Microstructure evolution and softening processes occurring during annealing of hot deformed Ni-30Fe austenite

The current work investigates the microstructure evolution and softening processes that take place during annealing of an austenitic Ni-30Fe model alloy subjected to hot deformation in the dynamic recrystallization (DRX) regime. The substructure of the deformed matrix grains largely comprised organized microband arrays, though that of the DRX grains consisted of more random, complex subgrain/cell arrangements. This substructure disparity was also reflected by the distinct difference in the mechanism of post-deformation softening taking place during annealing of the deformed matrix and DRX grains. In the former, the recrystallization process took place through nucleation and growth of new grains fully replacing the deformed structure, as expected for the classical static recrystallization (SRX). The corresponding texture was essentially random, in contrast to that of the DRX grains dominated by low Taylor factor components. The microbands originally present within the deformed matrix grains displayed some tendency to disintegrate during annealing, nonetheless, they remained largely preserved even at prolonged holding times. During annealing of the fully DRX microstructure, a novel softening mechanism was revealed. The initial post-dynamic softening stage involved rapid growth of the dynamically formed nuclei and migration of the mobile boundaries in correspondence with the well-established metadynamic recrystallization (MDRX) mechanism. However, in contrast to the deformed matrix, SRX was not observed and the sub-boundaries within DRX grains rapidly disintegrated through dislocation climb and dislocation annihilation, which led to the formation of dislocation-free grains already at short holding times. Consequently, the DRX texture initially became slightly weakened and then remained largely preserved throughout the annealing process.

Texture evolution and softening processes in an austenitic Ni-30Fe alloy subjected to hot deformation and subsequent annealing

The current work has investigated the texture development in an austenitic Ni-30Fe model alloy during deformation within the dynamic recrystallization (DRX) regime and after post-deformation annealing. Both the deformed matrix and DRX texture displayed the expected FCC shear components, the latter being dominated by the low Taylor factor grains, which was presumably caused by their lower consumption rate during DRX. The deformed matrix grains were largely characterized by organized, microband structures, while the DRX grains showed more random, complex subgrains/cell arrangements. The latter substructure type proved to be significantly less stable during post-deformation annealing. The recrystallization of the deformed matrix occurred through nucleation and growth of new grains fully replacing the deformed structure, as expected for the classical static recrystallization (SRX). Unlike the DRX grains, the SRX texture was essentially random. By contrast, a novel softening mechanism was revealed during annealing of the fully DRX microstructure. The initial post-dynamic softening stage involved rapid growth of the dynamically formed nuclei and migration of the mobile boundaries in line with the well-established metadynamic recrystallization (MDRX) mechanism, which weakened the starting DRX texture. However, in parallel, the sub-boundaries within the deformed DRX grains progressively disintegrated through dislocation climb and dislocation annihilation, which ultimately led to the formation of dislocation-free grains. Consequently, the weakened DRX texture largely remained preserved throughout the annealing process.

Substructure and texture characteristics of the deformed matrix and dynamically recrystallized grains in a Ni-30%Fe austenitic alloy

The substructure and crystallographic texture characteristics of both the deformed matrix and dynamically recrystallized (DRX) grains were investigated in a Ni-30%Fe austenitic model alloy subjected to hot torsion. Deformation was performed at a temperature of 1000°C using strain rates of 1, 0.1 and 0.01 s-1, which produced a range of DRX grain sizes. Electron back-scattered diffraction and transmission electron microscopy were employed in the investigation. Both the deformed matrix and DRX grains revealed the texture components expected for simple shear deformation by crystallographic slip. The texture of DRX grains was dominated by low Taylor factor components as a result of their lower consumption rate during growth of these grains. There was a marked difference in the substructure characteristics between the deformed matrix and DRX grains regardless of the grain size and orientation. The deformed matrix substructure was largely characterized by organized, banded subgrain arrangements with alternating misorientations. By contrast, the substructure of DRX grains was generally more random in character and displayed complex, more equiaxed subgrain/cell arrangements characterized by local accumulation of misorientations. Based on the experimental observations, a mechanism of the distinct substructure development within DRX grains has been proposed.

Textures and compressive properties of ferromagnetic shape-memory alloy Ni48Mn25Ga22Co5 prepared by isothermal forging process

A ferromagnetic shape-memory alloy Ni₄₈Mn₂₅Ga₂₂Co₅ was prepared by the induction melting and isothermal forging process. Dynamic recrystallization occurs during the isothermal forging. The deformation texture was studied by the neutron diffraction technique. The main texture components consist of (110)[112] and (001)[100], which suggested that in-plane plastic flow anisotropy should be expected in the as-forged condition. The uniaxial compression fracture strain in the forged alloy reaches over 9.5%. The final room-temperature fracture of the polycrystalline Ni₄₈Mn₂₅Ga₂₂Co₅ is controlled mainly by intergranular mode.

Effect of substructure characteristics on the dislocation annihilation process during post-deformation annealing

Two distinct substructures were produced in a Ni-30Fe austenitic model alloy by different thermomechanical processing routes. The first substructure largely displayed organized, banded subgrain arrangements with alternating misorientations, resulting from the deformation at a strain just before the initiation of dynamic recrystallization (DRX). By contrast, the second substructure was more random in character and exhibited complex subgrain/cell arrangements characterized by local accumulation of misorientations, formed through DRX. During the post-deformation annealing, the latter substructure revealed a rapid disintegration of dislocation boundaries leading to the formation of dislocation-free grains within a short holding time, though the former largely preserved its characteristics till becoming replaced by growing statically recrystallized grains.

Microstructure evolution and softening processes in hot deformed austenitic and duplex stainless steels

The microstructure evolution and softening processes occurring in 22Cr-19Ni-3Mo austenitic and 21Cr-10Ni-3Mo duplex stainless steels deformed in torsion at 900 and 1200 °C were studied in the present work. Austenite was observed to soften in both steels via dynamic recovery (DRV) and dynamic recrystallisation (DRX) for the low and high deformation temperatures, respectively. At 900 °C, an "organised", self-screening austenite deformation substructure largely comprising microbands, locally accompanied by micro-shear bands, was formed. By contrast, a "random", accommodating austenite deformation substructure composed of equiaxed subgrains formed at 1200 °C. In the single-phase steel, DRX of austenite largely occurred through straininduced grain boundary migration accompanied by (multiple) twinning. In the duplex steel, this softening mechanism was complemented by the formation of DRX grains through subgrain growth in the austenite/ferrite interface regions and by large-scale subgrain coalescence. At 900 °C, the duplex steel displayed limited stress-assisted phase transformations between austenite and ferrite, characterised by the dissolution of the primary austenite, formation of Widmanstätten secondary austenite and gradual globularisation of the transformed regions with strain. The softening process within ferrite was classified as "extended DRV", characterised by a continuous increase in misorientations across the sub-boundaries with strain, for both deformation temperatures.

Ultrafine grain formation in a Ti-6Al-4V alloy by thermomechanical processing of a martensitic microstructure

In the current study, ultrafine equiaxed grains with a size of 150 to 800 nm were successfully produced in a Ti-6Al-4V alloy through thermomechanical processing of a martensitic starting microstructure. This was achieved through a novel mechanism of grain refinement consisting of several concurrent processes. This involves the development of substructure in the lath interiors at an early stage of deformation, which progressed into small high-angle segments with increasing strain. Consequently, the microstructure was gradually transformed to an equiaxed ultrafine grained structure, mostly surrounded by high-angle grain boundaries, through continuous dynamic recrystallization. Simultaneously, the supersaturated martensite was decomposed during deformation, leading to the progressive formation of beta phase, mainly nucleated on the intervariant lath boundaries.

Deformation behaviour of a commercial pure titanium alloy during hot compression testing

The flow curve behaviour and microstructure evolution of commercially pure titanium (CP-Ti) through uniaxial hot compression was investigated at 850 °C and a strain rate of 0.1/s. Electron back scattered diffraction (EBSD) was employed to characterize the microstructure and crystallographic texture development for different thermomechanical conditions. The stress-strain curves of CP-Ti alloy under hot compression displayed a typical flow behaviour of metals undergoing dynamic recrystallization (DRX), which resulted in grain refinement. The critical strain for the onset of DRX was 0.13 using the double differentiation analysis technique. It was also revealed that the texture was markably altered during hot deformation. © (2014) Trans Tech Publications, Switzerland.

Microstructure evolution of martensitic Ti-6Al-4V alloy during warm deformation

The microstructure evolution of martensitic Ti-6Al-4V alloy was investigated through uniaxial hot compression at 700°C and a strain rate of 10-3 s-1. A combination of scanning electron microscopy observation in conjunction with high resolution electron back scattered diffraction (EBSD) was used to characterize the microstructure in detail. The development of the microstructure displayed continuous fragmentation of martensitic laths with increasing strain (i.e. continuous dynamic recrystallization), concurrently with decomposition of supersaturated martensite resulting in the formation of equiaxed grains. At a strain of 0.8, an ultrafine equiaxed grained structure with mostly high angle grain boundaries was successfully obtained. The current work proposes a novel approach to produce equiaxed ultrafine grains in a Ti-6Al-4V alloy through thermomechanical processing of a martensitic starting microstructure. © (2014) Trans Tech Publications, Switzerland.

A Novel Mo and Nb Microalloyed Medium Mn TRIP Steel with Maximal Ultimate Strength and Moderate Ductility

The multi-phase, metastable, and multi-scale (M3) constitution of a novel transformation-induced plasticity (TRIP) steel (Fe-0.17C-6.5Mn-1.1Al-0.22Mo-0.05Nb, wt pct) was designed through thermodynamic calculations combined with experimental analysis. In this study, Mo and Nb microalloying was used to control the fraction of retained austenite and its mechanical stability during tensile deformation and to improve the yield strength. Thermodynamic calculations were developed to determine the critical annealing temperature, at which a large fraction of retained austenite (~38 pct) would be obtained through the effects of solute enrichment. The experimental observation was in good agreement with the predicted results. According to the critical annealing temperature, such an ultrafine (<200 nm) M3, microstructure with optimum mechanical stability was successfully achieved. The results of this work demonstrated the superior performance with improved yield strength of 1020 to 1140 MPa and excellent ductility (>30 pct), as compared with other TRIP steels. Both angle-selective backscatter and electron backscatter diffraction techniques were employed to interpret the transformation from the deformed martensitic laths to the ultrafine austenite and ferrite duplex structure.

Constitutive analysis of hot deformation behavior of a Ti6Al4V alloy using physical based model

The effect of deformation parameters on the flow behavior of a Ti6Al4V alloy has been studied to understand the deformation mechanisms during hot compression. Cylindrical samples with partially equiaxed grains were deformed in the α+β phase region at different thermo-mechanical conditions. To develop components with tailored properties, the physically based Estrin and Mecking (EM) model for the work hardening/dynamic recovery combined with the Avrami equation for dynamic recrystallization was used to predict the flow stress at varying process conditions. The EM model revealed good predictability up to the peak strain, however, at strain rates below 0.01s^-1, a higher B value was observed due to the reduced density of dislocation tangles. In contrast, the flow softening model revealed higher value of constants a and b at high strain rates due to the reduction in the volume fraction of dynamic recrystallization and larger peak strain. The predicted flow stress using the combined EM+Avrami model revealed good agreement with the measured flow stress resulted in very low average absolute relative error value. The microstructural analysis of the samples suggests the formation of coarse equiaxed grains together with the increased β phase fraction at low strain rate leads to a higher flow softening.

Microstructure evolution during warm deformation of low carbon steel with dispersed cementite

The microstructure evolution and mechanical behavior during large strain of a 0.16%C-Mn steel has been investigated by warm torsion tests. These experiments were carried out at 685°C at equivalent strain rate of 0.1 s . The initial microstructure composed of a martensite matrix with uniformly dispersed fine cementite particles was attained by quenching and tempering. The microstructure evolution during tempering and straining was performed through interrupted tests. As the material was reheated to testing temperature, well-defined cell structure was created and subgrains within lath martensite were observed by TEM; strong recovery took place, decreasing the dislocation density. After 1 hour at the test temperature and without straining, EBSD technique showed the formation of new grains. The flow stress curves measured had a peculiar shape: rapid work hardening to a hump, followed by an extensive flow-softening region. 65% of the boundaries observed in the sample strained to ε = 1.0 were high angle grain boundaries. After straining to ε = 5.0, average ferrite grain size close to 1.5 μm was found, suggesting that dynamic recrystallization took place. Also, two sets of cementite particles were observed: large particles aligned with straining direction and smaller particles more uniformly dispersed. The fragmentation or grain subdivision that occurred during reheating and tempering time was essential for the formation of ultrafine grained microstructure.

Estudo microestrutural em mineralização aurífera do tipo-veio hospedada em zona de cisalhamento: caso do depósito Sertãozinho, província Borborema, NE do Brasil

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Aspectos texturais do magmatismo e tramas da tectônica impostas ao Granito Chaval na Zona de Cisalhamento Santa Rosa, extremo Noroeste da Província Borborema

A região Noroeste da Província Borborema apresenta uma diversidade de corpos graníticos de natureza e evolução tectônica diversificadas, do Paleoproterozoico ao Paleozoico, com maior incidência relacionada ao Neoproterozoico e alojamento em diferentes fases da orogenia Brasiliana. Um desses exemplos é o Granito Chaval, que representa um batólito aflorante próximo à costa Atlântica do Ceará e Piauí, intrusivo em ortognaisses do Complexo Granja e supracrustais do Grupo Martinópole. Ele é, em parte, coberto por depósitos cenozoicos costeiros e rochas sedimentares paleozoicas da Bacia do Parnaíba. O Granito Chaval tem como característica marcante a textura porfirítica, destacando-se megacristais de microclina, em sienogranitos e monzogranitos, e outras feições texturais/estruturais de origem magmática, Essas permitiram interpretar sua evolução como de alojamento relativamente raso do plúton, conduzido por processos de cristalização fracionada, mistura de magmas com fluxo magmático e ação gravitacional em função da diferença de densidade do magma, levando à flutuação e ascensão de megacristais de microclina no magma residual, com alojamento de leucogranitos e pegmatitos nos estágios finais da evolução deste plutonismo. Por outro lado, em toda a metade Leste do plúton, encontra-se um rico acervo de estruturas tectógenas de cisalhamento, relacionada à implantação da Zona de Cisalhamento Transcorrente Santa Rosa, que levou a transformações tectonometamórficas superpostas às feições magmáticas, as quais atingiram condições metamórficas máximas na fácies anfibolito baixo. Cartograficamente, foram individualizados três domínios estruturais em que estão presentes uma gama de variações petroestruturais do Granito Chaval, sejam feições texturais/estruturais ígneas e tectônicas. As rochas plutônicas foram deformadas e modificadas progressivamente à medida que se dirige para Leste, no qual as rochas mudam-se para tonalidades mais escuras do cinza e os processos de cominuição e recristalização dinâmica reduzem, progressivamente, a granulação grossa desses granitos bem como o tamanho dos fenocristais para dimensões mais finas, mantendo-se suas características porfiroides. Desse modo, a trama milonítica se torna evidente, acentuando-se ao atingir a porção principal da Zona de Cisalhamento Transcorrente Santa Rosa. Como principais feições estruturais, destacam-se extinção ondulante forte; encurvamento e segmentação de cristais; geminação de deformação; rotação de cristais; microbudinagem; foliação anastomosada, inclusive S-C; lineação de estiramento; formas amendoadas de porfiroclastos, fitas e folhas de quartzo e recristalização. Os produtos desses processos de cisalhamento resultam na formação de protomilonitos, milonitos e ultramilonitos. Essas faixas miloníticas representam os locais de maior concentração da deformação, por isso é possível acompanhar progressivamente suas modificações texturais e mineralógicas, configurando uma sequência clássica de deformação progressiva heterogênea, por cisalhamento simples, em condições frágil-dúctil e dúctil. O alojamento do Granito Chaval aconteceu no final do Criogeniano (aproximadamente 630 Ma) e pode ser interpretado como magmatismo sin a tardi-tectônico em relação ao evento Brasiliano. O processo de cisalhamento que gerou a Zona de Cisalhamento Transcorrente Santa Rosa se formou nos incrementos finais da deformação de uma colisão continental em um sistema de cavalgamento oblíquo, em que se edificou o Cinturão de Cisalhamento Noroeste do Ceará, devido ao extravasamento lateral de massas crustais em fluxo dúctil acontecido no final da orogenia Brasiliana no Noroeste da Província Borborema.