Fabrication And Properties Of In Situ Particulate Reinforced Titanium Matrix Composites

Titanium matrix composites?TMCs?have high specific strength,high specific modulus,more outstanding high-temperature creep resistance and oxidation resistance as well as higher thermal stability and thermal fatigue strength than titanium alloys,so that they have been considered as one of the most potential structural materials in aviation and aerospace fields.TMCs prepared by in-situ synthesis have the advantages of clean interface,good adhesion and high thermodynamic stability,and have become a research hotspot in recent years.However,the size and morphology of the reinforcements in the composites and the distribution characteristics of these reinforcements in the matrix generally have great influence on the properties of the composites.Therefore,it is of great significance to investigate the formation mechanisms of the in-situ reinforcements and their influence on the properties of the composites for the development and application of high-performance in-situ particulate reinforced TMCs.This study was aimed at preparing high-performance TMCs with excellent hot workability,good oxidation resistance and high wears resistance.TMCs were prepared by in-situ synthesis of powder metallurgy.Ti-7Al alloy??-titanium alloy?was selected as the matrix of TMCs,and Ti-7Al-B?TiB/Ti?and Ti-7Al-B-C?TiB+TiC/Ti?particulate reinforced TMCs were prepared by high-energy ball milling and reactive hot-pressing sintering processes.On the basis of the formation mechanism of in-situ particulate reinforcements,the high-temperature compression rheological mechanism and the high-temperature oxidation,friction and wear mechanism,low-cost and high-performance TMCs were designed and developed,which could provide theoretical and technological basis for industrial applications of TMCs.The research in this study mainly includes the following aspects:?1?High-energy ball milling and reactive hot-pressing sintering processes were studied,and in-situ particulate reinforced TMCs with fine grain particles were successfully prepared.By virtue of the in-situ reaction powder metallurgy method,near-fully dense TiB/Ti and?TiB+TiC?/Ti particulate reinforced TMCs with fine grain size of the matrix and the particulate reinforcements were prepared by the high-energy ball milling and reactive hot-pressing sintering processes.The microstructure analysis showed that the particulate reinforcements were dispersed in the matrix and had clean interfaces and good binding with the matrix.The high-energy ball milling could refine Ti,Al and B?C?powders,so solid solution and partial mechanical alloying were realized during this process.As a result,fine grain microstructure and TiB reinforcements with diameter below 1 ?m were obtained in TiB/Ti TMCs.On the other hand,due to the amorphous structure of sintering powders formed by high-energy ball milling,the growth of the reinforcements was hindered by the atom ordering process in the sintering process,so that the TiB whiskers had an average diameter of less than 100 nm and the equiaxial TiC particles had an average diameter of about 2 ?m in the?TiB+TiC?/Ti TMCs.Meanwhile,TiB and TiC could hinder the movements of grain boundaries and restrain the growth of matrix grains,thus refining the matrix microstructure.?2?The rheological behavior and microstructural evolution of the composites at high temperatures were studied.The high-temperature rheological behavior of TiB/Ti and?TiB+TiC?/Ti revealed the rheological stress variation rules with the deformation temperatures and strain rates.The peak flow stress decreased with the increase of deformation temperature,and increased with the increase of strain rate.The lower the temperature,the more greatly the peak stress varied with strain rate;the lower the strain rate,the more slightly the peak stress decreased with temperature.The high-temperature deformation processes of TiB/Ti and?TiB+TiC?/Ti composites were hot activation processes,and the high-temperature thermal deformation activation energies were 166.02 and 208.23 k J/mol-1 respectively.Dynamic recovery was the main softening mechanism of the two composites,while dynamic recrystallization played a supporting role.Based on the rheological stress variation rules,the high-temperature deformation constitutive equations and the flow stress equations of the TiB/Ti and?TiB+TiC?/Ti composites were established.In the high-temperature hot compression process,the TiB and TiC reinforcements in the composites strongly hindered the dislocations,so that the dislocations accumulated at the boundaries and formed dislocation subgrains;meanwhile,the rotation of the reinforcements in the matrix would generate phase difference of the lattices on two sides,which also formed subgrains.These subgrains could be used as the cores of dynamic recrystallization,and provided favorable conditions for dynamic recrystallization.The microstructure of the composites at high temperatures was stable,and the matrix grains and TiB and TiC reinforcements did not grow up obviously after deformation at high temperatures.These ensured the good mechanical properties of TMCs after hot working process,and provided the possibility for applications of these TMCs at high temperatures.?3?The friction and wear properties of the composites and their friction and wear mechanism were studied.Friction and wear studies showed that under the load of 20-50 N and with the sliding velocity of 0.3-1.2 m/s,the wear weight loss of Ti-7Al alloy,TiB/Ti and?TiB+TiC?/Ti composites all increased with the increase of load and sliding velocity.TiB and TiC reinforcements improved the wear performance of TiB/Ti and?TiB+TiC?/Ti composites,and the wear weight loss of the TMCs significantly reduced by about 57% and 27% respectively compared with that of Ti-7Al alloy within the scope of the experimental conditions.The main wear mechanisms of Ti-7Al alloy were oxidation peeling wear and abrasive wear,those of TiB/Ti composite were lighter oxidation peeling wear and abrasive wear,and those of?TiB+TiC?/Ti composite were chip micro cutting wear and fatigue peeling wear.?4?The high-temperature oxidation resistance of the composites was studied,and the oxidation mechanism and the growth mechanism of the oxide films were also elucidated.High-temperature discontinuous oxidation experiments of Ti-7Al alloy,TiB/Ti and?TiB+TiC?/Ti composites were carried out at 700?,800? and 900? for 60 h.The results showed that the oxidation products of these three testing materials were only TiO₂ and Al2O3,and that continuous oxide films with uniform thickness could be formed.The oxide films were of complex multi-layer structures,and could be divided into TiO₂ layer,high Al2O3 layer,TiO₂ layer and TiO₂+Al2O3 mixed layer from outside to inside according to the element distribution.In the process of high-temperature oxidation,the oxidation product of Al,Al2O3,had thermodynamic advantages in oxidation reaction,and the oxidation product of Ti,TiO₂,had advantages in growth kinetics;the competition between Al2O3 and TiO₂ played a leading role in the formation of the complex structures.Oxidation kinetics curves of Ti-7Al alloy,TiB/Ti and?TiB+TiC?/Ti composites were all similar to the parabolic shape.To be specific,the speed of weight gain was fast in the initial stage,but the oxidation rate decreased gradually with the increase of oxidation time.Oxidation weight gains of the two composites were both lower than that of the matrix alloy.The oxidation resistance of?TiB+TiC?/Ti composite was the best,and the oxidation weight gains at 700?,800? and 900? for 60 h reduced by 15%,21.5% and 24.5% respectively compared with those of Ti-7Al alloy at the same temperatures.It was considered that the small grain size of the?TiB+TiC?/Ti composite and the abundant fine reinforcements could provide a large number of short-range fast channels for the diffusion of Al ions?grain boundaries and phase interfaces?,so that more Al ions in a wider scope could participate in the oxidation reaction in early oxidation stage to form higher density Al2O3 layer.As a result,the diffusion process of Ti and O in the subsequent oxidation process could be greatly suppressed,thereby reducing the oxidation rate.