The Study Of Hypo Eutectic Al-Ni Alloy Solidification Microstructure Under High Pressure

In this paper, the microstructure of Al-1.87wt%Ni alloy solidified underhigh-pressure have been investigated. The microstructure character and the phasecomposition in Al-1.87wt%Ni alloy have been analyzed.the XRD diffraction pattern of Al-1.87wt%Ni alloy solidified under highpressure(1GPa,2GPa,3GPa) shows that the phase composed by-Al+Al3Ni. But withthe increase of pressure Solid solution phase-Al rising on the whole, content of IMCphase Al3Ni content is declining. With the increase of pressure the solid solubility ofsolid solution increased.The microstructure of the Al-1.87wt%Ni alloy solidified under high pressure haschanged a lot. After the Al-1.87wt%Ni alloy solidified under1GPa and2GPa, the wholesolidification interface has four kinds of microstructure. Compared with themicrostructure solidified under normal pressure, the first kind of microstructure， theprimary dendrites are much more developed. The microstructure on second area undervarious pressures, solid solution becomes continuous, eutectic structure gathered in thegrain boundary with discontinuous, under1GPa, the partial second area segregationphenomenon is obvious. Part of the grain size composition reached the eutecticcomposition. Then eutectic transformation happens, generate all eutectic structure.When the pressure is2GPa, the second area after solid solution appeared block Al3Niphase on the solid solution matrix. the second district structure under1Gpa and2GPa,the solid solution matrix to be cut off by the narrow grain boundary which consist byeutectic structure, and there are most of the eutectic structure gathered distributed in thematrix with the shape of squares or globular. With the increase of the pressure the forthdistrict structure eutectic structure are all spherical eutectic, all the dendrite of thecrystal has a tendency to shift towards the equiaxial structure. After3GPa pressure,there are only two kinds of microstructure on the cross section. Under differentpressures, on the solid solution matrix, the distribution of Ni element in various regionare uneven.Al-1.87wt%Ni alloy solidified in liquid nitrogen, the phase is consists of-Al+Al3Ni. The-Al phase volume fraction increased by15%, IMC content decreases. Andthe eutectic microstructure obviously refined. The phase content of Al-1.87wt%Ni alloysolidified in graphite mould did not change，-Al phase volume fraction increased byabout9%, content of IMC decrease. Solid solution dendrite become the equiaxial dendrites, also eutectic structure becomes very small and distributed in the grainboundary. The alloy, after it was heated to625℃and keep the temperature for30h.Phase composition also has not changed, the-Al phase volume fraction increased withthe time longer. Solid solubility is also increased compare with the alloy solidifiedunder normal pressure, and the grain boundary become narrow, eutectic structuregathered and Al3Ni grew up into a ball shape.