A high-resolution-electron-microscopy study of a γ/γ′-α directionally solidified eutectic alloy

The microstructure of a /- directionally solidified (DS) eutectic alloy with a nominal composition of Ni-30.26Mo-6.08Al-1.43V (wt%) was investigated by means of high-resolution electron microscopy (HREM) and analytical electron microscopy. The -fibres exhibited a typical morphology with a rectangular cross-section and they displayed the Bain orientation relationship (OR) with the / matrix; that is, [001][001] and (110)(010). Misfit dislocations and lattice strain fields existed at the / interface for different habit planes; that is, (110)(010) and (100)(110) were analysed. EDAX (Energy dispersive X-ray) analysis showed that the composition of the -phase was approximately Ni₄(Mo, Al, V); it contained 90° rotational domains of Ni₃(Mo, Al, V) with a DO₂₂ structure and Ni₂(Mo, Al, V) with a Pt₂Mo structure.     

Effect of swaging on the 1000‡C compressive slow plastic flow characteristics of the directionally solidified eutectic alloy γ/γ'′-α

Swaging between 750 and 1050 C has been investigated as a means to introduce work into the directionally solidified eutectic alloy /gg- (Ni-32.3 wt % Mo-6.3 wt % Al) and increase the elevated temperature creep strength. The 1000 C slow plastic compressive flow stress-strain rate properties in air of as-grown, annealed, and worked nominally 10 and 25% materials have been determined. Swaging did not improve the slow plastic behaviour. In fact large reductions tended to degrade the strength and produced a change in the deformation mechanism from uniform flow to one involving intense slip band formation. Comparison of 1000 C tensile and compressive strength-strain rate data reveals that deformation is independent of the stress state.     

Dendritic coarsening of γ′ phase in a directionally solidified superalloy during 24,000 h of exposure at 1173 K

Dendritic coarsening of γ′ was investigated in a directionally solidified Ni-base superalloy during exposure at 1173 K for 24,000 h. Chemical homogeneity along different directions and residual internal strain in the experimental superalloy were measured by electronic probe microanalysis (EPMA) and electron back-scattered diffraction (EBSD) technique. It was indicated that the gradient of element distribution was anisotropic and the inner strain between dendrite core and interdendritic regions was different even after 24,000 h of exposure at 1173 K, which influenced the kinetics for the dendrite coarsening of γ′ phase.     

Fatigue crack propagation behaviour of unidirectionally solidified γ/γ′-δ eutectic alloys

Fatigue crack propagation (FCP) studies were conducted on a series of/- (Ni-Nb-Al) alloys by subjecting them to cyclic four-point bending loads at room temperature. The aluminium contents of the alloys investigated ranged from 1.5% to 2.5% by weight, with the niobium contents adjusted to maintain controlled eutectic microstructures. In addition to studies of as-grown alloys, heat treatments were performed on several of the alloys to determine the effect of resultant changes in microstructure on FCP behaviour. Crack growth rates from approximately 2×10^–6 to 10^–3 mm/cycle were recorded as a function of the crack tip stress intensity factor range. The growth rates for the heat-treated alloys differed little from the as-grown FCP behaviour. Comparison with other published results indicated that the addition of aluminium was beneficial to FCP resistance, although the level of aluminium addition (within the investigated range of 1.5 to 2.5% by weight) did not influence the crack growth rates. Based on a comparison with previously reported results, chromium additions were seen to have a detrimental effect on FCP behaviour. Fractographic studies revealed the superior fatigue behaviour of the/- eutectic composite to be a result of repeated grain boundary delamination as the crack progressed through the microstructure.     

Effect of multiple strain-anneal cycles on the 1000° C creep behaviour of γ/γ′-α

An investigation of the influence of multiple strain-anneal cycles on the 1000° C creep behaviour of the directionally solidified eutectic alloy /- has been undertaken. Cycles consisted of swageing at room temperature or 900° C by about 5 to 10% per pass followed by annealing at 900° C, and were repeated to total strains of approximately 10, 30 and 50%. Transmission electron microscopy (TEM) of strain-annealed materials revealed that three-dimensional dislocation networks were introduced into the matrix, but very little work remained in the fibres. Constant-velocity creep testing indicated that all thermomechanical processing schedules improved the creep strength for strain rates 2 x 10^–6sec^–1; however only strain-annealing with a total of 13% work at room temperature (RT13) improved the behaviour at strain rates 2 x 10^–7sec^–1. The advantage of RT13 processing over as-grown materials at lower strain rates was confirmed by constant-load creep testing. It was also shown that 900° C annealing slightly improves the 1000° C creep properties in comparison to as-grown alloys. TEM of crept materials indicated that the active creep mechanism had been changed from dislocation pile-ups at fibres in as-grown alloys to dislocations being stopped by sub-boundaries in the matrix for RT13.