Bimodal Fine Grained Titanium Prepared By High-energy Ball-milling And Spark Plasma Sintering

Pure titanium has low density, high specific strength, excellent corrosion resistance andbiocompatibility, however, it exhibits relatively lower strength, which limits its use forstructural and functional application. To produce pure titanium with high strength and goodductility, we proposed to prepare fine grained titanium bulk material with a bimodal grain sizedistribution by high-energy ball-milling and spark plasma sintering in this paper. Researchwas focused on the following content:High-energy ball-milling was used to prepare nanocrystalline titanium powders. Duringthe ball-milling process, with the increasing of milling time, the morphology of titaniumpowders changed from original sphericity to bulky flakes at the initial stage, and graduallyturned into broken tiny particles. TEM result showed that the grain size of titanium powdersball-milled for10h ranged from several tens to hundreds of nanometers.The not fully ball-milled mixed powders containing5h and10h ball-milled titaniumpowders were sintered by spark plasma sintering technology. The effects of sinteringtemperature, holding time and heating rate on the microstructure and mechanical properties ofthe sintered samples were researched. The density, compressive yield strength, fracturestrength and plastic strain increased with the increasing of sintering temperature. When thesintering temperature was800℃, the sintered sample were nearly fully densified, whichshowed a bimodal grained microstructure with a laminated distribution consisting of finegrained regions with grain sizes less than2μm and coarse grained regions with grain sizesmore than5μm. When the sintering temperature reached900℃, recrystallization occurred inthe sample, forming composite microstructure containing the matrix of micrometer gradeequiaxed grains and some embedded coarse flaky phase.The heating rate and holding time had few influence on the density and compressiveplastic strain of the sintered samples. With the increase of heating rate, the fine grainedregions of the sintered samples grew slightly coarser, and the compressive yield strengthtended to decrease gradually. With the increasing of holding time, although the fine grainedregions of the sintered samples grew coarser slightly, as the sample were sintered more andmore completely, consequently its compressive yield strength increased.The effect of ball-milling time on the microstructure and mechanical properties of thesintered samples were researched. Results showed that the microstructure of the samplessintered with titanium powders ball-milled for various time under the same sintering temperature obviously changed. When the milling time increased from0h to50h, the sinteringtemperature of titanium powders dramatically decreased, the microstructure of the sinteredsamples transformed gradually from coarse grain to bimodal grain, then to fine grains, thevolume fraction of fine grains increased gradually, and the compressive yield strength andfracture strength of the sintered sample increased contemporarily, while the plastic strain ofthe sintered sample decreased dramatically. The fracture morphology of sintered samplestended to change from a mixed fracture of cleavage and toughness to the single cleavagefracture.Microstructure and mechanical properties of the samples sintered with titanium powdersball-milled for50h with addition of0~30wt.%content of titanium powders ball-milled for5hwere studied. With the increase of the content of titanium powders ball-milled for5h, thevolume fraction of coarse grain regions in the sintered sample increased gradually, thecompressive yield strength of the sintered sample decreased consequently, while the plasticstrain increased slightly. The fracture morphology changed from the single cleavage fractureto a mixed fracture of cleavage and toughness, and the area of the tough tearing regionincreased gradually.When the mixed not full ball-milled titanium powders were used as raw material, thesintering temperature was800℃, the sintering time was4min and the heating rate was100k/min, bimodal-grained titanium with optimized comprehensive mechanical propertiescould be achieved,the compressive yield strength, fracture strength and ductility of thesintered samples arrived at860MPa,2028MPa and28%, respectively, which is comparablewith the casting–forging Ti-6Al-4V alloy...