Temperature dependence of yielding and work-hardening rates in magnesium oxide single crystals

The mechanical properties of MgO single crystals have been analysed for their yielding and work-hardening behaviour as a function of purity and deformation temperature. The Fleischer theory accounts for the observed temperature dependence of the critical resolved shear stress well. Depending on temperature and impurity content, the MgO single crystals are shown to exhibit two-stage work-hardening behaviour, with the hardening shown to be athermal in nature. At and below room temperature, a temperature-dependent contribution results from dislocation-point defect interactions, while the primary athermal contribution arises out of dislocation-dislocation dipole interactions. The CRSS and work-hardening behaviour of MgO single crystals indicates a close relationship to the deformation characteristics of NaCl-structure alkali halides.     

Cyclic hardening/softening behaviour in AZ31B magnesium alloy based on infrared thermography

The work-hardening/softening behaviour of AZ31B magnesium alloy during high cycle fatigue was investigated. The superficial temperature evolution during fatigue tests was used as a criterion for the different levels of work-hardening/softening. The microstructures under different cycles were observed by transmission electron microscope. Tensile test (with post-fatigue) was conducted to quantify the work-hardening/softening behaviour which showed that high dislocation density after cyclic loading lead to high tensile strength. The temperature evolution of the specimens with different levels of work-hardening/softening during tensile tests is related to the microstructures; the results indicated that the temperature rise of the specimen with high density dislocation was lower. Microstructures after tensile tests showed that high dislocation density after cyclic loading would lead to high twinning density.     

Temperature Dependent Work-hardening Behaviour in an Fe-Mn-Si-Cr-Ni Shape Memory Alloy

The work-hardening behaviour in an Fe-Mn-Si-Cr-Ni alloy has been investigated using tensile test at different temperatures and TEM observation. It was found that besides the intersection of εmartensite, the intersections of ε martensite with stacking fault and the cross-slip of dislocation which is difficult to occur in the alloy with low stacking fault energy are also important factors to the temperature dependent work-hardening behaviour.     

New method for measuring the non-elastic work-hardening rate of solid polymers

Probing the non-elastic deformation of solid polymers in the pre-yield range has proved to be a very sensitive test of the structural response to plasticity nucleation. A work-hardening rate parameter,K, deduced from repeated stress relaxations was thus introduced to quantify this behaviour. It is shown that considerable improvement and more extensive information may be gained by measuring the work-hardening rate by the so-called direct method from a simple stress-non-elastic strain plot. A comparison is given of the two methods in the case of unsaturated polyester resins, together with a careful evaluation of potential sources of discrepancy.     

Studies on the stress-strain relations of dual-phase steels

The correlations of the work-hardening exponent,n, with quenching temperature, martensite volume-fraction (MVF) and solute concentration in ferrite are discussed and derived for dual-phase steel. The flow stress of dual-phase steel at low strain is suggested to be expressed by the combination of the terms due to plastic deformation in ferrite and elastic deformation of martensite. Previous experimental results are compared with the behaviour suggested by this theoretical work. In addition, an expression for the work hardening exponents at moderate strains and at the onset of necking are also theoretically suggested.     

Effect of inclusion orientation upon acoustic emission-microstructural relationships in ferritic steels

A comparison of the acoustic emission, mechanical properties and fracture behaviour of three model low-alloy steels containing manganese sulphide inclusions of varying orientation has been made as a function of tempering conditions. The majority of the inclusions existed in elliptical clusters whose major axis was aligned with the rolling direction and whose plane lay on the rolling plane. Both the yield region acoustic emission activity and ductility in the through-thickness (short transverse) sample orientation were reduced compared with the longitudinal, and to a lesser extent, transverse orientation samples. These effects are shown to be a consequence of inter-inclusion cluster shear localization in material of high yield stress and low work-hardening capacity. Because of the wide range of yield strengths and work-hardening capacities used in this study, the results extend our insight of the interactions between inclusion distribution, stress state and work-hardening capacity during ductile fracture of this class of materials.     

Observations on regenerated chitin films

Chitin films regenerated from shrimp shell waste were studied in order to determine stress-strain behaviour and tensile strengths as related to preferential chain orientation.Plastic deformation of the chitin films was observed to cause a decrease in the transmission of visible light and the stress-strain behaviour could be described approximately by a linear Bingham model incorporating work-hardening.The properties of chitin films were observed to deteriorate both in strength and dyestuff affinity over a 30-year period.     

Ageing and work-hardening behaviour of a commercial AA7108 aluminium alloy

In the 7xxx aluminium alloying system several mechanisms influence the hardening behaviour of the alloys, e.g. particle size and distribution, dislocation density and alloying elements in solid solution. This work is an experimental study of ageing and work-hardening considering a commercial AA7108 alloy in the as-cast and homogenized condition. Tensile specimens have been exposed to a solution heat treatment and a two-step age-hardening treatment with varying time at the final temperature. The tensile data for the different tempers have been evaluated in elucidation of already existing models based on a one-parameter framework. The precipitate size and distribution have been further investigated in the transmission electron microscope for a selection of tempers, and the influence of these parameters on the work-hardening behaviour has been discussed.     

Work-hardening behaviour of a heat-treatable AA7108 aluminium alloy deformed to intermediate strains by compression

This work is an experimental study of ageing and work-hardening considering various heat treatments of an AA7108 aluminium alloy in the as-cast and homogenized condition. Specimens have been exposed first to a solution heat treatment and a then to a two-step age-hardening to obtain different tempers. These tempers have been tested in an upsetting test applying state-of-the-art cold lubrication allowing intermediate strains to be reached before the onset of barrelling. The measured work-hardening response has been analyzed by fitting an extended Voce equation to the experimental stress–strain curves. With increasing overageing the stage III part of the stress–strain curve shrinks, and for the most overaged condition studied here, the stress–strain curve has reached a linear stage IV already at a strain of about 0.3. Interestingly, the slope of the stage IV curve then is lower than commonly reported for Al alloys. The microstructure and texture of the deformed material have been further investigated for a selection of tempers, and their influence on the work-hardening behaviour has been discussed.     

Some aspects of the static and dynamic compressive behaviour of β-brass single crystals

The compressive behaviour of β-brass single crystals has been investigated in uninterrupted static and dynamic tests and in interrupted tests using static-static, static-dynamic, dynamic-dynamic and dynamic-static loading sequences. Static and dynamic strain-rates were 1.5×10^?4 and 3.2×10³ sec^?1 respectively. Slip traces on the statically deformed crystals were wavy and deformation occurred by single slip on either (ī01) [111] or (¯211) [111] or by a transition mode involving both (ī01) [111] and (¯211 [111]. Except for anomalous behaviour in the dynamic reload following static preload the dynamic slip traces were straight with deformation occurring by multiple slip on four {110} planes involving two 〈111〉 directions. It is shown that there is no direct causal relationship between the lower work-hardening rate and level of flow stress and the crystallography of slip in dynamic deformation. The work-hardening rate and flow stress in static and dynamic loading are rather determined by the dynamics of the deformation. The differences in the substructural features as observed by transmission electron microscopy arise principally from the differences in the slip modes and cannot be interpreted as controlling the stress-strain behaviour. The low work-hardening rate and flow stress in dynamic deformation is believed to be due to the production of short-lived disorder. The absence of a/2〈111〉 dislocations in thin foils is explained in terms of the fast reordering reaction in β-brass.     

Work-hardening rate of glassy polymers: evolution with curing of thermo-set resins

A new parameter, the work-hardening rate,K, is introduced to characterize the non-elastic deformation behaviour of glassy polymers. Related to the defects nucleation in the preyield stage, this parameter is shown to be a very sensitive probe of the structural evolution of materials. The variation ofK with cross-linking in the curing of a polyimide resin is reported.     

Superconducting properties of the composite-processed Nb3Sn superconductor with the Cu-Sn-Zn matrix

Effects of the Zn addition to the Cu-Sn matrix of the Nb₃Sn composite tape have been investigated by measuring the matrix work-hardening behaviour, the rate of Nb₃Sn layer formation and the pertinent superconducting properties. The Zn addition drastically enhances the diffusion rate of Nb₃Sn formation at each Sn level in the matrix examined, leading to sufficient superconducting properties even at a low Sn level of 3.5 at. %; a sample containing 3.5 at. % Sn and 15 at. % Zn in the matrix exhibits a critical temperature and critical current density comparable with those of samples at a Sn level of 7 at. %. The upper critical field obtained for a sample with the 6 at. % Cu-6 at. % Sn-4 at. % Zn matrix beyond 200 kOe. The work-hardening of the composite matrix is found to be essentially a function of Sn level, and insensitive to the Zn addition.     

Design and characterization of a lamellar nanostructure in a low C steel

A fully lamellar ferrite/cementite nanostructure was designed in a low C steel by using a specific thermal treatment. The strengthening of such microstructure has been investigated as a function of prestrain by rolling up to a deformation of 300%. As in usual pearlitic structure, its work-hardening shows no saturation and its elongation to fracture remains rather constant instead of decreasing drastically as conventional steels. The hardening by a similitude effect is thus not the privilege of pearlitic steels. Nevertheless, its lower initial work-hardening rate at low strain compared to an equivalent pearlitic steel and a lower hardening potential at high strain let us suspect major differences in the nature and the behaviour of ferrite channels in relation to the morphogenesis of the microstructure. This study opens a new way to obtain low carbon ultra-high strength steel by a nanostructuration process using severe plastic deformations.     

An improved constitutive model for high-temperature flow behaviour of the Fe–Mn–Al duplex steel

The high-temperature flow curves of the Fe–Mn–Al duplex steel showed an uncommon yield-like behaviour and an abnormal dynamic recrystallisation behaviour that occurred at low temperatures rather than high temperatures. The interaction of strain partitioning and unsynchronised softening behaviour in δ-ferrite and austenite caused this peculiar flow behaviour. By discussing the stress exponent and apparent activation energy, respectively, at low and high temperatures, a modified hyperbolic sine function was developed to predict the characteristic stresses. By simplifying the material constant θ and compensating the microstructural evolution in the exponential saturation work-hardening law, an improved constitutive model was developed to predict the transient stress. The comparison between the experimental and calculated values confirmed a high prediction accuracy of this improved model.     

Influence of vanadium on grain boundary segregation of phosphorus in iron and iron-carbon alloys

The influence of vanadium on grain boundary segregation of phosphorus has been studied in iron and iron-carbon alloys by means of fracture experiments in a scanning Auger microprobe. The emphasis here is to study the effects of vanadium on the interaction processes operative under circumstances when structure in the interior of the grain (in the present case carbide formation) and grain boundary segregation form simultaneously. It is emphasized that to predict and analyse the behaviour of an alloy, it is important to consider atomic interactions both at the grain boundaries and in the grain interior and that between the constituents and the grain boundaries. The study suggests that the principal determining factor in the scavenging or retardation of migration of phosphorus to the grain boundaries is whether vanadium is present in the combined form (say, carbide) or is available in solid solution form. When vanadium is present in solid solution form, grain boundary segregation of phosphorus is low because of the chemical interaction of vanadium and phosphorus. However, as carbon is increasingly introduced in the alloy, vanadium now preferentially reacts with carbon in view of higher interaction for carbon as compared to phosphorus. A consequence of this is the increase in the grain boundary concentration of phosphorus. In such a situation the presence of excess carbon in addition to what is stoichiometrically required to precipitate the entire vanadium as vanadium carbides, serves as a palliative with regard to the reduction in the intergranular concentration of phosphorus. This palliative behaviour is explained in terms of the sitecompetition model. An effort is also made to examine the behaviour of segregating elements in terms of whole range of probable interactions (both at the grain boundaries and in the grain interior) and chemical interaction energies.     

Constitutive equation of a non-simple elastic plastic material

Summary The mechanical response predicted by the constitutive equation of a non-simple elastic material is considered in relation to the total strain behaviour of an elastic-plastic solid extensively deformed in the range of plastic strain. Both loading and unloading are considered in relation to the range of total elastic-plastic strain. In the absence of appropriate experimental studies, comparison of the predictions of the proposed constitutive equation of a non-simple elastic material, when applied to the work-hardening behaviour of the material, has been restricted to a study of the characteristic stress-strain behaviour of a strain hardening material. This has centred on the correlation of stress-strain curves characteristic of the mechanical response of a material tested in simple compression, simple torsion and pure shear with the object of obtaining a universal stress-strain curve.With 1 Figure     

Metallurgical aspects of steels designed to resist abrasion,and impact-abrasion wear

Many abrasion resistant steels rely on a martensitic microstructure to ensure hardness, which in general correlates with better wear performance. However, in practice the steel may be subjected to a complex combination of conditions where hardness alone may not be sufficient to ensure tribological performance. This review is a critical assessment of the mechanical and metallurgical parameters that control wear resistance of steel in impact-abrasion conditions, although relevant work dealing with abrasion has also been included. It is found, for example, that fracture toughness and work-hardening behaviour have a role in enhancing the wear resistance of hard steels.     

A model of the high-temperature deformation of metallic materials with ellipsoidal second-phase particles

A micromechanics model developed in the previous work which incorporated the effect of dynamic recovery by diffusion of atoms was applied to the interpretation of the high-temperature deformation of metallic materials with ellipsoidal second-phase particles. A theoretical discussion based on this model was made on the effect of several factors including shape, particle size, orientation and elastic modulus of second phase on the work-hardening behaviour of the materials at high temperature. A good correlation was found between the result of the calculations and those of the experiments obtained by the present authors or other investigators of several kinds of metallic materials with ellipsoidal second-phase particles. The dynamic recovery model used in this study can be applied to the understanding of high-temperature deformation behaviour or to the prediction of the possible recovery mechanism of the materials.     

Preparation, microstructure and mechanical properties of metal-particulate/glass-matrix composites

Composites with a borosilicate glass matrix containing different concentrations of vanadium particles were fabricated by powder metallurgy and hot-pressing. The mechanical properties and fracture behaviour of the composites were assessed by a range of techniques. Young's modulus, fracture strength in bending, and fracture toughness increased with vanadium content. By virtue of the good interfacial bonding and low residual internal stresses, an effective crack-particle interaction during fracture was achieved. The fracture toughness of composites containing 30 vol. % of vanadium inclusions was approximately 65 % higher than that of the unreinforced glass. Experimental values for the fracture toughness increment were in good qualitative agreement with the predictions of theoretical models in the literature. Extensive plastic deformation of the vanadium inclusions was not found, however. This was attributed mainly to the constraint imposed by the rigid matrix surrounding the particles and to possible embrittlement of the particles during composite fabrication at high temperatures. The brittleness index (B) of the composites was calculated and its relevance for characterisation of the ductile versus brittle behaviour of brittle-matrix composites is discussed.