among heat treatment processings and microstructureand mechanical properties of TC21 titanium alloy are also studied. The originallong-bar shape pre-α phase in basketweave gradually spheroidized as thesolutioning temperature increased. Solutioning at 903°C could keep themicrostructure in basketweave state after aging which can obtain excellentcombination of overall mechanical properties and damage tolerant properties.The aging alloy is composed of α+β phases, too. There is the fine niddle shapesecond precipitating α phase in β phases by TEM metallography method。Thestrength of TC21 alloy decreased when tensile plasticity increased after aging athigher temperatures. Aging at 570℃ to 590℃ can lead to fine and compact secondprecipitating α phase, and therefore the optimum overall mechanical properties anddamage tolerant properties can be improved. Aging time can also effect the amountand size and shape of the second precipitating α phase. The optimum aging time ofTC21 alloy should be about 4 hours.The heat treatment processing for TC21 titanium alloy forgings can be selected as900℃/1h,AC+(570℃-590℃)/4h,AC. For bars, the best heat treatment processingcan be selected as 900℃/1h,AC+(570℃-590℃)/4h, AC。Lastly, the microstructure design and the mechanism of strengthing andtoughening for high strength and damage tolerant titanium alloy has also beenanalysed and put forward in this paper. The study focus on two types ofmicrostructures for TC21 alloy: flake and basketweave. For flake microstructure thefracture profile micrograph shows a toughness fracture type, but the basketweavemicrostructure shows the combination of cross grain and along the grain interfacefracture. The strength and fracture toughness of flake microstructure are almostequivalent to that of basketweave microstructure, but the room temperatureplasticity of flake microstructure is lower than that of basketweave microstructureevidently. The fatigue crack propagation rate (da/dN value) of basketweavemicrostructure is lower than that of bimodal microstructure.
|
PDF Full Text Request