Short-range order structure and effective pair-interaction energy in Ni–Si alloys

The diffuse X-ray scattering from single crystals of Ni – 11.3 at.% Si and 12.3 at.% Si, both aged at 1293 K followed by a water quench, was measured at room temperature. The measured diffuse intensities were analyzed to determine the Warren–Cowley atomic short-range order (SRO) parameters αlmn. The 17 values of αlmn obtained were fitted in a 5 × 105 atom model crystal to simulate the SRO. It was found that the C16 and the C17 configurations are significantly enhanced in comparison to a random crystal. The pair-interaction energies Vlmn were obtained using an inverse Monte Carlo method from the αlmn parameters. Assuming that Vlmn are independent of temperature, the αlmn were calculated as a function of temperature on the basis of the Monte Carlo simulation. The curves of αlmn for 11.3 at.% Si showed a knee point at 1262 (19) K and for 12.3 at.% Si at 1325 (11) K. Both temperatures are higher by about 100 K than those of the (fcc/fcc + L12) phase boundary in an equilibrium phase diagram.