Influence of heat treatment modes on the properties of 56DGNKh (Cu20Ni20Mn2Cr) alloy

Alloys of the Cu – Ni – Mn system are used in many areas, and for some applications (watchmaking, dentistry, precision mechanics) they must have high hardness. A state of high hardness can be achieved by two-stage heat treatment – quenching and subsequent aging. To obtain a good set of performance characteristics, decomposition of the solid solution must proceed through a continuous mechanism, which can be regulated by additional alloying (for example, chromium) and aging parameters. In this work, we studied the influence of quenching and aging modes on microhardness of 56DGNKh (Cu20Ni20Mn2Cr) alloy. It was shown that quenching from temperatures of 700 – 750 °С provides higher microhardness values ​​than quenching from 800 °С. By varying the temperature and duration of aging, it was found that the maximum microhardness is observed at aging temperatures of 475 – 500 °С. Metallographic analysis shows that in this case, the supersaturated solid solution of Mn, Ni and Cr in copper decomposes into a less supersaturated solid solution and the precipitation of MnNi intermetallic particles occurs according to a conti­nuous mechanism. The change in microhardness of 56DGNKh alloy depending on the aging time is multi-stage: its increase at short exposures is replaced by a subsequent decrease at increasing exposure with a clearly defined maximum or “plateau” between these two parts of the graph, and this type of dependence is observed at all aging temperatures. X -ray diffraction phase analysis shows that during the aging process, concentration of the solid solution decreases and MnNi particles are formed, the crystal lattice period of which differs from the period of the solid solution by 50 pm. The observed patterns of changes in hardness during the aging process are explained from the standpoint of the general theory of decomposition of supersaturated solid solutions. The maximum increase in microhardness (up to 450 kgf/mm 2 versus 130 – 160 kgf/mm 2 in the state after quenching) is achieved at a coherent or semi-coherent interface between MnNi particles and a Ni-based solid solution. This is observed after quenching from 750 °С and aging at 475 °С for 10 h.