Strengthening mechanisms in Monel K500 alloyed with Al and Ti

Monel alloys containing 63Ni-30Cu (wt%) are often used in applications requiring simultaneously high strength and corrosion resistance. Additions of Ti, Al and C to Monel K500 lead to formation of TiC, Ni3Al and Ni3Ti particles, which provide precipitation strengthening effect following heat treatment. The traditional heat treatment schedule includes solution annealing above 1000 degrees C and aging in the 400-600 degrees C temperature range. However, no correlation exists between the alloy composition and the heat treatment schedule (holding temperature and time) required to obtain the optimum microstructure and mechanical properties. This may result in excessive alloying, energy loses during heat treatment, and higher product costs. In this work, we investigate the effect of solution annealing part of heat treatment schedule on microstructure (particularly, particle precipitation and grain growth), hardness and strength. For hot rolled samples, solution annealing followed by aging was shown to result in lower strength compared to aging without annealing. The analysis of strengthening mechanisms carried out utilising our theory for calculation of solute atom concentrations has explained the strength variation with heat treatment and has shown (i) a lower strength after annealing and aging being related to dissolution of fine (< 20 nm) TiC particles, (ii) Ti- and Al-rich precipitates to provide a larger strengthening than Ti and Al solute atoms, (iii) Al to be a more effective strengthening agent than Ti, and (iv) the majority of Al to remain in solution for both processing schedules, this indicates potential for mechanical properties improvement via optimisation of the heat treatment schedule aiming to generate more Al-rich precipitates.