Effect of B and Cr on elastic strength and crystal structure of Ni₃Al alloys under high pressure

Samples of Ni₃Al, Ni₃Al:B and Ni-Al-Cr super alloys were prepared by directional solidification method and their effect of alloying with ternary elements on the mechanical properties was investigated. In-situ X-ray diffraction studies were carried out on undoped Ni₃Al, Ni₃Al:B with boron 500 ppm and Ni-Al-Cr with 7.5 at.% of chromium super alloys at high pressure using diamond anvil cell. The results indicate that micro-alloying with B forms γ′-phase (L1₂structure), similar to the pure Ni ₃Al, while Ni-Al-Cr alloy consists of γ′ precipitates in a matrix of γ-phase (Ni-FCC structure). The crystal structure of all three alloys was stable up to 20 GPa. Micro alloying with boron increases bulk modulus of Ni₃Al by 8% whereas alloying with chromium has the opposite effect decreasing the modulus by 11% when compared to undoped alloy. Further, the elastic modulus and hardness of Ni₃Al, Ni₃Al:B and Ni-Al-Cr alloys were determined using the nano-indentation technique, in combination with compressibility data which enabled the estimation of shear modulus and Poisson's ratio of these alloys.