Research On Preparation Technology Of Powder Metallurgy TiB Reinforced Titanium Matrix Composites

Titanium matrix composites can effectively improve the wear resistance and comprehensive mechanical properties of matrix by introducing reinforcements.In particular,the density and coefficient of thermal expansion of TiB reinforcements are close to those of titanium,which can reduce the residual stress during hot working and avoid cracks due to temperature changes.Therefore,TiB reinforced titanium matrix composites have attracted attention in the titanium field.Powder metallurgy technology has unique advantages in composition design,near-netshape forming and microstructure control.However,titanium is sensitive to interstitial oxygen element,the critical oxygen content is low,so the oxygen content of powder metallurgy titanium is hard to control,and it is difficult to achieve full density at low sintering temperature.The introduction of reinforcements will also affect the plasticity of the composite at room temperature,resulting in unsatisfactory service performance.Although technologies such as spark plasma sintering and hot isostatic pressing have been adopted to improve performance,they usually lead to a large increase in production costs,which deviates from the original intention of low cost of powder metallurgy technology.For this reason,this study achieved the low-cost manufacturing of high-performance in-situ TiB reinforced titanium matrix composites through vacuum pressureless sintering and thermal deformation,and systematically studied the influence of different reinforcement precursor(ZrB2,CaB6)on the microstructure,phase,room temperature mechanical properties and wear resistance of titanium matrix composites.Firstly,low-oxygen HDH TC4 fine powder was selected as the raw material,and boron powder was used as the precursor of reinforcement.The TiB reinforcement can be synthesized by sintering at 1150℃.The oxygen contents of as-rolled samples are about 2200 ppm,which is significantly lower than that of the powder metallurgy TC4 alloy specified in ASTM B988-13 standard.The tensile strength of as-rolled TC4-0.1B is 1021.2 MPa,and the elongation is 10.3%.However,when the content of boron is further increased to improve the strength,hard particles are observed in the fracture surface,which worsens the ductility at room temperature.Therefore,it is necessary to find a suitable method to enhance the strength and suitable precursors.Some studies have shown that the high-strength pure titanium material prepared by adding TiO2 shows a room temperature elongation of more than 20%,which provides a new idea for improving the strength of the titanium matrix composite.Zirconium and titanium have similar properties,and there is a continuous solid solution zone in the Ti-Zr binary phase diagram.Based on this,TixZrO2 samples were prepared.It is found that sintering at 1100℃ for 120 min can obtain a high density of more than 99%.Zr and O are dissolved into titanium matrix.Zr has the effect of decreasing the phase transition temperature of Ti,which can promote the densification of titanium powder to a certain extent.The fine recrystallized grains generated by rolling improve the overall deformation ability of the material,so the ductility is greatly increased.The equivalent oxygen content of Ti-0.5ZrO2 is about 0.54%,the elongation is 28%,and the ultimate tensile strength is 736.8 MPa,showing excellent comprehensive properties.The elongation of Ti-1.5ZrO2(equivalent oxygen content is 0.7654 wt.%)reaches 9%,and its mechanical property at room temperature is equivalent to those of TC4 in ASTM B988.The most widely used TC4 alloy has also been studied.As-rolled TC40.75ZrO2 shows a tensile strength of 1180.2 MPa and an elongation of 7.3%.And the equivalent oxygen content is 0.5895 wt.%.The above results lay the foundation for the research of high-performance pure titanium matrix and titanium alloy matrix composites materials and reduce the difficulty of controlling the interstitial oxygen during the preparation of powder metallurgy titanium.The precursor ZrB2 was added to titanium to improve the low sintering density of the composite at low temperature by using the role of Zr element to promote sintering densification.During sintering,TiB is in-situ synthesized by ZrB2 reaction with the matrix,and zirconium atom replaces the position of titanium in the lattice.The average grain size of as-rolled composite is basically below 20 μm.Compared with pure titanium,the grain size of as-rolled Ti-2.0ZrB2 decreases by 65.5%,and less than or equal to 15 μm accounts for 52%.The ductility of the composite at room temperature is significantly improved by the high proportion of fine grains.The elongation of Ti-2.0ZrB2 is up to 21.4%.Compared with other reported boron sources,a small amount of ZrB2 can obtain composites with higher strength,showing better comprehensive mechanical properties.It is found that the dislocation substructure hardening plays the most important role among the different strengthening factors.Further,the feasibility of adding ZrB2 to prepare high-performance TC4 matrix composites was studied.The tensile strength of asrolled TC4-2.0ZrB2 was 1153.6 MPa,but the elongation was only 4.3%,which could not further increase the content of reinforcement to improve its wear resistance and hardness.So,it is necessary to reduce the content of solid solution atoms,especially the interstitial oxygen,to improve the plasticity of the composites at room temperature.Therefore,CaB6 is selected as the precursor of TiB for the preparation of TC4 matrix composites,which can avoid the introduction of foreign atoms,and calcium can also adsorb solid solution oxygen atoms in titanium matrix to form ternary calcium oxide.The effect brought by a small amount of CaB6 can significantly increase the ductility of the composite at room temperature,and the strength is also improved.The elongations of as-rolled TC4-0.1CaB6 and TC4-0.2CaB6 are more than 10%,and the tensile strengths are about 1150 MPa.The TC4-0.5CaB6 still show 1227.2 MPa strength and 8.5%elongation,which provides a solution to the problem of the application of HDH TC4 powder with high oxygen content above 3500 ppm.The hardness and room temperature wear resistance of TC4 matrix composites with high TiB content were further studied.The tensile strength of TC412 wt.%TiB prepared by this method can reach 1327.6 MPa,elongation 2.8%,hardness 42.5 HRC,wear rate 5.07 × 10-13 m3/(N·m),which is 33.6%lower than traditional ingot metallurgy TC4,showing excellent wear resistance.