Fabrication Of Bulk Ticunisnta Ultrafine/fine-grained Alloys By Mechanical Alloying And Subsequent Spark Plasma Sintering

Due to high performances in mechanical strength and hardness, ultrafine-grained crystallines and fine-grained crystalline composites are well recognized as two of the most promising materials in material science. In our previous studies, mechanical alloying (MA) and spark plasma sintering (SPS) were applied to synthesize ultrafine-grained Ti₆₆Nb₁₃Cu₈Ni_6.8Al_6.2 composites, which have good plasticity, as well as good mechanical strength around 2400 MPa. On these bases, MA and SPS were continued to be employed in this study, Ti₆₄Cu_11.2Ni_9.6Sn_3.2Ta_12.0 and Ti_58.8Cu_1.1Ni_3.2Sn_5.0Ta_31.9 were selected to investigate the influence of amorphous powder and processing parameter of SPS on microstructure and properties of their bulk materials.Firstly, MA was used to synthesize two powders: Ti₆₄Cu_11.2Ni_9.6Sn_3.2Ta_12.0 amorphous composite powder, whereβ-Ti(M=Ta,Sn),α-Ti(M=Cu,Ni) and an unknown phase disperse, and Ti_58.8Cu_1.1Ni_3.2Sn_5.0Ta_31.9 amorphous composite powder containing nano-crystallineβ-Ti(M) andα-Ti(M). After 70 h ball milling, the sizes of two alloy powders range from 20 to 30 ?m, the sizes of residual nano-crystallines are 10 nm approximately. After 70 h ball milling with a heating rate of 20 K/min, the glass transformation temperature, crystallization temperature and crystallized peak temperature of Ti₆₄Cu_11.2Ni_9.6Sn_3.2Ta_12.0 amorphous composite powder are 652 K, 731 K and 773 K respectively, the width of supercooled liquid region reaches 79 K. Meanwhile, their activation energy of glass transformation, crystallization and crystallized peak are 186.7, 354.6 and 304.4 kJ/mol. Prepared by 70 h milling, Ti_58.8Cu_1.1Ni_3.2Sn_5.0Ta_31.9 powder contain less none-crystalline phase. A weak exothermic peak in DSC tracing is presented. The composite with mainly amophous can be prepared while the ball milling time is further extended. At the same time, the glass formation ability of Ti₆₄Cu_11.2Ni_9.6Sn_3.2Ta_12.0 powder is stronger than Ti_58.8Cu_1.1Ni_3.2Sn_5.0Ta_31.9 powder.Subsequently, SPS with a sintering pressure of 50 MPa was applied to consolidate Ti₆₄Cu_11.2Ni_9.6Sn_3.2Ta_12.0 amorphous powders in order to synthesize high dense bulk fine-grained alloy. The shrinkage of flans is faster when the heating rate is increasing during the consolidation process. All bulk samples consist ofβ-Ti(M) and (Cu,Ni)Ti₂ and have few pores. The influences of holding time on the volume content of crystallized phase and the compressive mechanical properties of bulk samples exhibits significantly different in the different fabrication temperature. All phase sizes increase with increasing holding time less than 10 min, however, the phase sizes keep stable even holding time was continued to be prolonged. Theβ-Ti(M) phase decrease with increasing heating rate. Besides, fracture strength and elastic moduli have no clear relation with holding time. The bulk sample exhabits the best comprehensive mechanical properties by heating to 1253 K at 45 K/min and holding for 5 min, fracture strength reachs maximum 2204.9 MPa, elastic moduli of sample reachs 119.8 GPa, and the plastic strain reachs 2.5 %. The maximum fracture strength of bulk samples reachs 2267.4 MPa by heating to 1253 K at 90 K/min and holding for 15 min.When the sintering pressure reaches 50 MPa, Ti_58.8Cu_1.1Ni_3.2Sn_5.0Ta_31.9 amorphous powders consolidated by SPS were synthesized into bulk ultrafine-grained alloy. All these samples consist ofβ-Ti(M), (Cu,Ni)Ti₂ and a few uncertain phases. The density and microhardness of bulk alloys increase with increasing holding time in the same consolidation temperature. The highest density and relative density of bulk samples is by heating to 1297 K at 100 K/min holding for 15 min, their values are 8.94 g/cm3 and 99.77 % respectively. The consolidated sample reaches its maximum in micro-hardness 7.73 GPa by heating to 1297 K at 80 K/min holding for 15 min. All phase sizes increase with increasing holding time less than 5 min by heating to 1473 K at 150 K/min, however,β-Ti(M) stops growing even if time is further prolonged. By heating to 1473 K and holding for 15 min, the consolidated sample reaches its maximum in fracture strengths, 1890.9 MPa. By heating to 1297 K and holding for 5 min in various heating rate, the ultrafine-grained composites are obtained, whose grains sizes are from 200 to 400 nm, and the crystalline size is no relation with the heating rate. We find the 25 nm crystallines distribut in the trifurcate crystalline borderlines. The results of mechanical property reveal that fracture strengths and elastic module keep the same changing trend.