Metallurgical evaluation of heat-treated nickel-chromium base denture alloy
Abstract
In this study, a combination of solution treatment at 1050°C for 60 min, quenching, and age hardening at 900°, 750° and 600°C for 6, 60, 600 min was carried out on as-east nickel-chromium-beryllium base denture alloy (Ticonium Premium 100 "Hard"). Changes in the microstructure following heat treatment have been investigated by light microscopy, while the composition of the microstructural constituents was determined by energy dispersive spectroscopy (EDS). The changes in the overall and dendritic Vickers hardness, which accompanied various heat treatments, were determined. X-ray diffraction, transmission electron microscopy and selected area diffraction were conducted to obtain direct crystallographic information about the phases present in the as-cast, solution-treated and aged conditions. It was shown that heat treatment has a profound effect on the microstructure of the as-cast Ticonium Premium 100 "Hard" alloy. In the as-cast condition, the alloy exhibited an inhomogeneous dendritic structure with oriented needle-like (Widmanstätten) precipitates and a discontinuous interdendritic eutectic structure. Solution treatment produced a homogenized structure as a result of dissolution of almost all the precipitates present in the as-cast condition. Aging of the solution-treated and quenched alloy, at 750°C and 600°C resulted in the precipitation of very fine precipitates in the dendritic matrix, whereas overaging was observed with aging at 900°C. There was a direct correlation between changes in the microstructure and the microhardness. The solution-treated alloy exhibited the lowest Vickers hardness value (200.5 ± 6.5 kg/mm2 ), while the alloy aged at 750°C for 10 hours showed the highest value (359.1 ± 8.1 kg/mm 2 ). Aging at 750° and 600°C showed a continuous increase in hardness with increasing the aging time. On the other hand the alloy aged at 900°C exhibited an increase followed by a decrease in hardness with increasing aging time. Changing the aging temperature for a given aging time caused a more pronounced effect on the microhardness than changing the aging time at a given temperature. There was no significant difference between the overall hardness and the dendritic hardness. Transmission electron microscopy and selected area diffraction in combination with x-ray diffraction studies revealed that this alloy consists of three phases in the as-cast and as-cast and aged conditions, the γ (nickel rich) matrix phase, the NiBe eutectic phase and the γ' (Ni3 Al) precipitate phase. The strength and hardness properties of this alloy mainly depend on the precipitation of γ' , which precipitates coherently with the γ matrix phase.
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