Microstructure study of the hardening mechanism of Cr–Ni alloy deposits after flame heating for a few seconds

Cr–Ni alloy deposits with different chemical compositions could be obtained from a bath with trivalent Cr and divalent Ni ions. With a plating current density above 20 A dm− 2, amorphous Cr-rich alloy deposits were obtained, and crystalline Ni-rich deposits were achieved with an electroplating current density lower than 15 A dm− 2. The hardness of the Ni-rich deposit decreased gradually with increasing flame-heating time. On the contrary, the hardness of Cr-rich alloy deposit could be significantly increased from 550 Hv to 1450 Hv after flame heating for 3 s. The results of a microstructure study show that the precipitation of nano-sized carbon-related particles, possibly diamond-like particles, could be the main hardening mechanism of flame-heated Cr-rich alloy deposit. These particles have particularly high hardness values, and they distort the nearby Cr-rich lattice, which leads to obvious strain fields in the flame-hardened Cr-rich alloy deposits.