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.     

Mechanical alloying of copper with niobium and molybdenum

Copper has been mechanically alloyed with the bcc structure type elements Nb and Mo in order to obtain a material with high strength, good ductility and good electrical conductivity. In the case of alloying Cu with Nb a solid solution is formed during milling, whereas in the case of alloying Cu and Mo a fine distribution of Mo particles within the copper matrix is observed. The different behaviour of these two alloys is related to the large difference of the elastic constants of the bcc elements. Niobium precipitates from the solid solution and molybdenum particles coarsen during a subsequent heat treatment.     

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.     

The effect of deliberate aluminium additions on the microstructure of rolled steel plate characterized using EBSD

The use of aluminium as a deliberate alloying addition in steels has attracted increased attention recently as a possible replacement for Si in transformation-induced plasticity (TRIP) steels. In addition, some authors have suggested that it offers beneficial effects as a solid solution strengthener as well as galvanizability. In this work three low carbon (0.02 wt.%) manganese (1.4 wt.%) steels have been alloyed with very different aluminium contents (0.02, 0.48 and 0.94 wt.%) in order to study the effect of this alloying element on the final ferritic microstructure. Two different rolling schedules have been applied to these steels and the final microstructures have been characterized extensively by EBSD measurements. The results indicate that aluminium additions have a profound influence on ferrite grain size and the grain boundary misorientation distribution functions.     

Thermomechanical processing of aluminium-based particulate composites

Thermomechanical processing of aluminium-based particulate composites containing dispersions of various soft and hard particles such as graphite, zircon, glass, alumina and mica in aluminium alloy matrices, with a view to improving strength and ductility for structural applications, has been discussed. The existing literature on the subject has been critically reviewed and analysed, and broad guidelines for optimum thermomechanical processing have been presented. Considerable improvements in strength and ductility of these composite materials have been reported after rolling, forging and extrusion due to fragmentation/fibrization of particles together with the refinement of matrix microstructure, annihilation of defects such as porosity, and texture hardening, etc. The influence of process variables, and of volume fraction, size and morphology of the particles on the strengthening mechanisms, fracture toughness and work-hardening behaviour of worked composites has been discussed.     

Influence of hot forging and alloying with Al on the electrochemical behavior and mechanical properties of austenitic stainless steel

This paper introduces a new approach to overcome the deterioration of ductility of austenitic stainless steel (ASS) by Al alloying using vacuum induction furnace. The available data reported about the addition of Al to ASS shows that Al alloying increases the work-hardening property and deteriorates the ductility due to decreasing the austenite phase and increasing the ferrite phase. Therefore, austempering process was a vital stage to increase the austenite phase. The optimum conditions due to Al alloying have been determined. The promising samples were undergo corrosion tests using electrochemical impedance spectroscopy and cyclic voltammetry to investigate their performance and electrochemical behavior. Results showed that hot forging and alloying with Al improve the hardness and ductility but have an adverse effect on the pitting corrosion resistance.     

A study in the work-hardening behaviour of austenitic manganese steels

The work-hardening behaviour of Hadfield's steel and the effect of vanadium additions have been examined. At room temperature, deformation twinning plays a significant role. The rate of work-hardening was found to be sensitive to the presence of vanadium above 1 wt%. Transmission electron microscopy studies indicate that the work-hardening is due to a combination of structural defects. The mechanical behaviour of the alloys in compression can be described by a mathematical expression derived from the familiar Ludwik expression.     

Mechanical properties and strain-induced phase transformations of some high-strength manganese steels

Work has been carried out to determine the influence of composition and heat treatment on the mechanical properties of some high-strength manganese steels. These steels display impressive mechanical properties in the as-hot-rolled and solution-treated conditions. These impressive mechanical properties have been directly related to the strain-induced phase transformation which was monitored continuously using a magnetic reluctance techniquein situ and dynamically during the mechanical tests. Exceptional work-hardening rates were displayed by these steels exceeding that of Hadfield's manganese steel, which is itself known as a highly work-hardening material. They underwent considerable phase transformation during deformation. The results indicated that work-hardening effects probably arise from an increased dislocation density and stacking fault population which have been generated in the austenite/epsilon matrix during the strain-induced formation of lath martensite which itself also takes part in the work-hardening process.     

High Manganese Steel Alloying Process and Its Influence on Microstructure and Properties of the Steel

The influence of two kinds of alloying processes, adding Nb (or Ti) and N-Mn alloy as well as adding Nb (or Ti) and spraying N2, on microstructures and properties of a high manganese steel has been studied. It has been found that adding Nb(or Ti), accompanying with N-Mn alloy, is unfavourable to microstructure compactness of the high manganese steel, but adding Nb (or Ti)and spraying N2 into the melt is good for refining austenitic grain, forming a lot of hard particles and improving microstructure compactness. The mechanical properties of the high manganese steel have relation to the content of elements Nb or Ti. Its fracture mode will turn ductile fracture into brittle cleavage fracture gradually. By X-ray and TEM analysis, it is proved that the austenite can be transformed to deformation-induced α martensite after adding a certain amount of element Nb (or Ti). The microstructure transformation of alloying high manganese steels through deformation is one of methods for strengthening austenite matrix and increasing the work-hardening rate as well as improving antiwear property.     

Evolution of mechanical properties in a 7075 Al-alloy subject to natural ageing

A series of mechanical tests was used to characterise the evolution of yield strengths, r-values and work hardening during the natural ageing of a commercial 7075 aluminium alloy. The results show that there is a significant underlying change in anisotropy between the original O-condition and solution heat treated tempers. In the absence of recrystallisation, the abrupt change in anisotropy appears to have been caused by the modification of the precipitate during the solution heat treatment. The degree of induced anisotropy remained constant throughout the natural ageing process from 15 to 120 min. A series of phenomenological functions is presented for the purpose of interpolation between the original test data points. These empirical functions are shown to predict the yield behaviour of the naturally aged material within the observed experimental scatter.     

On the plastic zone and residual stresses within a strip and a half plane under a moving load

A quasi-steady solution has been obtained for the determination of the extent of plastic zones and the amplitude of the residual stresses in (1) an elasto-plastic strip lying on a rigid frictionless base and (2) a half-plane, under the first passage of a moving load. The load is assumed to be applied with a semi-elliptic pressure distribution which is of sufficient magnitude to cause plastic yielding in the body. A work-hardening material obeying von-Mises' yield criterion is considered. A numerical solution procedure, based on discretizing the plastic strain field in the coordinate system moving with the load into uniformly strained rectangular or semi-infinite elements has been developed. Numerical results are obtained for a strip with linear work-hardening material properties. The effect on the distribution of residual stresses created by the ratio of load half-width to strip thickness is also discussed.     

Hydrogen embrittlement studies of aged and retrogressed-reaged Al–Zn–Mg alloys

The hydrogen embrittlement (HE) of Al–4.35Zn–1.4Mg–0.059Zr alloys in different artificial aging tempers and after retrogression and reaging (RRA) treatments has been investigated by tensile testing hydrogen precharged specimens. The influence of RRA and hydrogen charging on the dislocation structure was studied by TEM. The under-aged temper was the most susceptible while the over-aged temper was the most resistant to HE. The RRA treatment improved the HE resistance of all the tempers. This has been attributed to the reduction in dislocation density upon retrogression and reaging. Flat fractography features near the surface of the hydrogen charged specimen have been correlated to the depth of hydrogen penetration. The hydrogen dislocation interaction and hydride cracking mechanism of HE have been addressed.     

Effect of partial mechanical alloying on the self-propagating high-temperature synthesis of Ni3Si

Self-propagating high-temperature synthesis (SHS) is a new method for economical processing of intermetallic compounds and ceramic materials, as well as composites based on them. On the other hand, mechanical alloying is an effective method for producing highly metastable and, therefore, reactive metal powders. In this paper an overview of partial mechanical alloying is given. The effect of partial mechanical alloying on the self-propagating high-temperature synthesis of Ni₃Si-compounds is studied. The influence of alloying time on powder characteristics, e.g. particle size distribution, is given. The effect of alloying time on the properties of Ni–Si composite powders and on the characteristics of the SHS process, e.g. propagation rate, is reported. Ni₃Si was chosen as the object for this study because of its corrosion and high-temperature oxidation resistance. Like other L1₂-type compounds, the strength of Ni₃Si shows an anomalous behaviour as a function of temperature, therefore, it has potential for high-temperature applications.     

Entwicklung mit Hilfe des Warmpreßverfahrens hergestellter Sinterstähle mit optimaler Warmstreckgrenze,Dauerfestigkeit und Kerbschlagzähigkeit – Teil 2

Development of Hot-Forged Sintered Steels with Optimum Yield Point at Higher Temperature, Fatigue and Impact Strength . The material characteristic values of hot-forged sintered steels are reported. Besides the usual properties, (ultimate tensile strength, yield strength and elongation at room temperature). The behaviour of materials at higher temperature is of great importance. Further, fatigue strength and impact strength must be considered. The influence of heat treatment on these properties were investigated and brought to a correlation with the formation of microstructures. The hot forging P/M preform requires a special attention on alloying elements and alloying techniques. Optimum properties results when using completely alloyed powders or mixed powders by employing special master alloys. This and further problems involving alloying techniques and particularly the behaviour of carbon as an alloying element is reported in details. The correlation between mechanical properties and fracture behaviour is discussed.     

The alloying behaviour and mechanical properties of polycrystalline Ni3Al (γ′ phase) with ternary additions

A survey of the results from a variety of techniques has shown that the substitution behaviour of alloying additions is primarily determined by electronic considerations. Thus, Si, Ti, V, Mn, Nb, Hf and Ta substitute for aluminium; Co and Cu substitute for nickel and Cr, Fe, W, and Mo substitute for both species. A number of conclusions have been drawn from an analysis of compression test data, the most significant of which was that the mechanical properties of γ′ depended on both the substitution behaviour of the alloying addition and the degree of non-stoichiometry. Considerable strengthening is only obtained when (1) the alloying addition substitutes for aluminium and has a large size misfit parameter, and (2) the alloy is aluminium-rich or stoichiometric.     

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.     

Entwicklung mit Hilfe des Warmpreßverfahrens hergestellter Sinterstähle mit optimaler Warmstreckgrenze,Dauerfestigkeit und Kerbschlagzähigkeit – Teil 1

Development of Hot-Forged Sintered Steels with Optimum Yield Point at Higher Temperature, Fatigue and Impact Strength . The material characteristic values of hot-forged sintered steels are reported. Besides the usual properties, (ultimate tensile strength, yield strength and elongation at room temperature). The behaviour of materials at higher temperature is of great importance. Further, fatigue strength and impact strength must be considered. The influence of heat treatment on these properties were investigated and brought to a correlation with the formation of microstructures. The hot forging P/M preform requires a special attention on alloying elements and alloying techniques. Optimum properties results when using completely alloyed powders or mixed powders by employing special master alloys. This and further problems involving alloying techniques and particularly the behaviour of carbon as an alloying element is reported in details. The correlation between mechanical properties and fracture behaviour is discussed.     

Development of nanocrystalline Fe–Co alloys using mechanical alloying

Abstract

Nanostructured alloys have considerable potential as soft magnetic materials. In these materials a small magnetic anisotropy is desired, which necessitates the choice of cubic crystalline phases of Fe, Co, Ni, etc. In the present work, Fe–50 at.-%Co alloys were prepared using mechanical alloying (MA) in a planetary ball mill under a controlled environment. The influence of milling parameters on the crystallinity and crystal size in the alloys was studied. The particle size and morphology were also investigated using SEM. In addition, a thermal treatment was employed for partial sintering of some of the MA powders. The crystal size in both MA powders and compacted samples was measured using X-ray diffraction. It was shown that the crystal size could be reduced to less than 15 nm in these alloys. The nanocrystalline material obtained was also evaluated for magnetic behaviour.     

Non-linear analysis for anisotropic materials

An elasto-plastic analysis of anisotropic work-hardening materials, using the finite element method is presented. The analysis is based on the generalized Huber-Mises yield criterion extended by Hill for anisotropic materials. General expressions for the anisotropic parameters in the yield criterion have been derived both for initial yielding as well as subsequent yielding in the case of work-hardening materials. The isoparametric ‘quadratic’ quadrilateral elements have been used for the analysis and the ‘initial stress technique’ has been adopted for the iterative solution of the non-linear problems. The results of the various numerical examples have been compared with the available solutions.