| As a kind of promising light-weight high-temperature structural material,y-TiAl matrix alloy has broad application prospects in aerospace,automotive industry and other fields.However,the alloy currently faces a series of problems which severely restrict its application,including room temperature brittleness,poor workability,insufficient high-temperature plasticity and strength.In recent years,the concept of material design has been deeply rooted in the reseachers' hearts,which leading to the rapid development of the technology and theory for preparing composites.To improve the above problems,the design of composites based on y-TiAl alloy has became a feasible means.In this paper,powder metallurgy technology was used to introduce Ti-6Al-4V alloy particles into Ti-48Al-2Cr-2Nb alloy matrix,which broke through the traditional perception that adding"soft phase" can only make composites tougher,to design a new type of y-TiAl-based composites that used "soft phase" to strengthen and toughen and to provide a theoretical basis for its applied research.The main conclusions of this study are as follows:(1)y-TiAl matrix composites with Ti-6Al-4V particles uniformly distributed in the Ti-48Al-2Cr-2Nb matrix was successfully prepared using Ti-48Al-2Cr-2Nb and Ti-6Al-4V pre-alloyed powders by vacuum hot pressing sintering and sheathing hot extrusion process.The matrix was composed of lamellar ?/?2 phase and equiaxed y phase,and the disperse phase was consist of lamellar ?/? phase.Good metallurgical bonding was observed in the interface area and no new phase was seen.The interface was mainly ?/?2 phase interface.(2)y-TiAl-based composites are temperature and strain rate sensitive.The tensile strength decreased with the increased temperature and the decreased strain rate.When the content of Ti-6Al-4V increased from 0 to 12 wt.%,the tensile strength continuesly increase,while the elongation increased first and decreased then.Among them,the composites with 8 wt.%Ti-6Al-4V showed the best mechanical properties,the high-temperature strength and toughness were improved simultaneously.(3)For y-TiAl matrix composite,plastic deformation occurred in Ti-6Al-4V first at the initial state of high-temperature deformation,while the matrix was still in the stage of elastic deformation.The activated dislocations in Ti-6Al-4V gathered at the grain boundry and could not move into the matrix,which resulted in the additional work hardening effects.Besides,crack growth in the composites could be effectively hindered by the Ti-6Al-4V alloy,and the hard and soft phases deformed together since the dynamic load had transferred from the hard matrix to the soft phase.Therfore,the strength and plasticity of composites improved synchronously.(4)The deformation of y-TiAl matrix composites under high temperature tensile was a thermally activated process.The constitutive relationship between stress,temperature and strain rate during the deformation process could be represented by a hyperbolic sine curve.The constitutive equation of the composites with Ti-6Al-4V content of 8 wt.% was:(?)(5)A constitutive equation based on hyperbolic sine form in the stage of work hardening-recovery softening and dynamic recrystallization was established through the study of high temperature compression behavior of ?-TiAl matrix composites.Meanwhile,the accuracy of the constitutive equation was also verified.The power dissipation diagram of the composites under different strains was established based on the dynamic material model.The hot processing diagram of the high temperature processing and forming performance of the composites was drawn according to the instability criterion.Then the optimal thermal processing window was determined.(6)According to the hot working drawing of the ?-TiAl matrix composites with 8 wt.%Ti-6Al-4V,the safe area of thermal processing was determined.The deformation temperature of 850??1020?,the strain rate of 1×10-1 s-1?1×10-3 s-1,the deformation temperature of 1020??1000?,and the strain rate of 3×10-1 s-1?1×10-3 s-1.It is a typical dynamic recrystallization zone of the alloy,with a peak efficiency of 47%.The material had a small instability area,that was the deformation temperature of 800??850?,the strain rate of 1×10-1 s-1?2×10-3 s-1,and the deformation temperature of 950??1100?,and the strain rate of 1×10-1 s-1?3×10-1 s-1. |
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