High Temperature Microstructure Research Articles

High temperature oxidation and microstructure of MoSi2/MoB composite coating for Mo substrate

The two-layer MoSi2/MoB composite coatings were developed using the halide activated pack cementation (HAPC) method on Mo substrate. Oxidation resistance property and microstructural evolution of the coatings at high temperatures were investigated. During oxidation exposure, the coatings exhibited a good oxidation resistance property. The mass gains of the coated specimens oxidized at 1200°C for 100h and at 1300°C for 80h were 0.270 and 0.499mg/cm2, respectively. Compared with the monolithic MoSi2 coatings, the transformation of MoSi2 phase in the MoSi2/MoB composite coatings was more sluggish at elevated temperatures. The growth rate constant of the Mo5Si3 layer in the composite coatings was two orders of magnitude lower than that of the Mo5Si3 layer in the monolithic coatings at 1300°C. The microstructural degradation of MoSi2 in the composite coatings at high temperatures was slowed by the introduced MoB layer. The MoB layer in the composite coatings is useful to prolong the service life of MoSi2 coatings at high temperatures.

Microstructure evolution and high-temperature mechanical properties of SiCf/SiC composites in liquid fluoride salt environment

High temperature mechanical properties and microstructure evolution of chemical-vapor-infiltrated SiC fiber reinforced SiC ceramic matrix composites (SiCf/SiC) in 46.5LiF-11.5NaF-42.0KF (mol.%, FLiNaK) eutectic salt were investigated. The results indicate that the corrosion of SiCf/SiC composites was accelerated with increase of corrosion temperature. Interlayer boundaries in SiC matrix with higher oxygen content were corroded preferentially. The F− formed SiF coordinate bond with SiC and replaced carbon, followed by the corrosion of SiC matrix. While single SiC fiber tensile strength suffered no depravation after corrosion, corrosion and damage of SiC matrix led to deteriorated high temperature mechanical properties of SiCf/SiC composites.

Deformation condition determination and strain analysis: Application of microstructural and microthermometry study of the Zamanabad Shear Zone (East of Iran)

Microstructural analysis and microthermometry are useful methods for determining the deformation evolution. To address this issue, rheological behavior of quartz, feldspar and calcite in veins and host rocks during deformation, are presented in the mylonite zone of the dextral reverse Zamanabad Shear Zone (ZSZ), in northern part of Sistan Suture Zone (SSZ), in east of Iran. Microstructure evidences revealed two evolution stages of high and low temperature deformation. Quartz microstructures in the ZSZ show abundant evidences for early high-temperature plastic deformation (e.g. Bulging recrystallization (BLG)) which are as microstructures with SW directed ductile shearing in the central parts of the ZSZ. This shear zone shows progressively decreasing strain away from the central of shear zone toward the wall. High-temperature microstructures are overprinted partly or completely during shearing by the later low-temperature deformation (e.g. Pressure solution, fractures, veinlets). Microstructural observations of veins (quartz and calcite) confirms the results of microstructures in the host rock, as quartz veins occurred from peak metamorphic conditions (<400°C) and then in lower P–T conditions have been formed calcite veins (~250°C). According to microthermometric studies, two primary fluid groups are observed in quartz veins: (1) fluids trapped during peak deformation conditions, with higher-salinity, They were initially trapped at ~300–400°C, (2) smaller fluids by trapping of low-salinity inclusions at ~240–180°C that related to subsequent phases of shear zone exhumation in lower deep. Microthermometry results and microstructural analysis indicate deformation under lower greenschist facies conditions for the ZSZ, and then exhumation of the early of high-temperature rocks within regime of ductile-brittle transition to brittle.

Near-surface microstructure improvement for die-upset Nd-Fe-B magnets with an enhanced maximum energy product

Abstract A Dy-Cu eutectic injection process accompanying by hot deformation had the potential to fabricate the high-performance die-upset magnets. The good fluidity of Dy-Cu eutectic liquid accelerated the formation of near-surface texture and suppressed largely the grain growth during subsequent heat treatment. By the high-temperature microstructure evolution, the maximum energy product of injected magnets was substantially increased by 17.8%, from 45 to 53 MGOe. The near-surface microstructure of injected magnets was studied to clarify the effect of inhomogeneous Dy-Cu distribution on the texture and grain size.

Investigation on compression behavior of TZM and La2O3 doped TZM Alloys at high temperature

Mechanical properties of Titanium-zirconium-molybdenum (TZM) and La2O3 doped TZM alloys under compression were tested at 1000°C and 1200°C. Microstructure of TZM and La2O3 doped TZM alloys after compressing was characterized by scanning electron microscopy. The effects on La2O3 doping on the high temperature deformation behavior and microstructure evolution of the TZM alloy were analyzed. Results show that La2O3 doping can refine the grain size of TZM alloy. La2O3 doping changes fracture model of TZM alloy. TZM alloy exhibits mainly intergranular fracture, while the La2O3 doped TZM alloy exhibits both intergranular and transgranular fracture mode.

Novel thermal insulating and lightweight composites from metakaolin geopolymer and polystyrene particles

In this work, new foamed thermal insulation geopolymer composite based on polystyrene particles (PP) and metakaolin was developed. Compressive strength, flexural strength, high temperature resistance and microstructure were evaluated. The experimental results show that compressivestrengthand flexural strength of the thermal insulation geopolymer composite decrease with increasing polystyrene particle content. However, it still exhibits considerable and sufficient strength. The dry density and thermal conductivityalso decrease as polystyrene particle content increases due to the contribution of polystyrene particles with low density. The floatation of the thermal insulation geopolymer composite on water surface indicates the relatively low density and a good quadratic function relationship can be found between thermal conductivity and dry density. Furthermore, the dense interfacial transition zone indicates the high compressive strength and flexural strength of thermal insulation geopolymer composites. The cumulative intrusion volume corresponding to the porosity decreases and the critical pore diametersshift to lower values with addition of polystyrene particles. Geopolymer composites gain strength after exposure around 400°C, and it suffers dramatic strength loss after 800°C temperature exposure especially for the 100% polystyrene particles addition specimen.

Comparison of high temperature performance and microstructure for foamed WMA and HMA with RAP binder

The foamed warm-mix recycled asphalts and hot mix recycled asphalt with different reclaimed asphalt pavement (RAP) binder contents (0wt%, 20wt%, 40wt%, 60wt%, and 80wt%) were prepared in laboratory. The influence on the viscosity-temperature performance, rheological properties, temperature sensitivity, fatigue resistance, and microstructure were investigated by conventional tests, Brookfield viscosity tests, Dynamic Shear Rheometer (DSR) tests, and Fourier Transform Infrared Spectroscopy (FTIR) tests. The results show that the RAP binder content has great effect on the performance of foamed warm mix recycled asphalt and hot mix recycled asphalt, and there is different change between foamed warm mix recycled asphalt and hot mix recycled asphalt. Compared to foamed warm-mix asphalt, foamed warm-mix recycled asphalt provides better high temperature performance and temperature sensitivity. Adding the RAP binder compromise the fatigue resistance of asphalt and with the increase of RAP binder content, the fatigue resistance of asphalt becomes worse. And the high temperature performance and temperature sensitivity of hot mix recycled asphalt is better than that of foamed warm mix recycled asphalt, while the fatigue resistance is on the contrary. By the microstructure analysis, the foaming process could slow down the aging degree of asphalt, and the aging degree of hot mix recycled asphalt is deeper than that of foamed warm mix recycled asphalt. The optimum RAP binder content of foamed warm mix recycle asphalt is more than that of hot mix recycled asphalt.

High temperature magnetism and microstructure of ferromagnetic alloy Si1−xMnx

The results of a detailed study of magnetic properties and of the microstructure of SiMn films with a small deviation from stoichiometry are presented. The aim was to reveal the origin of the high temperature ferromagnetic ordering in such compounds. Unlike SiMn single crystals with the Curie temperature ~30 K, considered Si1−xMnx compounds with x = 0.5 +Δx and Δx in the range of 0.01–0.02 demonstrate a ferromagnetic state above room temperature. Such a ferromagnetic state can be explained by the existence of highly defective B20 SiMn nanocrystallites. These defects are Si vacancies, which are suggested to possess magnetic moments. The nanocrystallites interact with each other through paramagnons (magnetic fluctuations) inside a weakly magnetic manganese silicide matrix giving rise to a long range ferromagnetic percolation cluster. The studied structures with a higher value of Δx ≈ 0.05 contained three different magnetic phases: (a)—the low temperature ferromagnetic phase related to SiMn; (b)—the above mentioned high temperature phase with Curie temperature in the range of 200–300 K depending on the growth history and (c)—superparamagnetic phase formed by separated noninteracting SiMn nanocrystallites.

Rheological weakening of high-grade mylonites during low-temperature retrogression: The exhumed continental Ailao Shan-Red River fault zone, SE Asia

We present a detailed case study of an exhumed continental strike-slip fault zone, the Ailao Shan-Red River (ASRR) strike-slip fault zone, to investigate how deformation promotes strain localization, and how the weak second phases and fluids trigger rheological weakening during retrogression near the ductile to brittle transition during exhumation. Along the ASRR strike-slip fault zone, in the Diancang Shan (DCS) metamorphic massif, high-temperature ductile deformation (D1) pervasively occurred during shearing and exhumation since late Oligocene. The high-temperature microstructures and textures are in part or entirely altered by subsequent low-temperature shearing (D2) since late Miocene, which is under the conditions of frictional-viscous transition of K-feldspar (ca. 450°C) during further exhumation to the upper crustal levels. The formation of D2 microstructures and shear bands overprinted high-temperature intracrystalline plasticity phases (D1) in mylonitic rocks. Depending on the main rock-forming minerals, the results also demonstrate that the brittle–ductile transition involves a combination of different deformation mechanisms and possible rheological paths. In quartz-rich rocks, quartz was deformed in the dislocation creep regime and records transition of microfabrics and slip systems during decreasing temperature, which lasted until retrogression related to exhumation. As a result, grain-size reduction associated with fluids circulating within the ASRR strike-slip fault zone at brittle–ductile transition leads to reaction and texture weakening. Rheological weakening is the consequence of the syntectonic deformation, fluid flow, reaction softening, reaction creep and textural softening. The hydrous fluids resulted in hydration of silicates. Decompression occurred during shearing and as a result of tectonic exhumation. All these results demonstrate that the exhumation through the ductile to ductile-brittle transition involves a combination of different deformation mechanisms, rheological transition features and feedbacks between deformation, decreasing temperature and fluids in the ASRR fault zone.

Porosity analysis and oxidation behavior of plasma sprayed YSZ and YSZ/LaPO4 abradable thermal barrier coatings

In this research, the high temperature oxidation behavior, porosity, and microstructure of four abradable thermal barrier coatings (ATBCs) consisting of micro- and nanostructured YSZ, YSZ-10%LaPO4, and YSZ-20%LaPO4 coatings produced by atmospheric (APS) method were evaluated. Results show that the volume percentage of porosity in the coatings containing LaPO4 was higher than the monolithic YSZ sample. It was probably due to less thermal conductivity of LaPO4 phases. Furthermore, the results showed that the amount of the remaining porosity in the composite coatings was higher than the monolithic YSZ at 1000°C for 120h. After 120h isothermal oxidation, the thickness of thermally growth oxide (TGO) layer in composite coatings was higher than that of YSZ coating due to higher porosity and sintering resistance of composite coatings. Finally, the isothermal oxidation resistance of conventional YSZ and nanostructured YSZ coating was investigated.

Microstructure and mechanical properties of Fe-doped NiAl–28Cr–6Mo eutectic alloys

Room temperature and high temperature microstructural and mechanical properties of arc melted NiAl–28Cr–6Mo eutectic alloys doped with 0.1% Fe, 0.2% Fe and 0.5% Fe (mole fraction) were investigated. The homogenization heat treatment of the alloys was conducted at 1300 °C in Ar atmosphere. Microscopic analyses, hardness measurements, XRD measurements and compression tests were used to characterize the alloys. As-cast and homogenized alloys exhibit fine cellular eutectic structures with coarse intercellular eutectic structure. The increase in the content of Fe results in coarsening eutectic layers and the decrease in eutectic cells. All alloys have very high compressive stress and strain at room temperature. The addition of Fe has small negative impact on the strength and ductility of the alloys at room temperature. However, the addition of Fe increases the high temperature strength of the alloy. High temperature XRD patterns show that peaks shift to lower Bragg angles. This indicates that the lattice parameter of the alloys increases.

Effect of Al on the Microstructure and Mechanical Properties of HK40 Heat-Resistant Steel at High Temperature

The high-temperature mechanical properties and microstructure of HK40 heat-resistant steel with different content of Al were investigated. The results from scanning electron microscope and transmission electron microscope showed that a large amount of spheroidal and dispersed γ′ phase were observed HK40 steel with 4.72wt.% and 7.10wt.% Al. The diameter of γ′ phase decreases from about 1.5μm to 50nm after solution treatment of 1200°C for 5h. The results of short term tensile test showed that tensile strength at 900°C decreased and the elongation was improved with increasing Al content. The oxides in the alloy with 4.72wt.% and 7.10wt.% Al were more uniform and finer than that in the alloy with and without 1.68wt.% Al.

Evaluation of High Temperature Properties and Microstructural Characterization of Resistance Spot Welded Steel Lap Shear Joints

AbstractJoining of thin sheets (0.5 mm) of stainless steel 304 and 17-4PH through resistance spot welding is highly challenging especially when joint is used for high temperature applications. Various combinations of stainless steel sheets of thickness 0.5 mm are spot welded and tested at room temperature as well as at high temperatures (800 K, 1,000 K, 1,200 K). Parent metal as well as spot welded joints are tested and characterized. It is observed that joint strength of 17-4PH steel is highest and then dissimilar steel joint of 17-4PH with SS-304 is moderate and of SS-304 is lowest at all the temperatures. Joint strength of 17-4PH steel is found to be &gt;80% of parent metal properties up to 1,000 K then drastic reduction in strength is noted at 1,200 K. Gradual reduction in strength of SS-304 joint with increase in temperature from 800 to 1,200 K is noted. At 1,200 K, joint strength of all combinations of joints is found to be nearly same. Microstructural evaluation of weld nugget after testing at different temperatures shows presence of tempered martensite in 17-4PH containing welds and homogenized structure in stainless steel 304 weld.

Densification and properties of magnesia-rich magnesium-aluminate spinel derived from natural and synthetic raw materials

Magnesia rich magnesium aluminate spinel (MgO: Al2O3=2:1) was developed by reaction sintering of Indian natural magnesite of Salem region as well as from synthetic caustic magnesia with calcined alumina. Dilatometric study of the green compacts was carried out to evaluate the spinelisation and sintering behaviour of both the samples. Green samples were heat treated between 1400 to 1600°C and characterised in terms of densification behaviour, high temperature flexural strength, microstructure and phase development. Spinel and periclase are the major phases in both the samples, where as forsterite is found only in the sample developed from Indian magnesite due to presence of silica as impurities.

Effect of Thermal Pretreatment on the Corrosion of Stainless Steel in Flowing Supercritical Water

The effect of high-temperature microstructure degradation (thermal ageing) on the corrosion resistance of austenitic stainless steels in supercritical water (SCW) was evaluated in this study. Mill-annealed (MA) and thermally treated (TT) samples of Type 316L and Type 310S stainless steel were exposed in 25 MPa SCW at 550°C with 8 ppm dissolved oxygen in a flowing autoclave testing loop. The thermal treatments applied to Type 316L (815°C for 1000 hr + water quench) and Type 310S (800°C for 1000 hr + air cool) were successful in precipitating the expected intermetallic phases in each alloy, both within the grains and on the grain boundaries. It was found that a prolonged time at relatively high temperature was sufficient to suppress significant compositional variation across the various intermetallic phase boundaries. This paper presents the results of the gravimetric analysis and oxide scale characterization using scanning electron microscopy (SEM) coupled with X-ray energy-dispersive spectroscopy (EDS). The role played by the fine precipitate structure on formation of the oxide scale, and thus corrosion resistance, is discussed. The combined role of dissolved oxygen and flow (revealed by examining the differences between Type 316L samples exposed in a static autoclave and in the flowing autoclave loop) is also addressed. It was concluded that formation of intermetallic phase precipitates during high-temperature exposure is not likely to have a major effect on the apparent corrosion resistance because of the discontinuous nature of the precipitation.

γ-Phase transformation, dynamic recrystallization and texture of a forged TiAl-based alloy based on plane strain compression at elevated temperature

In order to investigate the real or close-to-real high temperature microstructures of a TiAl-based alloy during rolling, low-cost plane strain compression tests were carried out at elevated temperature. γ as a dominant and plastic phase was selected as the main research object. It was found that fine γ lamellae precipitated in α2 phase deformed at 1200°C, with a total engineering strain of 60%. The precipitation of γ lamellae was mainly attributed to deformation-induced phase transformation at high temperature. Additionally, a certain misorientation angle (~89°) was observed in γ phase of all the samples and related to discontinuous dynamic recrystallization. Discontinuous dynamic recrystallization was promoted by pseudo and true twin boundaries which increased with increasing temperature and total strain. As regards γ-phase textures, the cubic textures related to recrystallization seemed to be readily weakened because of the γ→α2 and α2→γlamella transformations, while R texture was not significantly influenced. In addition, the cubic components and R component were respectively weakened and enhanced with increasing temperature and total strain. In short, enhancing or weakening of textures was mainly associated with phase transformation and recrystallization which were operated by deformation conditions.

Thermal Stir Welds in Titanium

Although conventional friction stir welding (FSW) has proven unsuccessful in joining thick sections of alpha and near-alpha titanium alloys, thermal stir welding, a variant of the FSW process in which an external heat source is used to preheat the workpiece, is demonstrated to be able to reliably join 12.3-mm-thick plates of CP titanium. This paper describes the microstructures and textures that develop in these thermal stir welds. The observed microstructure was used to reconstruct the high-temperature microstructure and texture present during the welding process and therefore reveal the genesis of the welding structures.

Effect of microstructure morphology on the high temperature tensile properties and deformation in directionally solidified NiAl-Cr(Mo) eutectic alloy

High temperature tensile properties and microstructure morphologies of directionally solidified NiAl-Cr(Mo) alloy under different compositions and withdrawal rates were investigated. Tensile strength of 513.8MPa was obtained in the directionally solidified NiAl-36Cr-6Mo hypereutectic alloy at 1000°C, which is the highest value reported in the NiAl-Cr(Mo) alloy until now. In addition, perfect cellular eutectic alloy solidified at fast withdrawal rate possessed higher tensile strength and elongation than planar eutectic alloy solidified at low withdrawal rate when they had the same composition. This is because bonding strength between eutectic cells in the perfect cellular eutectic alloy was high, which resulted in high crack propagation resistance and harmonious deformations in the intracellular and intercellular regions. The results above may change the locked-in view that only regular eutectic microstructure morphology grown with planar solid/liquid interface could be used in the eutectic in-situ composites. Perfect cellular microstructure morphology grown in a far fast rate could be also used, which would result in high efficiency in the industrial production. Refined microstructure, larger volume fraction of strengthening phase and dispersion strengthening of second phase particles were contribute to the improvement of high temperature tensile strength of the directionally solidified NiAl-Cr(Mo) alloy. However, the bonding strength of interface was obviously different in the alloys with different microstructure morphologies, which had significant effect on the tensile strength. When bonding strength of phase interface was low, the other strengthening factors could not be able to play their due roles, thus leading to relatively low tensile strength.

Fine-scale precipitation in the high-entropy alloy Al0.5CrFeCoNiCu

The high-entropy alloy Al0.5CrFeCoNiCu has been shown to consist of two stable, face-centred cubic solid solutions at temperatures approaching its solidus: one rich in Cr, Fe, Co & Ni (dendritic) and the other rich in Cu (interdendritic). While some studies have suggested that the high-temperature microstructure may be metastably retained to room temperature through rapid cooling, evidence of phase decomposition has also been reported. In this study, fine-scale precipitation has been observed in samples of Al0.5CrFeCoNiCu that have been rapidly cooled after casting, and water quenched following ageing for 1000h at 1000°C. Contrary to previous reports, in the as-cast state, the two face-centred cubic phases, as well as an L12 phase, were found in both dendritic and interdendritic areas, with the dendritic areas having undergone a spinodal decomposition. After ageing and quenching, L12 precipitates were found in both dendritic and interdendritic areas, and precipitates of the Cr-, Fe-, Co- and Ni-enriched face-centred cubic phase were found in the Cu-rich interdendritic regions. Given the nature of the heat treatments applied, the results suggest that precipitation in the alloy is rapid and cannot be avoided, even when the material is cooled quickly to room temperature.

Experimental reinvestigation of Al–Co phase diagram in vicinity of Al13Co4 family of phases

The work is focused on the experimental reinvestigation of the Al–Co phase diagram in vicinity of the Al13Co4 family of phases. For this purpose several alloys were prepared with bulk compositions ranging between Al71Co29 and Al77Co23. Experimental samples processed of the as-cast alloys were annealed for 330 h at various temperatures between 1070 and 1150 °C and subsequently rapidly cooled to fix their high-temperature microstructures. The annealing temperatures were selected with respect to the earlier published Al–Co phase diagrams. In the investigation X-ray diffraction, differential scanning calorimetry, and scanning electron microscopy including both energy dispersive X-ray spectroscopy and electron backscatter diffraction were used. In the microstructures of as-annealed alloys various combinations of B2-AlCo, Al5Co2, Z-Al3Co, Y1-Al13Co4, Y2-Al13Co4, M-Al13Co4, O′-Al13Co4, and solidified liquid were identified. It was found out that the earlier published single-phase Y-area is comprised in fact of monoclinic Y1-Al13Co4 and orthorhombic Y2-Al13Co4 lamellas. From the thermodynamic point of view, Y1-Al13Co4 and Y2-Al13Co4 are stable and metastable phases, respectively. The O′-Al13Co4 phase was identified at 1070 °C in the alloys where the occurrence of O-Al13Co4 was expected. A modified partial Al–Co phase diagram was also proposed with respect to the experimental results obtained.