The Effects Of Element Alloy On The Microstructure And Property Of The NiAl Alloys

The effects of element alloy on the microstructure and mechanical property of the NiAl alloys were studied. The microstructure and phases composition were investigated by analysing methods of metallography, SEM, Energy Spectrometer and X-rays diffractometer. The hardness and high temperature compressive properties were tested.The directionally solidified NiAl-Cr(Mo)-Hf alloy is the eutectic alloy, and with the Heusler (Ni₂AlHf) phase at the cell boundary. The microstructure of DS NiAl-Cr(Mo)-Hf alloy is composed of the black gray NiAl substrate and grey white lamellar Cr(Mo) phase. The NiAl matrix and Cr(Mo) phase have both the second phase separation after heat treatment, and the Heusler (Ni₂AlHf) phase at the cell boundary disappears, but the lamellar microstructure of NiAl and Cr(Mo) does not basically change, and Cr phase has the accumulated tendency. At the strain rate of 2.78×10^-3 s^-1, the compression yield strength of DS NiAl-Cr(Mo)-Hf alloy reduces along with temperature increasing.The microstructure of DS Ni-30Al-5Mo-0.5Hf alloy is composed of the dark gray and gray matrix Ni₃Al phase and the bright white Mo phase. The transverse microstructure of the alloy is the dendritic NiAl matrix, Ni₃Al phase is distributed in the strip or block of dendritic rooms, Mo phase and the phase distribution of Hf between dendrites and branches in the department. The longitudinal microstructure of the alloy is Ni₃Al phase distribution of a large number of bars in the dendrites of NiAl, the dendrite by the bright white of the Mo phase and large irregular Ni₃Al phase. After the heat treatment, the alloy does not change the basic morphology, but NiAl and Ni₃Al phase matrix are the second-phase precipitates.The microstructre of directionally solidified NiAl-Cr(Mo)-Hf(Ho) is composed of NiAl and Cr(Mo) phase, The element of Hf with the white crystallology along the Cr(Mo) phase distribution,and exists in the eutectic cell boundary. Element of Ho is dispersive and irregular. With withdrawal rate increasing, the size of fir-tree crystal cell reduces, the lamellar spacing also decreases.After heat treatment, the size of fir-tree crystal cell increases with same withdrawal rate, and the NiAl phases precipitate Cr(Mo) phases and Cr(Mo) phases precipitate NiAl phases.The three directionally solidified NiAl base alloys do not basically change after the high temperature heat treatment,But in the same temperature,with the withdrawal rate decrease, The directionally solidified NiAl-Cr(Mo)-Hf(Ho) alloy hardness elevated .