J. Lapin

Directional solidification of intermetallic Ti–46Al–2W–0.5Si alloy in alumina moulds

Abstract The microstructure and mechanical properties of intermetallic Ti–46Al–2W–0.5Si (at.%) alloy directionally solidified in alumina moulds were studied. After directional solidification the microstructure consisted of regular α 2 - and γ-lamellae, elongated B2 particles, Al 2 O 3 particles and fine Ti 5 Si 3 precipitates. Formation of transient cellular-dendritic structure was observed at the growth rates of 2.78×10 −6 and 5.56×10 −6 m/s. Higher growth rates ranging from 1.39×10 −5 to 1.18×10 −4 m/s resulted in a regular dendritic structure where the primary dendrite arm spacing decreased with increasing growth rate according to relationship λ 1 ∝ V −0.24 . The interlamellar spacing decreased with increasing growth rate according to the relationship λ ∝ V −0.46 . The volume fraction of Al 2 O 3 particles increased with increasing temperature of the melt and reaction time. Increasing growth rate resulted in increasing microhardness and compression yield stress of DS ingots. A linear relationship between the microhardness and compression yield stress was determined. The yield stress increased with decreasing interlamellar spacing according to the Hall–Petch relationship.

Effect of Al2O3 particles on mechanical properties of directionally solidified intermetallic Ti-46Al-2W-0.5Si alloy

Abstract The effect of Al2O3 particles on microhardness and room-temperature compression properties of directionally solidified (DS) intermetallic Ti–46Al–2W–0.5Si (at.%) alloy was studied. The ingots with various volume fractions of Al2O3 particles and mean α2–α2 interlamellar spacings were prepared by directional solidification at constant growth rates ranging from 2.78×10−6 to 1.18×10−4 ms−1 in alumina moulds. The ingots with constant volume fraction of Al2O3 particles and various mean interlamellar spacings were prepared by directional solidification at a growth rate of 1.18×10−4 ms−1 and subsequent solution annealing followed by cooling at constant rates v varying between 0.078 and 1.889 K s−1. The mean α2–α2 interlamellar spacing λ for both DS and heat-treated (HT) ingots decreased with increasing cooling rate according to the relationship λ∝ ν −0.46. In DS ingots, microhardness, ultimate compression strength, yield strength and plastic deformation to fracture increased with increasing cooling rate. In HT ingots, microhardness and yield strength increased and ultimate compression strength and plastic deformation to fracture decreased with increasing cooling rate. The yield stress increased with decreasing interlamellar spacing and increasing volume fraction of Al2O3 particles. A linear relationship between the Vickers microhardness and yield stress was found for both DS and HT ingots. A simple model including the effect of interlamellar spacing and increasing volume fraction of Al2O3 particles was proposed for the prediction of the yield stress.

On the parameters affecting the formation of iron aluminides during pressure-assisted infiltration of aluminium into a preform of steel fibres

The parameters affecting the formation of iron aluminides during reactive infiltration of iron-based fibre preforms in a squeeze-casting equipment were investigated. The volume fraction of intermetallic phases was found to increase when the fibre volume fraction and size decreased. The increase of the temperatures of the die, aluminium and preform allowed avoiding premature checking of the plt forms, reducing the gradient of reaction and increasing the extent of exothermic reactions. As-infiltrated material with overall chemical composition corresponding to Fe3Al was studied by SEM, X-ray diffraction and TEM. The unreacted iron fibres were surrounded by iron-rich FeAl phases. The single-phase grains adjacent to the boundary layer were identified as Fe2Al5 whereas the grains with lamellar morphology were identified as consisting of a mixture of FeAl and FeAl2 lamellae. Heat treatment of such an as-infiltrated material resulted in the formation of a fine-grained structure composed of FeAl and Fe3Al phases

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.

Microstructure and mechanical properties of a directionally solidified and aged intermetallic Ni–Al–Cr–Ti alloy with β-γ′-γ-α structure

Abstract Microstructure and mechanical properties were investigated in a directionally solidified (DS) Ni–21.7Al–7.5Cr–6.5Ti (at.%) alloy. The dendrites of the as-grown alloy were composed of β(B2)-matrix (NiAl), coarse γ′( L 1 2 )-particles (Ni 3 Al), fine γ′-needles and spherical α(A2)-precipitates (Cr-based solid solution). The majority of fine γ′-precipitates was found to be twinned. The interdendritic region contained γ(A1)-matrix (Ni-based solid solution) separating ordered domains of γ′-phase and fine lath-shaped α-precipitates. Ageing in the temperature range 973–1373 K decreased the volume fraction of dendrites from about 50 vol.% measured in the as-grown material to about 38 vol.% in the material aged at 1373 K for 300 h. During ageing in the temperature range 973–1273 K the γ-phase transformed to the γ′-phase in the interdendritic region. This transformation was connected with precipitation of lath-shaped α-precipitates. Ageing at higher temperatures of 1373 and 1473 K resulted in stabilisation of the γ-phase and precipitation of spherical γ′-particles in the interdendritic region. Ageing at 973 K significantly increased the microhardness, hardness and decreased room-temperature tensile ductility. Neither ageing nor finer dendritic microstructure were found to be effective in increasing the ductility of the alloy. The measured tensile ductility up to 1.1% can be attributed to the effect of extrinsic toughening mechanisms operating in the β-phase such as blunting and bridging of cracks by the α- and γ′-precipitates.

Microstructural stability, hardness and compressive behaviour of directionally solidified intermetallic Ni3Al-based alloy with γ/γ′-β structure

Abstract Microstructural stability, hardness and compressive behaviour were investigated in directionally solidified (DS) Ni–20.2Al–8.2 Cr–2.44 Fe (at.%) alloy. The as-grown lamellar γ(Al)/γ′(Ll2)-β(B2) structure of DS alloy is found to transform to a γ/γ′-α(A2) structure during annealing in the temperature range 1023–1173 K. This annealing is connected with precipitation of spherical α-Cr particles within the lamellae and lath-shaped α-Cr particles in the interlamellar γ/γ′-region. The size of the lath-shaped α-Cr precipitates depends on the annealing temperature. However, these precipitates are found to be relatively stable with respect to their size and morphology during annealing for 100–600 h at given temperature. Annealing at temperatures higher than 1210 K stabilises β-lamellae in the microstructure. The volume fraction of transformed lamellae is found to follow ΔVl∝t1/n law with n=4 and the activation energy for lamellae transformation is determined to be Q=236 kJ/mol. The kinetics of lamella transformation is proposed to be governed by diffusion along disordered regions bounding growing ordered domains of γ′-phase around the lamellae. The peak hardness and γ/γ′-region microhardness values are reached after 100 h annealing at 1023 K. The compressive yield stress increases with increasing temperature, reaching a peak value at about 1000 K, and rapidly decreases at higher temperatures. The quasi-steady strain-hardening rate decreases with increasing temperature in the temperature range 773–1273 K. Both yield stress and strain-hardening rate depend on strain rate.

On the control of the residual porosity in iron aluminides processed by reactive squeeze-infiltration of aluminium into a preform of steel fibres

An original approach for the synthesis of iron aluminides of the type FeAl1-x is presented, based on the use of a squeeze-casting equipment for inducing the reactive infiltration of Al into a porous preform made by sintering continuous Fe-based fibres. The casting is subsequently homogenised by heat treatment in the solid state. Preforms with fibre volume fractions ranging between 40 and 80% (fractions that correspond to stoichiometries close to FeAl and Fe3Al. respectively) are prepared using steel-wool fibres with various diameters. Small fibre diameters and high processing temperatures are found to be beneficial for obtaining, after squeeze casting a low-porosity material with a large amount of uniformly distributed reaction compounds. However, the preform thickness that can be properly infiltrated is limited to about 6 mm. The homogenisation of the intermetallic phase is complete after only 5 h at 1000 degreesC. A major problem to be solved is the heterogeneity of the porosity distribution in the sample after homogenisation. This heterogeneity results from an insufficiently uniform distribution of the fibre volume fraction

Microstructure and mechanical properties of wrought aluminium alloy prepared by recycling of aluminium matrix composites reinforced with Inconel 601 fibres

Abstract The microstructure and mechanical properties of Al-2.2Cu-1.7Mg-1.1Ni-1.1Fe-0.9Si-0.3Zn-0.2Cr-0.2Mn-0.2Ti (wt.%) alloy prepared by recycling of composite scrap containing Inconel 601 fibres in aluminium matrix were investigated. The structure of recycled alloy consisted of α-Al dendrites and a multiphase interdendritic region. Transmission electron microscopy and energy dispersive X-ray spectroscopy showed that the interdendritic region contained coarse Al 9 Fe(Ni, Si), Al 7 Cu 4 Ni, Al 4 CuMg 5 Si 4 , (Cu, Al) 2 Mg and Mg 2 Si intermetallic phases distributed in the α-Al matrix. Heat treatments resulted in additional hardening of the alloy through the precipitation and growth of fine θ-Al 2 Cu, S-Al 2 CuMg and β-Mg 2 Si intermetallic particles in the α-Al matrix. Peak microhardness, hardness and yield strength values were reached after 20, 12 and 8 h ageing at 180, 190 and 200°C, respectively. During the early stages of plastic deformation both intermetallic particles in the interdendritic region and small precipitates in the α-Al matrix contributed to the high strain hardening of the alloy. The breakage of coarse intermetallic particles resulted in a considerable reduction of the strain hardening exponent and quasi-steady strain hardening rate. The alloy failed by the coalescence of cavities formed within broken intermetallic particles and small voids formed around the precipitates.