E. Cerri

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.

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.

Interpretation of constant-load and constant-stress creep behavior of a magnesium alloy produced by rapid solidification

Abstract The creep behaviour of an Mg–Zn–Ca–Ce–La alloy produced by rapid solidification was investigated by means of constant-load creep tests carried out at 498, 523 and 548 K. The analysis of strain rate as a function of applied stress suggests that in the ranges of stress and temperature studied, the creep mechanism is glide on basal plane controlled by cross-slip on non-basal planes or dislocation glide limited by the nucleation of kinks. This behaviour contrasts with previous results obtained in pure Mg where cross-slip on non-basal planes was the rate-controlling mechanism only at higher temperatures. The different behaviour of this alloy is attributed to the very fine grain size typical of rapidly solidified material. The enhancement in strain rate observed in the present alloy can be attributed to grain-boundary sliding accommodating the strain produced by dislocation creep mechanisms (glide on non-basal plane controlled by cross-slip or dislocation glide limited by the nucleation of kinks) that operate in the subgrain interior.

An evaluation of the creep properties of two Al-Si alloys produced by rapid solidification processing

Tensile creep tests were conducted on two Al-Si alloys produced by rapid solidification: an Al-Si-Ni-Cr alloy and an Al-Si-Cu-Fe alloy, designated alloys A and B, respectively. The creep curves of these two alloys in the temperature range from 493 to 573 K were markedly different, with alloy A exhibiting a normal creep curve with a very short tertiary region and alloy B exhibiting an extended tertiary stage associated with strain localization. The minimum creep rates varied, with the applied stress raised to exponents of ∼9.0 and ∼8.5 for the two alloys, respectively. The hardness of alloy B decreased with time during the creep testing, but there was little or no change in the hardness of alloy A. These differences in the creep and hardness characteristics are attributed to the evolution of precipitates within the two alloys during creep testing. A detailed analysis shows that, over the temperature range examined experimentally, alloy A exhibits a creep strength that is superior to conventional Al-based alloys and comparable to, or even higher than, some SiC-reinforced Al composites.

Significance of continuous precipitation during creep of a powder mettallurgy aluminum alloy

Abstract Experiments were conducted to evaluate the creep properties of a 2024 aluminum alloy fabricated by powder metallurgy processing. The creep curves exhibit a minimum creep rate followed by an extended tertiary stage prior to failure. Using the values of the minimum creep rates, the apparent stress exponents are high and variable suggesting the presence of a threshold stress. Observations using transmission electron microscopy (TEM) reveal the occurrence of a continuous precipitation of fine particles during the tests. Although the density of these particles is dependent upon the testing conditions, quantitative measurements show that their average size is of the order of ∼ 60 nm under all conditions. A temperature compensated time is introduced to describe the evolution of fine particles during testing, and this permits the development of a relationship which can be used to estimate the density of the particles under any selected conditions.

Constitutive Equations for Mg Alloy Hot Work Modeling

Constitutive equations for hot working are of great importance in optimizing forming processes to balance reductions in preheating and force, to avoid defects and to improve properties. Flow curve shapes and. constitutive parameters are affected by variations in composition, in homogenization, in grain morphology and significantly in texture. Confidence in published analyses is enhanced by existence of many data for the same or similar alloys. In this paper, constitutive equations have been collected for commercial Mg alloys from torsion, compression and tension tests in the range from 10-3 to 10 s-1 and 180 – 500°C. Some data were determined by the authors’ but more came from published reports; in some cases they have been re-calculated in a common manner. The deformation and restoration mechanisms that control the flow curve shape and the material parameters are summarized. Microstructure investigations of strained samples are illustrated. Applications to extrusion or rolling are discussed; comparisons to Al are made as appropriate.

Friction Stir Welding of Ti-6Al-4V Alloy

Titanium (Ti) and its alloys are used extensively in aerospace industry where there is a critical need for material with high strength-to–weight ratio and high elevate temperature properties. Friction stir welding (FSW) is a new solid state welding process in which a cylindrical–shouldered tool with an extended pin is rotated and gradually plunged into the joint between the workpieces to be welded. The material is frictionally heated to a temperature at which it becomes more plastic but no melting of the blanks to be welded is reached therefore the presence of defects typically observed in and close to the welding seam is strongly reduced. The final result is the improvement of the mechanical performances of the welded joints even in some materials with poor fusion weldability. In this paper the authors analyze the microstructure of FSW joints of Ti-6Al-4V processed at the same travel speed (50 mm/min) and at different rotation speed (300-500rpm). The microstructure of base material (BM) is not homogenous. It is characterized by distorted α/ β lamellar microstructure together with smashed zone of fragmented β layer and β retained grain boundary phase. The BM has been welded in the as received state, without any previous heat treatment. The microstructure of the transverse section of joints is not homogeneous. Close to the top of weld cross sections a much finer microstructure than the initial condition has been observed while in the center of the joints the microstructure is mixed and less refined.