Phase transformations in nickel- and titanium-based alloys during chemical heat treatment

The structure of surface areas and cores of Ni-40Cr-4Al and Ti-6Al-5V alloys in the process of thermal and chemical heat treatment - nitriding, oxidation and carburizing was studied by X-ray diffractometry, optical microscopy and hardness measurement. Ni-40Cr-4Al alloy after quenching contains two supersaturated solid solutions based on fcc-Ni and bcc-Cr and a small amount of compound Ni3Al; subsequent aging leads to decrease in the degree of supersaturation of fcc-Ni and increase in the amount of bcc-Cr and Ni3Al. Nitriding leads to the formation of continuous CrN layer 3-4 µm thick on the surface, decomposition of Ni3Al and formation of AlN in underlying layer 100-140 µm thick. After nitriding high microhardness value of CrN (up to 1060 HV) and a slight decrease from 740 to 650 HV in the subsurface layers due to the transformation of Ni3Al to AlN were found. The surface layer of Ti-6Al-5V alloy is oxidized by erosion cutting in water, but the oxides formed during this process do not influence on subsequent diffusion saturation. During nitriding, oxidation, carburizing of Ti-6Al-5V alloy, diffusion layers are formed. They are characterized by the increased amount of α-solid Ti solution and formation of the surface intermediate phases. The thickness of the hardened layers is 8-20 μm. The hardness of these layers is 1000 HV for oxidation, 1000 HV for nitriding, 570 HV for carburizing.