Materials Science and Technology

Processing of Al–19·4Si alloy by high intensive electron beam has been carried out, and multiple increase in fatigue life of the material has been revealed. Investigations of structure and surface modified layer destruction of Al–19·4Si alloy subjected to high cycle fatigue tests to fracture have been carried out by methods of scanning electron microscopy. The factors responsible for the increase in fatigue life of Al–19·4Si alloy have been revealed and analysed.

Precipitation takes place when the austenite stainless steel is heated to a high temperature. This is found significantly different when the electropulsing is implemented during the heat treatment. Considerable less number density and much smaller particle size of precipitates are formed in the sample treated with electropulsing. Electropulsing helps to dissolve precipitates. The effect is not due to Ohm heat. Instead, it is attributed to the electric current induced change of thermodynamic sequences of the phases and the electric current accelerated mass diffusion.

The influence of structure-phase features of nanocrystalline and amorphous alloys and electric current modes on the electroplastic effect under tension is investigated. Grain size refinement up to nanoscale, occurrence of the second phases, and amorphization in alloys lead to decreases in or the full disappearance of the electroplastic effect. In nanocrystalline alloys with reverse thermoelastic martensite transformation, the introduction of current pulses suppresses stress jumps downwards induced by the display of electroplastic effect and causes active stress jumps upwards connected with the shape memory effect.

Using transmission electron microscopy and differential scanning calorimetry, the microstructure and martensitic transformation of the Ti_50·0Ni_50·0 alloy after cold rolling and after rolling with a pulse current were compared. A structural relaxation effect was observed in the alloy during rolling with current. The effect is manifested in the more slow kinetics of the structure formation and less intensive suppression of martensitic transformations at rolling with current compared to cold rolling with some deformation degrees.

In this paper, microstructure evolution in metals induced by high density electric current pulses (ECPs) treatment, such as grain refinement, formation of oriented microstructure and redistribution of inclusions, is carefully reviewed. The mechanism for these microstructure evolutions is discussed. It is suggested that there are usually several factors working during ECP: Joule heating (including temperature rise, the heating rate and the cooling rate), stress (including thermal compressive stress, skin effect, electron wind force) and free energy change caused by the current. The Joule heat and the stress usually affect the dynamics, enhance the microstructure evolutions rate, while the free energy change affects the thermodynamics and lowers the barrier of the phase transformation or recrystallisation. The experimental results indicate that high current density ECP provides an effective approach to refine the microstructure of polycrystalline materials and to improve the mechanical properties of the materials.

The refinement of inclusions in molten steel induced by a continuous electric current pulse was investigated at 1823 K. The results revealed that due to the application of electric current, the melted sulphide inclusions in molten steel were refined. Analysed from the thermodynamic theory, the refinement mechanism was ascribed to the decrease in the system free energy that resulted from the formation of the refined sulphide inclusions in molten steel at 1823 K. Hence, the electric current pulse treatment may be a new method to refine inclusions in molten metallic materials in the future.

A priori derivation for the extra free energy caused by the passing electric current in metal is presented. The analytical expression and its discrete format in support of the numerical calculation of thermodynamics in electric current metallurgy have been developed. This enables the calculation of electric current distribution, current induced temperature distribution and free energy sequence of various phase transitions in multiphase materials. The work is particularly suitable for the study of magnetic materials that contain various magnetic phases. The latter has not been considered in literature. The method has been validated against the analytical solution of current distribution and experimental observation of microstructure evolution. It provides a basis for the design, prediction and implementation of the electric current metallurgy. The applicability of the theory is discussed in the derivations.

Electropulsing was introduced into the turning process of the AISI 304 stainless steel. The results indicate that the main cutting force, microhardness and axial roughness on machined surface are reduced dramatically under appropriate electropulsing parameters. For AISI 304 stainless steel, the electropulsing applied in turning process can improve the plastic deformation ability in cutting area, increase lubricating property and change friction state between the tool and the workpiece in the forming process of the chip. It causes the arising of plastic stripping tear based on pure shear plastic deformation and brings obvious changes on morphologies of the machined surface and the corresponding chip.

An ultrasonic electric surface modification treatment was employed to improve the surface properties of 2316 stainless steel. The surface properties of the specimens after conventional cutting, ultrasonic surface modification treatment and ultrasonic electric surface modification treatment were characterised respectively. A grain refinement layer was formed on the specimen's surface after ultrasonic electric surface modification treatment. The average grain size on the top surface was refined into the submicrometre or nanometre scale. This is caused mainly by two aspects: one is the accumulation of initial tiny particles during deformation, and the other is that the ferrite is smashed into pieces due to microfatigue damage. Moreover, it was found out that the specimen after ultrasonic electric surface modification treatment at four times had shown the optimal surface properties.

Effects of electric pulses on the annealing hardening of submicrometre grained (0.1 μm < D < 1 μm) Cu processed by heavy cold rolling were investigated. Annealing induced hardening occurred after moderate annealing, which can be attributed to the dislocation source hardening rather than aging hardening, based on the fact that the ultrapure Cu showed higher hardening effect than commercial pure Cu. Electric pulses induced enhancement in the hardening effect for ultrapure Cu, but not for commercial pure Cu. The TEM investigations show that dislocation recovery was enhanced by electric pulses for ultrapure Cu, but not for commercial pure Cu due to the pinning effects of impurity, and negligible growth in both cases due to the short treatment time.

The effect of electropulsing assisted ultrasonic impact treatment (EUIT) on the mechanical properties and microstructure evolution of S50C steel welded components has been investigated. The present paper presents the application of a relatively new post-weld treatment method to eliminate the residual stress and improve the surface mechanical properties. The results show that EUIT exhibits better surface modification capability than does conventional ultrasonic impact treatment. After EUIT, plastic deformation layer with strengthened grains formed on the sample surface, and residual tensile stress was converted into residual compressive stress.

Large scale 30Cr2Ni4MoV ingots play a crucial role in nuclear power plants. Shrinkage and carbon segregation are the most common defects in the manufacture of these large scale ingots. Large scale ingots have very low cooling rates. In the present work, an experimental method was employed to achieve similarly low cooling rates with the aim of simulating the solidification process of large ingots in smaller 30Cr2Ni4MoV ingots.Thus, we examined the effect of surface pulsed magneto-oscillation (SPMO)on solidification structure in a laboratory setting. Our experimental results showed an SPMO treated ingot with less carbon segregation and a smaller shrinkage cavity than in an untreated one. Finally, the action mechanism was analysed by numerical simulation.

In the present work, pure aluminium melt was treated using electric current pulse. The effect of current density on the solidification microstructure was investigated by means of wire netting technology. It was found that wire netting could not only effectively stop crystal nucleus to enter other zones but also has relatively small effects on the heat transfer and convection in melts. Theoretical analysis and ANSYS simulation suggested that variation of current density in different localised zones is a key factor influencing the microstructural features. Increasing the current density is favourable for the formation of equiaxed grains.

Tensile tests with pulse current for AZ31 alloy under room temperature and different voltages were carried out. Pulse current influence on the critical condition of initial dynamic recrystallisation for AZ31 alloy was investigated. The critical value was determined by one-parameter approach and validated by metallographic observation. The results showed that, with current voltage increased, the ultimate strength decreased, and the amount of strain hardening before reaching the ultimate strength decreased. When the voltage increases, high dynamic recrystallisation degree can be obtained, but due to the high electroplastic effect, the dynamic recrystallisation grain grows up and the material mechanical property deteriorates. The microstructure observation proved that the critical condition for initial dynamic recrystallisation can be determined with the one-parameter approach.

Phase transformation with a moving interface occurs under far from local equilibrium conditions when the interface moves with sufficiently high velocity. This deviation cannot be adequately described by the classical irreversible thermodynamics with diffusion equation of parabolic type because it assumes local equilibrium hypothesis. The local non-equilibrium diffusion model has been developed to take into account the deviation from local equilibrium during binary alloy solidification using the hyperbolic diffusion equation. The model introduces a finite propagation velocity of concentration disturbances in the bulk liquid V_D as the characteristic diffusion parameter and predicts a sharp transition from diffusion controlled to diffusionless and partitionless solidification at V = V_D due to the deviation from local equilibrium. This review does not intend to be extensive, but rather to illustrate the main advances of the local non-equilibrium diffusion approach in rapid alloy solidification. Applications of the local non-equilibrium model to other phase transformation processes such as solid–solid transformation, colloidal and multicomponent alloy solidification have been briefly discussed. The parallels and possible combinations of the model with molecular dynamics, phase field, phase field crystal and cellular automata methods have been considered.

The mechanisms involved in the abnormal grain growth of the iron based oxide dispersion strengthened alloys are analysed in the present work. Its microstructural evolution takes place at high temperatures (0.9T_m) and is characterised by an initial submicrometre size microstructure and a strong < 110>|| rolling direction (RD) texture that evolves into a few extremely coarse grains (mm sizes) with < 112>||RD orientation. The analysis of the observed grain boundaries has been completed by molecular dynamics simulations. Microstructure evolution consists of an extended recovery process, followed by an abnormal grain growth stage, consequence of the orientation pinning mechanism and the proximity to a symmetric tilt boundary family between the < 110>||RD and < 112>||RD grains.

A fatigue model based on entropy generation is presented and validated through experiments. This model is purely physical and combines statistical mechanics with thermodynamic laws applied at a local scale. The model does not require an empirical damage surface or phenomenological constitutive modeling constants. Damage evolution parameter varies from 0 to 1. As it is for the irreversible internal entropy production, this parameter is a non-decreasing quantity that increases with the degradation of a material.

A unified treatment of interdiffusion in various reference frames is presented in view of the consistency between Nernst–Planck, Onsager and Darken formalisms. A present discussion involves (i) material, (ii) laboratory and (iii) Rth component reference frames. It is shown that when the fluxes and forces are linearly interrelated, one can define a symmetric matrix of phenomenological coefficients. Its explicit form is derived. Problem is generalised to the case of different molar volumes that satisfy Vegard law, which presents entirely new result. An evidence is given that the Onsager fluxes can be equivalently treated as the fluxes due to the material drift and diffusion, as accounted in Darken treatment. The entropy production is the same within both formalisms.

Solidification, as the key step in physical metallurgy, plays a decisive role in tuning the various properties of materials. From a thermodynamic perspective, the solidification processes can be considered as the evolution of non-equilibrium systems, where the metastable melts become the stable solids with lower free energy. In contrast to equilibrium thermodynamics, which focus on the static equilibrium states, irreversible thermodynamics is a powerful tool to describe the evolution of non-equilibrium systems and has been successfully applied to various fields in materials science. In the present paper, we review the basic philosophy for the phenomenological irreversible thermodynamics, the methods to obtain the governing equations for the evolution of multicomponent solidifying systems and the potential applications to other metallurgical phenomena.

The effective mobility approach is compared with the kinetic energy approach in terms of sharp interface modeling and phase-field modelling of non-equilibrium solute diffusion upon rapid solidification of binary alloys. The two approaches are equivalent for modelling of long range solute diffusion in bulk phases, but only the effective mobility approach can introduce the non-equilibrium solute diffusion effect to short range solute diffusion at a sharp interface or within a diffuse interface. Addition of the kinetic energy terms results in an unreasonable non-bilinear expression of the flux and thermodynamic driving force in the free energy production of interface migration or phase field propagation, whereas the effective mobility approach allows the thermodynamic extremal principle workable.

The plastic deformation of multiphase steels is described employing an irreversible thermodynamics formulation. Transformation induced plasticity and dual phase grades are described within a single theoretical framework. The approach describes the plastic deformation of each individual phase in terms of the evolution of dislocation density, subject to dissipative mechanisms associated to dislocation generation, glide and annihilation. The collective behaviour of the ensemble of phases into a single microstructure is ensured through a self-consistent approach based on the iso-work approximation. The parameterised model shows very good agreement with several alloys studied experimentally and available in the literature.

In this paper, we describe a simple methodology that offers a familiar constitutive description of plasticity in terms of the dislocation density evolution as an outcome of the approach based on the thermodynamics of irreversible processes. We further demonstrate that the dislocation density evolution approach can organically predict the critical strain corresponding to the Considère instability point. Finally, we show that the fractal dimension (FD) of the dislocation population of a deforming material can be integrated in the proposed modelling framework and, consequently, the FD behaviour can be traced, providing insights in the evolution of the dislocation structure in the course of deformation.