Properties and strengthening mechanisms of nickel matrix composites reinforced with high-speed steel particles

Nickel matrix composites reinforced with T15 high-speed steel (HSS) particles were prepared using powder metallurgy. A systematic investigation was conducted into the effects of the sintering temperature and T15 HSS particle content on the microstructure and properties of the composites. The results indicate that the grain size of the nickel in the composites was effectively refined by the addition of T15 HSS particles in comparison to the pure sintered nickel. It was also observed that the T15 HSS particles, after sintering at all the used temperatures (850, 900 or 950 °C), were diffusion-bonded to the nickel matrix. There were two distinct layers between the reinforced particles and the nickel matrix: the solid solution of elements in nickel and the FeNi3 intermetallic compound, whose thickness slowly grows with the increase in sintering temperature. Also, as the sintering temperature was incremented, the relative density and hardness of the composites gradually rose. When sintered at 950 °C, the Ni+20 wt.%T15 composite achieved a maximum hardness of 135 HB, which was about 52 % higher than that of the pure sintered nickel. The introduction of an increasing amount of T15 HSS particles combined with sintering resulted in a rise in the yield strength of the sintered composites. At all the investigated temperatures, as the T15 HSS particle content was increased, the compressive strength of the composites also gradually grew. Nonetheless, as the sintering temperature was raised from 850 to 950 °C, the compressive strength of the composites initially increased and then decreased. The composite containing 20 wt.%T15 HSS particles sintered at 900 °C achieved the highest compressive strength of 445 MPa, which was about 50 % higher than that of the pure sintered nickel. Additionally, the primary contributions of strengthening mechanisms such as load transfer, grain refinement and thermal expansion mismatch to the mechanical properties of the Ni+T15 HSS composites were analyzed.