Bimodal Ultrafine-grained Ti-6Al-4V Alloy Prepared By High Energy Ball Milling And Spark Plasma Sintering

One of strategies to improve the plasticity of ultrafine-grained materials is to introducesome volume fraction of micron-sized grains to obtain bimodal ultrafine-grained materials.Ultrafine-grained Ti-6Al-4V material with a bimodal grain size distribution was prepared byhigh-energy ball-milling and spark plasma sintering in this paper. Research was focused onthe following content:Nanocrystalline Ti-6Al-4V powders was prepared by high-energy ball-milling. Duringthe ball-milling process, particle size of powders first increased then decreased, crystallinesize decreased and impurity contents increased with the increase of milling time. The averagecrystalline size of powders without PCA addition was reduced to nano-meter scale aftermilling time was10h, and reached16nm when milling time was50h. It was found that PCAcould accelerate the nanocrystallization and amorphization process of ball milled powders.Amorphous phase appeared in powders with PCA addition when milling time was30h.The effect of sintering parameters on the microstructure and properties of the sinteredsamples was studied. With the increase of sintering temperature, density of sintered samplesincreased gradually, microstructures were transformed gradually from ultrafine-grained,bimodal grained, and then to coarse grained structure, the compressive yield strength andultimate strength increased first and then decreased, but the plastic strain increasedcontinuously. With the increase of heating rate, density of sintered compacts decreasedslightly, the grains of different regions became coarser, and the volume fraction of coarsegrains increased observably, the compressive yield strength, ultimate strength and plasicstrain decreased slightly. With the increase of holding time, density of sintered compactsincreased gradually, the grains of different regions grew coarser slightly, and the compressiveyield strength decreased but the plasic strain increased.The effect of ball-milling PCA on the microstructure and properties of sintered sampleswas researched. Microstructure of samples sintered with powders with and without additionof PCA both contained fine equiaxial α+β structure, coarse recrystallized equiaxial α grainsand the regions with precipitation of fine globular β phases, but the distribution andproportion of each region were somwhat different. The samples sintered with the addition ofPCA obtained higher compressive yield strength and ultimate strength, with the plasic strainunchanged or lower, compared with those without addition of PCA.The effect of ball-milling time on the microstructure and properties of the sinteredsamples was investigated. Samples sintered with powders ball-milled for more than10h showed bimodal ultrafine grained structure. When the ball milling time increased from0h to50h, the average grain size of ultrafine-grained region first decreased and then increased, butthe volume fraction of coarse grains increased and then decreased. The compressive yieldstrength and ultimate strength increased markedly, while the plastic strain to failure decreasedquickly with the increase of milling time.For mixed powders (80%ball milled powder and20%unmilled powders), when thesintering temperature increased, the density of sintered compact was increased gradually, themicrostructure changed from bimodal-grained to coarse-grained structure. The compressiveyield strength first increased and then decreased because of grain coarsening, while the plasticstrain continuously increased with the increase of sintering temperature. The fraturemorphology of sintered samples consisted of smooth shear band and shear dimples, and thefraction of shear dimples increased as the sintering temperature increased.With the increase of unmilled powders content in the mixed powders, the volume fractionof coarse grain regions in sintered samples increased and their densities were reduced. Withthe content of unmilled powders increasing from0wt%to20wt.%，the yield strength ofsintered compacts didn’t decrease obviously, but the plastic strain increased from22%to28%.When the content of unmilled powders reached30wt.%, both the compressive yield strengthand the plastic strain decreased as a result of the increasing porosity in sintered samples. Withthe increase of the unmilled powders content, the proportion of shear dimples graduallyincreased in the fracture surface of sintered samples.When ball milling time was10h, mixed powders (10h:0h=4:1) were used as raw material,the sintering temperature was850℃, the sintering time was4min and the heating rate was100℃/min, a high-density bimodal ultrafine-grained Ti-6Al-4V alloy could be achieved. Therelative density of sintered compact reached99.5%, the microsturcture consist ofultrafine-grained α+β equiaxed structure with an average grain size of0.87μm and28%coarse grained α phases and α+β basketweave structure whose grain size was larger than5μm.The compressive yield strength, ultimate strength and plastic strain to failure of the sinteredsamples arrived at1306MPa,1656MPa and28%, respectively, showing optimizedcomprehensive mechanical properties.