The stress dependence of the activation volume was studied over a range of strains in polycrystalline alpha brasses of various grain sizes containing 10, 15, 20, and 30 at.-%Zn, and in polycrystalline Cu of 4N and 5N purities at 77, 200, and 290 K. The data show that solute atoms are effective obstacles to dislocation glide only up to small percentage values of uniaxial strain; conventional work hardening, i.e. forest cutting, determines the flow stress at higher strains.
Argon atmosphere pressure had no effect on the nodularization of graphite in magnesium treated cast iron, while it had considerable influence on the graphitization and graphite nodule number. With increasing argon atmosphere pressure up to 145 atm, the graphitization was retarded and the graphite nodule number reduced. The degree of nodularization became maximum at a magnesium content of about 0.15 mass-% and thereafter decreased as the magnesium content increased. For values >0.15 mass-%Mg, meshlike and flaky graphite was formed in the region adjacent to tiny particles which proved to be magnesium droplets. The effect of magnesium on the graphitization was opposite to that on the nodularization of graphite, i.e. the graphitization was suppressed with increasing magnesium content up to about 0.15 mass-% and was promoted when the magnesium content exceeded 0.15 mass-%.
The shape of the strain transient following a sudden stress change (drop or increment) during steady state creep has been investigated for AI, AI–Mg alloys, AI–Li alloys, Cu, and Zr−1 wt-%Nb over a range of temperatures. The strain transients were classified as N type, for which the initial creep rate is high but decreases with time eventually reaching a constant value, or I type, for which the creep rate gradually increases to a constant value. The shape after a stress increment can be used as a guide to the rate controlling process of creep with pure metals and Class 2 alloys in which recovery is rate controlling showing an N type transient, and Class 1 alloys in which viscous glide is rate controlling showing I type transients. The shape of the strain transient after a sudden stress drop gives no clear indication of the rate controlling mechanism since, in general, pure metals and Class 1 and Class 2 alloys each show I type transients.
Strain softening in a wrought Pb–Ca–Sn battery alloy is explained by the formation of shear bands during subsequent deformation. It is shown that these bands develop as a result of geometric softening, but once they exist the localized deformation in these bands causes shearing and subsequent dissolution of precipitates producing a substantial loss of strength. Adiabatic heating in the shear bands promotes localized dissolution of precipitates and recrystallization. As the reduction in rolling is increased the number of shear bands increases while their strength decreases. This leads to a decrease in maximum tensile strength and an increase in elongation to fracture.
The present paper shows the results of high temperature experiments performed on Cu single crystals. The crystals were deformed by rolling at 900°C with the rolling plane either parallel to {123} or 14° off the {111} crystal plane. In the former, slip events were evenly distributed in the samples. No recrystallization was then observed. For the crystals which were 14° off the {111} plane, the deformation was non-uniform and numerous shear bands were formed. It was found that the density of shear bands increased with strain, and recrystallization initiated within the shear bands. A model of dynamic recrystallization in crystals deforming by the shear band mode is discussed in terms of the structural features revealed by optical and electron microscopy.
An investigation has been made of the constitution of the Ni rich portion of the Ni-Al-Mo-Ta system using alloys lying in the 85 and 75 at.-%Ni sections of the system. Observations are reported of alloys in the as-cast condition and also after annealing at 1523 K; the results of electron probe microanalysis, X-ray diffraction, and microscopical examination of the annealed alloys are presented as isothermal sections. The phases encountered in equilibrium in the composition range studied are as follows: the Ni based solid solution γ and intermetallic compounds based on Ni3Al(γ′), Ni3Ta(δ), and Ni6TaAI(η). The 85 at.-%Ni section is predominantly single phase γ at 1523 K. In the 75 at.-%Ni section, γ enters into equilibrium with γ′ and δ;γ′ enters into equilibrium with γ, η, and δ to form γ + γ′ + δ and γ′ + η + δ phase regions. In the two phase γ + γ′ region, lattice parameter mismatch values lie in the range 0.56–0.81%. In the as-cast condition, some alloys show regions which are interpreted as containing one or more intermetallic compounds based on the Ni-Mo system.
AbstractThe creep behaviour of a series of alloys based on IN 597 (Nimonic 101) and with varying volume fractions of γ′ precipitate has been studied at 800°C in the standard condition of heat treatment and for two grain sizes. In order to assess the long-term structural stability of alloys of this type, which have high contents of Cr, creep tests were also carried out on material overaged for 5000 and 10000 h at 850°C. Optical and electron metallography have been used to examine the microstructural characteristics of the materials. The results show a consistent pattern of increasing creep resistance with increasing volume fraction of γ′ precipitate, although the effect was more significant in the coarse grained material. After overaging, the creep resistance and the life to rupture of the alloys were reduced, and acicular particles, believed to be γ′ phase, were observed in the microstructure. Some limitations of current mechanisms describing dislocation–particle interactions are discussed with respect to creep deformation.
An Al-6 wt-%Ni alloy containing a volume fraction of 0.1 of 1.1 μm dia. particles was deformed 95% by cold rolling and annealed. Recrystallization was nucleated in the vicinity of the particles, and the orientation of the new grains was investigated by X-ray and electron diffraction. The orientation of the recrystallization nuclei was found to be almost random, with the majority of nuclei being misoriented by 15–45° from the adjacent matrix, and this was interpreted in terms of nucleation within the deformation zone at the particles. In the fully recrystallized material, however, the texture was significantly sharper than in the deformed material, and this was ascribed to the more rapid growth of grains of selected orientations.
AbstractPrecipitates which form on {111} planes of the Al-Cu-Mg alloy lattice when small amounts of silver are added are newly established as having a hexagonal structure with a = 0.496 nm and c/a = 1.414. The [0001] direction in the precipitate is aligned parallel to a 〈111〉 direction in aluminium with 〈1010〉 parallel to 〈110〉, giving close registry of the respective crystal lattices. A small addition of zinc reduces the size of precipitate but does not change the crystal structure.
AbstractExperiments were made to determine the influence of the substructure of hot deformed low alloy steel austenite on the structure and mechanical properties of shear type γ → α transformation products. Four steels having 0.16–0.20%C and additions of Mo, V, and B were used. Immediately before γ → α transformation, austenite was deformed by rolling according to a programme simulating the reduction schedule in industrial plate mills. It was found that the increase in strength owing to the effect of austenite substructure on the structure of shear type γ → α transformation products was small and did not exceed 10%.
Results are given of the equilibrium measurements between liquid Fe containing Si and Al and calcium aluminosilicate melts (with 5%Si02) saturated with CaO at 1600°C and with CaAl4O7 at 1650°C. By incorporating the experimental results of this work with the available data on oxide activities and other slag-metal reactions, the equilibrium relations are derived for Si-Al redox slag-metal reaction. Comments are made on the ladle desulphurization of liquid steel by treatment with lime saturated aluminate slags.
AbstractFactors affecting the cleavage fracture stress and the fracture toughness K 1C in steel have been examined. A statistical method based on carbide induced cleavage fracture is proposed, by which it is possible to estimate the fracture toughness and study the effect of different variables. The predictions made are in excellent agreement with experimental results for a variety of microstructures.
AbstractThe structure and morphology of Mo2C and VC resulting in secondary hardening of high purity alloys Fe-Mo-C and Fe-V-C during aging at 550°C are studied by electron diffraction, electron and field ion microscopy, X-ray measurements of α Fe matrix unit cell dimensions, and Mössbauer spectroscopy. It is stated that in the early stages of aging, when a sharp decrease in hardness occurs, the process takes place in two directions simultaneously, namely, the formation of cementite particles and the formation of globular clusters of alloying element atoms, the latter being mainly on dislocations and at grain boundaries. During the period of hardness increase, cementite particles are dissolving, and the content, shape, and location of alloying element clusters are changing. The alloying element clusters are mixed zones of M and C atoms, disclike in shape, the discs lying on {100}α planes. Special carbides having the fcc structure are observed at peak hardness in all the alloys investigated. The fcc structure is formed inside Mo2C and VC irrespective of the type of structure in the equilibrium phase. On overaging, the fcc structure of Mo2C changes into the equilibrium (hcp) structure, the particles of which are needlelike.
Considering the similarities between structural distribution and deformation behaviour of martensite–ferrite dual phase steels with composite materials reinforced by short, discontinuous fibres, a new expression for the tensile strength of these steels was suggested using shear lag analysis, i.e.σbDP=1K(β23+0⋅65)σbMVM+σbF(1−VM)where σbDP, σbM, and σbF are the tensile strengths of the steel, the martensite, and the ferrite phases respectively, β is the aspect ratio of martensite islands, K is the strength ratio of martensite to ferrite σbM/σbF, V M is the volume fraction of martensite, and (1 – V M) is the volume fraction of ferrite. The experimentally measured strengths of a variety of dual phase steels are shown to be in good agreement with those calculated using this equation. Problems concerning the load transfer from ferrite to martensite and the degree of utilization of martensite strength are discussed.
Processes taking place in the course of annealing of deformed ferrite–austenite stainless steel have been examined by means of light and electron metallography. Deformations of 23, 43, and 85% were obtained by rolling at room temperature. It has been found that ferrite and austenite recrystallize discontinuously irrespective of the magnitude of the deformation. Ferrite recrystallization is preceded by extensive recovery processes. Initially this induces the formation of elongated subgrains, whose growth results in a rapid spheroidization and nucleation of recrystallization. The recovery of austenite before recrystallization is far more limited and, in general, does not lead to the formation of a subgrain structure. The preferred regions of recrystallization nuclei formation in ferrite are shear bands and microbands, whereas in austenite the nucleation of recrystallization takes place at the intersections of deformation twins and in shear bands. The size of ferrite and austenite grains after recrystallization decreases with increasing deformation. After a given deformation, the ferrite grain size is much larger than that in austenite.
The use of only the thermodynamic approach in the investigation of the phenomena of temper embrittlement and impurity segregation is insufficient for the complete understanding of such processes. To explain correctly those phenomena, it is suggested that the phase transformation theory and the data from structural studies for different stages of aging be used in conjunction with the thermodynamic method. Such an approach makes it possible to explain the effects of each solute upon the process of impurity segregation.
When a metal is deformed cold, a small fraction of the energy expended in its deformation is stored in the crystal lattice associated with crystal defects and their elastic strain fields. There is evidence that grains having different orientations store different amounts of strain energy, although the data available are restricted to just a few grain orientations or are averages over a zone of orientations. A method is described which allows a more complete measurement of the orientation dependence of the stored strain energy of cold work, and is illustrated for the case of 50% cold rolled copper.
Observations made defining the conditions under which Al halo formation occurs on primary Si and angular Si in pure Al-Si and Sr modified Al-Si alloys during directional solidification with a temperature gradient of 125K cm−1 in the liquid are described. Halo formation is explained in terms of the competitive growth of eutectic Si and primary Al phase.
The effects of the addition of rare earth metal (REM) on the stress relief cracking susceptibility of low alloy martensite have been examined by slow strain rate tensile tests. The partial dissolution of MnS by austenitization before quenching at temperatures as high as 1573 K promoted the intergranular decohesive mode of fracture in hot tensile tests, and the stabilization of free S atoms by the addition of REM increases ductility at temperatures between 823 and 890 K and the fracture morphology changes to that of intergranular microvoid coalescence. However, in higher temperature deformation in the range from 890 to 973 K, the addition of REM reduces ductility and increases the fraction of intergranular microvoid coalescence at the expense of the transgranular ductile fracture mode. This can be explained in terms of AlN precipitation in relation to the dispersion of fine MnS particles.
