Microstructure Evolution Of Cu_x[Ni₃Cr₁] And Cu_x[Ni₃Fe₁] Alloys And Their Effects On The Alloy Properties

As a passivating and strengthening element,Cr or Fe is added to Cu-Ni binary alloy to improve its strength,resistance to seawater corrosion,oxidation resistance,fabricability and weldability,etc.,which makes the Cu-Ni-Cr and Cu-Ni-Fe alloys widely employed,such as valves,pump castings,condenser tubes in marine atmospheres and potential high temperature anode materials.Both of them belong to spinodal decomposition structure alloys.Several earlier studies showed that spinodal decomposition of the Cu-Ni-Cr and Cu-Ni-Fe alloys formed two fcc structures with the same structure but different compositions.In this system,a lot of researches on the two alloy systems are mainly focused on the effects of the aging technology on the morphological,lattice constant,hardness and tensile strength.However,only a few scholars carry out systematic study on the microstructure evolution and its effects on properties of the alloys.In the present paper,Cu-Ni-Cr and Cu-Ni-Fe alloys as the object of study with the fixed Ni/Cr or Ni/Fe atom ratio to 3/1 and varying Cu content were prepared.The microstructure evolution of Cu_x[Ni₃Cr₁]?x=1,2,3,4,5,7,9 and 12?and Cu_x[Ni₃Fe₁]?x=4,7,9 and 12?alloys alloys were systematically studied over a large composition range.Then the surface morphology,phase structure,composition,hardness,tensile properties and the resistivity were characterized by optical microscope?OM?,X-ray diffraction?XRD?,scanning electron microscope?SEM?,transmission electron microscopy?TEM?,electron probe micro analysis?EPMA?,energy dispersive spectrometer?EDS?,Vickers hardness tester,universal testing machine,variable temperature resistance meter.The results show that:1.Cu_x[Ni₃Cr₁]?x=1,2,3,4,5,7,9 and 12?alloys have a typical spinodal decomposition structure.All alloy samples have taken place spinodal decomposition after solid solution at1073 K for 5 h and aging at 773 K for 4 h.The mismatch of the two decomposition phase's Cu-rich??₁?and Ni-rich??₂?would significantly affect the shape of the?₂ precipitates.The?₂phase exists as cubic when the mismatch degree???varied from 0.62%to 0.75%and appears as lath-shaped when the?ranging from 1.08%to 1.35%.The lath-shaped?₂ phase decreased with the increase of x.The?_b gradually decreased from 818 MPa to 490 MPa,the?_s decreased monotonically from 810 MPa to 344 MPa.For the?,it increased from 2.5%to25.6%but dropped sharply to 11.7%at x=9.The Vickers hardness has a maximum of 245 HV at x=3.The proportion of cubic?₂ phase increased continuously would enhance the strength of the alloy,while the lath-shaped precipitation showed better corrosion resistance due to the high packing density.Therefore,the appropriate x can be selected according to the actual service conditions.In addition,the state of Cr would affect the microstructure and performance of the alloys.The solid solution Cr could promote the spinodal decomposition,while the large amount of Cr-rich precipitate in the grain boundaries would have a negative effect on the mechanical performance.No Ni₂Cr phase appeared in the whole alloy samples.2.The results show that the spinodal decomposition is the main strengthening method in Cu_x[Ni₃Fe₁]?x=4,7,9 and 12?alloys after solution treatment at 1073 K for 5 h and aging at723 K for 4 h.With the increase of x,tensile strength,yield strength and hardness decreased due to the decreased of Ni-Fe richer phase.The lath-shaped precipitation showed better corrosion resistance owing to its high packing density.For the aged alloys with x=4,7,9,a small amount of?'-Ni₃Fe was precipitated,which had no obviously effect on the hardness and tensile properties.But the?'-Ni₃Fe phase decompose at about 773 K,resulting in a slight decrease in the resistivity.For the aged alloy with x=12,a large number of?-Fe were precipitated in the grain boundaries.