MICROSTRUCTURE AND MECHANICAL-PROPERTIES OF MG33-XSIXNI67 INTERMETALLIC COMPOSITE ALLOYS

The microstructure, microhardness, compressive strength and fracture t oughness of the in situ intermetallic composite alloys in the Ni-rich corner of the ternary Mg-Si-Ni system were studied. The compositions w ere selected according to the formula Mg33-xSixNi67 (x = 0-33 at.%), i .e. along the constant Ni line from MgNi2 to Ni2Si intermetallics. Aft er casting and subsequent homogenization at 1000 degrees C for 100 h, the ternary solid solution delta'-Ni-2(Si, Mg) is the primary (matrix) phase in the x = 12, 17 and 19 at.% alloys. It is based on the delta- Ni2Si intermetallic which can accommodate up to 22.1 +/- 0.6 at.% Mg. Its Vickers microhardness (at 100 gf load) is 860 kgf mm(-2) vs. about 640 kgf mm(-2) for delta-Ni2Si, indicating a solid solution strengthe ning effect by Mg. The compressive fracture stress of all the in situ intermetallic composite alloys behaves according to the composite rule of mixtures being dependent on both the volume fraction of the strong er phase and its inherent strength (hardness). Chevron-notch beam (CNB ) fracture toughness increases with increasing volume fraction of mino rity phases (microconstituents) such as Ni(Si), MgNi2 + Ni and Ni5Si2. The highest CNB K-lc value recorded is 9.6 +/- 1.1 MPa m(1/2). The te rnary solid solution (Mg, Si/Ni-2 based on the MgNi2, intermetallic se ems to be detrimental to the fracture toughness.