Study On Preparation And Heat Treatments Of TiC_p/Ti(Cr) Composites By Alloying And Compositing

Titanium matrix composites are widely used in aerospace and medical fields due-to their excellent mechanical properties and high temperature resistance,but the high cost coming from the usw of Ti alloy powder matrix and the poor interfacial bonding between reinforcements and matrix are the key barriers to their further applications.In this paper,pure Ti and Cr₃C₂ powders are used as raw materials to prepare titanium matrix composites with high toughness and low cost by simultaneous alloying and compositing,which are accomplished by the solid solution decomposed Cr in Ti matrix and the in-situ reaction of decomposed C with Ti matrix,respectively.Firstly,TiC/Ti（Cr）composites with different TiC contents as 1,2,4,6,8 and 10 vol%,where the corresponded Cr contents of 1.41,2.80,5.43,7.91,10.25 and 12.48 wt%,（abbreviated as TMC1～TMC10 according to TiC volume content）were prepared by rapid hot pressed sintering combined with hot forging process.The effects of TiC content and Cr concentration on the microstructure,phase composition and mechanical of the composites were investigated,and then the strengthening mechanism of composite materials was discussed.Subsequently,TMC4 with a two-phase matrix and TMC8 with an all-β matrix were heat-treated,and the change of microstructure and its effect on mechanical properties during heat treatments were studied.The main conclusions of this paper are as follows:1.In the hot pressed sintering process of Ti-Cr₃C₂ system,Cr₃C₂ decomposes into Cr and C atoms,and then Cr atoms dissolve into Ti matrix rather than reacting with Ti to form intermetallics while C in-situ react with Ti to form TiC particles,which can inhibite the growth of matrix grains.With the increase of Cr₃C₂,the Cr solid solutioned in the Ti matrix also increases,leading the matrix phase changes from α to α+β and finally to β gradually,viz.the matrix structure changes from Widmannstatten structure to a mixed structure,and then to β equiaxed structure.As a result,the hardness of the composites increases firstly due to the increase of TiC_p and’the solid solution strengthening of Cr,and then decreases since the soft property of β phase itself.2.The tensile test results at room temperature show that,with the increase of Cr₃C₂ content,namely the increase of TiC_p content and Cr solid solubility,the strength of the composite increases first and then decreases,while the plasticity decreases first and then increases.The fracture type gradually changes from ductile fracture to brittle fracture and then to ductile fracture.Thestrengthening factors in TiC_p/Ti（Cr）composites as well as their influencing degree on the improvement of strength are as follows:solution strengthening>fine crystal strengthening>load transfer effect of TiC_p.The decrease of plasticity is mainly due to the fracture and removal of TiC and the supersaturation of matrix phase,while the formation of bcc structure β matrix in Cr solution is the fundamental reason for the recovery of plasticity of TMCs with high Cr content.When the TiC content is 8vol%（corresponding to 12.48wt%Cr）,a good balance between tensile.strength and elongation can be obtained,which are 1531MPa and 5.2%,respectively.3.It is found that the needle strip α cluster in the TMC4 matrix becomes coarsening after solution treatment at 700℃,which significantly reduced the strength but increased the elongation,but the shear formation of α" laminates in β matrix at this condition worsens the tensile properties of TMC8.When the solution temperature enters the β single-phase zone,the matrix consists of a large amount of β and a small amount of α’ and α" phases,and the increase of hardness with the increase of solution temperature can be ascribed to the formation of α’ by the decomposition ofα".After solution quenching at 900℃,the amount of supersaturated solid solution α’ in TMC4 increases,leading to the occurrence of local stress concentration and brittle fracture.Compared with that before heat treatment,even though the strength of TMC8 is slightly reduced,its plastic toughness is significantly improved by 92.48%.The β matrix with high strength and toughness along with the load transfer effect of TiC_p make the material achieve an excellent comprehensive performance,which are tensile strength of 1420MPa and elongation of 10%.4.During the subsequent aging process,the phase transition at 300℃ and 400℃ is a continuation of α" decomposition into α’ at the solution stage.The increase of α’ content is the reason that the hardness reaches the maximum but the elongation deteriorates sharply after aging treatment at 400℃.After aging at 500℃,fine and dispersed α+β mixed phase precipitate from the supersaturated phase and metastable phase in the matrix,and then the elongation of TMC4 increases by 31.82%compared with that before heat treatment.Nanoscale Laves phase TiCr2 precipitate after aging at 600℃,which deteriorate the plastic toughness of the material.During the aging process,the decomposition of α",α’ and β" of supersaturated phases has a significant strengthening effect on the material.The tensile strength of TMC8 after aging treatment reaches 1800MPa,which is 19.2%higher than that before heat treatment.