E. Evangelista

Comparative hot workability of 7012 and 7075 alloys after different pretreatments

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.

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

Abstract 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.

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

AbstractThe 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.

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

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.

Study of hot workability of a heat treated AA6082 aluminum alloy

Abstract The high-temperature plasticity of an AA6082 aluminium alloy was investigated in a wide range of temperatures and strain rates. The peak stress dependence on strain rate and temperature was analysed by means of the conventional constitutive equation and by means of a modified form, where the stress was substituted by the difference between stress and a threshold stress.

Analysis of the creep behaviour of a thixoformed AZ91 magnesium alloy

Abstract Experiments were conducted to determine the creep response of a thixoformed Mg–Al–Zn AZ91 alloy in the temperature range between 120 and 180°C. The initial microstructure of the alloy consisted of large globular dendrites of α phase (solid solution of Al in Mg) surrounded by a divorced eutectic of β phase (Mg 17 Al 12 ) and particles of α phase rich in Al. The minimum creep rate ( e m ) dependence on applied stress σ and temperature T was analysed in detail. The high stresses applied in this study resulted in power law breakdown, leading to the use of the equation e m ∝ sinh α 0 σ n exp (−Q/RT) instead of the conventional power law. The stress exponent n was close to 5 with α 0 =0.024 MPa −1 . The activation energy for creep Q was close to 140 kJ mol −1 . These results, and the comparison with creep data obtained by testing the same material after solution treatment, led the authors to conclude that in the considered temperature regime, creep of the thixoformed AZ91 is controlled by high temperature climb of dislocations.

Review of creep behaviour of AZ91 magnesium alloy produced by different technologies

Abstract The available studies of the creep behaviour of AZ91 alloy produced by die casting, ingot casting, or thixoforming are reviewed in the present paper. Differences in microstructure and creep response are analysed and discussed with a view to providing a unitary, comprehensive description of the creep behaviour of this material. In particular, the minimum creep rate dependence on applied stress is described by means of a modified form of power law, which takes into consideration the effects of grain size and of intragranular precipitates interacting with dislocations. Analysis of the data obtained from the die cast alloy indicates that, in this material, creep is controlled by climb. Based on this observation and on theoretical considerations, as well as on the microstructural similarities, it is concluded that creep is controlled by climb also in thixoformed and ingot alloys. The effect of grain size is then quantified, and the strengthening effect of intragranular precipitates is described by introducing the threshold stress concept.

Energy dissipation efficiency in aluminum dependent on monotonic flow curves and dynamic recovery

In the hot working of Al, the flow curves are usually monotonic, reaching saturation at a lower strain εs and stress σs as temperature rises and strain rate declines. Microstructural examination confirms that the dislocation density rises to a steady-state level through formation of an equiaxed subgrain substructure with constant dimension that is larger for lower stress. The energy dis-sipation efficiency estimated by dynamic materials modeling for flow curves of the above type is the result of dynamic recovery, not of dynamic recrystallization, which is characterized by flow curves with a peak and marked softening to a steady-state regime.

Hot workability of 2304 and 2205 duplex stainless steels

Abstract The duplex stainless steels 2304 and 2205 were subjected to torsion testing over the range of temperatures from 1000 to 1200 °C and strain rates from 0.1 to 5 s-1 to characterize their hot working behaviour. The flow curves exhibit a peak followed by a decline towards a steady state that is attained at high temperatures and low strain rates. The peak stresses are fitted to a sinh-Arrhenius constitutive equation with activation energies of 536 and 406 kJ/mol, respectively. The optical microstructure shows elongated austenite regions in a ferrite matrix that increase in volume fraction with a rising temperature. The austenite phase becomes more elongated as the fracture strain rises indicating that both phases codeform in the temperature regime studied. During hot deformation, the ferrite effectively undergoes dynamic recovery, thus its substructure consists of well polygonized subgrains. In contrast, dynamic restoration processes are much suppressed in the austenite phase. The austenite substructure contains dense irregular dislocation networks and the dynamic recrystallization is observed only at very high temperatures. The hot ductility of the steels is limited by the cracking at the ferrite/austenite interphase boundaries; however, it improves as the temperature rises and the strain rate declines reaching maximum true strains of about 1.2 and 3, respectively. Present results indicate that the hot workability of 2304 and 2205 duplex stainless steels can be improved modestly by multistage testing. This is attributed to the static restoration processes (static recovery of ferrite and static recrystallization of austenite) which reduce internal stress concentrations but can not inhibit interphase boundaries from nucleating the cracks. On a soumis les aciers inoxydables duplex 2304 et 2205 à des essais de torsion dans la gamme de températures de 1000 à 1200 °C et à des vitesses de déformation de 0.1 à 5 s-1 afin de caractériser leur comportement de travail à chaud. Les courbes d’écoulement exhibent un pic suivi par un déclin vers une asymptote qui est atteinte à températures élevées et à basses vitesses de déformation. On ajuste les pics de contrainte à une équation constitutive de sinh-d’Arrhénius avec des énergies d’activation de 536 et 406 kJ/mol, respectivement. La microstructure optique montre des régions allongées d’austénite, dans une matrice de ferrite, dont la fraction volumique augmente avec une élévation de la température. La phase d’austénite s’allonge à mesure que l’effort de rupture s’élève, indiquant que les deux phases se co-déforment dans le régime de température étudié. Lors de la déformation à chaud, la ferrite subit effectivement une récupération dynamique, donc sa sous-structure consiste en sous-grains polygonaux bien formés. Par contraste, les procédés de restauration dynamique sont plutôt supprimés dans la phase d’austénite. La sous-structure de l’austénite contient des réseaux denses et irréguliers de dislocations et la recristallisation dynamique est observée seulement à des températures très élevées. La ductilité à chaud des aciers est limitée par la fissuration à l’interface ferrite/austénite; cependant, elle s’améliore à mesure que la température s’élève et que la vitesse de déformation décline, atteignant des déformations réelles maximales d’environ 1.2 et 3, respectivement. Les résultats courants indiquent que l’on peut modestement améliorer la malléabilité à chaud des aciers inoxydables duplex 2304 et 2205 par essais en plusieurs étapes. Ceci est attribué aux procédés de restauration statique (récupération statique de la ferrite et recristallisation statique de l’austénite) qui réduisent la concentration de contraintes internes mais qui ne peuvent pas empêcher l’interface de générer des fissures.

Constitutive equations for hot deformation of an Al-6061/20%Al2O3 composite

The hot deformation response of an Al-6061+20%Al2O3 particulate composite was investigated by means of torsion tests. An analysis of the flow stress dependence on strain rate and temperature gave a stress exponent of n?5.6 and an activation energy of Q?150 kJ mol?1. This value of Q is reasonably close to the anticipated value for self-diffusion in Al. The results are in excellent agreement with other studies carried out on the hot deformation of composites but the phenomenological nature of the constitutive equation does not permit an unambiguous determination of the rate-controlling flow mechanism. Nevertheless, it is shown that an identical constitutive equation may be used to describe the creep of composites under both shear and hot torsion conditions. It is concluded that the mechanism controlling hot deformation and creep at high stresses is the climb of dislocations.