This paper investigates the fracture behavior of FeAl and Ni3AI matrix composites with ceramic continuous fibers 8.5–10 μm in diameter. When stress is applied to these composites, multiple-fracture of fibers predominately occurs before matrix cracking, because the load carried by the fibers reaches their fracture strength. Fragments which remain longer than the critical length can provide significant strengthening through load bearing even though fiber breaking has occurred. The ultimate fracture strength of the composites also depends on stress relaxation by plastic deformation of the matrix at a crack tip in the multiple-fractured fibers. Ductilizing of the matrix by B doping improves the ultimate strength at ambient temperatures in both composites. However, their mechanical properties at elevated temperatures are quite different. In the case of Ni3AI matrix composites, embrittlement of the matrix is undesirable for high strength and reliability at 873–973 K.

Transmission electron microscopy (TEM) studies of dislocation structures in single crystals of TiAl containing 54.5 or 54.7 at% Al deformed at different temperatures revealed the occurrence of slip on ½〈112]{ {111} at room temperature and near the peak of the yield stress anomaly (YSA). Measurements of the corresponding yield stresses revealed the existence of a YSA for this type of slip. Weak-beam TEM showed the presence of locks at room temperature for 30° dislocations and at high temperatures for edge dislocations. Both types of locks involve dissociation on two intersecting { 1111} type planes, driven by reduction in elastic strain energy. The edge dislocation dissociation at high temperatures involves both climb and glide.

NiAl alloys offer significant advantages as structral materials in gas turbine applications due to a unique range of physical and mechanical properties. The high melting point and low RT plasticity of NiAl alloys present special problems for mold materials. Typical shell molds for advanced superalloys are very close to there application limits. On the one hand it is necessary to keep the SiO2 level as low as possible, but on the other hand the mold has to be strong enough for the cast process and weak enough to allow for the contraction of the metal during the solidification. The result of this research is the application of a SiO2 shell mold for a heat shield. To scale up these results to production and quantities will be the further work.

In order to study the deformation processes of reaction-sintered and hot pressed MoSi2, compression tests have been performed and the microstructure of the deformed samples has been investigated by optical, scanning and transmission electron microscopy. At a low strain rate of 2.5× 10−7 S−1 permanent deformation has been achieved at 1250°C. The deformation behaviour is controlled by different microprocesses within different temperature ranges. Below 1000°C, hardening occurs beyond the upper and lower yield points. Inside the grains, dislocations glide at sufficiently high stresses in a homogeneous way as well as in localized slip bands. A plateau in the dependence of the yield stress on temperaturecorresponds to the flow stress anomaly of easy slip systems in single crystals. However, since there are not enough independent slip systems to satisfy the van Mises criterion, the material does not deform homogeneously in all grains. Intergranular and intragranular cracks, which are formed after the yield point, carry an increasing part of strain at decreasing temperatures. Above 1000°C, softening occurs after the yield point. A grain boundary phase flows viscously and carries most of the deformation. Since the grains do not deform themselves, intergranular cracks develop also at high temperatures.

The microstructures of as cast and air quenched Ti48Al48Cr2Nb2 alloys are analysed by transmission electron microscopy. Statistical analyses on the orientation variants and on the orientation relationships at the interfaces are presented. The results are used to have a better understanding on the formation mechanism of the lamellar microstructure.

Compression tests were performed on specially-oriented PST TiAl crystals to which strain gages were attached in order to precisely measure the three axial strains in the samples during deformation. It was found that all samples deform by plane strain except when the compression axis is nearly perpendicular to the lamellar planes. This measurement technique was used to determine the orientations in which the various slip systems are operative in PST samples, and this, in turn was employed to explain the shape of the flow stress vs. orientation curve. An important component of this explanation is the fact that slip of ordinary dislocations and twinning are closely coupled in this material, implying that the twinning must occur relatively slowly-at about the same rate that ordinary dislocations move. Atomistic calculations have shown that a slow twinning process is to be expected in this material at low temperatures, perhaps even by a thermally activated process.

A detailed understanding of the structure of ordered intermetallic systems is difficult at best, causing a serious disconnect in the typical process-structure-properties approach to alloy development. Basic information, like the site substitution schemes of various alloying elements, partitioning behavior in multiphase alloys, and the dependence of these phenomena with concentration and higher order alloying additions is necessary to predict and understand the effect of various alloying schemes on the physical and mechanical properties of a material. It is only recently that theoretical methods can begin to provide useful insight in these areas, as most current techniques suffer from strong limitations including the type and number of elements that can be considered and the crystallographic structure of the resulting phases. The Bozzolo-Ferrante-Smith (BFS) method for alloys was designed to overcome these limitations, with the intent of providing a useful tool for the theoretical prediction of fundamental properties and the structure of multi-component systems. The role or potential contributions of theoretical procedures like the BFS method to the alloy design process are discussed with a specific emphasis on work that has been conducted on NiAl-based alloys. After a brief description of the method and its range of applications, we will concentrate on the usefulness of BFS as an alloy design tool. The theoretical determination of site substitution schemes for individual as well as collective alloying additions to NiAl, the resulting behavior with respect to solubility limits and second phase formation, and the concentration dependence of the lattice parameter will be demonstrated.

The relationship between microhardness and grain size was investigated for nanocrystalline and coarse-grained stoichiometric intermetallic NiAl. The nanocrystalline NiAl specimens were synthesized through mechanical alloying with a high-energy Spex 8000 shaker mill and consolidated by shock compaction at a peak pressure of 4–6 GPa, to 83% dense compacts. The nanocrystalline NiAl compacts were also sintered at 1073, 1173 and 1473 K for 2 h. The Vickers hardness of consolidated and sintered NiAl was determined by microhardness testing, and the grain size and microstructure were investigated with transmission electron microscopy. It was found that the hardness values increased with decreasing grain size of the NiAl alloy. The Vickers hardness values were approximately 650±16, 690±6, 800±43 HV, respectively for NiAl with grain-sizes corresponding to approximately 27±18, 11±6 and 9±6 nm. The possible strengthening mechanisms operating in NiAl are discussed.

Temperature dependence of yield stress and operative slip system in Ni3Nb single crystals with the DOa structure was investigated in comparison with that in an analogous L12 structure. Compression tests were performed at temperatures between 20 °C and 1200 °C for specimens with loading axes perpendicular to (110), (331) and (270).

(010)[100] slip was operative for three orientations, while (010)[001] slip for (331) and {211} <10 7 13> twin for (270) orientations were observed, depending on deformation temperature. The critical resolved shear stress (CRSS) for the (010)[100] slip anomaly increased with increasing temperature showing a maximum peak between 400 °C and 800 °C depending on crystal orientation. The CRSS showed orientation dependence and no significant strain rate dependence in the temperature range for anomalous strengthening. The [100] dislocations with a screw character were aligned on the straight when the anomalous strengthening occurred. The anomalous strengthening mechanism for (010)[100] slip in Ni3Nb single crystals is discussed on the basis of a cross slip model which has been widely accepted for some L12-type compounds.

The influence of the fully lamellar morphology and third phase β on the creep properties of near γ-TiAl intermetallics is presented. Specifically, the effect of improved microstructural control obtainableby a stepped cool, involving furnace cooling and air cooling from the a single phase, on creep resistanceis demonstrated for three near γ-TiAl intermetallics: binary Ti-48AI, ternary Ti-48A1–2W and Ti-47A1–2Nb-IMn-0.5W-0.5Mo-0.2Si. The results indicate that appropriate stepped cooling can be used to reducethe lamellar interface spacing without the formation of Widmanstatten, feathery γ or γM structures, leadingto longer creep life and reduced creep strain rates. A second benefit of stepped cooling is preventionof βformation during cooling from the α phase, allowing controlled β precipitation during aging at 950 C. Creep tests on variously aged Ti-48A1–2W indicate that β precipitation along lamellar grain boundariesimproves creep resistance. Development of a uniform fully lamellar structure in Ti-47A1–2Nb-1Mn-0.5W-0.5Mo-0.2Si significantly improves creep resistance. Applying the stepped cool to this alloy allowsthe precipitation of β and silicides to be controlled during lower temperature aging.

The bimetallic valves were produced using a new technique, IFFM (Impact Fused-Forging Modeling). Distribution of the alloy components as well as phase composition across the valve section was obtained by means of energy-dispersive analysis and X-ray diffraction. Optic and electron microscopy research of macro- and microstructure was conducted. High quality of the joint between the sintermetallic (TiAl) head and the titanium (VT-9 and VT-6 alloys) stem was demonstrated. As a result of the contact of the stem with the semi-solid intermetallics at the temperature above Tc, a polymorphous transformation was obtained in the upper part of the Ti-based stem. The globular microstructure in the intermetallic head in the area near to the joint of the two alloys was achieved. The results proved that the process allows production of complex shaped parts based on advanced alloys of high quality, featuring extra wear-resistance and strength.

Ir-based alloys with fcc-L12 two-phase coherent structure, which are called “refractory superalloys”, have good potentiality as structure materials used at ultra-high temperatures up to 2000 °C. Preliminary results showed that the refractory superalloys failed predominately by brittle intergranular fracture at room temperature even though they showed higher strength at that temperature. This paper will present the influence of nickel (Ni) addition on mechanical properties and fracture behaviors of one of these alloys, Ir-15at%Nb. The results indicated that Ni addition was beneficial to the compression ductility and strength of two-phase Ir-15at%Nb alloy when Ni content was below the optimum content.

The operative slip systems in the so-called PST alloy are determined by transmission electron microscopy. Both post-mortem analyses and in situ observations are presented. It is shown that glide of ordinary dislocations and twinning are the most easily deformation modes activated at yield. The role of the interfaces on the activation of these operative slip systems is then examined and discussed.

Nb addition to Ti3Al has been found to be effective to improve its oxidation resistance. Recently, the beneficial effect of nitrogen in the oxidation behaviour of Nb contained T3AI-based alloys has been noticed. In this paper, the oxidation behaviour of five alloys with different Nb content from 0 to 20 at. % exposed in room air at different temperatures (from 600 to 950,C) has been examined with the focus on the TiN formation in the oxide scale. No TiN is detected in the scales of binary Ti3Al under all temperatures tested. On the contrary, in alloys containing Nb, TiN is identified in the scales irrespective of the oxidation temperature. The amount of TiN formation differs with differentNb content. EPMA analysis of the cross section of the scales is performed and discussion is made accordingly.

A modified electrolytic etchant has been successfully used to observe dislocation etch-pits on {100} surfaces of single crystal Ni3AI containing Hf and B additions. Four-point bending tests have been used to obtain the dependence of dislocation velocity on resolved shear stress at room temperature. A comparison with earlier studies reveal that the rate of change of dislocation velocity with resolved shear stress, to a first approximation, is independent of alloying. Atomic level simulations have been performed using the embedded atom method (EAM) to study dislocation core structures and frictional stress in Ni3Al.

An alloy composed of L12-type Zn3Ti was investigated in terms of phase stability and mechanical properties. Zn and Ti powders were mixed at a composition of Zn-25mol%Ti using a ball mill in Ar, and an ingot was made by melting the powders. Optical microscopy, X-ray diffraction analysis and thermogravimetry - differential thermal analysis were carried out. Mechanical properties were investigated by Vickers hardness tests at room temperature (RT) and compression tests from RT to 703K in Ar. It is found that (1) the alloy is mainly composed of L12 Zn3Ti, (2) the alloy has weak positive temperature dependence of strength, and (3) normalized strength by melting point is comparable to that of L12 Al3Ti-Cr alloys. L12 Zn3Ti has HV178 and is brittle at RT. Reaction temperatures of Zn-rich portion of the Zn-Ti phase diagram were also reinvestigated and a peritectic-reaction temperature between Zn3Ti and liquid + Zn2Ti is determined to be at 880K.

The nature of defects in a quaternary Fe-40A1–0.7C-0.5B alloy was examined by transmissionelectron microscopy (TEM) techniques including weak beam microscopy and z-contrast imaging.Complex {001} stacking faults were observed in certain conditions in these alloys that were not previouslynoted in a ternary Fe-40A1–0.6C alloy. These faults exhibit a 1/2<001> displacement vectorthat lies in the plane of the fault and structurally misses an aluminum layer. Six variants of this faulttype are present. These faults are absent in brine-quenched and furnace-cooled alloys, but are readilyobserved in the air-cooled condition. In addition to the usual <111> superdislocations that are APBdissociatedinto 1/2<111> superpartials, dissociated <100> dislocations (into two 1/2< 100> partials)are also observed under certain conditions. Quenched and aged alloys show dissociated <001> loops.The absence of such defects in the B-free alloy and following certain heat treatments in the B-containingalloy, suggests that boron may be segregating to the cube planes and lowering the interfacialenergy on these planes. The presence of dissociated <001> loops in the quenched and aged alloy alsosuggests the formation of B-vacancy complexes that coalesce during aging, forming such dissociatedloops.

Fe-40A1, Fe-40A1–0.12B and Fe-40A1–5Ti were annealed at a variety of temperatures, water quenched to obtain different vacancy concentrations, and heated at 10 K/min to 973K in a differential scanning calorimeter. Either one or two exothermic peaks were observed, at 533–613K and 743–933K, for all the alloys, while an additional exothermic peak was observed at 653K-693K for Fe-40A1–5Ti. All the peaks shifted to lower temperature with increasing quenching temperature. It is shown that the highest temperature exothermic peak is associated with the annihilation of thermal vacancies. Analysis of this peak as a function of temperature yields vacancy formation enthalpies of 102±15kJ/mol, 104±15 kJ/mol and 81±15 kJ/mol for Fe-40A1, Fe-40A1–0.12B and Fe-40A1–5Ti, respectively. The lower temperature exothermic peaks may arise from vacancy rearrangement.

Room-temperature tensile properties of polycrystalline Ti–;47A1–2Mn–;2Nb alloy with nearly lamellar (NL) microstructures were investigated at the strain rates between 10-;5 and 1000 s-1 using a self-designed Split-Hopkinson tensile bar setup with a rotating disk and conventional testing machine. It was found that tensile ductility varies within a narrow range with the strain rate while dynamic strengths (σd) of the alloy are obviously higher than static strengths (σs). There exists a linear relationship between σs and the logarithm of the strain rate , and between σ d and the strain rate itself (). Fractography analysis indicated that the alloy fractured in a mixed mode of predominant transgranular cleavage and minor intergranular cracking under static and dynamic strain rates. Environmental effect was excluded from the main cause for the room-temperature brittleness of the investigated alloy.

The C15 NbCr2 + V Laves phase ternary system is studied by using a first-principles, self-consistent, full-potential total energy method. Equilibrium lattice parameters, cohesive energies, density of states and formation energies of substitutional defects are calculated. Results of all these calculations show that in the C15 NbCr2 + V compounds, V atoms substitute Cr atoms only.