Influence of Ni-P content on microstructure and mechanical properties of Fe-x(Ni+P)-1Cu alloys

The present paper is devoted to investigating the effects of various Ni-P contents on microstructure and related mechanical behavior of Fe-x(Ni+P)-1Cu alloys fabricated by semi-liquid phase sintering (SLPS) at 950 °C, with a special emphasis on strengthening effects of Cu nanoscale precipitates. Uniaxial compression testing and scanning electron microscope (SEM) were performed to correlate the microstructure with compression properties. Transmission electron microscope (TEM) studies were conducted to characterizing the ultrastructure of Fe-Ni-P-Cu alloy and accurately elucidating the strengthening effect of Cu nanoprecipitates. Experimental results show that the optimal Vickers hardness (465.4HV0.1), ultimate compressive strength (1920 MPa) and yield strength (1223 MPa) would be available provided that alloys contain appropriate amount of Ni-P (around 25.3 wt%). But excess Ni-P (> 28.5 wt%) and resulting coarse phosphide phase at grain boundaries had a detrimental effect on the ductility (21.1%) of Fe-Ni-P-Cu alloy. A quantitative expression had been proposed to elaborate the relationship between yield strength, porosity and component content of as-prepared alloys. It was confirmed that, due to a high level of Ni content, the ultra-fine and dense Cu precipitates obtained could strongly strengthen Fe-Ni-P-Cu alloys even they were air-cooled rapidly. The observed yield strength increment contributed by Cu precipitates coincided with theoretical arithmetic.