Angelo Fernando Padilha

Nucleation and growth during recrystallization

The evolution in the understanding of the recrystallization phenomena is summarized in this paper. Initially the main developments concerning recrystallization are presented from a historical perspective. Definitions and concepts involving recrystallization are presented regarding it as a solid-state reaction that occurs by nucleation and growth. The recrystallization nucleation mechanisms are subsequently discussed. Finally, the growth step is highlighted, emphasizing boundary and sub-boundary mobilities and the forces acting on the high angle grain boundaries that sweep the microstructure during recrystallization.

A Short review on wrought austenitic stainless steels at high temperatures: processing, microstructure, properties and performance

Wrought austenitic stainless steels are widely used in high temperature applications. This short review discusses initially the processing of this class of steels, with emphasis on solidification and hot working behavior. Following, a brief summary is made on the precipitation behavior and the numerous phases that may appear in their microstructures. Creep and oxidation resistance are, then, briefly discussed, and finalizing their performance is compared with other high temperature metallic materials.

Competition between recovery and recrystallization

Abstract The present work is an analysis of the influence of the rate of recovery on the Avrami–Johnson–Mehl–Kolmogorov equation which is meant to describe the kinetics of recrystallization. First, an equation for the recovery kinetics is given. This is entering the AMJK equation via a variable growth rate. The result is applied to experimental data on Ta. It explains ‘anomalous’ Avrami exponents and other effects.

Microstructural Refinement during Annealing of Plastically Deformed Austenitic Stainless Steels

The phenomena of strain hardening, strain induced martensite formation, recovery, martensite reversion and recrystallization have been studied in austenitic stainless steels of the AISI 304L and 316L types, after solution annealing, followed by rolling at different temperatures (-196, 25, 100 and 200°C) and subsequent annealing of the worked samples. Strain hardening and the percentage of α’ martensite formed showed strong dependency with the deformation temperature and with the austenite chemical composition. As expected, both strain hardening as well as the amount of the martensite formed was higher in the 304L steel and for lower temperatures. Reversion temperature of the α’ martensite was close to 550°C for both steels, independent of the amount of martensite. The 316L steel presented a higher resistance to recrystallization when compared to the 304L steel. The recrystallization temperature of both steels was about 150°C higher than the α’ phase reversion temperature. Rolling temperature did not influence significantly the recrystallization temperature. Proper thermal and mechanical treatments lead to interesting combinations of mechanical properties in both steels with values such as yield strength YS of about 1000 MPa, with an elongation around 10%.

Orientation effects during grain subdivision and subsequent annealing in coarse-grained tantalum

Abstract Noticeable orientation effects were observed in two neighboring grains in tantalum during cold rolling and further annealing. The stored energy varied markedly from grain to grain and, consequently, their response to annealing: grain A recrystallized in full extent while grain B was strongly softened by static recovery.

Grain subdivision and recrystallization in oligocrystalline tantalum during cold swaging and subsequent annealing

A coarse-grained ingot of high-purity tantalum was deformed by swaging at room temperature to a strain of 1.28. During annealing at 900°C for 30 min two neighboring grains were observed to behave quite differently. Electron backscattering diffraction (EBSD) results show noticeable differences in terms of the misorientations developed in both grains. The grain developing larger misorientations recrystallized much more readily than the other. The result is interpreted in terms of the differences in grain subdivision into strongly misoriented regions.

Recrystallization behavior of a cold-rolled niobium bicrystal

A high-purity coarse-grained niobium bicrystal was 70% cold rolled in multiple passes. Deformation occurred in an inhomogeneous manner in both grains giving rise to a banded structure. In consequence, highly misoriented boundaries were developed in the microstructure in a wide range of misorientations, many reaching about 55°. These boundaries act as effective nucleation sites for recrystallization. The microstructure of both grains was investigated using electron backscatter diffraction (EBSD) in the cold-worked state in order to quantify the misorientations (ψ) associated to these bands. Upon annealing at 800 and 900 °C, the new recrystallized grains were nucleated preferentially at deformation heterogeneities and in the vicinity of the prior grain boundary in this bicrystal.

Microstructural path of recrystallization in a commercial Al-Mn-Fe-Si (AA3003) alloy

The phenomenological analysis of recrystallization data is described. The concept of microstructural path is briefly introduced as well as some of its relevant equations. The correct equation for the impingement compensated chord length is introduced and used here for the first time. The theory is then applied to data from the recrystallization of a commercial Al-Mn-Fe-Si (AA3003) alloy. It is concluded that nucleation is site saturated and that the grain boundary velocity decreases with time. The analysis also shows that the shape of the recrystallized regions significantly deviates from spherical as the reaction progresses, probably because of the boundary/precipitates interaction.

Precipitation of a boride phase in 15% Cr-15% Ni-Mo-Ti-B austenitic stainless steel (DIN 1.4970)

Abstract For nuclear application of the steel 1.4970 its boron content and distribution are important due to the influence on mechanical and irradiation properties. Heats with 30 to 100 ppm boron have been investigated by phase and element analysis of chemically isolated precipitate residues and supplementary methods. Boron was found to be completely dissolved by solution treatment at 1373 K. During ageing the precipitation of a boride phase occurs, which was not previously reported for this steel. It was identified as M 2 B-type phase with orthorhombic structure and analysed as chromium boride with considerable iron content. For a heat with 45 ppm boron a time-temperature-curve of (Cr,Fe) 2 B phase precipitation was defined. Further structural, compositional and morphological features of the boride phase are presented and compared to literature results. Consequences of its existence are discussed.

On Cr2N precipitation mechanisms in high-nitrogen austenite

Precipitation mechanisms in a high-nitrogen (0.87 wt%) fully austenitic 25Cr–5Ni (wt%) steel have been studied after ageing at 900°C and 960°C. Ageing induced discontinuous precipitation of Cr2N from grain boundaries, discontinuous precipitation at twin fronts, continuous precipitation throughout the matrix, continuous precipitation in association with ferrite plate formation and also grain boundary precipitation in the form of both discrete precipitates and films. The observed morphologies at the onset of discontinuous precipitation from grain boundaries suggest a Tu–Turnbull initiation mechanism. The orientation relation between Cr2N and γ in well-developed lamellae is not strictly (111)γ//(0001)Cr2N and γ// Cr2N. The fact that the process shows some orientation tolerance, together with the lamellae irregular and curved interfaces, indicates that minimization of the system interfacial energy does not play a key role during growth. In this precipitation mechanism, twinning inside growing γ lamellae is associated with abrupt local changes in the growth direction. In the case of cooperative growth at incoherent twin fronts, the irregular habit surfaces of Cr2N point to a Fournelle–Clark initiation mechanism. At early precipitation stages, the interlamellar spacing for cooperative growth at twin fronts is similar to the one observed for cooperative growth from grain boundaries (∼100 nm). It is proposed that the low growth efficiency observed at twin fronts results from lower diffusivity conditions at twin fronts as compared with grain boundaries.