The present work addresses the effect of three high-temperature deformation modes, namely, torsion, uniaxial compression, and plane strain compression, on flow stress vs. strain curves, as well as on post-dynamic and static recrystallization of 316Nb austenitic stainless steel. Using a Hosford criterion instead of the classical von Mises criterion enables a unified description of the stress–strain curves obtained under different loading modes. This work also revealed that the loading mode had no significant effect on post-dynamic and static recrystallization phenomena. The amount of niobium atoms in solid solution might be preponderant in the control of recrystallization of 316Nb.

The size evolution of niobium carbonitrides Nb(C,N) and the evolution of the composition of an austenitic matrix in 316Nb stainless steel were simulated using DICTRA software. For the first time, the complete nine-element composition of steel was taken into account during isothermal and even anisothermal heat treatments. A reduced model was then proposed to optimize the calculation time for complex heat treatments. The change in the mean Nb content in austenite due to Nb(C,N) evolution during different heat treatments was studied. It qualitatively agrees with experimental data as obtained by electron probe microanalysis. Furthermore, the model was successfully applied to explain the effect of heat treatments on the recrystallization behavior of 316Nb steel during hot torsion tests. Moreover, the effect of the thermodynamic database and the number of alloying elements chosen was discussed. We showed that taking into account seven or even nine elements greatly improves the accuracy compared to usual simplified compositions. The proposed method can be useful in designing heat treatments promoting or conversely hindering recrystallization for a wide variety of Nb-bearing steels.

In this study, neutron diffraction measurements have been performed to study the phase transformations taking place during isothermal treatments of two 316Nb type austenitic stainless steels with different delta ferrite content. Kinetics of phase transformations during heating, isothermal hold at 900 degrees C and cooling have been monitored and the phase fractions quantified using Rietveld refinements. Two main transformation mechanisms are evidenced and quantified: the delta ferrite transformation into sigma phase and austenite and the sigma phase precipitation inside the austenitic matrix.Furthermore, the joint use of X-ray and neutron diffraction on a powder residue obtained by electroetching extraction allowed determining the sigma phase, delta ferrite and Nb(N,C) weight fractions as well as the precise N and C contents of the carbo-nitrides and the site occupancies in the sigma phase. (C) 2016 Elsevier B.V. All rights reserved.

The microstructure of grade X4CrNiMo16.5.1 stainless steel was studied at different scales. The chemical composition of the native passive film formed on the different phases was then determined at the micro-scale. The degree of homogeneity of the native passive film is discussed. Subsequently, the susceptibility to pitting corrosion of X4CrNiMo16.5.1 was quantified using the electrochemical microcell technique. The nature of precursor sites and the morphology of pits were investigated by combining scanning electron microscopy with Electron BackScatter Diffraction and potentiostatic pulse tests. The role of the microstructure and the cold-worked layer generated by polishing in pitting is discussed. (C) 2014 Elsevier Ltd. All rights reserved.

The microstructure of an electron beam weld of a medium strength Al–4.5%Zn–1%Mg (wt.%) alloy has been characterized in terms of solute element distribution, grain structure and fine-scale precipitates after a T6 post-welding heat treatment. It is found that the weld nugget consists of small grains, whose size (1–50μm) is heterogeneously distributed. The nugget composition is unaffected in Mg but depleted of 20% in Zn in the first run zone. This is shown to affect the fine-scale precipitate microstructure, which has been mapped in the weld cross-section using Small-Angle X-ray Scattering. It is shown that the nugget exhibits a precipitate size only slightly different from that of the base material after the post-welding heat treatment, and that the difference in volume fraction, much more significant, can be understood from the magnitude of the solute depletion. The relative precipitate sizes and volume fractions in the weld nugget and base material enable to understand effectively the corresponding microhardness levels.

Medium strength Al–Zn–Mg age hardening alloys are widely used when a low quench sensitivity is required, such as in welding applications. In this work we present a detailed characterization of the precipitate microstructures resulting from different quench rates from the solution treatment, and from the subsequent artificial ageing to the T6 state, in an Al–4.5Zn–1Mg (wt%) alloy. This work is carried out using differential scanning calorimetry, transmission electron microscopy and in situ small-angle X-ray scattering. It is shown that for quench rate between 5 and 200°C/min substantial heterogeneous precipitation is observed, nucleated on dispersoids and on grain boundaries, the former being of much larger size than the latter. During subsequent ageing, it is shown that the precipitation kinetics in the material unaffected by the quench-induced precipitates is independent on the quench rate used.

L'alliage d'aluminium 7020 a durcissement structural est particulierement bien adapte a la realisation de pieces soudees du fait de sa composition sans cuivre. Le soudage de cet alliage par faisceau d'electrons (FE) sous-entend de bien connaitre l'impact du procede sur les proprietes mecaniques du materiau. Cet article presente quelques resultats obtenus lors d'etudes menees sur la caracterisation des nanostructures en relation avec les proprietes de la liaison soudee.

An Al-3%Mg-0.25%Sc-0.12%Zr alloy was deformed by triaxial forging at 20-400°C up to strains of about 3. A study of its textural evolution reveals the tendency towards three symmetrical variants of a <110><1 10 ><001> component. This experimental observation is supported by a 3D spatially resolved crystal plasticity analysis. Samples strained at room temperature undergo grain fragmentation in the form of fine substructures and relatively weak textures. Conversely, at 300°C and above, more homogeneous intergranular deformation and rotations give rise to stronger textures. This eventually encourages grain coalescence and thus the development of interpenetrating “orientation chains”, creating a new type of microstructure. The influence of this texture development on the specific work hardening behaviour is discussed.

Two relatively simple schemes are described for the interactions of grain deformations during large plastic deformations with the aim of evaluating their influence on texture development. The stress transfer model basically assumes that there is some degree of stress transfer across the boundaries proportional to the boundary area. The reduced stress incompatibility model minimizes the stress incompatibilities between each grain and their surrounding grains These models assume 3D topological schemes using evolving truncated octahedra for the spatial distributions of the grains. They are applied to the cases of hot rolled and cross forged Al alloys. Both give quite similar predictions for texture development which are moderate improvements on the Taylor models, confirming that the incorporation of grain interaction effects can be useful for texture modeling without major modifications. Moreover, they can yield interesting results for local orientation effects and their influence on orientation stability; an example of cube grains hot rolled in different crystallographic surroundings is also treated.

The development of deformation substructure and texture has been studied up to large plastic strains in some simple Al base alloys by multiple forging. The experiments involve successive forging strains on near-cube samples along 3 orthogonal axes up to cumulative strains of 3 or more (and temperatures from 20 to 400°C). The alloys include the commercial AA 3103 (Al- 1%Mn) and a laboratory Al-3%Mn-Sc-Zr alloy for the high temperature tests. Some complementary experiments have been carried out on oriented single crystals of Al-0.3%Mn. During 3D cross forging of fcc metals a clear texture composed of three symmetrical components is formed; they are the 3 possible variants of the <110> <110> <100> crystal axes along the 3 forging axes. This macroscopic texture is demonstrated by X-ray pole figure analysis, EBSD mapping and confirmed by crystal plasticity (CP) simulations. At room temperature the alloys (particularly Al-Mn) exhibit significant grain refinement by grain fragmentation leading to "grain sizes" of less than 103m. However, at temperatures ≥ 300°C in the stable Al-3%Mn-Sc-Zr alloy the lattice rotations towards just 3 texture components leads to a high frequency of grain "fusions"; each grain becomes surrounded by 3-5 neighbours of the same orientation so that long interpenetrating chains of the texture components are formed; they are also confirmed by FEG-SEM EBSD and spatially resolved texture simulations. The behaviour of stable (Goss) and unstable (cube) single crystal orientations during the same deformation processing is also investigated and shown to agree with the CP simulations.

A new triaxial forging equipment has been designed and built to enable sequential forging of near cube samples up to strains of 3–4 and temperatures up to 450°C. This paper presents a technical and analytic description of the system and its application to a standard AA 3103 alloy. Some first results in terms of stress–strain curves, microstructures and textures are given.

The texture and microstructure evolutions of a triaxially forged Al–3% Mg–0.25% Sc–0.12% Zr alloy are described for strains up to 3 and temperatures in the range 20–400°C. A clear deformation texture develops and is composed of three symmetrical variants of a 〈110〉〈11¯0〉〈001〉 component. A three-dimensional spatially resolved crystal plasticity analysis also predicts the same texture evolution. At room temperature, significant grain fragmentation leads to substructure refinement and weaker textures. However, at 300°C and above, more homogeneous intragranular deformation and rotations lead to stronger textures and, ultimately, the formation of interpenetrating ‘orientation chains’ as a result of grain coalescence to common orientations, a new type of microstructure.

Multiple forging (MF) can be used to attain large plastic strains in bulk alloys by successive forging along three orthogonal directions to retain the initial sample shape. An original multiple forging technique enabling 3-D cross forging at constant temperature up to 500°C has been applied to two Al alloys (Al-1%Mn and Al-3%Mg-Sc,Zr). Their rheology, texture and microstructure evolution are compared with those obtained in plane strain compression (PSC). The results are interpreted in terms of slip activity behaviour during both deformation modes. They can also be correlated with the contributions of free dislocations and sub-boundaries.