Crystal structure of a long period stacking ordered (LPSO) phase newly found in Mg-Al-Gd ternary alloys was investigated by scanning transmission electron microscopy (STEM) and transmission electron microscopy (TEM). The Mg-Al-Gd LPSO phase was confirmed to be constructed with 6-layer structural blocks, which is similar to the case of the 18R-type LPSO phases in the other Mg-TM-RE alloys. Atomic resolution high-angle annular dark-field (HAADF) STEM imaging revealed that Gd atoms are enriched in four layers instead of two layers and are ordered in a long range within the 6-layer structural block.

Intermetallic γ-TiAl based alloys with additional amounts of the ternary bcc β phase attracted increasing attention in recent years due to their improved workability at elevated temperatures. At lower temperatures the ductile high-temperature β phase can transform to several ordered phases. However, actually available phase diagrams of these multiphase alloys are quite uncertain and the precipitation kinetics of some metastable phases is far from understood.

In the present study various transformations of the third phase are observed in situ by means of high-energy x-ray diffraction using synchrotron radiation. A Ti-45Al-10Nb (at.%) specimen is subject to a temperature ramp of repeated heating cycles (700 °C - 1100 °C) with subsequent quenching at different rates. Depending on the quenching rate reversible transformations of the B2-ordered βo phase to different ω related phases are observed in Ti-45Al-10Nb. At low quenching rates the hexagonal B82-ordered ωo phase is formed while at high quenching rates the metastable intermediate trigonal ω’’ phase can be preserved. The results indicate that the complete transformation from βo to hexagonal B82-ordered ωo consists of two steps which are both diffusion controlled.

The creep behavior of a new type of austenitic heat-resistant steel Fe-20Cr-30Ni-2Nb (at.%), strengthened by intermetallic Fe2Nb Laves phase, has been examined. Particular attention has been given to the role of grain boundary Laves phase in the strengthening mechanism during long-term creep. The creep resistance increases with increasing area fraction (ρ) of grain boundary Laves phase according to equation ε/ε = (1−ρ), where ε0 is the creep rate at ρ = 0. In addition, the creep rupture life is also extended with increasing ρ without ductility loss, which can yield up to 77% of elongation even at ρ = 89%. Microstructure analysis revealed local deformation and well-developed subgrains formation near the grain boundary free from precipitates, while dislocation pile-ups were observed near the grain boundary Laves phase. Thus, the grain boundary Laves phase is effective in suppressing the local deformation by preventing dislocation motion, and thereby increases the long-term creep rupture strength. This novel creep strengthening mechanism was proposed as “grain boundary precipitation strengthening mechanism” (GBPS).

In this study, the exothermic DSC peaks observed in rapidly-solidified Fe-44.9at.%Al ribbons and water-quenched Fe-48.5at.%Al single crystals were analyzed by Matusita’s method in order to discuss the kinetics of the agglomeration processes of supersaturated vacancies. Both the nucleation and morphological factors, n and m, respectively were approximately 3 for the rapidly solidified ribbons and were approximately 2 for the water-quenched single crystals. Based on the Matusita’s idea, the m values suggest the growth of 3-dimensional voids in the rapidly solidified ribbons and the growth of 2-dimensional dislocations loops in the single crystals due to the agglomeration of supersaturated vacancies. In addition, the n values suggest that the nuclei of voids and dislocation loops existed in as-quenched samples. These interpretations are in good agreement with the results of TEM observations.

The factors which influence the ductility of cast samples of TiAl-based alloys are briefly reviewed with emphasis on alloys where microstructural refinement has been used in an attempt to improve ductility. The grain size in cast samples of different TiAl-based alloys can be refined either by high additions of about 1at% boron, or by lower additions of about 0.2at%. In addition it is possible to refine the microstructure by massively transforming samples and heat treating the transformed samples in the (alpha + gamma) phase field to precipitate alpha. Significantly different ductilities are found in different alloys with similar grain sizes or with similar microstructures and the origins of the improvements in ductility and of these differences are discussed in this paper. The role of alloying elements in influencing the degree of order in alpha 2 and in turn influencing slip in alpha 2 is discussed.

Recently, Nb-Si alloys have attracted attentions as substitutional materials of Ni-based superalloys because of its low density and high melting point. For attaining good room temperature toughness of Nb-Si alloys, proposed is a microstructure-control technique by combining eutectic reaction (L->Nb+Nb3Si) and eutectoid reaction (Nb3Si->Nb+ Nb5Si3) for spheroidizing Nb5Si3 strengthening phase embedded in Nb matrix [1]. For the solid solution strengthening of Nb matrix phase W and Mo are very effective, but Nb3Si phase disappears by adding these elements of as small as 3 at%. In contrast, Ti and Ta stabilize Nb3Si phase. For a further alloy development, establishment of an alloy design based on the control of phase stability of Nb3Si is needed. In the previous study [2], it was revealed that the phase stability of Nb3Si can be controlled by selecting appropriate Ta/Mo ratio. In the present study, this approach is expanded to other combinations of stabilizing and destabilizing elements of Nb3Si, such as Ti and W, Ta and W, and Ti and Mo. Vickers hardness tests were conducted on the heat-treated samples to reveal effects of additives on mechanical properties of Nb-Si alloy.

The temperature dependence of yield stress and the associated dislocation dissociation in L12 intermetallic compounds are investigated in order to check the feasibility of the classification of L12 intermetallic compounds so far reported in terms of the planarity of core structures of partial dislocations with b = 1/2<110> and 1/3<112> on {111} and {001} glide planes. In contrast to what is believed from the reported classification, the motion of APB-coupled dislocations is proved to give rise to the rapid decrease in yield stress at low temperatures for Co3Ti and Co3 (Al,W). The temperature dependence of yield stress at low temperatures is newly interpreted in terms of a thermal component of solid-solution hardening, at least, for these two L12 compounds. We have proposed a new way to describe the yield stress–temperature curves of L12 compounds with three parameters (the athermal and thermal components of solid-solution hardening and the anomalous strengthening component) when the dislocation dissociation scheme is of the APB-type.

Mo-Si-B alloys respond to high temperature oxidation in two distinct stages. First, there is a transient stage with an initial high recession rate that corresponds to the evaporation of volatile MoO3 due to the oxidation of the molybdenum rich phases. The steady state stage of the oxidation begins when a borosilica layer that initiated in the transient period becomes continuous and protects the alloy from further rapid oxidation. Then, the oxidation rate is limited by oxygen diffusion through the borosilicate layer. In order to improve the oxidation performance of the Mo-Si-B alloys, it is necessary to minimize the transient stage. The three phases, Mo (solid solution), Mo3Si (A15) and Mo5SiB2 (T2), composing the Mo-Si-B alloys play different roles in the transient stage. The interaction of the three phases with a reduced microstructure scale can reduce considerably the transient oxidation stage. As a further approach to inhibit the transient stage, a kinetic biasing strategy has been developed to capitalize on the reactions between different phases to develop useful reaction products and alloy compositions that evolve toward a steady state of a compatible system. In order to achieve a compatible interface coating together with enhanced oxidation resistance, a pack cementation process has been adopted to apply diffusion coatings. Two areas are highlighted for successful coating applications on Mo-Si-B alloys and robust high temperature oxidation resistance: development of metal-rich silicide + borosilicide high-temperature coating and in-situ thermal-barrier + borosilica coatings.

The pseudoelastic behavior of Fe3Al and Fe3Ga alloys with the D03 structure is reviewed. In general, pseudoelasticity of shape memory alloys is based on a thermoelastic martensitic transformation. However, pseudoelasticity regardless of the martensitic transformation is found to take place in D03-ordered Fe3Al and Fe3Ga alloys. For instance, a 1/4<111> superpartial dislocation in Fe3Al alloys moves independently dragging an antiphase boundary (APB). During unloading, the APB pulls back the superpartial to decrease its energy resulting in pseudoelasticity, which is called “APB pseudoelasticity”. Moreover, D03-type Fe3Ga alloys were found to demonstrate three types of pseudoelasticity based on the dislocation motion, twinning and martensitic transformation depending on the chemical composition, degree of D03 order, loading axis, stress sense and deformation temperature. The mechanism of the pseudoelasticities in the D03-type intermetallics is discussed based on some in situ observations. The effects of chemical composition, deformation temperature and crystal orientation on the pseudoelastic behaviors are also discussed.

A series of porous aluminum-based materials are prepared by the reduction of solutions of metal chlorides with lithium powder in diethyl ether under dry argon. The reactants must be combined slowly, but either order of addition is used. The reduction of AlCl3 produces hollow Al balls composed of ~100 nm aluminum particles in nearly quantitative yield after the LiCl byproduct is washed away with dry tetrahydrofuran. Similar structures are formed when mixtures of AlCl3 and SiCl4, are reduced, except that the second component has the effect of reducing the Al nanoparticle size. Mixtures of AlCl3 with FeCl3 reduce to similar ball-like porous structures that are composed of Al, Fe, and Fe-Al intermetallic nanoparticles. When AlCl3 and ZnCl2 are co-reduced, flake-like nanoporous structures are obtained, and solutions of AlCl3 + VCl3 produce more compact nanoporous structures. Some side reactions involving ether cleavage that produces aluminum alkoxides and alkyls do occur, and the amount of side reaction is dependent on the identity of the second metal. The reduction of AlCl3 with excess (4 eq) Li powder produces LiAl nanomaterials. NMR shows the intermetallic compound LiAl to be the only Li-Al intermetallic present.

A microstructure-based FEM model that couples crystal plasticity, crystallographic descriptions of the B2-B19′ martensitic phase transformation, and anisotropic elasticity is used to simulate thermal cycling and isothermal deformation in polycrystalline NiTi (49.9at% Ni). The model inputs include anisotropic elastic properties, polycrystalline texture, DSC data, and a subset of isothermal deformation and load-biased thermal cycling data. A key experimental trend is captured—namely, the transformation strain during thermal cycling is predicted to reach a peak with increasing bias stress, due to the onset of plasticity at larger bias stress. Plasticity induces internal stress that affects both thermal cycling and isothermal deformation responses. Affected thermal cycling features include hysteretic width, two-way shape memory effect, and evolution of texture with increasing bias stress. Affected isothermal deformation features include increased hardening during loading and retained martensite after unloading. These trends are not captured by microstructural models that lack plasticity, nor are they all captured in a robust manner by phenomenological approaches. Despite this advance in microstructural modeling, quantitative differences exist, such as underprediction of open loop strain during thermal cycling.

The effect of microstructure on cold-rolling workability and tensile properties of Ni3Si (L12)−Ni3Ti (D024)−Ni3Nb (D0a) multi-phase intermetallic alloys was investigated. The cast alloys with different microstructures containing the D024 phase and/or the D0a phase particles in the L12 matrix were homogenized and then cold rolled. For the alloys with the microstructure consisting of coarse plate-like D024 particles in the L12 matrix, serious cracks initiated at the coarse D024 particles in the early stage of the cold rolling process and then propagated, resulting in failure of the rolled plate. On the contrary, for the alloys with the microstructure consisting of fine needle-like D024 precipitates and/or granular-shaped D0a particles, these second phase particles did not spoil the cold workability, leading to successful cold rolling to 90 % reduction. After 90 % cold rolling, the rolled sheets were fully recrystallized at 1173 K for 1 h, resulting in the formation of a fine-grained microstructure. The room-temperature tensile strength and the yield stress of the recrystallized sheet were remarkably enhanced compared with that of the unrolled alloys, possibly due to the fine-grained microstructure as well as the particle hardening. Also, the high-temperature tensile strength and the elongation were improved in the recrystallized sheets compared to an L12 single-phase Ni3(Si,Ti) alloy sheet. Consequently, it was found that the cold rolling and annealing process was beneficial to improve the tensile properties for the present multi-phase intermetallic alloys.

Refractory Metal-Intermetallic Composites (RMICs) based on the Nb-Si system have been considered as candidates for the next-generation high temperature materials (i.e. >1200°C). Ti, Cr and Hf have been shown to have beneficial effects on the oxidation resistance and mechanical properties of Nb-Si alloys. The present study has determined phase equilibria in the Nb-rich region of the Nb-Si-Ti-Cr-Hf system via the Calphad approach. The alloying effects of Cr and Hf on the microstructure of Nb-Si-Ti alloys are understood based on isothermal sections, liquidus projections, and solidification curves that were calculated from the thermodynamic models of the Nb-Ti-Si-Cr-Hf system developed in the present study. This work provides important guidelines on the development of new Nb-Si-Ti-Cr-Hf alloys.

An alternative high temperature structural alloy system based on the X-X3Si eutectic compositions of chromium and vanadium is put forward. These low-density (~6g/cm3) eutectics have a bcc solid-solution to increase alloy fracture toughness, and a A15 X3Si as the high temperature load-bearing phase. (½Cr,½V)-(½Cr,½V)3Si was used as the base alloy for further element additions, and is represented by the symbol 山 10at.% tantalum and aluminium were substituted for vanadium as quaternary and quinary alloy additions.

Microstructure, elemental phase partitioning, compression creep and oxidation results will be discussed. Cr-Cr3Si has a tidy, fine lamellar microstructure. Vanadium coarsens and destabilises the lamellae to a limited extent. Tantalum addition causes two distinct populations of eutectic to form; one population having finer lamellae than the other. Aluminium does not coarsen or destabilise the lamellar microstructure. High temperature compression tests at 1200°C and 1300°C show that 山 is stronger than the binary alloys, and of similar strength to the quaternary and quinary alloys.

Intermetallic TiAl-alloys can replace the heavier Ni-based superalloys in several high temperature applications with regards to their mechanical properties, however they can not be used at temperatures above 800°C in oxidizing environments for longer times because of insufficient oxidation resistance. Despite an Al-content of about 45 at.% in technical alloys, no protective alumina layer is formed because the thermodynamic stabilities of titanium oxide and aluminum oxide are of the same order of magnitude. Therefore a mixed TiO2/Al2O3-scale is formed which is fast growing so that the metal consumption rate is quite high. On the other hand the formation of a slow growing alumina layer is promoted by a fluorine treatment. This so called fluorine effect leads to the preferential intermediate formation of gaseous aluminum fluorides at elevated temperatures if the fluorine content at the surface stays within a defined concentration range. These fluorides are converted into solid Al2O3 due to the high oxygen partial pressure of the high temperature service environment forming a protective pure Al2O3 surface scale. In this paper results of high temperature oxidations tests of several technical TiAl-alloys will be presented. Different F-treatments e.g. dipping or spaying which are easy to apply have been used and their results will be compared. The mass change data of the F-treated specimens are always lower than those of the untreated ones. Post experimental investigations such as light microscopy, scanning electron microscopy and energy dispersive X-ray analysis reveal the formation of a thin alumina layer on the F-treated samples after optimization of the process while a thick mixed scale is found on the untreated samples. The results will be discussed in view of an optimized procedure and the future use of TiAl-components in high temperature environments.

The combination of the molecular beam epitaxy growth method with the in-situ reflection high energy electron diffraction measurements currently offers unprecedented control of crystalline growth materials. We present here a stoichiometric study of MnxSc(1-x) [x = 0, 0.03, 0.05, 0.15, 0.25, 0.35, and 0.50] thin films grown on MgO(001) substrates with this growth method. Reflection high energy electron diffraction and atomic force microscopy measurements reveal alloy behavior for all of our samples. In addition, we found that samples Mn0.10Sc0.90 and Mn0.50Sc0.50 display surface self-assembled nanowires with a length/width ratio of ~ 800 – 2000.

Large, defect-free single-phase samples of the hexagonal C14 NbFe2 and Nb(Fe,Al)2, and the cubic C15 NbCo2 Laves phases have been produced by a modified levitation melting technique. The compressive strength of NbFe2 and NbCo2 has been determined in dependence on the Nb content, that of Nb(Fe,Al)2 in dependence on the Al content. The binary phases did not show either a maximum (defect softening) or minimum (defect hardening) in strength when the Nb content was varied. Instead, for both phases an increase of the compressive strength with increasing Nb content is observed.

The half-Heusler compound ZrNiSn has a quite small solubility for Ni from the stoichiometric composition towards the Ni-rich direction since Ni atoms are not supposed to occupy the vacancy-site. Nevertheless, Co and Ir atoms preferably occupy the vacancy-site of ZrNiSn, which is contrary to the prediction that they would substitute for Ni sites. This implies that the phase stability of the compound gradually changes toward that of the Heusler compound Zr(Ni,M)2Sn (M = Co, Ir). It has been confirmed that there exists a two-phase field between half-Heusler Zr(Ni,Cox)Sn and Heusler Zr(Ni,Co)2Sn. The n-type thermoelectric property of ZrNiSn can be converted to p-type by the addition of Co and Ir within the compositional range of the half-Heusler phase. The occupation of vacancy sites by Co and Ir atoms leads to a drastic reduction in the thermal conductivity owing to the enhancement of phonon scattering. With further Co addition, the Heusler phase Zr(Ni,Co)2Sn alloys show metallic behavior.

New advanced multi-phase γ-TiAl based alloys (TiAl-Nb-Mo), so called TNM alloys, have been developed to promote hot workability and to allow easier processing by conventional forging. However, to control and stabilize the final microstructure, specific processing and further thermal treatments are required. In the present work we used mechanical spectroscopy techniques to obtain a better understanding of the microstructural mechanisms taking place at high temperature applying two different heat treatments. Internal friction spectra and dynamic modulus evolution have been measured in an inverted torsion pendulum up to 1220 K. A stable relaxation peak was observed in both cases at about 1050 K for 1 Hz. Spectra acquired at several frequencies between 0.01 Hz and 3 Hz allow us to measure the activation parameters of this peak. In addition, a high temperature background (HTB) has been observed. This HTB, which has been found to be dependent on thermal treatments, has been analyzed to obtain the apparent activation enthalpy, which seems to be correlated to the creep behavior. Finally, we discuss the relaxation peak and the HTB in terms of the microstructural evolution during thermal treatments.