Porous TiAl-based Intermetallics Synthesized By Thermal Explosion And Pore Formation Mechanism

TiAl-based intermetallics were considered as a novel inorganic porous materials due to its low density,high specific strength,good corrosion resistance and oxidation resistance at elevated temperatures and have the performance advantages of porous ceramics and metals.Thus,there is a promise to make this kind of new porous materials in large-scale industrial production and application with the preparation technology of simple,rapid and free from contamination.Titanium and aluminum powders were used as the raw materials in this paper with the similar particle size.The powders were firstly mixed with the Ti/Al molar ratio of 1:1 and experienced pressing and vacuum sintering process.Porous TiAl-based intermetallics were synthesized by thermal explosion?TE?reaction with different heating rates?1?/min,2?/min,5?/min,10?/min?.The effect of heating rate on expansion behaviors of powder assemblies,phase compositions,pore structures and oxidation resistance were investigated.Results showed that the actual temperature of specimen increased from 655-661??ignition temperature?to 1018-1136??combustion temperature?in a short time of 25-55 s.TE reaction in Ti/Al powder assemblies resulted in the formation of open-celled TiAl-based intermetallics with high porosity.The maximum open porosity of 59% was obtained in sintered bodies thermally treated at heating rate of 5?/min,which means the specimen experienced the highest combustion temperature and underwent the obvious volume expansion of 48%.Additionally,high porosity of porous TiAl₃ intermetallics were fabricated by thermal explosion with Na Cl particles as space holder material,from the Ti/Al molar ratio of 1:3.The actual temperature of specimen climbed rapidly from 667? to 1124?.XRD patterns showed that only single–phase TiAl₃ compounds were synthesized.As a consequence,porous TiAl₃ intemetallics with high open porosity?>75%?can be easily achieved when adding Na Cl particles above 30 vol.%.Hierarchical porous TiAl₃ materials displayed three pore structures,including large pores replicating from original Na Cl particles,small pores among the skeletons formed by TE reaction and tiny pores precipitated from particle skeletons.Meanwhile,we have explored that a variety of pore structures exist in the porous TiAl₃ skeletons,ranged from a few microns to dozens of microns and have somewhat different in sizes and shapes.Therefore,it is necessary to investigate the effect of sintering conditions on expansion behaviors,phase compositions and pore structures.XRD result showed that small amount of TiAl₃ phase formed and unreacted Ti and Al elements were still exist when sintering at 600? for 1h.The open porosity increased from 22.2% of green compact to 32.8% due to the reactive diffusion in the low-temperature sintering.While the temperature heating up to TE occurs,Al element disappeared and only single-phase TiAl₃ intermetallics were formed and a few amounts of Ti element existed in the TE products and the open porosity significantly increased to 58.7%,indicating that TE reaction is the dominated pore formation mechanism of porous TiAl₃ intermetallics.When continue to heating up to 1100? for 1h,the open porosity decreased to 51.2%,showing that high-temperature sintering caused densification and some particles had already grown up.Finally,we explained the sources of these pores as well as its formation mechanisms through the pore morphology analysis.