| According to the preparation method and the corresponding powder geometry,metallic powders can be classified into spherical powder by gas-atomization and irregular powder by high-energy ball milling.As for a given alloy composition,spherical powder and irregular powder have different physical properties parameters of surface energy,average particle size,viscosity and so on.During the study of powder densification behavior,it is difficult to isolate some physical property parameter and further investigate its influence on densification behavior and mechanism.As an advanced biomedical metallic material,?type titanium alloy has some advantages of excellent biocompatibility,high corrosion and wear resistance,the combination of high strength and low elastic modulus,etc.,and thus is regarded as the research hotspots.On this account,this dissertation selected the atomized fine and coarse spherical powders and the high-energy ball milled irregular powder as the research objects,and studied influence of powder properties on densification behavior,microstructure and mechanical property of biomedical?type titanium alloy.It is expected to reveal influence of powder properties on densification mechanism during SPS of Ti66.23Nb22.53Zr4.59Ta1.65Si5 alloy powder and fabricate high-strength and low-modulus biomedical?type titanium alloy.Firstly,a comprehensive impact factor f,that can be related to powder properties and reflect powder densification behavior,was derived based on the Frenkel's model.This factor,f,can be used in combination with activation energy Q for viscous flow to assess the densification behavior of the powders.Densification curves were obtained by SPS consolidation of the atomized spherical and milled irregular pure Ti,Ti-6Al-4V alloy and Ti66.23Nb22.53Zr4.59Ta1.65Si5 alloy powders under different heating rates and sintering pressures.Then,densification behavior and the values of f and Q were studied and calculated according to the temperature-instantaneous relative density curves and temperature-densification rate curves.Results show that the values of f and Q of the milled irregular powders are always lower than those of the atomized spherical powders,which determines the lower and higher relative density in the earlier and later stages compared to the atomized counterparts,respectively.With the increased heating rate and sintering pressure,the value of f and Q increases and decreases respectively,and thus enhance the powder densification behavior.Compared to the atomized coarse counterpart,the higher f and lower Q of the atomized fine powder decides its higher relative density during the whole consolidation process.Subsequently,influence of heating rate and sintering pressure on microstructure and mechanical property of the as-sintered bulk alloys by SPS of the milled Ti66.23Nb22.53Zr4.59Ta1.65Si5 alloy powder was studied.The higher the heating rate,the finer microstructures and higher mechanical properties.Mechanical properties of the as-sintered specimens do not change monotonically under increasing sintering pressure.Fine S2 phase is homogeneously distributed into the?-Ti matrix phase of all samples fabricated under different processing parameters.Under the sintering pressure of 50MPa with the heating rate of 150?/min to 970?and holding for 5min,the as-sintered bulk alloy possesses the best comprehensive mechanical properties.Correspondingly,the relative density,yield strength,fracture strength,fracture strain and compressive elastic modulus are 99.9%,1858.0MPa,2218.6MPa,18.5%and 39.3GPa,respectively.These values are superior to present biomedical alloys and are close to the human bones in elastic modulus.Finally,influence of heating rate,sintering pressure,sintering temperature and holding time on microstructure and mechanical property of the as-sintered bulk alloys by SPS of the atomized fine and coarse Ti66.23Nb22.53Zr4.59Ta1.65Si5 alloy powders was investigated.The distribution of S2 phase in the as-sintered bulk alloy from the atomized powders is quite different from that sintered from the milled counterparts,and can be divided into 3 areas:concentrated S2 phase area,sparse S2 phase area which may be network S2 phase area,and non-S2 phase area from the atomized coarse powder.The mechanical property of the as-sintered bulk alloys by SPS of the atomized alloy powders is mainly determined by the relative density and microstructure homogeneity.Higher heating rate,sintering pressure,sintering temperature and holding time could increase the relative density of the as-sintered bulk alloys,while lower heating rate,higher sintering pressure and sintering temperature and holding time could promote microstructure homogeneity.As for the influence of powder properties,the relative density of the as-sintered bulk alloy from the atomized fine powder is always higher than those of the coarse counterparts because the fine powder owns the higher f and lower Q.Besides,the microstructure of the as-sintered bulk alloys from the fine powder is more homogeneous relative to the coarse counterpart.The tensile fracture morphology of the as-sintered bulk alloys from the fine powder shows ductile fracture characteristics of dimples while some cleavage planes are discovered in the coarse counterparts.Therefore,under the sintering pressure of 50MPa with the heating rate of 100?/min to 1170?and holding for 5min,the as-sintered bulk alloy from the atomized fine powder has the best comprehensive mechanical property.Correspondingly,the relative density,yield strength,fracture strength,fracture strain and compressive elastic modulus are99.9%,829.7MPa,2088.8MPa,53.1%and 31.7GPa,respectively.These values are superior to those from the coarse counterparts.Noted that,the as-sintered bulk alloy from the atomized powder exhibits excellent tensile mechanical properties,such as elastic modulus of about 61.6GPa,yield strength of 858.0MPa,fracture strength of 925.5MPa,and elongation of 10.6%so on.These values are superior to those of equivalent biomedical?type titanium alloys. |
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