Influence Of Double-stage Aging Process On The Microstructure And Mechanical Properties Of TC4 And TC18 Titanium Alloys

The consistency between strength and plasticity of titanium alloy could be achieved by double-stage aging heat treatment.In this paper,TC4 and TC18 titanium alloys were used as research objects,and the microstructure of the alloys were adjusted by means of double-stage aging heat treatment.Combined with X-ray diffraction（XRD）,metallography（OM）,scanning electron microscopy（SEM）,transmission electron microscopy（TEM）and mechanical properties tests,the effect of parameters of heat treatment process on the microstructure and mechanical properties of the alloy was studied.The main contents and results are as follow:（1）Firstly,the solution treatments of TC4 and TC18 titanium alloy with different holding temperature,time and cooling methods were conducted.The mechanical properties of the samples under different solution conditions were tested and optimized.Under the optimized solution condition the microstructure of the alloy was better.The optimized solution condition is 950℃-1h-WQ for TC4 alloy and780℃-1h-AC for TC18 alloy.Under these processes,the alloy has good compatibility of strength and plasticity.Under the optimized solution condition the tensile strength（σ_b）,HV₅,area shrinkage（Z）,elongation（δ）are 984MPa,336,14.94%,8.34%for TC4 alloy,and 949MPa,344,11.61%,8.34%for TC18 alloy.（2）Under the optimized solution processes mentioned above,the effects of single-stage aging on the microstructure and mechanical properties of the two alloys were investigated with varied holding temperature,holding time and cooling methods.The results show that the comprehensive mechanical properties of the two alloys are better under the condition of air cooling（AC）than water quenching（WQ）.For TC4 alloy,under the single-stage aging condition of 600℃-4h-AC,which has the best comprehensive mechanical properties（σ_b:1082MPa,HV₅:367,Z:18.45%,δ:9.03%）,the volume content of equiaxedαis 58.08%and the width of lamellarαis 0.92μm.For TC18 alloy,under the single-stage aging condition of 450℃-4h-AC,which has the highest strength and hardness（σ_b:1506MPa,HV₅:453,Z:1.94%,δ:0.71%）,the width of grain boundaryαand intragranularαare the smallest,which are 0.86μm and0.4μm respectively.（3）On the basis of the single-stage aging process mentioned above,the effects of the ascending and descending sequence of two-stage aging on the microstructure and mechanical properties of the two alloys were investigated.Compared with ascending aging condition,the descending aging condition has smaller equiaxedαand larger lamellarαin the microstructure of TC4 alloy.Theσ_band HV₅of TC4 alloy under descending aging condition（988MPa,338）are lower than that of ascending aging condition（998MPa,350）,and theδof descending（11.12%）is higher than that of ascending（6.4%）.For TC18 alloy,there’s little effect of the double-stage aging ascending and descending sequence on the strength,and the plasticity increases with longer pre-aging holding time.（4）During the heat treatment of TC18 alloy by 600℃-8h-450℃-4h and 600℃-4h-450℃-8h double-stage aging conditions,theα’has been precipitated in the pre-aging stage and coarsened in the second-aging stage（the width increased from 80±5nm to 100±5nm）.At the same time,smallerα’phase is precipitated along theα’and interlaced with theα’.Therefore,the change of the width of grain boundaryαand intragranularαcould not completely explain the variation of mechanical properties.62 Figures,29 Tables,105 References...