Microstructural peculiarities of aluminides of nickel, titanium, and cobalt

Abstract

Scanning electron microscopy, electron probe microanalysis, X-rays diffractometry, and microhardness measurements were used for studies of diffusion zones in systems Al-Тi, Al-Co, and Al-Ni formed by means of diffusion couples method in a temperature range of 1,300-1,350 С. It was shown that diffusion zones have the multilayer structure with numerous peculiarities. For a number of intermetallic compounds formed in all three systems there were revealed their temperature shifts using the electron probe microanalysis methods and confirmed with X-rays diffractometry. The correlation between the individual intermetallic layers and their microhardness numbers are not always evident. For some aluminides the microhardness can reach 7,060 ± 1,200 MPa (in globular area of Ti5Al11 and TiAl) and 4,938 MPa (in multilayer area of titanium aluminide TiAl2). For very thin (about 10 μm) layers in multilayer area the microhardness has been measured in the first time 4,000 MPa (TiAl) and 4,450 MPa (Ti3Al). The obtained microhardness numbers demonstrated reasonable coincidence with previous published results. Microhardness in Al-Ni system was measured as 5,200 ± 500 MPa (for β-NiAl) and 5,300 ± 860 MPa (for γ΄-Ni3Al). For cobalt aluminide СоАl from Аl-Со system the microhardness showed numbers 3,900 ± 200 MPa, whereas for СоАl3 it was higher up to 6,600 MPa. The intermetallic compound γ΄-Ni3Al considered as the most interesting from practical point of view had small (up to 10 μm) width whereas the next layer Ni3Al5 had the wider (100 μm) width. There were also revealed some globular Ti5Al11 having thin shells which consist of intermetallic Ti9Al2.