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.

Role of microstructure in the low cycle fatigue of multi-phase steels

The low cycle fatigue (LCF) behaviour of several commercially-produced multiphase steels was studied; including dual-phase (DP) and transformation induced plasticity (TRIP). In addition, a novel TRIP980 hybrid microstructure was examined that consisted of coarse ferrite grains along with low temperature bainite regions interspersed with retained austenite. Fully reversed strain controlled fatigue tests were conducted on the different steels to determine the cyclic stress response and strain to failure. The effects of the cyclic deformation on the microstructures were analysed using electron backscattered diffraction (EBSD) and X-ray diffraction (XRD). Results showed that the initial cyclic hardening behaviour and low cyclic softening ratio observed in the TRIP steels was not necessarily due to austenite to martensite transformation. Differences between the austenite transformation behaviour of the conventional and novel hybrid TRIP microstructures was related to the different surrounding phases and the size of the retained austenite.

Microstructure-property relationship of the high strength low alloy steels during tensile testing at high strain rates

Positive The influence of strain-rate on the room temperature mechanical properties of the Dual-Phase and Transformation Induced Plasticity (TRIP) steels was investigated.The results showed that both the plastic strain, and strength properties increased with increasing strain rates at high strain rates.At strain rates lower than approximateil 1s ~ (-1) the properties no longer have an advantageous proportionality to strain rate and remain strain rate neutral.Possible explanations are offered for trends exhibited, in terms of thermal and athermal considerations, in relation to the respective microstructures of the two steels

Effect of thermomechanical processing on the microstructure and retained austenite stability during in situ tensile testing using synchrotron X-Ray diffraction of NbMoAl TRIP steel

In this work we compare and contrast the stability of retained austenite during tensile testing of Nb-Mo-Al transformation-induced plasticity steel subjected to different thermomechanical processing schedules. The obtained microstructures were characterised using optical metallography, transmission electron microscopy and X-ray diffraction. The transformation of retained austenite to martensite under tensile loading was observed by in-situ high energy X-ray diffraction at 1ID / APS. It has been shown that the variations in the microstructure of the steel, such as volume fractions of present phases, their morphology and dimensions, play a critical role in the strain-induced transition of retained austenite to martensite.

Data gathered during investigations into the evolution of hot working stress and microstructure in Mg-3Al-1Zn

The data is the result of hot deformation tests conducted on magnesium alloy AZ31. It includes stress strain data for a range of deformation conditions and different initial microstructures. It also includes data for the developed grain size and the degree of dynamic recrystallisation.

Orientation relationship, texture and microstructure in electromagnetic processed materials (EPM)

In this work, some of our recent results in microstructure, texture and orientation relationship resulting from the application of an external high magnetic field during diffusional and non-diffusional phase transformation in both steel and functional metallic materials have been summarized. A 12-T magnetic field was applied to the diffusional decomposition of austenite in 0.81C-Fe alloy and martensitic transformation of a Ni-Mn-Ga magnetic shape memory alloy. For the 0.81C-Fe alloy, it was found that the magnetic field induces the formation of proeutectoid ferrite and slightly enhances the <001> fiber component in ferrite in the transverse field direction. The magnetic dipolar interaction between Fe atoms in the transverse field direction accounts for this phenomenon. The magnetic field favors the formation of pearlite with Pitsch-Petch 2 (P-P 2) and Isaichev (IS) orientation relationships (OR) between the lamellar ferrite and cementite. For the Ni-Mn-Ga magnetic shape memory alloy, the magnetic field makes the martensite lamellas to grow in some specific directions with their c-axes [001] orientated to the field direction and transverse field direction.

Optimizing oxygen transport through La0.6Sr0.4Co0.2Fe0.8O3-δ hollow fiber by microstructure modification and Ag/Pt catalyst deposition

This work compares the oxygen permeation fluxes of five different La0.6Sr0.4Co0.2Fe0.8O3−δ membranes (e.g. disk, conventional hollow fiber, modified hollow fiber, Ag- or Pt-deposited hollow fiber membranes) to elucidate the dominance of a particular oxygen transport limiting step (e.g., bulk-diffusion or surface reaction) within each of these membranes. At 900 °C and 100 mL min–1 helium gas sweep rate, the oxygen fluxes for disk, conventional hollow fiber, modified hollow fiber, Ag-deposited modified hollow fiber, and Pt-deposited modified hollow fiber membranes are 0.10, 0.33, 0.84, 1.42, and 2.62 mL min–1 cm–2, respectively, denoting enhanced performance in this sequential order. More than 300% enhancement of fluxes is evidenced by modifying the geometry from disk to conventional hollow fiber. This is attributed to the thickness reduction from 1 mm to 0.3 mm, thus implying bulk-diffusion and surface reaction as the jointly limiting transport step for this disk membrane. In contrast to a conventional hollow fiber that has a sandwich cross-sectional structure (e.g. dense center layer sandwiched by two finger-like layers) as well as dense outer and inner circumference surfaces, the modified hollow fiber has only one dense layer in its outer circumference surface with finger-like porous layer extending all the way from outer cross-sectional part to the inner cross-sectional part. This microstructural difference, in turn, provides substantial reduction of membrane thickness and enlarges surface reaction area for modified hollow fiber (relative to conventional hollow fiber), both of which contributes to the reduced bulk-diffusion and surface reaction resistance; evidenced by almost 250% oxygen flux enhancement. To enhance the performance even further, catalyst (e.g., Ag or Pt) deposition on the outer circumference surface of modified hollow fiber can be utilized to reduce its dominating surface reaction resistance. While both catalysts increase the oxygen fluxes, Pt reveals itself as the better candidate relative to Ag due to melting-induced aggregation and growth of Ag at 950 °C.