Complex Deformation Behaviors Of Net-structure TA15 Based Alloy At High Temperatures

To reduce weight and improve flying distance,lightweight and high-strength materials servicing at 600-800°C become an imperative requirement to replace superalloys for high-speed vehicle.Netlike-morphology titanium based alloys can suppress grain-boundary weakness and improve performances at high temperatures because of adding reinforcements on grain boundaries.The evolution of distributed reinforcements will affect the performances of net-structure titanium based alloys after complex deformation.In this dissertation,using the spherical TA15 powders and prismatic TiB₂ particles as raw materials,the alloy was prepared by reactive hot pressing sintering.For revealing the deformation behavior and microstructure evolution during complex hot deformation,simple compression,multiaxial compression,ring compression,and upsetting-extrusion were carried out at high temperatures.The obtained results provide theoretical guidance for processing complex-shaped parts of this alloy.In order to solve the problem of poor plasticity and improve the strength,the preparation technique of net-structure TA15 based alloy was developed.Firstly,TiB₂particles were uniformly adhered to the surface of TA15 powders using low-energy milling,which made the TA15 powders maintaining intact and free from break.Then,the mixed powders were synthesized by reactive hot pressing.The distribution of synthesized TiB reinforcements showed 3D network characteristic.In addition,the initial microstructure of matrix consisted of coarse primary?phase lamellae and residual?phase layer.The boundary between matrix and reinforcements was clear.It was noted that the alloy exhibited excellent tensile properties at room and elevated temperatures.The macroscopic fracture surface exhibited a honeycomb-like feature,and the pulling-out of reinforcements from the matrix could also be observed.The microstructure of matrix could be regulated by heat treatment,however,the plasticity was not obviously improved.Simple compression of this alloy was conducted to study the deformation behavior at high temperatures.The results revealed that the deformation temperature,strain rate and strain could affect the flow stress and microstructure.The flow stress was greater at higher deformation temperature and larger strain rate.Furthermore,the fraction of primary alpha phase decreased with the increasing of deformation temperature.It was also of interest to note that the flow stress curves were very sensitive to strain rate,namely the flow curves exhibited typical work hardening features at high strain rate and substantial flow softening occured at low strain rate.At different strains,the flow stress curves exhibited diverse strain hardening rates due to combined effects of work hardening and softening.The morphology of the net microstructure was related to strain and became more flattened with the increasing of strain.The softening fraction of the interrupted deformation increased with increasing holding time and deformation temperature or decreasing strain rate.In particular,the fine grains of the matrix exhibited relative homogeneity,and the flow stress of each step exhibited relatively lower values under deformation conditions of 120 s/980°C at 0.1 s^-1.The equation of stress-strain curves was established to provide material parameters for numerical simulation.Multiaxial compression of the alloy was carried out at high temperature to optimize the mechanical properties.The tensile strength and elongation exhibited homogeneous distribution in the deformed billet,and at ambient temperature,they increased by 8%and163%,respectively,when compared with those of the as-sintered alloy.The microstructures of the matrix was composed of primary alpha phase and transformed beta phase consisting of lamellas/needles of a secondary alpha phase and residual beta phase.The results showed that the precipitation of the secondary alpha phase was triggered by omega-assisted heterogeneous nucleation.The morphology of net microstructure was flattened after hot deformation,which made the connectivity of adjacent matrix increased.Thus,more plastic deformation was accommodated before fracturing and further resulted in improved elongation.The ring compression and upsetting-extrusion were performed to analyzing the complex deformation behavior of this alloy.The numerical simulation exhibited the process of evolution for net microstructure,which was also observed during the experimental Process.The obtained results revealed that the shape of deformed net microstructure was closely related to deformation types,and the relationship between shape parameter of the deformed net microstructure and the strain was built.