Investigation Of Microstructure Evolution Research Articles

Investigation of microstructural evolution under neutron irradiation in Eurofer97 steel by means of small-angle neutron scattering

Small-angle neutron scattering (SANS) has been utilized to investigate in Eurofer97 steel (9Cr, 0.01C, 1W, 0.2V Fe bal wt%) the microstructural effect of neutron irradiation at 300 °C up to a dose level of 8.4 dpa. For each irradiated sample an unirradiated reference was measured to distinguish as accurately as possible the actual effect of the neutron irradiation. The SANS measurements were carried out at the D22 diffractometer at the High-Flux Reactor of the Institut Max von Laue–Paul Langevin, Grenoble, France. Analysing separately the nuclear and magnetic SANS components obtained after subtraction of the reference from the irradiated sample it appears that the microstructural inhomogeneities produced under such irradiation conditions are non-magnetic ones, such as microvoids. Their size distributions are presented and compared with those previously obtained for the same steel irradiated at 2.5 dpa: with increasing the dose, the volume fraction is increased by a factor of 2 roughly, while the average size of these inhomogeneities remains nearly unchanged.

Microstructure evolution of YBCO films deposited by advanced TFA-MOD process

We have investigated microstructure evolution of YBa2Cu3O7−y (YBCO) films through the two steps of heat-treatments in the advanced TFA-MOD process, by means of transmission electron microscopy. In the calcination process, precursor films including nanopores are formed with shrinkage of the film after a remarkable increase of the thickness caused by gas generation due to thermal decomposition of metalorganic salts in the starting solution. During the crystallization process, the densification and shrinkage of the film occurs after agglomeration of nanopores and unreacted phase particles such as Y2Cu2O5, CuO and Ba–O–F in the precursor films. Before starting of YBCO epitaxial growth, the densification and shrinkage of the film occurs. The YBCO films are epitaxially grown with remaining the unreacted phase particles in the film, subsequently pores are generated again by a reaction of these unreacted particles to form YBCO accompanying with the volume reduction. In conclusion, it is important to control the densification of precursor films and the coarsening of the unreacted phase particles in the crystallization process, in order to fabricate YBCO final films with fine crystallinity and high IC values.

Investigation of microstructure evolution in Pt-doped TiO 2 thin films deposited by rf magnetron sputtering

Pt-doped titanium dioxide or titania (TiO 2) films were grown by rf magnetron sputtering and then annealed in the conventional thermal annealing (CTA) process. Raman spectroscopy was used to characterize the structure of the films deposited. The effect of sputtering parameters was studied in focus of the nucleation sites energies (influenced by the substrate temperature) and substrate bombardment energies (influenced by the sputtering pressure or rf power). The X-ray diffractions technique was used to investigate the structural variation after the films were annealed at different temperatures. It was found that 0.75% Pt-doped TiO 2 film exhibits better thermal stability and smaller grain sizes than 0.35% Pt-doped TiO 2 film, suggesting that the suppression of crystallization can be expected with a proper increase of Pt doping level. And the obtained optical transparency higher than 80% even after annealing has demonstrated the films’ prospect for future developments.

Three-Dimensional Atom Probe Investigation of Microstructural Evolution during Tempering of an Ultra-High-Strength High-Toughness Steel

The evaluation of the tempering response of an ultra-high-strength, high-toughness (UHSHT) steel revealed that samples austenitized at 900 °C and tempered for 4 and 8 hours at 485 °C had similar yield strengths but a ∼50 pct increase in fracture toughness for the 8-hour temper. The results of our investigations of microstructural origins of this difference, using the nanometric scale resolution of the three-dimensional atom probe (3DAP) suggest that nanoscale strengthening precipitates, essentially carbides and intermetallic clusters (containing primarily Cr and Mo atoms), are present in both samples. The chemical compositions of the particles in the two samples were found to be similar, but clear evidence of differences in physical attributes of the precipitates, such as particle size, morphology, and interparticle spacing, are seen.

Recovery of HPT-Processed Iron Studied by Orientation Imaging Microscopy

A comprehensive investigation of microstructure evolution upon annealing in the temperature range between 100°C and 450°C in SPD-processed nanocrystalline iron had been conducted using transmission and orientation imaging microscopy, and XRD analysis. The asprocessed microstructure was typical of severely deformed metals consisting of grains with a mean size of 110 nm, each subdivided by a network of subgrain boundaries. Additionally, measurements of microstrains using XRD found a very high microstrain level of 0.003 in this material. After annealing at 200°C a drastic decrease of microstrains was observed; whereas no notable changes in the size of grains and subgrains as well in misorientation spectra have been revealed. Thus we relate this decrease of microstrains with recovery of non-equilibrium grain boundaries. Annealing at higher temperatures hadn’t led to further decrease of microstrains, but resulted in slight subgrain growth from 65 to 78 nm accompanied by increase of mean misorientation angle from 5° to 6.3°at 450°C. This indicates the occurrence of coalescence of subgrains as additional recovery mechanism.

Modeling of Microstructure and Microsegregation in Solidification of Multi-Component Alloys

Driven by industrial demand, extensive efforts have been made to investigate microstructure evolution and microsegregation development during solidification of multicomponent alloys. This paper briefly reviews the recent progress in modeling of microstructures and microsegregation in solidification of multicomponent alloys using various models including micromodel, phase field, front tracking, and cellular automaton approaches. A two-dimensional modified cellular automaton (MCA) model coupled with phase diagram software PanEngine is presented for the prediction of microstructures and microsegregation in the solidification of ternary alloys. The model adopts MCA technique to simulate dendritic growth. The thermodynamic data needed for determining the dynamics of dendritic growth are calculated with PanEngine. After validating the model by comparing the simulated values with the prediction of the Scheil model for solute profiles in the primary dendrites as a function of solid fraction, the model was applied to simulate the microstructure and microsegregation in the solidification of Al-rich ternary alloys. The simulation results demonstrate the capabilities of the present model not only to simulate realistic dendrite morphologies, but also to predict quantitatively the microsegregation profiles in the solidification of multi-component alloys.

Investigation of microstructural evolution and mechanical properties during two-step ageing treatment at 115 and 160 °C in an Al–Zn–Mg–Cu alloy pre-stretched thick plate

The microstructure evolution and age-strengthening characteristics of an Al–Zn–Mg–Cu alloy thick plate during a two-step ageing treatment at 115 °C and 160 °C were studied. The results show that the first step ageing treatment contributed to a major increase of tensile strength, the UTS value achieves 569 MPa after ageing for 7 h at 115 °C, both the GPI and GPII zones are dominant precipitates under this ageing condition. During the second step ageing at 160 °C, the peak strength value is achieved after a short ageing time, and then the strength decreases obviously, the UTS value reaches 493 MPa after ageing for 36 h. The microstructure in the matrix of the alloy at this ageing stage contains mostly metastable η′ phase and stable η phase. The grain boundary precipitates in two-step aged alloy were coarse and sparsely distributed, and clear precipitation free zone was observed.

Analysis of Microstructure Evolution in the Forging Process of a Windmill Main Shaft

The paper presents the results of an investigation of microstructure evolution in hot-deformed steel in the process of forging of large forgings. The objective is to create a feedback between the microstructure and the material flow stress in the finite element modelling. Using a forging of a windmill main shaft as an example, the description of the technological process is presented, focused on phenomena occurring in the microstructure during hot-forging. An investigation of the microstructure during and after forging of a shaft was performed and the results are presented in the paper. The analysis include the metallographic work after the last stage of forging and simulations of microstructure evolution during subsequent stages of the production cycle, based on temperature changes derived from industrial practice. Predictions of microstructure evolution allowed plotting changes of grain size and recrystallized volume fractions versus time during forging and reheating operations.

Grain refinement and grain coarsening of undercooled Fe–Co alloy

A bulk Fe–30 at. %Co alloy melt is undercooled in molten glass slag to investigate microstructure evolution under various undercoolings. Within achieved undercoolings, there are two grain refinement processes and one coarsening process: the first refinement transition from coarse dendrite to granular crystals occurred in low undercooling range, the second one was the refinement of directional fine dendrite to sphere-like particles in a medium undercooling, and the last was grain coarsening with the increase of metastable dendrite cores in a large undercooling range. The mechanisms of both grain refinements and grain coarsening process are discussed. Grain refinements are both caused by dendrite remelting, whereas grain coarsening results from metastable phase dendrite cores, which increase interface energy of the system and lead to transfer of grain boundary and coalition of grain and conduce grain coarsening finally.

Investigation of microstructural evolution during nitridation of aluminium using simultaneous thermal analysis

AbstractAluminium powders with different average particle sizes were synthesised into aluminium nitride in nitrogen atmosphere using simultaneous thermal analysis (TG/DTA). Experiments showed that reaction of Al powders started below the melting point of aluminium (660°C) in the solid–gas state, however most of the reaction took place above the melting point, in the liquid–gas state. Further conversion of Al to AlN took place up to 1200°C. The influence of particle size on the nitridation process was demonstrated by the use of different Al powders. Microstructural characterisation showed that two kinds of whisker were formed in the original Al powder bed, one with wool-like morphology and the other with needlelike morphology and sharp changes of direction.

An investigation of microstructural evolution during equal-channel angular pressing

Experiments were conducted to investigate the development of an ultra-fine grain size during equal-channel angular (ECA) pressing of high purity aluminum with an initial grain size of ∼1.0 mm. The results show that, under ECA pressing conditions giving a strain of ∼1.05 on each passage through the die, the microstructure is reasonably homogeneous after a single pressing and consists of parallel bands of elongated subgrains, having an average length of ∼4 μm, and these subgrains are further divided by boundaries with very low angles of misorientation. Repetitive pressings were conducted on the same samples, up to a total of 10 passages through the die, with the samples pressed either without rotation (route A) or after rotating through 180° between each pressing (route C). It is demonstrated that the misorientations of the subgrain boundaries increase with repetitive pressings until ultimately both routes lead to a similar equiaxed ultra-fine grain size of ∼1 μm after 10 pressings, but the microstructural evolution is enhanced using route C where there is a more rapid transition into an array of high angle grain boundaries. The results suggest that, at least for high purity aluminum, an ultra-fine microstructure close to optimum may be obtained after only 4 pressings provided the sample is rotated through 180° between each pressing.

Microstructural Characterization of Materials for Nuclear Applications Using Small-Angle Neutron Scattering

The results of a small-angle neutron scattering (SANS) investigation of microstructural evolution in martensitic steels, developed for use in future fusion reactors, are presented. Specifically, modified martensitic steel DIN 1.4914 (MANET type), subjected to different thermal treatments, was studied. The separate analysis of the size distribution functions determined from the nuclear and magnetic SANS components provides information on the presence of different inhomogeneities: large carbides and very small (~1–nm) C–Cr elementary aggregates. In the latter case, since they are such small particles at the resolution limit of the technique, the general problem of extracting their SANS signal from the background has been addressed.

New microstructure-property approach: Quenching/tempering treatment in gamma TiAl alloy

Gamma based TiAl alloys are currently seen as potential materials for high temperature structural applications due to their high specific strength, stiffness at elevated temperatures, and good creep and oxidation resistance. However, their poor ductility at ambient temperature and its poor formability prevents their practical applications. In this paper, a detailed investigation of microstructural evolution and mechanical properties of a tempered massive gamma TiAl alloy has been undertaken. The isothermally forged alloy was also investigated to determine which provides better mechanical properties.