The current understanding of the fundamentals of recrystallization is summarized. This includes understanding the as-deformed state. Several aspects of recrystallization are described: nucleation and growth, the development of misorientation during deformation, continuous, dynamic, and geometric dynamic recrystallization, particle effects, and texture. This article is authored by the leading experts in these areas. The subjects are discussed individually and recommendations for further study are listed in the final section.

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Current issues in recrystallization: a review

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Constitutive equations including an Arrhenius term have been commonly applied to steels with the objective of calculating hot rolling and forging forces. The function relating stress and strain rate is generally the hyperbolic-sine since the power and exponential laws lose linearity at high and low stresses, respectively. In austenitic steels, the equations have been used primarily for the peak stress (strain) associated with dynamic recrystallization (DRX) but also for the critical and steady state stresses (strains) for nucleation and first wave completion of DRX. Since the peak strain is raised by the presence of solutes and fine particles, the stress is raised more than by simple strain hardening increase, thus causing a marked rise in activation energy in alloy steels. In contrast, large carbides, inclusions or segregates, if hard, may lower the peak strain as a result of particle stimulated nucleation. Due to the linear relation between stress and strain at the peak, flow curves can be calculated from the constitutive data with only one additional constant. Maximum pass stresses can also be calculated from a sinh constitutive equation determined in multistage torsion simulations of rolling schedules. Comparison is made between carbon, micro-alloyed, tool and stainless steels and to ferritic steels which usually do not exhibit DRX. Parallels to the effects of impurities and dispersoids on the constitutive equations for Al alloys are briefly discussed.

Constitutive analysis in hot working

Processes of severe plastic deformation (SPD) are defined as metal forming processes in which a very large plastic strain is imposed on a bulk process in order to make an ultra-fine grained metal The objective of the SPD processes for creating ultra-fine grained metal is to produce lightweight parts by using high strength metal for the safety and reliability of micro-parts and for environmental harmony In this keynote paper, the fabrication process of equal channel angular pressing (ECAP), accumulative roll-bonding (ARB), high pressure torsion (HPT), and others are introduced, and the properties of metals processed by the SPD processes are shown Moreover, the combined processes developed recently are also explained Finally, the applications of the ultra-fine grained (UFG) metals are discussed

Severe Plastic Deformation (SPD) Processes for Metals

Microstructural characteristics of friction stir processed (FSP) 7075 Al samples in different regions behind the pin tool were investigated. Furthermore, the microstructure evolutions during friction stir welding (FSW)/FSP has been revealed. Multi-mechanisms, including dDRX, grain growth, dislocation introduction, dynamic recovery (DRV) and cDRX, were found to be inherent in the process at different stages. Because inhomogeneous plastic deformation was introduced by the process, individual grains in the final microstructure have undergone different evolution mechanisms. These are either from growth of initially recrystallized grains or are the result of cDRX of subgrains formed during DRV. Grains within the processed region exhibit different densities of dislocations and are in various degrees of recovery.

Microstructure evolution during FSW/FSP of high strength aluminum alloys

Hot torsion tests, in the range 250–450 °C and 0.05–5.0 s−1, were performed on AlZnMgCu alloys (7012 and 7075), which had been direct chill cast, homogenized and precipitation treated to give fine, well-dispersed precipitates. Additional tests were conducted on material that had been extruded, solution treated or precipitation treated at deformation temperature. The peak flow stress was related to the strain rate by the hyperbolic sine equation; the activation energy for precipitated alloys was close to that of the bulk self-diffusion of pure aluminium. For solution-treated metal, the peak stress was very high at low temperatures due to dynamic precipitation; as a consequence, the activation energy was about 50% higher than that of precipitated alloys. The ductility was almost independent of temperature in the investigated range, but decreased with rising strain rate. The ductility of the extruded alloys was almost double that of the as-cast material, with the exception of the solution-treated material where, at low temperature, the ductility of the extruded alloy was lower. The original grains were elongated with precipitates on the boundaries. The dynamically recovered subgrains exhibited sub-boundaries with a high density of fine precipitates and an interior network of dislocations also tied to precipitates.

Comparative hot workability of 7012 and 7075 alloys after different pretreatments

The mechanical and microstructural properties of 2024 and 7075 aluminium alloys joined together by friction stir welding were analysed in the present study. The two materials were welded with perpendicular rolling direction and after were tested in tension at room temperature in order to analyse the mechanical response and to observe the differences with the parent materials, the tensile response of the material in longitudinal direction revealed an increase in strength respect to the transverse one. The cyclic fatigue tests were conducted in the axial direction with R = σmin/σmax = 0.1). The microstructure resulting from the FSW process was studied by employing optical and scanning electron microscopy.

Mechanical response of 2024-7075 aluminium alloys joined by Friction Stir Welding

An investigation of the effect of creep exposure on the microstructure of a 9Cr–1Mo alloy for steam tubing was performed. The samples were machined from a tube, austenised at 1323 K for 15 min and air cooled to room temperature, followed by tempering at 1023 K for 1 h. Creep tests were performed at 848, 873, 898 and 923 K for different loading conditions. The conventional power law was used to describe the minimum creep rate dependence on applied stress; the stress exponent was found to increase when temperature decreased. Transmission electron microscopy (TEM) of the crept samples showed that during creep both subgrain and particle size increased; the statistical analysis of the dimensions of the precipitates revealed a bimodal distribution of particles that coarsen during creep exposure at testing temperatures. A linear dependence of subgrain size on the inverse of the modulus compensated stress was used to describe the softening of the dislocation substructure. A similar relationship was found to be also valid for particle carbides.

Evolution of microstructure in a modified 9Cr–1Mo steel during short term creep

The creep behaviour and the microstructural evolution of a 9Cr–Mo–Nb–V (T91) steel were extensively evaluated by means of short term constant load creep tests and TEM analysis. Statistical analysis of the microstructural data revealed that the precipitated phases M23 C6 (where M is a metal, mainly Cr or Fe) and MX (where M is Nb or V, and X is C and/or N) were subject to coarsening during creep exposure. The coarsening law and its dependence on applied stress were identified, and the model was used to predict the magnitude of the Orowan stress at the time corresponding to the minimum creep rate. The minimum creep rate dependence on applied stress at 873 K was described by incorporating the threshold stress concept in a power law with stress exponent n = 5. In the resulting phenomenological model, the strengthening effect of the dispersed phases was thus expressed by a threshold stress proportional to the Orowan stress.

Interpretation of creep behaviour of a 9Cr–Mo–Nb–V–N (T91) steel using threshold stress concept

Among cast aluminium alloys, 319 ranks as one of the commercially important alloys used in automotive applications, on account of its excellent casting characteristics and good mechanical properties. It has become one of the candidates for shaping the aluminium alloys in the semisolid state or thixocasting. In this study, thixocast bars of 319 aluminium alloy were heat treated in T4, T5 and T6 conditions, and the microstructural evolution was followed by optical and scanning electron microscopy. Electrical conductivity and hardness measurements were also performed on aged samples to follow the precipitation process. After aging, samples were prepared for tensile testing at room temperature, to study the effect of heat treatment on the mechanical properties. Longitudinal sections of tensile-tested samples were examined to identify the failure mechanism. The rupture propagates in the eutectic region or where Si particles are present, leading to a fracture of the particles themselves. The mechanical properties of the thixocast samples are, in some cases, higher than those obtained from traditionally cast 319 alloys.

Emanuela Cerri

Papers

Comparative hot workability of 7012 and 7075 alloys after different pretreatments

Mechanical response of 2024-7075 aluminium alloys joined by Friction Stir Welding

Evolution of microstructure in a modified 9Cr–1Mo steel during short term creep

Interpretation of creep behaviour of a 9Cr–Mo–Nb–V–N (T91) steel using threshold stress concept

Effects of thermal treatments on microstructure and mechanical properties in a thixocast 319 aluminum alloy