Microstructure and Properties of Laser Deposited and Wrought Alloy K-500 (UNS N05500)

Abstract

Feasibility tests were performed by manufacturing transition joints between Nickel-200 (Ni-200) and Alloy K-500 (K-500) with direct laser deposition. Both sharp and functionally graded interfaces were manufactured with no apparent issues. In addition, Alloy K-500 specimens were manufactured with direct laser deposition to analyze the isothermal hardening response of Alloy K-500. The laser deposited and wrought materials were precipitation hardened at three different temperatures (600 °C, 650 °C, and 700 °C) and various times. A relationship between the hardness and the volume fraction of Ni3(Ti,Al) was developed, and the subsequent analysis showed that the corresponding Johnson-Mehl-Avrami time constants (or n-values) ranged from 1.1 to 1.3 at an aging temperature 600 °C, and decreased to values ranging from 0.6 to 0.66 at an aging temperature of 700 °C. For site saturation and spherical precipitates, the expected n-value is 1.5. This analysis showed that the mechanism for precipitation changed as a function of temperature. The results of the analysis at higher temperatures were rationalized by the possibility of the decay of quenched-in vacancies, precipitation along the grain boundaries, and precipitation along the dislocations. It was, however, difficult to confirm these possibilities with transmission electron microscopy (TEM), since imaging the precipites is difficult at very early aging times. Finally, it was determined that the heat treatment schedule of laser welded or laser clad Alloy K-500 should be similar to that of wrought Alloy K-500.