Influence of addition of TiAl particles on microstructural and mechanical property development in Invar 36 processed by laser powder bed fusion

Invar 36 is well known for its extremely low thermal expansion coefficients at low temperatures. However, its relatively low strengths limit its application as a load-bearing structural material. In this study, powder particles of a titanium aluminide (Ti-48Al-2Cr-2Nb) were mixed with Invar 36 powder particles which were then processed by laser powder bed fusion (L-PBF) with the aim of developing intermetallic nano-particles reinforced Invar 36 by taking advantage of the rapid and localized solidification characteristics of L-PBF. It was found that the addition of 1.5 wt% TiAl powder particles into Invar 36 through L-PBF did not cause pronounced influence on porosity level and grain size but led to mild increase in texture level. It, however, led to significant formation of nano-sized spherical Al2O3 and cuboidal γ-TiAl precipitates which were homogeneously distributed in the matrix. It also resulted in segregation of Ti+Al and formation of Al2O3, γ-TiAl and TiNi precipitates at the bottom of solidified melt pools. The nano-sized Al2O3 and γ-TiAl particles were found to have acted as effective dislocation motion barriers during deformation and caused great improvement in both 0.2% yield strength and ultimate tensile strength. The (Ti, Al)-segregated and precipitates-enriched bottom regions of solidified melt pools, however, have acted as preferential crack initiation sites and caused a certain reduction in ductility. Consumption of a small amount of Ni from the matrix induced by the above precipitation behaviour together with increased texture level has caused a certain compromise in thermal expansion properties. Nonetheless, the newly formed material still demonstrates low thermal expansion coefficients at low temperatures and significantly improved tensile strengths.