Phase equilibria in the nickel-silicon-magnesium system and fracture toughness of selected in-situ intermetallic composites.

摘要

The phase equilibria in the Ni-Si-Mg system and mechanical properties such as fracture toughness and yield strength of selected ternary and binary composites in the system have been studied.; The Ni-Si-Mg ternary phase diagram has been established after homogenization and slow cooling to room temperature. The isothermal section of the phase diagram at 900°C in the Ni-rich region was also established after isothermal annealing at the temperature followed by water quenching. The following intermetallic phases have been observed: (a) four new ternary intermetallic phases, designated as ν, ω, μ, and τ, (b) a ternary intermediate phase, Mg(Ni,Si)₂ based on the binary MgNi₂ phase containing Si, (c) three ternary intermetallic; phases, η, κ, and ζ, previously reported by the present authors [96Son, 98Son1], and (d) Mg₂SiNi₃ (Fe₂Tb type), previously reported by Noreus et al. [85Nor].; The fracture behaviour and fracture toughness of binary and ternary intermetallic phases and composites using chevron-notched bend specimens (CNB) have been studied. Single or near single phase intermetallic alloys such as η, Ni ₂Si, and MgNi₂ showed low average fracture toughness values such as ∼2.0 MPa.m1/2, ∼3.0 MPa.m1/2, and ∼6.0 MPa.m1/2, respectively.; Environmental effects on fracture toughness have been investigated for single or near-single phase alloys and selected intermetallic composites. No environmental effects were observed for near-single phase η, single phase Ni₂Si, and most of the selected in-situ composites. Fracture toughness of a single phase Ni₃Si also does not seem to be affected by the test environment.; Indentation microcracking pattern and indentation fracture toughness of binary and ternary intermetallic phases in the Ni-Si-Mg system were studied. It is shown that the determination of the crack system as being either Palmqvist or halfpenny by simple polishing away of the indented surface is unreliable due to the existence of the core zone (crack-free zone) with compressive stresses. Comparing the fracture toughness values obtained by indentation method (1.3–1.8 MPa.m1/2) with those obtained by bulk CNB specimens (1.7 MPa.m 1/2) for the η phase, the indentation fracture toughness values are in a good agreement with those obtained on the bulk materials. (Abstract shortened by UMI.)