【作者】 李颖；

【导师】 龙雁；

【作者基本信息】 华南理工大学 ， 材料加工工程， 2012， 硕士

【摘要】 纯钛密度低、比强度高、抗蚀性能与生物相容性较好，但由于纯钛强度较低，极大的限制了其作为结构和功能材料的应用。为了制备兼具高强度和高塑性的钛材，本文提出采用高能球磨结合放电等离子烧结技术来制备双尺度细晶钛，着重开展了以下几个方面的研究。采用高能球磨法制备了纳米晶钛粉。在高能球磨过程中，随着球磨时间的延长，钛粉先由球状变为粗大扁平状，然后逐渐破碎成细小的颗粒状，同时其晶粒尺寸不断减小。TEM分析表明：10h球磨钛粉的晶粒尺寸在几十纳米至数百纳米的范围内分布。采用放电等离子烧结技术对未充分球磨的混合钛粉（5h和10h球磨钛粉按1:4的质量比例混合）进行了烧结，研究了烧结温度、保温时间和升温速率对烧结试样显微组织和力学性能的影响规律。随着烧结温度的升高，烧结试样的密度、压缩屈服强度、断裂强度和塑性应变不断增加。当烧结温度为800℃时，烧结试样已接近全致密，其显微组织呈双尺度分布，其中细晶区的晶粒尺寸小于2μm，粗晶区的晶粒尺寸大于5μm，二者呈层状交替分布。当烧结温度达到900℃时，烧结试样发生重结晶，形成微米级等轴晶基体中包含粗大片状相的复合组织。升温速率对烧结试样的密度、压缩塑性应变的影响不明显，随着升温速率的增加，烧结试样的细晶区略有长大，其压缩屈服强度呈逐渐降低的趋势；随着保温时间的延长，虽然烧结试样中细晶区略有长大，但烧结试样的烧结更加充分，使其压缩屈服强度不断增加。研究了球磨时间对钛粉烧结试样显微组织和力学性能的影响。当球磨时间从0h增加至50h时，钛粉的烧结温度显著降低；钛粉烧结试样的组织由粗晶→双尺度晶粒→细晶逐步转化，其细晶区的比例逐渐增加，烧结试样的压缩屈服强度、断裂强度逐渐增加，而塑性应变不断降低，其断口形貌存在着由解理和韧性混合断裂向单一的解理断裂转变的趋势。研究了在50h球磨钛粉中添加0～30wt.%的5h球磨钛粉对烧结试样显微组织及力学性能的影响。随着5h球磨钛粉含量的增加，烧结试样显微组织中粗晶区的百分比会不断增加，其压缩断裂强度不断降低，但塑性应变逐渐增大；其断口形貌由单一的解理断裂转变为解理和韧性混合断裂，且韧性撕裂区的面积不断增大。当采用未充分球磨的混合钛粉、烧结温度为800℃、烧结时间为4min、升温速率为100k/min时，可得到力学性能最佳的双尺度细晶钛，其密度为4.489g/cm3（相对密度约99.7%）。烧结试样的压缩屈服强度达到860MPa，断裂强度达到2028MPa，塑性应变达到28%，与铸造－锻造Ti-6Al-4V合金的水平相当。更多还原

【Abstract】 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更多还原