Composition and microhardness of Si–Ge solid solution precipitates in Al–Si–Ge alloys solidified during centrifugation
Abstract The chemical composition of precipitates, the Knoop microhardness of the precipitates and of the matrix of Al–Si–Ge alloys solidified during centrifugation at 7 ×  g were investigated. The initial content of (Si + Ge) was (10 + 10), (20 + 20) or (25 + 25) at.%. The Knoop microhardness of the ingots varied with height in the ingot. This variation is attributed to sedimentation of Si–Ge during solidification. Chemical analysis showed that the maximum Si:Ge ratio in the precipitates for Al–10 at.% Si–10 at.% Ge alloy was about 92:8, and decreased with increasing Si and Ge content in ingots. The Si:Ge ratio and Knoop micro hardness also varied throughout each precipitate. The composition was constant along precipitates of alloy solidified without centrifugation. A variation in Knoop microhardness along the precipitates was attributed to this composition variation. The variation of the maximum Si:Ge ratio in the precipitates in ingots of different composition solidified with centrifugation is attributed to nonequilibrium conditions of solidification induced by decreased mass transport in the melt adjacent to the growing precipitates. The variation of Si and Ge along the Si –Ge precipitates is created by convection of the melt during solidification with centrifugation.
Resistance to shape refinement of precipitates in Al-(Si,Ge) alloys during thermal cycling
The present work addresses the response of Si-Ge precipitates in Al-0.5Si-0.5Ge (at. pct) to thermalcycling treatments of the sort known to refine the shapes of Ge precipitates in binary Al-Ge alloys. Alloys aged at 250 °C contained both small, platelet precipitates on {111} planes and larger, equiaxed precipitates that were heavily twinned. Thermal cycling between 250 °C and 360 °C led to partial or complete dissolution of the platelets. However, the equiaxed particles coarsened at an essentially constant shape; the shape refinement that led to untwinned, single-variant octahedral precipitates in binary Al-Ge did not occur. The apparent reason is the heavy twinning of the Si-Ge precipitates, which produces particles with a nearly spherical shape and rounded, incoherent interfaces and is, hence, a viable mechanism for relaxing the large misfit strain of the precipitate structure. The twinned particles undergo normal coarsening at an essentially constant shape. After thermal cycling, the precipitates contain Si and Ge in the approximate ratio of 70Si-30Ge, which is in the composition range expected for the cycling temperature.
Effect of ageing on the microstructure and mechanical behaviour of a directionally solidified Ni3Al-based alloy
Abstract The effect of ageing in the temperature range from 1023 to 1373 K on the micro-structure and mechanical behaviour of a directionally solidified (DS) Ni3Al-based alloy modified with additions of chromium and iron was investigated. The microstructure of the as-grown alloy consisted of well-aligned and equally spaced lamellas composed of β(B2) intermetallic compound NiAl (Cr, Fe), some β′(L10) martensite and spherical α-Cr precipitates. The matrix consisted of γ′(L12) intermetallic compound Ni3Al (Cr, Fe), γ-phase (Ni-based solid solution) and lath-shaped α-Cr precipitates. Ageing at 1123 and at 1173 K was found to be the most effective in transforming the unstable lamellae to γ′-phase and α-Cr precipitates. The change of microstructural characteristics such as volume fraction of lamellae, size, morphology and distribution of γ′-phase, γ-phase and α-Cr precipitates significantly influenced the room-temperature yield strength and elongation of DS alloy after ageing. The strain-hardening exponent varied with the ageing temperature between 0.30 and 0.46 and the quasi-steady work-hardening rate between 2710 and 5340 MPa. In the specimens with the lowest amount of disordered regions, the strain-hardening exponent was found to be 0.46 and the quasisteady work hardening rate was determined to be 3340 MPa.