Study On The Microstructure And Mechanical Properties Of Ti-Al-Fe-V-(Mo,Cr) System Alloys

In order to meet the mechanical requirement for titanium alloys in the field of aerospace, two types of high strength and high toughness near P-type titanium alloy were designed in the present study based on the currently-used Ti-1023, VT22and Timetal555. The corresponding nominal composition was Ti-3Al-2Fe-8V-1.5Mo and Ti-3Al-1Fe-4V-4.5Cr. Quasi-static tensile properties testing, fracture toughness testing, optical microscopic observation, scanning electron microscopic observation and phase diagram calculation were applied to systematically study the mechanical properties and microstructure under various heat treatment conditions. Moreover, the influence of microstructure on the mechanical properties was analyzed, based on which heat treatment procedure was optimized to obtain a good match of strength, ductility and fracture toughness. The present results provide reference and basis for further investigation and application of the two designed alloys.When solution treated in (α+β) phase, both designed alloys mainly consist of metastable β phase and spherically shaped ap phase. While keeping solution time constant, increasing solution temperature will lower the volume fraction of ap phase and transform its shape from strip to globular. While keeping solution temperature constant, prolonging solution time will make the spherically shaped ap phase grow larger and merge into rod. When solution treated above the β-transus, both alloys consist of solely equiaxed β grains, and grain size grows easily with the solution temperature rising.After (α+β) phase solution treated and aged, the microstructure mainly consists of ap phase, as phase and residual metastable P phase. Under the same aging condition, rising solution temperature will increase the length-width ratio of αs phase and decrease the corresponding thickness. Under the same solution condition, rising aging temperature will transform the shape of as phase from dot to acicular and increase the corresponding length-width ratio. However, prolonging the aging time will decrease the length-width ratio of as phase and increase the corresponding thickness, which will coarsen the αs phase significantly.After β phase solution treated and aged, the microstructure mainly consists of αs phase and residual metastable β phase. Tiny as phase precipitates along grain boundaries while keeping certain orientation relationship with the P matrix, and the as phase within the β grains are arranged in a staggered manner. Rising solution temperature will decrease the size of as phase and narrow the grain boundary as phase.If the alloys are only solution treated without aging, rising solution temperature will decrease the strength and increase the plasticity. After aging, the strength will be significantly enhanced by dispersion strengthening effect. Under the same aging condition, rising solution temperature will increase the strength and decrease the plasticity. Under the same solution condition, rising the aging temperature and prolonging the aging time have an effect of both decreasing the strength and increasing the plasticity.In order to obtain high strength-to-density ratio and excellent strength/toughness combination, the designed alloys are suggested to be solution treated at (Tp-40)-(Tp-20)℃for1hour and quenched in water, after which the aging treatment should be performed at550℃for8hours or600℃for2-8hours. As for the fracture toughness, it increases with solution temperature under the same aging condition, and decreases with aging temperature and time under the same solution condition.After heat treatment optimization, for Ti-3Al-2Fe-8V-1.5Mo the corresponding tensile strength, elongation and fracture toughness can be controlled in the range of1185～1340MPa,10～16.5%and62.3～80MPa-ml/2; for Ti-3Al-1Fe-4V-4.5Cr the corresponding tensile strength, elongation and fracture toughness can be controlled in the range of1145～1250MPa,9.5～14.5%and57.0～95.1MPa-m1/2respectively. As a result, the designed alloys possess high strength-to-density ratio and good strength/toughness combination.