| Particle-reinforced in-situ synthsized titanium matrix composites have become a rational candidate for manufacturing aerospace structural parts because of their high specific strength,high specific modulus,low manufacturing cost,strong oxidation resistance and good creep properties.However,the titanium matrix composites still have high deforming resistance,narrow hot processing window and defects along the interface between ceramic particles and matrix after deformation.The thermal hydrogen treatment uses hydrogen as temporary alloying element during hot processing of titanium alloys.After hydrogenation,hot processing and dehydrogenation,the materials can achieve the effect of refining the structure and reducing the deforming resistance without affecting the final mechanical properties.In view of the shortcomings of traditional thermal hydrogen processing,such as low efficiency,long time-consuming and can only treat thin materials,this paper chooses the method of melt hydrogenation technology,which means melting materials in hydrogen-argon gas mixture,by which the preparation and hydrogenation of composite are completed at the same time.Since hydrogen directly participates in the solidification process of composites,the changes on the matrix structure,distribution and morphology of the reinforcements in the composites caused by hydrogen will affect the subsequent hot processing and microstructure after deformation.This paper will focus on the above issues and the problems are systematically studied.Firstly,melt hydrogenation was used to successfully prepare Ti-6Al-4V alloy with TiB and TiC particles in different hydrogen partial pressure in hydrogen-argon gas mixture,and volume fraction of ceramic particles was 5%.The phase identification results confirmed there were only ?-phase,?-phase,TiB and TiC in ingot,no hydride phase was generated.Microstructure results of the composites ingot revealed that the distribution of ceramic particles showed a gradient change from top to bottom.The ceramic particles had approximate network distribution at the primary ?-phase grain boundary in the top section,which were uniformly dispersed at the center,while the primary ?-phase at the bottom was affected by the directional heat flow in the form of columnar crystals,and the ceramic particles distributed at the grain boundary to form a strip-like distribution.The distribution of ceramic particles in the composites with different hydrogen content showed that the distribution changed from homogeneous dispersion distribution to approximate network distribution as the hydrogen content increased.The main reason was that hydrogen caused overheat on the melt surface and activated the diffusion of atoms,which in turn increased the the growth rate of the primary ? phase and leaded to the coarsening of grains.The influence of hydrogen on the morphology of the ceramic particles was observed.Hydrogen increased the length-diameter ratio of TiB and the size of TiC particles.The main reason was that hydrogen promoted the diffusion of atoms in the melt,which led to the growth of TiB along the preferred growth direction,while TiC grew into larger-sized equiaxed particles.The effect of hydrogen on the bonding interface between the ceramic particles and matrix was observed,and it was found that hydrogen did not significantly change the atomic bonding at the TiB-matrix interface,whereas TiC-matrix interface had a molecular-level transition layer,and hydrogen promoted the diffusion of C atoms into the matrix,thereby reduced the thickness of the transition layer.Through thermal dynamic simulation experiments,the high temperature deformation behavior of Ti-6Al-4V alloy and composites under the same hydrogen content and hot deformation conditions were studied.It was found that when the deformation temperature was higher(800°C-850°C),the hydrogen induced softening effect on Ti-6Al-4V was stronger,but at lower temperatures(700°C-750°C),the flow stress first decreased and then increased with increasing hydrogen content.Hydrogen induced both of the softening and hardening of the composite.When the deformation was below 850°C,the hydrogen induced softening on the matrix was weak,and the network-like distribution of the ceramic particles increased the flow stress due to the grain boundary strengthening effect,and the hardening was the dominant effect.When the temperature was 850°C,hydrogen induced improvement on ?-phase content,DRX and dislocation mobility,the stronger softening effect of hydrogen on the matrix reduced the flow stress of the composites,and the softening was the dominant effect.The dynamic material model was used to establish the constitutive equation for the high temperature deformation of composites.The calculated results showed that the thermal activation energy of the composites before and after hydrogenation was 339.65 kJ/mol and 286.5 kJ/mol,respectively.Hydrogen narrowed the destabilization zone during hot processing of composite,and expanded hot processing window and ideal deformation zone.The microstructure of deformed Ti-6Al-4V alloy and composites under same hydrogen content and deforming conditions were observed.It was found that hydrogen promoted the fragmentation and spheroidization of the lamellar layers,and accelerated the decomposition of the lamellar layers and generation of new grains.The crush and redistribution of the ceramic particles after deformation of the composites were observed.After hydrogenation,the crushed ceramic particles distributed more homogeneoulty,which was mainly due to hydrogen increasing the high temperature flowing ability of matrix alloy.TEM and EBSD results on DRX,dislocations and phase content after hot deformation of composites showed that hydrogen slightly increased the ?-phase content,significantly increased the DRX volume fraction,and increased the proportion of low-density dislocation regions.The interface after hot deformation of the composite was observed,hydrogen significantly reduces defects at the interface.It was believed that defects came from incoordination of deformation between matrix and ceramic particles and crush of ceramic particles.Positive effect of hydrogen on interface can be attributed to improved flowing ability of matrix alloy at elevated temperature and promotion on migration of grain boundaries of discontinuous DRX of matrix alloy,thereby matrix alloy can more effectively occupy defects along the interface. |
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