Effects of P on microstructures and tensile properties in Ni17Cr and Ni17Cr15Fe model alloys

In the present work, two model alloy systems which were NiCr (0Fe) and NiCrFe (15Fe) alloys with different phosphorus (P) contents were prepared and subjected to the same heat treatment. The influence of P on microstructures and 650 °C tensile properties in two model alloys was investigated. The results showed that the precipitation of intergranular α-Cr phases was inhibited by the addition of P and the addition of Fe made α-Cr phases tend to precipitate in granular form rather than lamellar form at grain boundaries. Meanwhile, the increase of P content promoted the grain growth for 0Fe alloys while had little effect for 15Fe alloys, and the proportion of Σ3 grain boundaries of the two model alloys both decreased with the increase of P content. The 650 °C yield and tensile strength of 0Fe alloys showed a faster decreasing trend with increasing the P content while the elongation both increased significantly. The main grain boundary strengthening mechanism gradually changed from the pinning effect of α-Cr phases to the Σ3 grain boundary strengthening and P atoms segregation strengthening with increasing the P content. Thus, the fracture mode transformed from single intergranular fracture to mixture of transgranular and intergranular fracture with the increase of P content. In addition, the intragranular strengthening effect of Fe and P also contributed to the strength of 15Fe alloys, which reduced the decline rate of tensile strength.