Flat duplex stainless steel acquires its characteristic banded microstructure during its processing sequence, which ends with a final annealing step aimed at fine tuning the phase ratio to equal parts ferrite and austenite. Besides phase fraction, the grain structures, shapes and morphologies of both phases evolve too during this final step. In the present, it was shown that recrystallization is mostly complete at the end of heating and phase equilibration shortly after. As a consequence, chemical and elastic driving forces are rapidly depleted while the sizes and shapes of phases keep evolving, driven solely by capillarity. Using quantitative measurements that enable decorrelation of coarsening and globularization, it is shown therein that the grain structure of the material as well as the annealing temperature play a crucial role in the intensity of those two phenomena. A high grain boundary density enhances the kinetics of both. A higher annealing temperature, which leads to faster mass transport, favors the increase of the characteristic sizes of the microstructure over its morphological evolution.

The design of the dissimilar metal weld investigated here is aimed at applications in the steam generator of a sodium-cooled nuclear reactor, with a multi-decade lifespan in demanding operational conditions. It consists in a narrow-gap joint between 2.25Cr-1Mo low-alloy steel and an austenitic alloy using a nickel-based alloy 82 as filler material. This study focuses on understanding the microstructural and micromechanical evolution in the near fusion boundary region between the low-alloy steel and the nickel alloy filler metal during post-weld heat treatment, using notably electron probe micro-analysis and nano-indentation. The difference in matrix phase and chemical composition between the two alloys leads to a large difference in chemical potential for carbon, which is mobile at the post-weld heat treatment temperature. A number of fine-scale characterization techniques were used to assess the gradient of composition, hardness, and microstructures across the fusion boundary, both as-welded and after post-weld heat treatment. This complete analysis permits to highlight and understand the main microstructural and micromechanical changes occurring during post-weld heat treatment and opens the way to their long term study in service conditions.

In ferrite/austenite dissimilar metal welds, carbon can diffuse from the ferritic low-alloy steel to the austenitic weld metal during heat treatment and thermal aging due to the large carbon and chromium content differences between the two alloys. This carbon influx leads to the precipitation of carbides in the high-alloy weld metal, near the fusion boundary, which can affect the properties of the joint. This precipitation was investigated using multiple characterization techniques: electron probe microanalysis, small angle X-ray scattering and atom probe tomography. The studied joint consisted of low-alloy steel and stainless steel base metals that were arc welded using a nickel-based filler metal. Given the heterogeneous microstructure of such joints, a microstructure navigation strategy was implemented to ensure all characterizations were carried out in the same regions of interest and the associated data could later be correlated. The precipitates were found to be mostly M23C6. The precipitation was quantified using two different methods that gave comparable results, whose differences highlight the capabilities and limitations of each technique. Precipitate fractions are higher following thermal aging but remain far from equilibrium in all cases, indicating that precipitation kinetics should be accounted for to model the microstructural evolution of such dissimilar metal welds more accurately.

We examine by transmission electron microscopy the precipitation state in the as-built microstructure of a hot-cracking sensitive nickel-based superalloy fabricated by laser powder bed fusion. Most observations are carried out on carbon extraction replica to isolate the precipitate signals from those of the matrix. Chemical maps measured by energy dispersive X-ray spectrometry are compared to phase and orientation maps obtained on the same precipitates by automated crystal orientation mapping. The chemical analyses are completed by electron energy-loss spectrometry measurements made on thin foils. The large fields of view investigated allow for achieving a representative picture of the second-phase particles in the as-built microstructure. A large variety of nano-particles is found: (Ti,Nb)(C,N) carbonitrides, (δ) Al2O3 alumina, (Cr,Mo)3B2 and (Cr,Mo)5B3 borides, and the intermetallic phase Ni7Zr2. The precipitates themselves are also complex in terms of their inner structure: carbonitrides can present a core-shell structure of composition and are frequently twinned, alumina particles serve as nucleation sites for carbonitride aggregates, and several crystallographic forms of borides may coexist next to each other. These observations provide the necessary information to retrace the solidification path of the microstructure during fabrication and to discuss the precipitation mechanisms under the extreme processing conditions associated with laser powder bed fusion.

Detailed characterization of the near fusion-boundary region of 18MND5/Alloy 52 dissimilar metal weld joints was performed to investigate the effect of thermal aging on the microstructure and mechanical properties and to quantify the diffusion of carbon into the weld metal. It was shown that the microstructural features of dissimilar metal welds, such as the size of the partially mixed zone in the weld and the adjacency to micro-segregations in the low-alloy steel have an effect on the peak carbon concentration in the enriched region of the weld. Aging conditions (time and temperature) influence the position of this concentration peak in the weld. Those trends were supported by thermodynamic calculations.

Super duplex stainless steel grades are modern stainless steel grades combining high specific mechanical properties with excellent corrosion resistance, especially to pitting corrosion. Their very specific microstructure consists in a mix of ferrite and austenite, usually at a 50/50 ratio, arranged in layers. This ratio is finely adjusted by a final annealing step during which, this fine, strongly banded microstructure evolves towards a coarser, globularized morphology in a matter of seconds. To determine the effect of annealing time and temperature on the final morphology, the kinetics of this morphological evolution have been quantitatively assessed in commercial and model alloys. In particular, stacks of austenite and ferrite with compositions found in the super duplex microstructure were assembled using HIP to produce an ideal super duplex microstructure with infinite, smooth bands. The obtained results suggest that grain boundaries and phase interfaces play a key role in the morphological changes. A phase field model of the polycrystalline two-phase microstructure was developed to simulate the observed phenomenon and rationalize the influence of the different parameters. Furthermore, the change in morphology of the phases was quantitatively analyzed against the change in mechanical properties. TEM and EBSD observations allow to relate these variations to the arrangement of dislocations in the microstructure. The present study affords a better understanding of the effect of final annealing on the microstructure of super duplex and opens new possibilities to tailor their properties.

We report initial results on high-temperature deformation of a complex metallic alloy, the gamma-Mg17Al12 phase, isostructural with alpha-Mn. Although, like all complex metallic alloys, the gamma-Mg17Al12, phase is brittle at room temperature, a high deformability accompanied by a strong softening was observed above 573 K (similar to 2/3melting point). Transmission electron microscopy investigation indicates that dislocation climb should be involved in the deformation mechanisms. Similarities with the deformation behaviour of quasicrystals and metallic glasses are discussed. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Hardening precipitation frequently occurs in Mg - rare earth ( RE) alloys after heat treatment in the 150 - 200 degrees C range. Early stages of precipitation have been studied in detail by transmission electron microscopy in two Mg - RE alloys ( Mg - Y - Gd and Mg - Y - Nd). Two types of structures may be involved in the precipitation sequence: a DO19 phase and the so- called orthorhombic beta' phase. The structural relationship between DO19 and beta' phases has been established in underaged and overaged states from the observations at peak ageing. We show that the earliest precipitates play a key role in the selection of phases developing in overaged states. Depending on the habit plane of the precipitates present in the early states, either the DO19 or the beta' phase will grow in further ageing. The Mg - Y - Gd and Mg - Y - Nd alloys illustrate the different microstructures resulting from such selection. Due to the selective growth of the beta' phase, the Mg - Y - Gd alloys are characterized by a fine scale microstructure which provides improved mechanical properties.

Early stages of precipitation in a Mg–Y–Nd based alloy aged at 150 °C have been studied using TEM and small angle X-ray scattering (SAXS). The former brings information concerning nature, morphology and size of precipitates, and the latter adds qualitative and quantitative information concerning populations of precipitates in terms of size and volume fraction. Precipitation at 150 °C involves formation of DO19 monoplanar precipitates, which further develop into the β″ and β′ phases having platelet and globular morphologies, respectively. TEM observations on samples aged at 150 °C reveal the formation mechanism of the bco-β′ structure by the ordering of monoplanar DO19-β″ precipitates. Additional examinations at 250 °C revealed the DO19-β″→bco-β′ transformation, as well as β1 precipitates. Estimation of the volume fraction deduced from SAXS is discussed on the basis of the TEM results.