Search

The equiaxed Ni-based superalloy René 108 was subjected to short-term annealing at five temperatures between 900 °C and 1100 °C. The phase composition, phase lattice parameters, microstructure, stereological parameters, and chemical composition of γ′ precipitates were investigated by thermodynamic simulations, X-ray diffraction, scanning and transmission electron microscopy, and energy-dispersive X-ray spectroscopy. Analysis of the γ and γ′ lattice parameters using the Nelson-Riley extrapolation function showed that the misfit parameter for temperatures 900 °C to 1050 °C is positive (decreasing from 0.32 to 0.11 pct). At 1100 °C, the parameter becomes negative, δ = − 0.18 pct. During the short-term annealing, γ′ precipitates dissolution occurred progressing more rapidly with increasing temperatures. The surface fraction of γ′ precipitates decreased with increasing temperature from 0.52 to 0.34. The dissolution of γ′ precipitates did not only proceed through uninterrupted thinning of each individual precipitate, but also included more complex mechanisms, including splitting. Based on transmission electron microscopy, it was shown that after γ′ precipitates dissolution, the matrix close to the γ/γ′ interface is strongly enriched in Co and Cr and depleted in Al.

The effect of yttrium additions on the glass formation of the Zr50Cu40Al10 alloy with high oxygen content was investigated. Unlike other reports showing the positive effect of alloying Zr-based alloys with a few percent of rare earth elements, we examined the Y-to-oxygen ratios. As calculated, a stoichiometric amount of yttrium (2808 at ppm) is required to bind all the oxygen (4212 at ppm) to form the Y2O3. Additionally, the influence of half (1404 at ppm) and double stoichiometric (5616 at ppm) levels of yttrium, with respect to the measured oxygen content to be bound was evaluated. The Y-doped alloys were synthesized by arc melting, followed by suction casting into copper molds with conical and rod cavities. The critical diameter of the undoped alloy was below 3 mm, while stoichiometric and double stoichiometric yttrium concentrations allowed to obtain 5 and 7 mm diameter glassy samples, respectively. Microstructure observations confirmed the scavenging effect of yttrium, leading to the formation of a cubic ( $$ Ia\bar{3} $$ I a 3 ¯ space group) yttrium sesquioxide. The glass forming ability of the alloys was additionally determined based on differential thermal analysis, allowing to propose the best indicator for the studied Zr-Cu-Al alloys.

The change of γ′ phase morphology in quenched Ni-based superalloy strengthened by γ′ precipitates was investigated by means of scanning electron microscopy and X-ray diffraction. Lattice parameters of γ and γ′ phases and subsequently the δ parameter were calculated. Relatively high content of gamma prime formers resulted in a substantially high volume fraction of precipitates in the dendrite cores. The volume fraction of γ′ phase was significantly decreased due to dissolution of precipitates in the surrounding matrix, as a result of short-term subsolidus exposure. The lattice parameter of γ′ phase was higher than that of γ phase, which results in a positive δ parameter in fully heat treated (FHT) and also FHT+ Quench conditions. For (200) and (220) reflections the γ/γ′ misfit was lower in the FHT + Q sample.

Fe–23wt.%Mn–3wt.%Si–3wt.%Al alloy was cast, homogenized at 1150ºC, hot-rolled at temperatures between 1200ºC and 900ºC and next cold-rolled from 5% up to 40% reductions in thickness. Microstructure and texture of this alloy, which has a low stacking fault energy, were defined after cold-rolling. Investigation of transmission electron microscopy and X-ray diffraction showed that mechanical twinning and martensitic transformations (γfcc→εhcp and γfcc→εhcp→α′bcc) took place during cold-rolling. The crystallographic Shoji-Nishiyama (S-N) {00.2}ε║{111}γ, ε ║ γ and Kurdjumov-Sachs (K-S) {111}γ║{101}α’, γ║α’ relations between martensite (ε, α’) and austenite (γ), were found in the coldrolled material.

The present research is part of a project concerning texture and microstructure evolution upon deformation and annealing in a series of various duplex stainless steels. This article shows a comparison of deformation behavior in two grades of duplex steels subjected to cold-rolling over a wide range of reductions. Both steels under examination showed clear differences in chemical composition and their preliminary treatment included different processes of hot plastic working. Optical microscopy was used to examine changes in morphology and formation of a ferrite–austenite band-like structure upon cold-rolling. Transmission electron microscopy was applied to analyze microstructural evolution and deformation behavior within the ferrite and austenite bands. Additional research included texture measurements for both component phases. Special attention was paid to the ferrite–austenite co-deformation and the structural effects resulting from strain localization at high rolling reductions.

The 17Cr–9Ni–3Mo precipitation hardened (PH) steel was processed after re-melting using different cooling rates including copper plate chilling and the melt-spinning. The effect of different cooling rates and the sub-zero treatment on the microstructure of the 17Cr–9Ni–3Mo steel was studied. The microstructure and the phase composition of the steel was investigated using light microscopy (LM), scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS), transmission electron microscopy (TEM), the X-ray diffraction (XRD) and Mossbauer spectroscopy. The results show that the microstructure of 17Cr–9Ni–3Mo steel after rapid solidification consists of austenite and δ-ferrite. The quantity of austenite increases with the increase of cooling rate. Sub-zero treatment of the samples cooled at the rate of 4 × 104 K/s and 2 × 105 K/s reduces the quantity of austenite as a result of austenite → martensite transformation. For the samples cooled at the rate of 2 × 106 K/s the quantity of austenite in the microstructure does not change when compared to the state before and after sub-zero treatment. The model for the formation of microstructures during rapid cooling was proposed.

Microstructural changes in the age-hardenable Fe-28wt.%Mn-9wt.%Al-1wt.%C steel during ageing at 550°C for various times have been investigated by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The steel was produced in an induction furnace and the ingot, after homogenization at 1150°C for 3 hours under a protective argon atmosphere, was hot-rolled and subsequently cold-rolled up to 23% reduction. The sheet was then aged at 550°C for various times in an argon atmosphere and cooled in air. XRD analysis and TEM observations revealed a modulated structure and superlattice reflections produced by spinodal decomposition, which occurred during ageing at 550°C. Theexistence of satellites suggests that either (Fe, Mn)3AlCx carbides were formed within the austenite matrix by spinodal decomposition during cooling or chemical fluctuactions occurred between the (Fe, Mn)3AlCx carbides and the austenitic matrix.

This paper presents structure investigations of the rapidly cooled Fe66Cu6B19Si5Nb4 alloy. A proper selection of chemical composition enabled in-situ formation of the amorphous-crystalline composite during the melt spinning process. Liquid phase separation into the Fe-rich and the Cu-rich phases was confirmed. The microstructures of alloy, melt-spun from 1723 and 1773 K, are composed of the Fe-rich amorphous matrix and Cu-rich spherical crystalline precipitates. For the higher melt-ejection temperature, no coarse precipitates could be observed. Amorphous nature of the Fe-rich matrix was confirmed by presence of a broad diffraction maximum on the X-ray diffraction patterns, a halo ring on the electron diffraction pattern as well as presence of exothermic effects, related to the crystallization of the Fe-rich amorphous matrix, in the differential scanning calorymetry. Beside presence of copper, revealing positive heat of mixing with iron, relatively large supercooled liquid region, was noticed.

The paper presents microstructures and mechanical properties of the melt-spun Fe30Cu32Si13B9Al8Ni6Y2 and Fe44Cu18Si13B9Al8Ni6Y2 alloys. It was found that liquid phase separation of the initially homogeneous melt occurred due to a positive heat of mixing between two major elements. The microstructures of the melt-spun ribbons were composed of the Fe-rich amorphous and the Cu-rich crystalline phases. A significant effect of the melt ejection temperature and the chemical composition (Fe and Cu content) on microstructures of rapidly solidified alloys was revealed. The microstructures of ribbons melt-spun from the miscibility gap region were non-uniform. On the other hand the microstructures of ribbons melt-spun from homogeneous melt temperature region were composed of the spherical precipitates distributed within the matrix. Superior hardness values of the examined ribbons melt-spun from the same temperature were found for the alloy with higher iron content. An increase of the melt-ejection temperature in the homogeneous melt region resulted in hardness decrease in case of the Fe30Cu32Si13B9Al8Ni6Y2 alloy and its increase for the Fe44Cu18Si13B9Al8Ni6Y2 ribbons.

The microstructure in the new COST martensitic 12% chromium steel VM12 developed for advanced power stations operated around 625 – 650°C has been investigated in the ‘as-received condition’ and after long-term creep exposure at 625°C up for around 29,000 h. Quantitative TEM analyses of VM12 steel microstructure were undertaken to determine the dislocation density within the sub-grain, the width of the martensite laths/sub-grains and the particle parameters (shape, size, distribution). Phase identification was performed using electron diffraction and X-ray spectrometry.The TEM results showed that thermal exposure and creep deformation at 625°C caused some changes of the VM12 steel microstructure. The statistical quantitative analyses of the microstructural parameters, showed that the sub-grain size and the size of M23C6 precipitates slightly increased after exposure up to 29,000 h. In the creep-tested specimens, microstructural changes were much more pronounced than in the thermally-exposed specime...

The aim of the work was to produce the amorphous/crystalline composite with uniform distribution of fine crystalline soft phase. Silver–copper–titanium Ag 20 Cu 30 Ti 50 alloy was prepared using 99.95 wt% Ag, 99.95 wt% Cu, 99.95 wt% Ti that were arc-melted in argon atmosphere. Then the alloy was melt spun on a copper wheel with linear velocity of 33 m/s. Investigation of the microstructure for both arc-melt massive sample and melt-spun ribbons was performed with use of scanning electron microscope (SEM) with EDS, light microscope (LM) and X-ray diffraction. The thermal stability was evaluated by differential scanning calorimetry (DSC). The properties such as Young modulus and Vickers hardness number before and after crystallization of the amorphous matrix were measured with use of nanoindenter. The microstructure was investigated by transmission electron microscope (TEM). It was found, that the alloy has a tendency for separation within the liquid state due to the miscibility gap which resulted in segregation into Ti–Cu–Ag matrix and Ag-base spherical particles after arc-melting. During rapid cooling through the melt spinning the Ag 20 Cu 30 Ti 50 alloy formed an amorphous/crystalline composite of fcc silver-rich spherical particles within the amorphous Ti–Cu–Ag matrix.

In the microstructures of slowly and rapidly cooled liquid of the immiscible alloy Fe30Cu32Ni10Si13Sn4B9Y2 two distinct regions were observed following arc melting and slow cooling, confirming that liquid/liquid phase separation had occurred. Rapid cooling from a temperature within the liquid immiscibility gap, melt spinning, resulted in an amorphous/crystalline composite, formed from the previously melted Fe- and Cu-rich regions, respectively. Transmission electron microscopic studies of this melt-spun ribbon revealed the glassy nature of the Fe-rich matrix, as well as of the Fe-rich spheres formed within the previously existing Cu-rich liquid.

Analytical electron microscopy, supported by microchemical and microdiffraction techniques, was applied to assess the microstructural changes accompanying semisolid molding of a creep-resistant magnesium alloy with nominal chemistry of Mg–5%Al–2%Sr. In an ingot precursor, the Sr containing precipitates exhibited a morphology of Al 4 Sr platelets within eutectic colonies and Al–Mg–Sr films, both residing at grain/cell boundaries, while sub-boundaries were occupied by fine Mg 17 Sr 2 plates. Trace quantities of Mg 17 Al 12 were present as plate-shaped continuous precipitates within selected grain/cell interiors. Semisolid processing of mechanically comminuted ingots generated thixotropic morphologies with globular solids of α-Mg, having well-developed sub-structures. The quickly-solidified liquid fraction, enriched in Al and Sr, led to the formation of increased volumes of a lamellae type Al 4 Sr phase, surrounding the secondary α-Mg. Traces of Mg 17 Al 12 still existed, predominantly as irregular shape compounds located mainly in grain interiors. Both the primary solid and high melting point precipitates of Mn–Al modified with Sr, acted as nucleation substrates for secondary α-Mg grains. The possible implications of semisolid processing on the creep behavior of magnesium alloys are discussed.

Microscopical techniques were used to provide the microstructural details and identify mechanisms governing phase and morphological transformations during the heating of Mg–8%Al–2%Zn pellets in solid and semisolid states. It was found that an as-extruded matrix of equiaxed α-Mg grains with a twinning substructure, was thermally unstable and experienced complete recrystallization after reheating to 200 °C. The precipitates of the Mg 17 Al 12 phase and augmented concentrations of alloying elements within migrating grain boundaries and triple junctions played a key role in transformation of the equiaxed grains into thixotropic structures during partial melting. A direct link exists between sizes of equiaxed grains in the solid state and unmelted particles in the semisolid slurry. Although the morphology of primary solid particles did not change during the melting progress, the rate of particles’ coarsening at high temperatures and their internal microstructure in subsequently solidified alloy were influenced by the solid–liquid ratio. The importance of these findings for semi-solid injection molding practice is emphasized.

In the present work, the 316L steel+2%TiB2 powders are sintered after milling by High Pressure-High Temperature (HP-HT) process. Studies cover the effect of sintering parameters on the development of microstructure and properties. The microstructural evolution of 316L steel-2TiB2 composites is characterized by the techniques of X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). It is found that the structure of a mixture of the 316L steel+2%TiB2 powders after milling and the structure of subsequent composites sintered by HP-HT is composed of a two-phase matrix including austenite (γ) and martensite (α'). The phase analysis and calculations of the martensite (α') content have revealed the highest content of this constituent in the matrix of 316L steel+2% TiB2 composites obtained by sintering at a temperature of 900 °C.

Thixomolding 1 , an emerging semisolid technology, was used to process an AZ91D magnesium alloy under experimental conditions designed to yield from 5 to 60% of the primary solid particles. The thixotropic microstructures obtained were characterized in detail and linked to the corresponding tensile properties. An increase in primary solid content was accompanied by its larger microchemical and microstructural inhomogeneity expressed by Al and Zn segregation, sub-micron precipitates of Mg 17 Al 12 and eutectic islands. At the same time, the size of α-Mg grains within the eutectic mixture was reduced. For the volume fraction of the primary solid up to about 20%, the tensile strength and elongation remained at the level of 240 MPa and 4.5%, respectively. A further increase of the primary solid caused a reduction in both strength and ductility. The fractographic analysis revealed a correlation between the primary solid content and the morphology of the decohesion surface. It is concluded that for alloys with a solid fraction below approximately 20%, the internal structure of the primary solid and the eutectic mixture control the properties. For a large volume of unmelted fraction, the interface between the primary solid and the eutectic mixture is a key factor which controls the tensile properties of the thixomolded alloy.

Thermal evolution of the crystallographic texture and microstructure in Ni electrodeposits, with a specific application for replication of optical recording devices, was examined. As-deposited microstructure was comprised of bimodal grains with a size of 4 and 0.1 μ m and the 〈100〉 fibre texture with inhomogeneous strength across the deposit thickness. The 〈100〉 texture was unstable and transformed during annealing to the 〈211〉 fibre. The texture transformation was accompanied by grain growth and deposit softening and the temperature range for rapid changes of all parameters was between 623 and 673 K. A numerical analysis of texture data was conducted to identify the type of Ni grain boundaries, which control grain growth and texture transformation. The microstructural observations suggested that the growth of grains with new orientation starts in fine-grained regions and controls the thermal stability of the entire deposit. The results are discussed in terms of the mechanism of texture transformation during annealing.

Japanese 9% Cr steel containing 0.5% Mo and 1.8% W (P92) has been investigated. Quantitative microstructural analyses using TEM of thin foils and extraction double replicas have been carried out after different austenitization and tempering treatments and after creep deformation at 600 and 650°C. Statistical quantitative analyses were undertaken to determine the dislocation density within the martensite laths, the width of the martensite laths/subgrains and the size and distribution of the carbide and carbonitride precipitates. Correlation of the results of the microstructural investigation with the creep rupture properties allowed the stability of the microstructure during the high temperature exposure to be assessed. The consequences of microstructural changes for the extrapolation of creep rupture data to design lifetimes are considered. Taking into account the strengthening mechanisms which remain effective in long term testing, the 100000 h stress rupture strength of P92 at 600°C is estimated at 115 MPa.

Austenitic stainless steels are substantially harder during hot-rolling than either ferritic or mild steels. The objective of this study is to verify the possible correlation between the edge-crack formation during hot-rolling and the presence of {delta} ferrite in austenitic stainless steel. Hot-rolled plates of austenitic stainless steels, examined at room temperatures, contain up to 9% of {delta} ferrite in austenitic matrix. The distribution of ferrite in steel plate is inhomogeneous: the highest ferrite content is located in the vicinity of the plate edge. Moreover, the content of {delta} ferrite changes irregularly across the plate thickness. The results obtained from analysis of several plates suggest a correlation between the maximum content of {delta} ferrite in steel microstructure and the length of the edge-crack formed during hot-rolling: the higher the volume fraction of ferrite, the longer the edge-crack.

Microstructural investigations of the steel 9Cr0.5Mo2W (NF616), a Japanese steel developed for advanced power station components, have been carried out by optical microscopy and by transmission electron microscopy, using thin foils and extraction replicas prepared after normalization at 970–1145°C and tempering at 715–835°C. After normalization, the steel exhibited a martensitic structure with only small amounts of retained austenite. During tempering recovery and precipitation processes occurred which reduced the hardness of the martensite. Elongated martensite laths were observed within the prior austenite grains, the boundaries of which were decorated with M 23 C 6 precipitates. In addition to the M 23 C 6 precipitates, three types of MX (where X is C and/or N) precipitate were detected; spheroidal Nb-rich carbonitrides, plate-like V-rich nitrides and complex particles consisting of the plate-like particles nucleated on the spheroidal ones. Statistical analyses of the microstructure were carried out using both thin foils and extraction replicas to determine the subgrain size and the particle size distributions and relate them to the heat treatments.