Effect of prior cold work on age hardening of Cu-4Ti-1Cd alloy

This research is part of a project whose scope was to develop high strength ternary alloys based on Cu-Ti system with the primary aim of substituting them for toxic and expensive Cu-Be alloys. In this pursuit, age hardening behaviour of Cu-4Ti-1Cd alloy has already been investigated and the present paper reports the investigations on the influence of prior cold work by rolling of 50, 75 and 90% on the age hardening of a Cu-4Ti-1Cd alloy using hardness and tensile tests and optical as well as transmission electron microscopy. As a result of cold work followed by aging, hardness of the alloy increased from 237 Hv in solution treated condition to 425 Hv on 90% cold work and peak aging. Similarly, yield and tensile strengths of the alloy reached maxima of 1037 and 1252 MPa respectively on 90% deformation and peak aging. The microstructure of the deformed alloy exhibited elongated grains and deformation bands. The maximum strength on peak aging was obtained due to precipitation of ordered, metastable and coherent β^l, Cu₄Ti phase in addition to high dislocation density and deformation twins. Both hardness and strength of the alloy decreased on overaging due to the formation of incoherent and equilibrium β, Cu₃Ti phase. However, the morphology of the discontinuous precipitation was changed to globular shape due to large deformations and overaging.     

Deformation behaviour of Al–Cu–Li alloy containing T1 precipitates

In the present work, a systematic investigation of crystallographic texture evolution and strain hardening behaviour was undertaken to comprehend the deformation behaviour in the presence of T₁ (Al₂CuLi) precipitates. Characteristic texture components symbolising multiple slip condition such as Copper and S were observed upon rolling which is in contrast with other Al alloys containing shearable precipitates. Strain hardening ability was also observed to be remarkably high in the presence of T₁ precipitates. The texture and strain hardening results are compared with another age hardenable Al alloy (Al–Mg–Si alloy) containing shearable precipitates to clearly bring out the difference in the nature of T₁ precipitates.     

Influence of cold rolling degree and ageing treatments on the precipitation hardening of 2024 and 7075 alloys

In the present work, the precipitation hardening of 2024 and 7075 aluminum alloys is investigated as a function of cold rolling degree, ageing time and temperature using Vickers microhardness measurements and differential scanning calorimetry (DSC). It is found that a variation in such parameters can improve the hardness and plays an important role in the precipitation hardening process. At specific ageing temperature, the large cold rolled 7075 alloy exhibits two peaks of hardness. Moreover, for both alloys, the increment of hardness during ageing decreases with increasing the cold rolling degree. While in some cases microhardness measurements give impression that the precipitation reaction is slowed down by deformation, DSC analysis indicates that the precipitation is much accelerated since only a slight deformation decreases strongly the temperatures of reactions. However, the degree of cold rolling does not play a crucial role.     

An approach to modeling time-dependent creep and residual stress relaxation around cold worked holes in aluminium alloys at room temperature

Creep behaviour of aluminium alloys is also observed at room temperature. As a result, a relaxation occurs of deliberately introduced beneficial residual stresses around fastener holes, before the relevant structural component is subjected to exploitation. Therefore, to adequately asses the life-time of the component with cold worked holes, it is necessary to quantify this relaxation. In this paper a combined iterative approach for building a time-dependent creep constitutive model of aluminium alloys at room temperature has been developed in order to be used in finite element (FE) simulations of the cold hole working process. The approach is based on an experimental study of the change in diameters of cold worked holes through mandrel cold working method and a subsequent series of FE simulations of the cold working process and of the following creep behaviour to determine the necessary equivalent stresses in the constitutive model. The obtained creep constitutive model has been founded on the power-law model. The model parameters A, n and m have been determined on the basis of a developed by the authors algorithm. The approach has been illustrated on D16T aluminium alloy widely used in the airspace industry. The material behaviour in the plastic field has been described by the nonlinear kinematic hardening model, obtained through a uniaxial tensile test. Both constitutive models have been used in FE simulations of the cold working processes and of subsequent residual stress relaxation around the cold worked open holes due to creep at room temperature. On the base of the FE results, mathematical models describing the residual stress relaxation have been obtained. Thus, the residual stresses are adequately evaluated immediately before introducing the structural component in operation.     

Effect of cobalt additions on the age hardening of Cu-4.5Ti alloy

The effects of 0.9 and 1.8 wt% cobalt additions on the age hardening behaviour of Cu-4.5Ti alloy have been investigated. It has been observed that though Co addition results in the refinement of grain size and the Cu-Ti-Co alloys exhibit age hardening (giving rise to peak hardness on aging at 400°C for 16 hours), the peak hardness as well as the corresponding yield and tensile strengths were found to decrease with increasing cobalt content. The electrical conductivities of 0.9 and 1.8 wt% Co alloys were found to be 6% and 10% International Annealed Copper Standard (IACS) and 7% and 14% IACS in solution treated and peak aged conditions, respectively. Like in the binary Cu-Ti alloys, precipitation of ordered, metastable and coherent Cu₄Ti(¹) precipitate was found to be responsible for maximum strengthening in these alloys. In addition, coarse intermetallic phases of Ti and Co, viz. Ti₂Co and TiCo particles have been observed in all the conditions studied. The inferior mechanical properties of Cu-Ti-Co alloys compared with those of the binary Cu-Ti alloys are attributed to the depletion of Ti from matrix, which is consumed to form Ti₂Co and TiCo phases.     

Precipitation hardening of Cu – Ti – Zr alloys

Abstract

The influence of age hardening temperature and time on the hardness, tensile properties, electrical conductivity, and microstructure of Cu – 4Ti – 0.1Zr and Cu – 3Ti – 0.1Zr alloys has been investigated. The resulting microstructure of these alloys suggests that zirconium addition prohibited the formation of compositional modulations in the solution treated condition. These alloys exhibited maximum hardness and strength on peak aging at 450°C for 24 h by the formation of a coherent and metastable Cu₄Ti phase (β') in modulated structure while overaging occurred by the formation of equilibrium phase β-Cu₃Ti. The electrical conductivity of both the alloys increased moderately on aging. Unlike in an earlier study of binary Cu – Ti and some ternary Cu – Ti – X alloys, overaging did not cause any discontinuous precipitation in the Cu – Ti – Zr alloys investigated. Modulated structure formed on peak aging persisted on prolonged aging at 450°C for 80 h or at 500°C for 8 h.     

Kaltgewalztes Flachzeug aus unlegierten weichen Stählen — Nach DIN 1623 Blatt 1

Cold-Rolled Flat Products of Plain Carbon Mild Steel According to DIN 1623 Sheet 1 . Cold-rolled flat products of plain carbon mild steel and among the most important finished products made by iron and steel works and rolling mills as well as being among the most important semi-products for industrial manufacture. Metallurgical and rolling developments during the course of the last twenty years have resulted in considerable improvements in the main cold forming properties of sheet and strip. The chief demand made of manufacturers of cold-rolled flat products is for sheet and strip which has been fully annealed and rerolled for good cold formability. Because of the various stresses on materials in the different forming processes, it is difficult to make a reliable prediction from test results about the behaviour of the material during actual forming operations. The new edition of sheet 1 of DIN 1623 prescribes the conventional test methods and their results as regards ensuring cold formability. The Standard does not include the additional means of using numerical values for vertical anisotropy r, work hardening exponent n and the maximum drawing ratio (ßo)_max.     

Effect of thermomechanical treatment on spray formed Cu – Ni – Si alloy

Abstract

The heat treatment response of a spray formed Cu – 2.4Ni – 0.6Si (wt-%) alloy has been investigated. The spray formed alloy was given various thermomechanical treatments prior to isothermal aging. These treatments included solutionising and/or cold rolling with different reductions in original thickness. The variation in hardness and electrical conductivity of the alloys was measured as a function of the aging time. The results indicated the highest peak hardness value of ~250 kg mm^-2 for the alloy aged after solution treatment and cold rolling to 40% reduction in thickness, compared with the maximum hardness of 220 kg mm^-2 for specimens aged directly in the as spray formed condition. However, the electrical conductivity after aging was observed to be a maximum of 65%IACS (International Annealed Copper Standard) in specimens cold rolled to 80% reduction in thickness before aging. The aging response was observed to accelerate with the degree of cold working. Optical, scanning electron and transmission electron microscopy were used for microstructural characterisation of the materials. Precipitation of the second phase was observed to dominate in deformation bands. The alloy showed evidence of discontinuous precipitation, particularly when the alloys were cold rolled before aging. The onset of discontinuous precipitation led to a drastic deterioration in hardness of the alloys. The precipitation behaviour of the alloy is discussed in the light of microstructural characteristics associated with various processing conditions of the alloy.     

Controllable fabrication of a carbide-containing FeCoCrNiMn high-entropy alloy: microstructure and mechanical properties

Previous studies have reported that high carbon contents in FeCoCrNiMn high-entropy alloys lead to carbides precipitating from the alloys. Typically, carbides are used to improve the strength of alloys but also lead to decreased ductility. However, the strength and ductility of alloys can be improved when carbides shape, size and distribution are carefully controlled. Therefore, a carbide-containing FeCoCrNiMn alloy with 2?at.-% carbon was prepared by arc melting, and its microstructure and mechanical properties were further tuned by cold rolling with subsequent annealing treatment. The yield strength and uniform elongation of the resultant alloy were excellent, reaching 581?MPa and 25%, respectively, due to the additive combination of various strengthening mechanisms, such as solid-solution hardening, grain-boundary hardening and precipitation hardening.     

变形量对NiTi形状记忆合金相变行为及恢复性能的影响

通过低温DSC测试仪、金相显微镜和拉伸试验机,研究NiTi形状记忆合金冷轧变形量对马氏体相变行和形状恢复率的影响。结果表明：冷轧变形生成的应力诱发马氏体和取向不同的丝织构及高密度位错,使得马氏体相变时界面迁移的阻碍作用增加,抑制了相变的发生;随冷轧变形量的增大合金的形状恢复率降低。     

The effect of hot-rolling on chill-cast Al,Al-2 wt % Ni and Al-4 wt % Ni alloys

The effect of hot-rolling on the mechanical properties and microstructure of directionallysolidified hypoeutectic Al-Al₃Ni alloys has been studied. Chill-cast hypoeutectic alloys were produced by casting into pre-heated mild-steel moulds placed on copper chills. The chill-cast Al-2 wt% Ni and Al-4 wt% Ni hypoeutectic alloys can be hot-rolled at 500 C to reductions of greater than 95%. Deformation is achieved by deforming the aluminium-rich dendrites in the rolling direction, followed by interpenetration of the Al₃Ni fibres into the dendrites resulting in a homogeneous microstructure. The variations of room-temperature tensile properties for the chill-cast hypoeutectic alloys were measured as a function of reduction of thickness during hot-rolling. The ultimate tensile strength and yield strength increase during rolling because of increasing Al₃Ni fibre alignment, homogeneous dispersion of the Al₃Ni fibres throughout the Al matrix, and work hardening in the Al matrix. The as-chill-cast alloys have strengths which agree with the composite law of mixtures for a combination of Al dendrites and Al-Al₃Ni eutectic. After hot-rolling, the alloy strengths can be predicted from discontinuous fibre reinforcement theory.     

Influence of alloying and secondary annealing on anneal hardening effect at sintered copper alloys

This paper reports results of investigation carried out on sintered copper alloys (Cu, 8 at%; Zn, Ni, Al and Cu-Au with 4 at%Au). The alloys were subjected to cold rolling (30, 50 and 70%) and annealed isochronally up to recrystallization temperature. Changes in hardness and electrical conductivity were followed in order to investigate the anneal hardening effect. This effect was observed after secondary annealing also. Au and Al have been found to be more effective in inducing anneal hardening effect.     

Prediction of asymmetric cyclic‐plastic behaviour for cyclically stable non‐ferrous materials

Investigation on asymmetric cyclic‐plastic or ratcheting behaviour of non‐ferrous materials has received relatively little attention compared with ferrous materials. Ratcheting behaviour of materials is generally simulated using isotropic‐kinematic hardening models; however, for materials showing cyclically stable response, isotropic hardening is often not accounted for the constitutive modelling. A methodology based on Chaboche's isotropic‐kinematic hardening (CIKH) model with the consideration of genetic algorithm for optimization of initial estimates of the CIKH parameters is used in this study. The investigated plastic responses incorporate both symmetric strain‐controlled hysteresis loops and ratcheting behaviour. The suggested approach satisfactorily predicts the reported plastic response of cyclically stable non‐ferrous alloys based on aluminum, zirconium, and titanium.     

Multipass cold rolling and resultant mechanical properties of a Cr-containing nickel aluminide

The difficulties encountered in fabricating Ni₃Al-based intermetallic alloys into final structural components, due to their limited workability as a result of their inherent high yield strength and low ductility at elevated temperatures, are an important issue that have restricted the commercial applications of these materials. The Osprey spray deposition process is capable of delivering near-net-shape preforms, thereby avoiding the technical problems related to the hot working of these materials, e.g. hot rolling of slabs. The present work concerns an investigation of the cold rollability of a chromium-containing Ni₃Al intermetallic alloy produced with the Osprey process. The sliced preform with a thickness of 7 mm was successfully cold rolled through multipasses into sheets with a thickness of 0.7 mm and a good surface finish. The material has been found to have a high working hardening rate at room temperature. The maximum total reduction permissible without resulting in rolling defects is 30%. Thus, for larger reductions, intermediate annealing between rolling passes is necessary and it has been optimized to be at 1100°C for one hour. The repeated cold rolling and the recrystallization occurring during intermediate annealing change the initial microstructural features and grain size of the Osprey-spray-deposited material. The cold-worked and annealed intermetallic sheets with a thickness of 0.7 mm have a yield strength of 570 and 730 MPa and a elongation value of 33 and 7%, at room temperature and at 700°C, respectively. Fractography shows a transition from the transgranular fracture mode at low temperatures to the intergranular fracture mode at temperatures above 650°C.     

Cold worked Cu-Fe-Cr alloys

The aim of this project was to investigate the properties of copper rich Cu-Fe-Cr alloys for the purpose of developing a new cost effective, high-strength, high-conductivity copper alloy. This paper reports on the influence of cold work. The age hardening response of the Cu-0.7%Cr-2.0%Fe alloy was minimal, but the resistance to softening was superior to that reported for any commercial high-strength, high-conductivity (HSHC) copper alloy with comparable mechanical and electrical properties. For example, an excess of 85% of the original hardness of the 40% cold worked alloy is retained after holding at 700°C for 1 hour, whereas commercial HSHC Cu-Fe-P alloys have been reported to soften significantly after 1 hours exposure at less than 500°C. The Cu-0.7Cr-2.0Fe alloy would therefore be expected to be more suitable for applications with a significant risk of exposure to elevated temperatures. Optical microscope examination of cold worked and aged microstructures confirmed the high resistance to recrystallization for Cu-0.7%Cr-2.0%Fe. The Zener-Smith drag term, predicting the pinning effect of second phase particles on dislocations in cold worked microstructures, was calculated using the precipitate characteristics obtained from TEM, WDS and resistivity measurements. The pinning effect of the precipitate dispersions in the peak-aged condition was determined to be essentially equivalent for the Cu-0.7%Cr-0.3%Fe and Cu-0.7%Cr-2.0%Fe alloys. A lower recrystallisation temperature in the Cu-0.7%Cr-0.3%Fe alloy was therefore attributed to faster coarsening kinetics of the secondary precipitates resulting from a higher Cr concentration in the precipitates at lower iron content.     

Fascinating microstructural evolution during consolidation and remarkable hardening behaviour of micro alloyed Al-Cu-Ni alloys

Ultrafine B2 NiAl and vacancy ordered phase (Al₃Ni₂ type) (Cu) evolve during consolidation of mechanically alloyed Al-Cu-Ni alloys without and with micro alloying addition of Zr and or Ti and results in high hardness of the alloys. Al-Cu-(Ni15)-Zr alloy consolidated by spark plasma sintering exhibits a remarkable hardness value of 7.2 GPa. Wide range of hardness values of the alloys indicates a significant effect of composition as well as synthesis route. Hardening behaviour through point defect hardening has been demonstrated in terms of constitutional vacancy concentration as well as solute dislocation interaction. Lattice parameter, bulk density, porosity anddislocation density of the alloys have been measured to understand the hardening mechanism further. TEM analysis and wear behaviour are carried out to justify the phase formation and strengthening mechanism of the alloys, respectively.     

Inhomogene Verformungserscheinungen bei Schwingbeanspruchung des Vergütungsstahls 42 CrMo 4

Inhomogeneous Deformation Processes During Cyclic Loading of Quenched and Tempered Steel SAE 4140 The cyclic deformation behavior of a normalized steel SAE 4140 in the loading range σ_a < R_eS is characterized by softening and hardening phenomena. Cyclic softening processes are combined with inhomogeneous plastic deformations and the propagation of a fatigue Lüdersband. The following hardening process occurs homogeneously. At stress amplitudes σ_a < R_p0.2 the quenched and tempered SAE 4140 worksoftens continuously until crack initiation. Plastic deformation processes are extremely inhomogeneous and are concentrated on small fatigue zones. The inhomogeneity increases with decreasing tempering temperatures und stress amplitudes.     

Effects of thermo-mechanical processing and trace amount of carbon addition on tensile properties of Cu–2.5Fe–0.1P alloys

The effects of thermo-mechanical processing, including intermediate aging treatment and/or solution heat treatment, and a trace amount of carbon (C) addition were studied on tensile behavior of Cu–2.5Fe–0.1P alloys. In this study, Cu–2.5Fe–0.1P alloy sheets without and with a carbon content of 0.05 wt.% were cast and subsequently rolled and thermo-mechanically treated following various processing routes. The introduction of intermediate aging treatment between cold rolling improved the tensile strength of Cu–2.5Fe–0.1P alloys. Solution heat treatment prior to aging was proved to be detrimental on the tensile strength, probably due to recovery and recrystallization causing the complete loss of work hardening during previous cold rolling. The present study also suggested that two-step aging is more effective in improving the strength of Cu–2.5Fe–0.1P alloys than one-step aging. The effect of C addition on improving the tensile strength of Cu–2.5Fe–0.1P alloys was real but marginal, probably due to the limited solubility of C in Cu–2.5Fe matrix. The effects of intermediate heat treatments between cold-rolling processes on tensile properties of Cu–2.5Fe–0.1P specimens with and without C addition are discussed based on optical, scanning electron microscope (SEM) and transmission electron microscope (TEM) micrographs, and SEM fractographs.     

Low-cyclic fatigue behaviour of an Al–Cu–Mg–Ag alloy under T6 and T840 conditions

The low-cyclic fatigue (LCF) behaviour of an AA2139 alloy belonging to the Al–Cu–Mg–Ag system was investigated under T6 and T840 conditions. The T840 treatment involves cold rolling with a 40% reduction prior to ageing, and this was effective in increasing the tensile strength of the alloy. Under cyclic loading at total strain amplitudes (ε_ac) of ±0.4 to ±1.0%, the mechanical behaviour is defined as the prevalence of elastic over plastic deformation processes under both the T6/T840 conditions. The initial weak hardening during one to two cycles of loading at ε_ac?>?0.55% and an insignificant softening upon following the cyclic loading to fracture was observed for the T6/T840 conditions. The LCF behaviour of the alloy under the T6/T840 conditions is described by the Basquin–Manson–Coffin relationship.

This paper is part of a Themed Issue on Aluminium-based materials: processing, microstructure, properties, and recycling.     

Constitution and age hardening of Al-Sc alloys

Aluminium-rich alloys from the Al-Sc system were examined to determine the form of the equilibrium phase diagram and to obtain information on age hardening of chill cast alloys. Samples containing up to 8.75wt% Sc were examined using thermal analysis and optical microscopy. This work indicated a eutectic type of phase diagram with a eutectic temperature of about 665° C and a eutectic composition of about 0.6wt% Sc. The scandium-rich primary phase was found to be ScAl₃ which is f c c with a lattice parameter of 0.4105nm. Chill cast samples of a 1 wt% Sc alloy were examined for their age hardening behaviour over the temperature range of 225 to 360° C. A maximum hardness of 77 VHN was obtained after ageing at 250° C for 3 days. This hardness was retained after ageing for a total of at least 12 days. The hardening precipitates were ScAl₃ which were observed to form via a discontinuous precipitation mechanism. The ScAl₃ precipitates were observed to have a parallel orientation relationship with the matrix.