The effect of fine M2C particles on the recrystallization temperature and high temperature strength of warm rolled Fe3Al base alloys was investigated. Fe-27Al-1.2C-2Cr-xMo (x: 0.3, 0.9) alloys (in at.%) were arc melted, warm rolled and annealed. TEM observations have revealed that fine M2C particles were present in the alloy containing 0.9% Mo but not in the alloy with 0.3% Mo after warm rolling. The recrystallization temperature increased from 740 °C to 810 °C when the Mo content is increased from 0.3 to 0.9 due to the presence of fine M2C particles. Tensile tests conducted on annealed samples with fine sub-grained matrix have shown that the introduction of fine M2C particles is effective to enhance the proof stress at 600 °C.

Two-phase intermetallic alloys composed of geometrically close packed (GCP) Ni3Al (L12 phase) and Ni3V (D022 phase) have attractive mechanical properties at high temperature, and are therefore considered to be used as high temperature structural materials. In this study, the effect of Ta and Re addition on the microstructure and hardness of two-phase intermetallic alloys was investigated. The addition of Ta remarkably enhanced the hardness due to solid solution hardening of the constituent phases. On the other hand, the addition of Re retarded the formation of the two-phase microstructure, resulting in the lowest hardness in the solution treated condition. By aging at 1223 K, the Ni solid solution in the Re added alloy decomposed to Ni3Al and Ni3V, accompanied by precipitates of a Re-rich phase. Consequently, the hardness rapidly increased with increasing aging time. Simultaneous addition of Ta and Re induced very fine precipitates of a Re-rich phase after aging, and consequently resulted in a higher hardness than by the addition of Ta or Re alone.

The effects of Al, Cr and Sn on segregation, microstructure, phase stability and hardness of Nb-24Ti-18Si-5X (X = Al, Cr, Sn, at%) alloys were studied. The microstructure of the as cast alloys with Cr, Al and Sn respectively contained (Nb,Ti)ss, Nb3Si, αNb5Si3 and C14-NbCr2 Laves, (Nb,Ti)ss and βNb5Si3 and (Nb,Ti)ss, Nb3Sn and Nb5Si3. The microstructures of the heat treated alloys with Al and Cr (1500 oC/100 h) contained (Nb,Ti)ss and αNb5Si3 and the alloy with Sn (1200 oC/100 h) contained (Nb,Ti)ss, Nb3Sn and αNb5Si3. Compared with Al and Cr, alloying with Sn enhanced the stability of the as cast microstructure, caused strong macrosegregation of Si and Ti, suppressed the segregation of Ti in the (Nb,Ti)ss that was promoted by Al and Cr, had the strongest effect on the macrohardness of the cast and heat treated alloys and on the vol% of the Nbss. All three alloying additions promoted the transformation of βNb5Si3 to αNb5Si3 during heat treatment and decreased the hardness of Nb5Si3 in the as cast alloys with Sn having the strongest effect and Al the weakest. After the heat treatment the hardness of Nb5Si3 increased in the alloys containing Cr and Sn and decreased in the Al containing alloy with Cr having the strongest effect.

The refractory element Ta was added to L12-type Ni3(Si,Ti) intermetallic alloys in order to substitute for Ti. The microstructure and the mechanical properties of the alloys were investigated as a function of the Ta content. All the alloys were doped with 50 wt.ppm boron to suppress intergranular fracture. The alloys containing up to 5 at.% Ta showed an L12 single-phase microstructure, while the alloys containing more than 5 at.% Ta exhibited a two-phase microstructure consisting of Ni3Ta particles in the L12 matrix. At room-temperature (RT), the hardness of the alloys with the L12 single-phase microstructure increased with increasing Ta content due to solid solution hardening of Ta. RT yield stress (0.2% yield strength) and tensile ultimate strength of the alloys with the L12 single-phase microstructure increased with increasing Ta content keeping a high level of tensile elongation. At high-temperature (HT), the positive temperature dependence of hardness was observed in all the alloys, irrespective of Ta addition. HT hardness was also enhanced by the addition of Ta. It was consequently demonstrated that Ta is a remarkable solid solution hardening element in the Ni3(Si,Ti) alloys.

Iron aluminides show many interesting properties, but still show relatively poor ductility at room temperature and only moderate creep resistance at temperatures above about 600ºC. Processes of severe plastic deformation have been investigated for a wide range of ductile alloys over the past decade, but have hardly been considered for intermetallics. This presentation discusses two studies aimed at refining microstructure by the use of severe plastic deformation of iron aluminides. The first considers processing Fe3Al by heavy cold rolling, followed by annealing for recovery or recrystallization, with an objective of refining grain size to improve strength at the same time as ductility. The high strength and poor ductility of the work hardened material leads to a danger of cracking during rolling, which is a problem for manufacturing large quantities of healthy material. Suitable rolling and recovery treatments can, nevertheless, lead to strong materials with some plastic ductility. A different technique of multidirectional, high-strain and high-temperature forging applied to a boride-containing Fe3Al alloy produces a material with large grain size and refined dispersion of boride particles. These particles lead to a considerable increase in creep strength under conditions of moderate stresses at temperatures around 700ºC. This high-strain forging technique can be seen as an intermediate processing method between conventional wrought metallurgy and mechanical-alloying powder metallurgy. This technique offers the possibility to improve high temperature behaviour of such intermetallics containing second-phase dispersions, and can be scaled to produce large quantities of high-quality material.

Dual two-phase intermetallic alloys based on the Ni3Al-Ni3V pseudo-binary alloy system have been reported to display high phase and microstructure stabilities and good mechanical properties at high temperature and are therefore considered to be used as a next generation type of high temperature structural materials. The microstructure of the dual two-phase intermetallic alloys is composed of primary Ni3Al and the channel (eutectoid) regions consisting of Ni3Al+Ni3V. In this study, the microstructure of the channel regions was investigated by a transmission electron microscope (TEM). The contrasts of the channel regions showed a complicated microstructure in bright-field images. However, the electron beam diffraction consisted of a single set of patterns and the spots did not accompany streaks, indicating that crystallographic coherency among the constituent phases or the domains is very high. It was also shown that the lattice misfit between the a-axis of Ni3Al and the c-axis of Ni3V is larger than that between the a-axis of Ni3Al and the a-axis of Ni3V. From the dark-field observation, it was found that the c-axis of Ni3V domains in the channel regions is oriented perpendicular to the interface between primary Ni3Al and Ni3V. Therefore, it is suggested that the crystallographic orientation of Ni3V in the channel regions is aligned in the manner of lowering an internal stress caused by the lattice misfit between primary Ni3Al precipitates and Ni3V domains.

The aim of this study is two-fold: first we reexamine the thermodynamic stability of γ’-Co3(Al,W) phase in the Co-Al-W ternary system. Secondly, we investigate the effect of a fourth alloying element (Ti or Ta) on the thermodynamic stability of the γ’ phase through microstructure observation, DSC measurement and EPMA analysis. Coarsened areas with γ/CoAl/Co3W phases are formed after annealing at 900 ºC for 2000 h in Co-Al-W ternary alloys with different Al/W ratios, which confirms that the three phases are in equilibrium with each other and that the γ’ phase is metastable at this temperature. The addition of a fourth alloying elements does not drastically change the microstructure formed after long term annealing at 900 ºC, indicating that the alloying elements do not improve the stability of the γ’ phase.

Al-rich Ti-Al alloys attracted some attention during the past years due to the possibility of their application as light-weight, high-performance materials at elevated temperatures. The effect of the addition of Nb to Al-rich Ti-Al alloys has been studied for Ti36 Al62 Nb2 by a combined approach of transmission electron microscopy (TEM) techniques for unraveling the structure and composition at the nanoscale. Structural analyses on as-cast ternary alloys revealed the presence of h-TiAl2-, Ti3Al5- and γ-TiAl-type phases. After heat treatment, phase transformations like the replacement of the metastable h-TiAl2-type by the stable r-TiAl2-type were identified. Additionally, changes of the microstructural features like the formation of interfaces with different orientation relationships are apparent. The orientation and interfacial relationships involved are compared to those of binary Ti-Al alloys rich in Al.

The martensitic microstructure of shape memory alloys is an aggregate of self accommodating plate groups. The principal character of this aggregate is its demonstration of pseudoplasticity, wherein a macroscopic shape change is brought about by extensive rearrangement and reorientation of the self accommodating martensite variants, and that of microstructural reversibility, wherein the polyvariant microstructure transforms back to the original grain of the parent phase after pseudoplastic deformation. These aspects of the shape memory intermetallic alloys are intriguing and, to a great extent, unsolved. The aim of this paper is to show that the underlying crystallographic interrelationships of the self accommodating microstructure of intermetallic alloys are responsible for the observed effect. The paper will discuss the relation between autocatalytic nucleation and self accommodation, the relation between microstructural reversibility and intervariant interfaces of the martensitic microstructure and the manifestations of microstructural irreversibility using results from the microstructural examination of the self accommodating microstructures.

Mo-base silicide alloys take advantage of their outstanding intrinsic properties, notably the high melting point and, thus, their excellent mechanical and creep strength. We demonstrate how the processing route influences the microstructure and consequently the mechanical and oxidation behaviour. Therefore two fabrication routes, a powder metallurgical (PM) and a zone melting (ZM) process, both starting from elemental powders, were used to prepare several Mo-Si-B alloys with varying chemical compositions. While PM processing leads to an ultrafine microstructure with a continuous Mo solid solution (“α-Mo”) matrix and embedded particles of the two intermetallic compounds Mo3Si and Mo5SiB2, the directionally solidified (ZM) materials possess a coarse grained structure composed of an intermetallic matrix with dendritic islands of α-Mo. A comparative assessment of the mechanical behaviour of the alloys utilizing both the Vickers indentation fracture (VIF) technique and three-point bending tests emphasizes the beneficial effect of a continuous Mo matrix resulting in increased room temperature fracture toughness and a reduction of the brittle-to-ductile-transition-temperature (BDTT). Likewise, the positive effect of the fine grained and homogeneous microstructure on oxidation performance is shown by the evaluation of mass change during heat treatment at 1100°C.

The effects of solute and vacancy segregation on APB migration in Ti3Al, and their dependence on composition, have been investigated by using a phase-field simulation in which vacancy distribution is taken into account. Al-atoms are depleted and vacancies segregate at APB in stoichiometric Ti3Al (Ti-25Al), whereas Al-atoms segregate and vacancies are depleted in Alrich one (Ti-28Al). The simulation indicates that APB in Ti3Al migrates much faster in Ti-25Al than in Ti-28Al with the effect of vacancy segregation whereas it migrates slightly faster in Ti-28Al than in Ti-25Al in the absence of the effect of vacancy segregation.

The effects of silanising using the coupling agent γ-glycidoxpropyltrimethoxysilane on microstructural stability and magnetic properties of Sm-Co powder particles have been investigated. The silanisation provides structural stability by improving the oxidation resistance at 400oC for 10 hours. The untreated particles undergo microchemical changes by redistribution of alloying elements which mainly accumulate in parallel black and grey streaks in the interior of the particles. The silanised particles after heat treatment show coercivity of 836 Oe and the untreated particles show a much lower coercivity of 376 Oe. The difference in magnetic properties of uncoated particles is caused by diffusion of oxygen and microstructural instability.

One of the main driving force for the development of advanced structural materials is weight saving especially in the transportation industry in order to reduce CO2 emission. The utilization of gamma aluminides, as good candidates for aerospace applications, is strongly related to the development of a cost-effective and robust processing route, as far as possible. It is well established that the processing route, i.e. cast, wrought or PM, has a dramatic effect on the microstructure and texture of gamma-TiAl alloys. Therefore, significant microstructural variations through post-heat treatments coupled with compositional modifications can only guarantee a proper balance of desired properties. However, a number of metallurgical factors during the processing steps can contribute to some scattering in properties. This review will highlight several critical process variables in terms of the resulting g-TiAl microstructures. Of primary importance is the as-cast texture which is difficult to control and may contribute to prefer some alternative processing routes to ensure a better repeatability in mechanical results. Some innovative processing techniques for controlling the structure will then be presented. The main point which will be discussed in this paper is whether an approach leading to a robust process would not be at the expense of the high performance of the structural material.

Elastic properties, thermal expansion and deformation behavior of Cr5Si3 with the D8m structure were investigated using single crystals. From the values of Cauchy pressures as well as the ratio of the polycrystalline bulk modulus (B) to shear moduls (G) estimated from single-crystal elastic constants (cij), deformation behavior of Cr5Si3 is expected to be relatively brittle compared to Mo5Si3 with the same crystal structure. However, plastic deformation of Cr5Si3 is confirmed above 900 ~ 1100 °C depending on the loading axis orientations.

The effect of grain boundary (GB) and matrix precipitates on high temperature strength was investigated in Fe3Al base alloys containing Cr, Mo and C. Tensile tests were conducted at 600°C for three types of microstructures consisting of: (I) film-like κ phase precipitates covering GBs and fine M2C particles in the matrix, (II) only fine M2C particles in the matrix and (III) no second-phase particles in the matrix. It was found that κ films on GBs are more than twice as effective as finely dispersed M2C particles for improving the proof stress.

In this study, the AC magnetic permeability of polycrystalline Fe81Ga19 alloy (Galfenol), without crystal orientation under both the bias applied fields and frequencies, was investigated by the method of measuring inductance of ring specimens. The results showed that the AC permeability of the alloy can reach more than 160 Gs/Oe under the conditions of low frequencies or quasi-static state. The permeability decreased with the increase of frequency. When the frequency was higher than 6 kHz, the permeability decreased slowly, and gradually stabilized with the increase of frequency. When applying a little of parallel bias magnetic field, the permeability decreases obviously with the increase of frequency. But applying a perpendicular bias magnetic fields, the permeability of the only initial point of the measuring frequency decreases a little compared to the permeability without bias field.

The site preference of ternary additions in GCP (geometrically close-packed) Ni3X-type compounds with D0a structure was determined from the direction of the single-phase region of the D0a phase in the reported ternary phase diagrams. The thermodynamic model based on the Bragg-Williams approximation, which is based on the change in heat of formation of the host compound by a small addition of ternary solute, was applied to predict the site preference of ternary additions. The heat of formation used in the thermodynamic calculation was derived from Miedema’s formula. Good agreement was obtained between the thermodynamic model and the result of the literature search.

Plane wave ab initio density functional theory (DFT) calculations of the B2 NiTi (100), (110), and (111) surfaces, the B2 and B19´ phases of NiTi, and the supercell structures of NiTi, Ni4Ti3 and Ni3Ti are reported. Electronic energies from the electronic structure calculations are used to assess relative stability of the different surface and supercell geometries.

In order to understand the deformation and fracture behavior of Nb-Si alloys, in-situ observation was conducted during bending of small specimens at room and high temperatures. Nb-Si alloy ingots containing 18.1 at.%Si, 1.5 at.%Zr and 100 ppmMg were prepared by arc melting, followed by uni-axial solidification in an optical floating zone apparatus and a heat-treatment to obtain Nb/Nb5Si3 two-phase microstructure. Chevron-notched specimens with a dimension of 1x2x10mm were used for in-situ observation of bending tests under a confocal laser scanning microscope (CLSM) at room temperature and at 1140 °C. At room temperature the Nb-Si alloy shows a fracture toughness of 8 MPa m1/2 and the crack propagation velocity seems to be not uniform, presumably due to the ductile Nb. At 1140 °C the toughness of the alloy was about 20 MPa m1/2 and slower plastic deformation prior to the cracking was observed. The SEM observation of crack surfaces revealed that plastic deformation of Nb enhances the toughness of the alloy.

A Mg-Zn-Y alloy including a Mg12ZnY intermetallic compound exhibits excellent mechanical properties as compared to conventional magnesium alloys. The superior mechanical properties of this alloy seem to originate from the Mg12ZnY intermetallic compound; however, the mechanical properties of Mg12ZnY itself have not yet been fully investigated owing to the small size of this compound. In this study, a microfracture test was performed to investigate the fracture properties of the Mg12ZnY intermetallic compound. The material used in this test was a Mg88Zn5Y7 alloy. Micro-sized cantilever specimens composed of Mg12ZnY, with dimensions of 10 × 20 × 50 μm3, were prepared selectively isolated from the Mg88Zn5Y7 alloy using focused ion beam (FIB) machining. Notches with a width of 0.5 μm and a depth of 5 μm were also introduced into the micro-sized specimens. Microfracture tests were performed using a mechanical testing machine for microscale materials. The fracture toughness values (KQ) of Mg12ZnY were 1.2−3.0 MPam1/2. TEM observations indicated that the KQ values were dependent on the crack orientation in Mg12ZnY, with the higher KQ values correlating with cracks propagating parallel to the c-axis of Mg12ZnY. This suggests that the fracture toughness of Mg-Zn-Y alloys can be improved by controlling the orientation of the Mg12ZnY compound.