B. Ramaswami

Crystallization behaviour of metglass 2826A by isochronal and isothermal annealing

Abstract Crystallization of Metglass 2826A by isochronal annealing showed a strong dependence on scan rate over the range of 1.4–48.7 K/min. At scan rates less than 9.2 K/min, crystallization of Metglass 2826A occurred by primary crystallization in a two stage process leading to both MS-I and MS-II type phases. A third (SIII) transformation in the crystalline state was also noted at a scan rate of 9.2 K/min. At scan rates of 22.5 and 48.7 K/min, MS-I and MS-II crystallization were concurrent and inseparable. Isothermal annealing revealed the presence of a low temperature polymorphic phase transformation corresponding to a supersaturated solid solution (SS) phase.

Fatigue deformation of copper single crystals containing alumina particles

Abstract The spatial and Burgers vector distribution of dislocations after fatigue deformation of copper single crystals containing alumina particles is described as a function of cumulative plastic strain and of plastic strain amplitude, γp. In constant plastic strain amplitude fatigue tests, the dislocation microstructure consists of a uniform distribution of dipoles at low cumulative strains. The dipoles cluster together in the form of walls and cells at large cumulative strains in the saturation stage. The forest dislocation density increases with cumulative strain. The saturation stress and the fatigue hardening rate increase with γp. The dislocation cell regions become increasingly dominant at the expense of dislocation walls with increasing γp. The experimental results are discussed in terms of the “cell-shuttling” model of fatigue deformation.

Corrosion of FeCrSiB and FeCrNiB metallic glasses containing various chromium concentrations

Abstract Anodic potentiodynamic polarization curves in 0.1 N H 2 SO 4 and 1 N H 2 SO 4 at 30°C were obtained for amorphous alloys having the following atomic compositions: Fe 83− x Cr x Si 5 B 12 with x = 4.1, 6.2 and 8.3 at.%, and Fe 62.5− y Ni 17.5 Cr y Si 9 B 11 with y = 2.1, 2.5 and 9.4 at.%. An increase in the chromium content increased the corrosion resistance of these alloys. Since X-ray scattering intensity studies on the quenched alloy samples showed no significant effect of chromium concentration on the structure functions, it is concluded that the composition and not the amorphous structure is the important criterion with respect to corrosion properties. The surface morphology of each amorphous alloy, which was polarized for 1 h in 0.1 N H 2 SO 4 at a fixed potential in the passive or transpassive region, was studied using scanning electron microscopy. Pitting was found at transpassive potentials in all the alloys and was promoted by the rough surface present in the as-quenched amorphous ribbons. Auger analysis of the Fe 53.1 Ni 17.5 Cr 9.4 Si 9 B 11 amorphous alloy indicated boron segregation near the surface of the as-quenched alloy and chromium depletion in the transpassive film.

Nature and morphology of oxide particles in internally oxidised CuAl alloys

Abstract The dispersed phases produced by two different methods of internal oxidation of Cu−0.035 wt.% Al single crystals were studied by electron diffraction. The Rhines pack method of internal oxidation gave a bimodal distribution of large 500–1000 A triangular γ-alumina plates and small sub-150 A ordered α-alumina particles. However, in single crystals oxidised by the two-furnace method two distinct regions were visible. The central region contained only small particles identical with those observed in the Rhines pack crystals. The outer section, covering 10–20% of the cross-section of the crystal, contained a coarse distribution of thin faceted α-alumina platelets, 1–5 μm in diameter. These α-alumina platelets were ordered and contained a high density of antiphase boundaries or shear faults.

Plastic deformation of Cu3Au single crystals containing alumina particles

Abstract The plastic deformation of ordered and disordered Cu3Au single crystals with and without a dispersion of alumina particles was investigated by stress-strain measurements and slip-line studies using optical microscopy. The dislocation microstructure was studied as a function of shear strain by transmission electron microscopy. The yield stress of disordered Cu3Au-Al2O3 crystals is given by the sum of the matrix flow stress (τ1) and the particle bypassing stress (τ2) indicating that this is a system containing a low density of strong obstacles and a high density of weak obstacles. In the ordered Cu3Au-Al2O3 crystals, the experimentally observed yield stress gives a better agreement with √τ12 + τ22 than with τ1 +τ2, showing that this system contains obstacles of equal strength or a combination of strong and medium strength obstacles. Orowan by-passing and cross-slip occur at the dispersed particles during tensile deformation by unit dislocations in the disordered and by superdislocations in the ordered Cu3Au-Al2O3 single crystals. The work hardening behaviour of ordered and disordered Cu3Au-Al2O3 crystals result from the superposition of contributions from geometrically necessary dislocations and statistically stored dislocations as well as the characteristic distribution of these dislocations. Whereas the initial stage of deformation is dominated by geometrically necessary dislocations in ordered Cu3Au-Al2O3 crystals, it is dominated by statistically stored dislocations arranged in dipole and multipole arrays in disordered Cu3Au-Al2O3 crystals. In both systems the later stages of deformation are controlled by the statistically stored dislocations.

The effect of particle size on the yield stress of Cu-Al2O3 crystals

Abstract The tensile behaviour of internally oxidized coppers-aluminum alloy single crystals has been studied as a function of oxide particle size and volume fraction. The results indicate that crystals containing only small ( A ), coherent oxide particles, exhibit tensile characteristics comparable to precipitation-hardened systems containing deformable coherent particles. The yield stress data obtained for crystals containing large triangular non-coherent alumina platelets (400–800 A) agree well with the theoretical predictions of the Orowan mechanism. However, crystals containing the same volume fraction of much smaller particles exhibit yield stresses an order of magnitude lower than required by the Orowan mechanism. It is concluded that the small, coherent, alumina particles are sheared or fractured at yield. The larger, noncoherent, alumina platelets are by-passed by dislocations, consistent with the Orowan yield criterion for non-deformable particles.

Fatigue softening of AlMg single crystals

Abstract The fatigue softening of Al-0.7 at.% Mg single crystals was studied by cyclic deformation at a constant plastic shear strain amplitude of ±0.01 of specimens prestrained in tension. The changes in the defect microstructure occuring during fatigue softening were studied by transmission electron microscopy. The results show that the saturation flow stress in independent of the amount of prestrain in tension. The cell size along the Burgers vector on the primary slip plane at the saturation stage is almost the same after either fatigue hardening or fatigue softening. The cell walls containing tangles of primary and secondary dislocations formed during prestraining in tension change into a regular array of primary and secondary dislocations after fatigue softening. The fatigue softening occurs by the dissociation of dislocation barriers such as Cottrell-Lomer and Hirth locks and the climb of dislocations enhanced by the point defects created during fatigue deformation. The defect microstructure and saturation stress are dependent only on the plastic shear strain amplitude and independent of the amount of prestrain before cyclic deformation of Al-0.7 at.%Mg single crystals.

Annealing of deformation substructure in fatigued silver single crystals

Abstract The annealing behavior of dislocation substructures and point-defect clusters produced by fatigue deformation was studied in single crystals of silver. Crystals fatigued to saturation in alternate tension and compression at a constant plastic strain amplitude of ±0.02 were annealed at different temperatures, and the resulting microstructural changes were studied by transmission electron microscopy. High-temperature annealing (> 500°C) of the dislocation cell structure results in almost complete annihilation of the dislocation cell walls with no indication of polygonization and recrystallization. Annealing at intermediate temperatures results in a partially annealed structure consisting of dislocation tangles formed by primary and secondary dislocations, several of which are held up at Lomer Cottrell barriers and point-defect clusters. A large number of dislocation dipoles and loops comprising the dislocation cell walls in fatigued specimens anneal out in the temperature range 300–500°C. Upon annealing, the point defects produced during fatigue deformation acquire various configurations which include Frank loops, stacking fault tetrahedra, and prismatic loops.

Effect of gaseous HCL on sulphur segregation and intergranular embrittlement in 321 stainless steel

Abstract Investigation of the susceptibility of Ti stabilized austenitic stainless steel to environmental degradation in argon, synthetic flue gas and synthetic flue gas with 0.2 vol.% HC1 at 813 K was performed. The slow strain rate technique was used over the range of strain rates ( \ ge) of 10−4–10−7 s−1. Microvoid coalescence was observed in both argon and synthetic flue gas environments. Auger analysis of TiC precipitates after exposure to the latter two environments revealed high concentrations of S associated with the carbide; no S was detected in the matrix or grain boundary. The addition of HCl promoted intergranular failure at \ ge = 10 −6 and 10−7 s−1. Chlorine was found to accumulate at the metal-oxide interface. Sulphur was now located both in the TiC carbide and the grain boundary. A reaction sequence is proposed in relation to hydrogen effects on grain boundary segregation in austenitic stainless steels.

Order-strengthening in the ternary alloy Cu2NiZn

Abstract The order-strengthening mechanism in Cu 2 NiZn was studied by comparing stress-strain curves and electron micrographs of corresponding dislocation structures in disordered, partly ordered and ordered samples. At the low strain level superdipoles were frequently observed showing a whole range of interdislocation distances, which were compared with the calculated equilibrium distance between the dislocations based on a zero force consideration.