Elevated temperature tensile properties of powder metallurgy Ni3Al alloyed with chromium and zirconium

The tensile properties of hot extruded powders of Ni-24.1Al, Ni-19.1Al-8.5Cr, and Ni-17.4Al-7.9Cr-0.5Zr have been evaluated from room temperature to 1000° C. These powder metallurgy materials have a fine grain size that results in relatively little increase in yield stress with increasing temperature compared to coarse-grained or single-crystal materials. The alloy containing chromium and zirconium shows greatly reduced dynamic embrittlement in the temperature range 600 to 800° C where the unalloyed aluminide exhibits brittle behaviour. The Cr- and Cr + Zr-containing alloys deform superplastically above 900° C. The mechanism of superplastic deformation appears to be predominantly grain-boundary sliding.     

The evaluation of hot-working characteristics of cobalt-based implant alloys

The hot-working characteristics of wrought Co-Ni-Cr-Mo implant alloy during ingot-to-billet conversion were evaluated using a Gleeble-2000A simulator. The hot tensile test at 700–1 320 °C was used to determine the optimum hot-working parameters at a strain rate equivalent to that of conventional press forging to ensure acceptable hot workability. Hot ductility and deformation resistance as a function of temperature can be clearly established. The fracture surfaces of the tensile specimens were examined to correlate them with the hot tensile ductility values at various temperatures. The poor ductility at temperatures above 1300 °C was attributed to the incipient melting of grain boundaries. The effect of temperature and strain rate on the flow-stress behaviour and microstructures were investigated by uniaxial compression testing in the temperature range 900–1200 °C and strain rate, , range of 0.01–10s^–1. The strain-hardening and steady-state behaviour were described from the measured true stress-true strain curves.     

Spinodal decomposition in Al/Zn alloys

The effect of spinodal decomposition on the mechanical behaviour of Al/Zn alloys was studied over the range of 30 to 60 wt % zinc. Two solution treatment temperatures, 365 and 435° C, were used; extensive ageing studies were carried out at 22 and 100° C, and limited tests were made on samples aged at other temperatures, 0, 55, and 200° C. The yield and tensile strengths were significantly increased by spinodal transformation, but ductility was seriously impaired. The tensile fracture was intergranular, with one exception, and was related to grain-boundary precipitation and a narrow denuded zone.Calculated yield strengths based on Cahn's analysis did not agree with those derived from the correct dislocation model. The wrong model did give fortuitous agreement. Spinodal hardening appears to offer a promising new hardening mechanism in aluminium alloys if the particular composition and treatment can be found to eliminate the serious lack of ductility.     

Texture changes during superplastic deformation of mechanically alloyed aluminium IN90211

Superplastic deformation of mechanically alloyed aluminium IN90211 was studied by texture analysis. The textures in three deformed specimens were investigated as a function of strain using the three-dimensional crystal orientation distribution functions (CODFs). The results for the two superplastically deformed specimens (425 °C, strain rate of 1 s^–1, stress near 50 MPa, and 475 °C, initial strain rate of 77 s^–1, about 110 MPa) indicate that at strains below about 2.0, the specimen deforms by grain-boundary sliding and single (or double) slip, and at larger strains the deformation is dominated by grain-boundary sliding, multiple slip and some recrystallization. At 475°C, 330s^–1, and stress near 160 MPa, the specimen was above the superplastic regime, and the resulting texture changes with deformation were markedly different from superplastic results, and quite unusual.     

Hot tensile properties and microstructural evolution of as cast NiTi and NiTiCu shape memory alloys

Hot tensile properties of as cast NiTi and NiTiCu shape memory alloys were investigated by hot tensile test at temperature range of 700–1100 °C using the strain rate of 0.1 s⁻¹. The NiTi alloy exhibited a maximum hot ductility at temperature range of 750–1000 °C, while the NiTiCu alloy showed it at temperature range of 800–1000 °C. It was found that at temperatures less than 750 °C, diffusion-assisted deformation mechanism was inactive leading to semi-brittle type of failure and limited ductility in both alloys. Also it was found that at temperature range of 800–1000 °C, dynamic recrystallization is dominant leading to high ductility. Likewise, the fracture surface of the specimens presenting the maximum hot ductility showed an ideal type of ductile rupture in which they gradually pulled out to a fine point. On the other hand, the decline in ductility occurred at the temperatures above 1000 °C was attributed to the liquid phase formation leading to interdendritic and intergranular type of fracture.     

Microstructure, tensile deformation and fracture behaviour of aluminium alloy 7055

The microstructure, tensile deformation and fracture behaviour of aluminium alloy 7055 were studied. Detailed optical and electron microscopy observations were made to analyse the as-received microstructure of the alloy. Detailed transmission electron microscopy observations revealed the principal strengthening precipitates to be the hexagonal disc-shaped η′ phase of size 2 mm×20 mm and fully coherent with the aluminium alloy matrix, the presence of spheroidal dispersoids, equilibrium grain-boundary η precipitates and narrow precipitate-free zones adjacent to grain-boundary regions. It is shown that microstructural characteristics have a profound influence on tensile deformation and fracture behaviour. Tensile test results reveal the alloy to have uniform strength and ductility in the longitudinal and transverse orientations. Strength marginally decreased with an increase in test temperature but with a concomitant improvement in elongation and reduction in area. No change in macroscopic fracture mode was observed with sample orientation. Fracture, on a microscopic scale, was predominantly ductile comprising microvoid nucleation, growth and coalescence. The tensile deformation and fracture process are discussed in the light of the competing influences of intrinsic microstructural effects, matrix deformation characteristics, test temperature and grain-boundary failure. This revised version was published online in November 2006 with corrections to the Cover Date.     

The relationship between intergranular cavitation and superplastic flow in an industrial copper base alloy

Intergranular cavitation has been observed during the superplastic deformation of a fine grain sized (1 m) Cu-2.8% Al-1.8% Si-0.4% Co alloy when tested at temperatures 500° C. High voltage electron microscopy revealed that the cavities could be nucleated at twin boundary/grain boundary intersections. The maximum elongation occurs at a higher temperature than that of the maximum strain-rate sensitivity and this is explained in terms of grain-boundary migration, at the higher temperature, which restricts the cavitation process. This explanation was put forward on the basis of texture analysis which was used to study the deformation characteristics at the temperatures of maximum elongation and strain-rate sensitivity. The final fracture mode is shown to change with test temperature: (i) at 400° C no cavitation occurs and fracture is by ductile rupture, (ii) at 500 to 550° C cavitation occurs and fracture is by the interlinkage of voids by an intergranular void sheet (IVS) mechanism and (iii) at 800° C grain growth occurs and fracture occurs by the propagation and interlinkage of grain-boundary cracks along the grain boundaries.     

The morphology and growth of creep cavities in α-iron

Electron fractography has been used to study the intergranular cavities formed in alpha-iron during slow tensile deformation at high temperatures. A minimum in ductility is observed at about 700° C: this coincides with conditions where grain-boundary sliding makes a maximum contribution to the overall deformation and where the morphology of the cavities tends to be dendritic and finely terraced. This is explained in terms of the gradient of chemical potential for vacancies which may develop at the cavity periphery during grain-boundary sliding. Under other testing conditions, planar growth is observed and the cavity surface is often faceted.     

Mechanical properties of a cold-rolled annealed HSLA steel

Hot rolled strips of an HSLA steel containing niobium and vanadium were cold rolled in the range 15 to 80% and annealed for 1 h at various temperatures from 400 to 700 ° C. The response to the amount of deformation and annealing temperature were studied in terms of changes in its hardness, tensile strength and ductility. Hardness and strength were observed to increase while ductility decreased with the amount of deformation. Although annealing of the steel up to 500 ° C did not show significant changes in its mechanical properties, a sharp decrease in hardness and strength and improvement in ductility were observed on annealing the steel at temperatures greater than or equal to 600 ° C. Microstructural studies showed complete recrystallization in the samples subjected to 60% deformation followed by annealing at 600 ° C.     

Hot ductility of a Fe–Ni–Co alloy in cast and wrought conditions

The hot ductility of Fe–29Ni–17Co alloy was studied in both cast and wrought conditions by hot tensile tests over temperature range of 900–1250 °C and at strain rates of 0.001–1 s⁻¹. Over the studied temperature range, the wrought alloy represented higher elongation and reduction in area as compared to the cast alloy. Dynamic recrystallization was found responsible for the higher hot ductility of the wrought alloy and the improvement of hot ductility of the cast alloy at high temperatures. At temperature range of 1000–1150 °C the wrought alloy exhibited a hot ductility drop while a similar trough was not observed in case of the cast alloy. It was also found that at temperatures of 1150–1250 °C the best hot ductility is achieved in both cases of cast and wrought alloy. The experimental data of flow stress were constitutively analyzed and the apparent activation energy of deformation was estimated to be 344 kJ/mol.     

The semi-solid tensile deformation behavior of wrought AZ31 magnesium alloy

The semi-solid tensile deformation behavior of wrought AZ31 magnesium alloy has been studied through applying a set of low strain rate (0.001 s⁻¹) hot tension tests at temperature range of 300–500 °C. The results indicated a ductility drop at ∼450 ± 25 °C. This was attributed to the occurrence of eutectic reaction (L → α + γ) and the partial melting of intermetallic γ phase. The ductility was started to improve by increasing the temperature to 500 °C. The latter was explained considering the effect of liquid phase on stress relaxation through accommodation of the grain boundary sliding phenomena. To further investigating the semi-solid tensile deformation behavior of the experimental alloy, the cavitations characteristics of the alloy were also examined.     

Mechanism of order-induced embrittlement in Ni-Ni4Mo alloys

Intergranular embrittlement of Ni-Mo alloys by long-range ordering to Ni₄Mo was examined for an off-stoichiometric alloy and a stoichiometric alloy of known impurity contents. Both tensile properties and corrosion resistance in HCl were measured as functions of exposure time at 700°C. Various techniques employed for microstructural characterization and microchemical analysis included analytical electron microscopy, X-ray diffractometry and Auger electron spectroscopy. During exposure at 700°C, the yield-strength maxima in both alloys were reached before the ductility minima. Homogeneous matrix ordering was observed to cause a moderate loss of ductility and the fracture mode remained to be transgranular. However, a considerable loss of ductility, intergranular embrittlement and extensive intergranular corrosion attack were found to be associated with heterogeneous grain-boundary ordering, which occurred by a discontinuous mechanism. Discontinuous ordering resulted in molybdenum-depleted zones alongside grain boundaries as evinced from microchemical analysis and localized corrosion attack. It was concluded that the observed intergranular embrittlement was caused by highly localized deformation in the molybdenum-depleted zones.     

Plastic behaviour of CuAl2

The plastic behaviour of CuAl₂ was studied by compression testing of single crystals and polycrystals in the temperature range 300–575 °C. While single crystals were grown from the melt by the Bridgeman technique, ingot and powder metallurgy routes were adopted for polycrystalline specimens. In addition to exploring their flow behaviour, the deformation mechanism was assessed through thermal activation analysis. It was observed that CuAl₂ failed in a brittle manner in compression below 375 °C and its ductility improved progressively with temperature. The brittle-ductile transition (BDT) temperature was influenced by the initial dislocation density but not by the grain size. The strong temperature dependence of flow stress and grain size strengthening effect as per the Hall-Petch relation, were dominant up to nearly the melting temperature of CuAl₂. The measured activation parameters for deformation suggest that the Peierls mechanism is rate controlling over the investigated temperature range.     

The deformation and fracture of austenitic heat-resisting steel with grain-boundary reaction nodules

The tensile properties in 21-4N austenitic engine valve steel containing pearlitic nodules due to the grain-boundary reaction have been investigated at room temperature. Some theoretical considerations on the deformation and fracture behaviour of this steel are also presented. In the steel with grain-boundary reaction nodules more than about 10% in area fraction, the ductility decreases considerably with their increase owing to the brittle fracture of rod-like precipitates in the nodules. Although the grain-boundary reaction also decreases the tensile stress and 0.2% proof stress, its effect is more noticeable on ductility than on strength. A theory is developed to explain the workhardening behaviour as well as the fracture mechanism using a model for composite materials and a good agreement between theoretical and experimental results is obtained.     

Plastic deformation of EVA, EVOH and their multilayers

The high strain rate, plastic deformation of multilayer films is analyzed in relation to the tensile properties of the components. The multilayers combine ethylene-vinyl acetate (EVA) blend surface-layers with an ethylene-vinyl alcohol (EVOH) core-layer; the volume of the EVOH layer is varied from 1% to 20%. Individually, the components exhibit markedly different tensile behavior in terms of yielding and ductility in the temperature range from 55°C to 95°C. The EVA blend deforms homogeneously whereas EVOH forms a neck at an extension rate of 10,000%/min. It is shown that the true stress-strain relationship of the components in the plastic region can be described with two parameters, the true yield stress and the strain hardening parameter. Multilayers deform homogeneously, as does the EVA blend. A simple rule of mixtures approach is used to describe the plastic behavior of the multilayers as a function of temperature, composition and moisture, and to predict whether or not deformation will be uniform.     

Cavitation and fracture of mechanically alloyed aluminium at high homologous temperatures

The tensile behaviour of mechanically alloyed (dispersion strengthened) IN90211 was characterized at strain rates between 0.0001 and 340 sec^–1 at temperatures between 425 and 475 ° C, At strain rates above 0.1 sec^–1, superplastic elongations were obtained (maximum elongation 525% at 475 ° C, 2.5sec^–1. Large elongations were possible due to the lack of cavitation, even though the strain-rate sensitivity was lower (m 0.25) than usually found in superplasticity. Cavitation was precluded by the morphology of the platelet-shaped grains in which low-angle subgrain boundaries were predominantly perpendicular to the tensile axis. Grain-boundary sliding was observed along high-angle grain boundaries which were generally parallel to the tensile axis. At the high homologous testing temperatures (0.76 to 0.81), concurrent grain-boundary sliding and lattice slip was made possible by the rapid lattice diffusivity and easy climb of lattice dislocations over dispersions in the matrix and grain boundaries.     

Influence of a transition metal oxide (CuO) on the superplastic behaviour of 8 mol% yttria-stabilised cubic zirconia polycrystal (8Y-CSZ)

The microstructure and superplastic deformation of fine-grained undoped 8Y-CSZ and 1 wt% CuO doped 8Y-CSZ have been investigated in tension in the temperature range 1503 to 1623 K and strain rate range 5 × 10^–5 to 1 × 10^–3 s^–1. Deformation of the undoped 8Y-CSZ was characterized by large strain-hardening with limited tensile elongations of 20%; this was mainly due to severe grain growth during deformation. The addition of a small amount of a transition metal oxide (CuO) resulted in a decrease in strain-hardening and enhanced tensile elongations up to 78%. The ductility enhancement in the CuO doped 8Y-CSZ was due to copper segregation to grain boundaries, thus facilitating grain boundary sliding. In addition, the enhanced ductility in the doped material was related to a reduction in flow stress which, in turn, suppressed cavitation and delayed fracture.     

Effects of heat treatments on the microstructure of a superplastic Ti3Al based alloy

The ability to develop superplastic properties in a two phase material depends mainly on three physical features: the volume fraction of the phases, their size (and morphology) and their composition (associated with their crystallographic structure). All these parameters are functions of the temperature of deformation. Therefore, their variation with temperature is required to determine the temperature domain in which superplastic properties can be expected and then to investigate the mechanisms of deformation induced in the superplastic regime. The material investigated in this study is an industrial Ti₃Al based alloy, to be formed by superplastic forming. From microstructural characterization of the material in as-received conditions, it is concluded that superplastic properties can be achieved with the provided microstructure. Microstructural transformation (variations in phase compositions, grain sizes and crystallographic structure) associated with annealing in the temperature range 890–1070 °C was investigated. The law of variation with temperature of phase volume fractions was determined by two independent methods.     

In-situ hot microhardness study of cavitation in superplastic 60/40 microduplex brass

The hardness of the individual phases of the 60/40 brass have been measured in situ over the temperature range 20 to 700° C. Activation energy values for the alloy and its phases were determined from the hot hardness data. Such data are utilized to understand the occurrence of cavitation during superplastic deformation of this alloy. It is also suggested how cavitation may be minimized by lowering the deformation temperature to 250° C where the alloy still exhibits large elongations.     

Superplastic behaviour of YBa2Cu3O7−x /Ag superconductors

High-temperature deformation characteristics of YBa₂Cu₃O_7–x oxide (YBCO) and YBa₂Cu₃O_7–x /Ag composite (YBCO/Ag) in uniaxial compression have been investigated. A compression test was carried out at temperatures from 780–930°C at initial strain rates between 10^–6 and 10^–4 s^–1. YBCO/Ag composites with fine, dense and equiaxed grains were compressed over 120% with no indication of failure at higher temperatures, and the strain-rate sensitivity exponent, m, was found to be about 0.42–0.46 between 890 and 930°C. They are considered to be one indication of superplasticity. The activation energy for deformation was 500–580 KJ mol^–1. The specimens suffered grain growth slightly during the deformation at 930°C and the majority of growth might be a function of exposure time, temperature and silver content, but each grain maintained the equiaxed shape after extensive superplastic deformation. This is consistent with a grain-boundary sliding mechanism. The silver at grain boundaries acts to decrease the activation energy for deformation and promote the grain-boundary sliding.