Investigation of the structure and properties of eutectic alloys of the Al – Ca – Ni system containing REM

Abstract

This study investigated the eutectic alloys based on aluminum containing small amount of rare earth metals (REM): 2La; 6Ca 3Ni – 2Pr. The compositions of the alloys were selected on the basis of previous studies of ternary Al – Ca – Ni and Al – Ni – Ce systems, taking into account the similarity of the structure of the Al – REM binary systems. Melting was carried out in an induc- tion furnace by RELTEC. Alloys were prepared on the basis of aluminum A99. Annealing of the samples at 550 о C for three hours was carried out in SNOL 8.2/1100 and SNOL 58/350 muffle electric furnaces. Calculation of Al – Ca – Ni – Ce systems at 6% Ca by means of Thermo-Calc (databases TTAL5, TCAL4), showed that primary crystals of the Al 3 Ni phase should be formed in the alloys of the selected composi- tions, however these crystals were not present. Using optical and scanning electron microscopy, the structure of alloys in the as-cast and heat-treated states was studied. It is established that in the process of non-equilibrium crystallization, the boundary of the phase region of existence of the aluminum solid solution significantly expands. Using micro- X -ray spectral analysis (MRSA), it was determined that during the equilibrium crystallization conditions in the Al – Ca – Ni – Ce system, rather than the binary Al 3 Ni the ternary Al 9 Ni 2 Ca phase is formed. The possibility of applying hot rolling to Al – Ca – Ni based alloys addi- tionally alloyed with Ce, La and Pr has been established, and the mechanical properties of hot-rolled samples have been obtained. Hot rolling was carried out at 500 o C. Rolling was carried out in five passes, the total degree of deformation in all cases was about 70%. Samples of the Al – 6Ca – 3Ni – 2Ce alloy were additionally rolled at a temperature of 550 о C. On the basis of a comparison of the mechanical properties and the microstructure of rolled products, it is assumed that the best mechanical properties are possessed by the samples of those alloys in which intermetallics have the smallest dimensions and are most evenly distributed in an aluminum solid solution. In particular, this demonstrates the Al – 6Ca – 3Ni – 2La alloy rolled at 500 o C and the Al – 6Ca – 3Ni – 2Ce alloy rolled at 550 o C. Zr) and estimating their mechanical properties. It is shown that metal powders are notable for good compactability on both single-action compacting and isostatic forming. Cold iso- static forming under pressure of 200 MPa permits to obtain briquettes with relative density of 65–68%. Sintering the briquettes at a temperature of 1873 K provides blank formation with porosity of 16 and 8% for Ti – 30.1Nb – 17.4Ta, Ti – 33.2Nb – 8.6Zr (wt.%) alloys, respectively. Sintering in vacuum of 1.33 Pa leads to formation of a gas-filled layer with heightened microhardness to a depth of 8 mm. Sintering in vacuum of 1.33·10 –2 Pa allows to avoid this phenomenon. Hot isostatic pressing of the sintered blanks at a temperature of 1193 K and pressure of 150 MPa guarantees obtaining practically porousless material (1% of pores). It is determined that Ti – 30.1Nb – 17.4Ta, Ti – 33.2Nb – 8.6Zr (wt.%) are characterized after sintering by the following values of the yield stress and the Young’s modulus: (cid:11) 0.2 = 444 ± 7 MPa, E = 57 ± 5 GPa and (cid:11) 0.2 = 570 ± 29 MPa, E = 62 ± 5 GPa, respectively. After HIP: (cid:11) 0.2 = 791 ± 16 MPa, E = 87 ± 4 GPa and (cid:11) 0.2 = 750 ± 50 MPa, E = 81 ± 1 GPa, respectively.