Study On Heat Treatment Process Optimization And Fatigue Properties Of High-strength Beta Titanium Alloy

Titanium alloy has come into the field of vision for its excellent mechanical properties,corrosion resistance and so on.The application range is all over the fields of aviation,space,ship and weapon manufacturing.High-strength beta titanium alloy has attracted more and more attention due to its high strength and toughness and excellent cold and hot processing properties.The alloy is a kind of high strength structural material,it will be subjected to alternating stress during serve.It will easily cause fatigue failure and pose a serious threat to the safety of life and property.Based on the optimization analysis of heat treatment of the new Ti-Al-V-Mo-Cr-Zr-Fe-Nb alloy,the low cycle and high cycle fatigue properties of the alloy were investigated under optimized heat treatment conditions.The cyclic stress-strain response behavior,fatigue life as well as the initiation and propagation mechanism of fatigue crack were analyzed.The results of the study are as follows:When the new Ti-Al-V-Mo-Cr-Zr-Fe-Nb alloy solution treated above or below the phase transition point,the microstructures both are single beta phase and homogeneous,and the grain boundary is clear.With the increase of the solution temperature,the larger the grain size,and about 40?m at 760?.After solution below the phase transition point and aging treated,the secondary alpha phase has the following characteristics:uniform and small short rods and long needles with 60 angles.After solution above the phase transition point and aging treated,only long-needle alpha phase is shown.The alloy has good match of strength and plasticity when solution treated at 740?760?/0.5h/AC and aging at 480?/12h/AC,the tensile strength more than 1350MPa,and the elongation more than 13%.The effect of different heat treatment methods on the microstructure and properties of the alloy was studied on the basis of the solution aging temperature of 740?760 ?/0.5h/AC+480?/12h/AC,and the differences in microstructure,mechanical properties and microhardness of the alloy after solution cooling,direct aging and double aging were analyzed.In this experiment,750?/0.5h/AC+480?/12h/AC is the best heat treatment system.The tensile strength of the alloy is 1467.0MPa,the yield strength is 1380MPa,the elongation is 11.5%,the section shrinkage is 30%,the hardness is 368HV.Under the condition of low cycle fatigue loading controlled by the external total strain 0.8%?1.8%,the alloy softens obviously at the beginning of the cycle and saturation at the later stage to achieve cyclic stability.With the increase of total strain,the low cycle fatigue life gradually decreases.The cyclic strain amplitude and fatigue life of high-strength beta titanium alloy satisfy the Coffin-Manson equation:??t/2 = 623.6×(2Nf)-1.184 +16.405 x(2Nf)-0.0522Under the low cycle fatigue condition,the fatigue crack initiation position is closer to the near surface with the increase of the strain,and it is a single fatigue source.There are a number of quasi cleavage planes in the extended area,and there are a large number of dimples.There are two cracks in the low strain,the fatigue life increases,and a large number of dimples in the transient zone show the plastic fracture morphology.The S-N curve of high-strength beta titanium alloy is very dispersed,and the fatigue life increases with the decrease of stress amplitude.The Zheng Xiulin formula is applied to nonlinear fitting of high cycle fatigue data,and the stress fatigue life prediction expression is:Nf =1.42×10/(Sa-535)2The theoretical fatigue strength of the alloy is 535MPa,which is 0.42 times the yield strength of the alloy.Under the high cycle fatigue condition,the fatigue crack sprouts on the surface of the specimen and is a single fatigue source and a large number of quasi cleavage planes and a small number of dimples in the extended area.With the decrease of the stress value,two cracks are produced in the extended area,and the fatigue life is obviously increased.There are a lot of dimple patterns in the instantaneous zone,and the fracture characteristics are plastic fracture.In low cycle fatigue,the crack initiation mechanism changes from the cracking of the surface slip band to the grain boundary cracking caused by the dislocation pile-up with the increase of the strain,and the initiation mechanism of the high cycle fatigue crack is the surface slip zone cracking.The low cycle fatigue propagation zone is mainly quasi cleavage plane,and there are a lot of dimples.The crack propagates forward in a transgranular way mixed by ductile and brittle.Under high cycle fatigue condition,with fatigue stress decreasing,fatigue striations are formed near the two cracks in the expansion zone,and the cracks extend along the strip mechanism.The instantaneous fracture zone is filled with equiaxed dimples,and there are many tiny holes in the dimples,indicating that the fracture failure mechanism of the alloys is micropore aggregation.