Crack propagation process in TiAl alloy PST crystal has been investigated by in situ straining TEM observation. It has been found that the influence of lamellar boundaries on the fracture behaviors is connected with the types of interfaces (twin, pseudo-twin and α2/γ interfaces). Even though secondary microcracks always nucleate at α2/γ interfaces due to stress concentration, the crack growth is cleavage in α2 and γ phases rather than along interlamellae. When a crack meet γ/γ interfaces, its propagation can be inhibited and crack tips blunt by the slide of the interfaces. It also deserves to be mentioned that ductility of PST crystals has connection with the rate of crack propagation..

The grain size effect on the creep behavior of hot-pressed monolithic molybdenum disilicide was investigated at 1200°C in a 19–255 MPa stress range. The creep-stress exponent, n, increased from 1 at low stresses to 4 at high stresses. The grain size exponent, p, varied from 0, to 3.5 and to 8 depending on the grain size, the creep-stress exponent, and creep history.

The zirconium silicide intermetallic compounds of 16H crystal structure are good materials for high temperature structural applications because of their high melting point and low density. Their shortcoming is low ductility. To increase ductility, new ternary zirconium silicide intermetallic compound of 16H crystal structure, Si3Y3Zr2, has been designed by substituting yttrium atoms into zirconium 6g point set sites.

Near-neighbor diagrams are used to examine the bonds in Si3Y3Zr2, whose bonds are more symmetric than other 16H binary intermetallic compounds.

Melting points of intermetallic compounds of 16H crystal structure depend upon the strain parameters of N4d-N4d(2-2) bonds. The melting point of Si3Y3Zr2 is estimated to be higher than that of Si3Zr5 (2500 K). It is predicted that Si3Y3Zr2 is a good material for high temperature structural applications.

The site preference of Al and elements in the 3rd row of the periodic table in Ni3Si was studied in a systematic way by using electronic structure calculations based on the local density approximation. Alloying elements in this intermetallic may occupy exclusively the Ni or Si sublattices, or may exhibit no particular site preference. By performing the calculations both for spin-polarized and non-spin-polarized cases, it was found that magnetism affects the computed site substitution behavior of Mn, Fe, and Co in Ni3Si. In the case of Fe, e.g., a preference for the Ni sublattice is computed when magnetic effects are ignored, but when magnetic effects are accounted for it is found that Fe has no significant site preference Specific trends regarding the site preference across the transition metal series are shown and discussed.

Two phase alloys based on the intermetallic system Nb5Si3/Nb system provide potentially superior combination of strength and toughness over their monolithic counterparts at high temperatures. To gain fundamental knowledge of the mechanical behavior of these materials, initial focus has been on the mechanical behavior of Nb5Si3/Nb laminates and composites at elevated temperatures. In this study, laminates have been fabricated at 32 at% Si with Nb, and a total laminate thickness of 2.1mm. Tests are being conducted on notched and unnotched tensile specimens. The unnotched specimens provide the overall strength and ductility of the laminate and a basis for comparison with notched specimens which provide information on crack propagation and ductile phase toughening effects. Results from samples with a variety of laminate spacings are being explored, and preliminary data will be presented.

Following a previous study in which we presented some microstructural aspects of meltspun ribbons having a composition close to Zr-25 at.% Al, we discuss now the crystallography of the phases observed in similar samples. We performed X-Ray diffraction analyses of ribbons and refined the observed crystallographic structures. We could identify a number of stable and metastable structures, according to the actual composition of the ribbons. We also estimated the percentage of each one of these phases. We did the same for some ribbons annealed at 750°C for several times. In this way we could follow the kinetics leading from the initial as-spun condition to the final one, featuring the L12 ordered Zr3Al, as the major phase, and other intermetallic phases of the Zr-Al phase diagram.

Since the L12 structured Al3Ti alloy exists only in a narrow compositional range, further alloying of the single phase Ll2 alloy to improve its property seems hardly successful. Developing two-phase or multiphase Al3Ti alloys may be an effective approach for strengthening and toughening. In this article, a new type of Al3Ti-based alloy which has a Ll2 matrix with precipitates of a second phase is reported. The quaternary alloys based on Al67Mn8Ti25, and modified with Nb additions, consist of an Ll2 matrix and DO22 second phase particles in the annealed state, but the second phase can be dissolved by solution treatment and precipitated during high temperature aging. Remarkable strenghtening and promising compressive ductility were exhibited by the experimental alloy. The influence of composition on the microstructure and properties of the alloys are reported also.

The electronic structure and total energy of the C15 NbCr2 phase have been calculated using the linear muffin-tin orbital (LMTO) method with the atomic sphere approximation (ASA). The total energy vs. volume curve, band structure, density of states and Fermi surface were obtained. The calculated results were used to examine several features of the C15 phase, including the elastic properties, phase formation and stability, and solubility range of the C15 phase. The theoretical results are compared to experimental studies on NbCr2 For example, the elastic moduli, phase stability, and homogeneity range have been determined with a variety of experimental techniques. Comparison of the experimental and theoretical results will be discussed.

The electronic structure and high pressure structural phase transition in thorium antimonide have been investigated using the tight binding LMTO method. We have calculated the total energies by reducing the cell volume for NaCl as well as CsCl structures using TBLMTO method. The total energy calculations reveal that ThSb undergoes a structural transition from NaCl to CsCl structure at 78 kbar. The calculated value of equilibrium cell volume and the cell volume at which phase transition occurs are found to have a good agreement with the experimental results.

In combustion synthesis, and in simultaneous reaction synthesis and hot pressing of Mo + 2 Si powders, a heterogeneous reaction occurs between liquid silicon and molybdenum powder. When the reactant mixture is heated at low-heating rates, Mo5Si3 is observed as a secondary phase along with MoSi2. Application of pressure during the reaction synthesis of MoSi2 eliminates the need to hot press a prealloyed powder. The technique can be applied to obtain composites and alloys of MoSi2 and various other transition metal silicides.

Nd2Fe14B is currently the best (highest energy product) hard magnet available. Like most magnetic materials, it is rather brittle. The fracture toughness of specimens of commercially available Nd2Fe14B magnets were measured using a chevron notch three point bend test. Values ranged from nearly 2 to 5.5 MPa√m. Consistent differences were found between the five different types of materials tested. In this initial survey, insufficient tests were done for good statistics to be obtained. However, it appears that this test will allow differences in fracture toughness between alloys with different compositions and fabricated by different production methods to be measured and tracked as improvements are made.

The site preferences of Ta, Mo, and W in A15 Nb3Al, the phase stability of off-stoichiometric A15 Nb3Al, and the atomic order of the Nb2Al D8b (σ) phase are investigated by performing electronic structure calculations based on the local density approximation. Mo, W, and Ta are found to have a preference for the Nb sublattice. Ta has the strongest site preference. It was shown that some recently proposed ground states for off-stoichiometric A15 phases are unlikely to occur in the Nb-Al system. Moreover, it was shown that the atomic order in the D8b Nb2Al phase contributes significantly to its stability.

A disadvantage to the application of Ti3Al is its poor high temperature oxidation resistance. It is found that the element Nb or(and) Si can greatly reduce the oxidation rate of Ti3A1. A hot stage in situ X-ray diffractometer was used to determine the formation sequence of the oxide layers. At 800°C, TiO2 as well as Al2O3 was detected on the surface of binary Ti3Al at the beginning of the oxidation process. The addition of 5 at % Si to Ti3Al alloy did not favor the formation of an A12O3 layer. On the contrary, it inhibited the onset of Al2O3 to nearly 20 hours from the start of the oxidation process at the temperature of 800°C. The Nb addition also did not promote the formation of Al2O3. TiO2 formed first on the surface of Ti3Al-11 at % Nb alloy while TiN and TiAl formed consequently. Al2O3 was further delayed to 20 hours from the beginning of the oxidation process. When the Nb addition increased to 15 at %, however, TiN and TiAl were not found. It is suggested that the favorable effect of Nb and Si to the oxidation resistance of Ti3Al alloy is not due to their promotion of A12O3 layer, but probably due to some other mechanisms, such as the formation of TiN which serves as a diffusion barrier and decreases porosity in the TiO2 layer.

The strength of the lightweight Mg(Al) alloys with Al concentration less than 10 wt% is due to controlled precipitation of the intermetallic phase Mg17Al12. This phase and its solid solution host have been characterised by 25Mg and 27Al NMR in terms of Knight shifts and nuclear quadrupole interactions for the distinguishable Mg and Al sites.

The recent progress made in our understanding of the phenomenology and mechanisms of environmental embrittlement in ordered intermetallics is reviewed by considering two model alloy systems of the L12 and B2 crystal classes (Ni3Al and FeAl). The poor ductility commonly encountered when these alloys are tensile tested in ambient air is due mainly to environmental embrittlement, in the absence of which, both alloys are now known to be quite ductile. Both H2O and H2, at levels found in ordinary ambient air, are found to cause environmental embrittlement, with the former usually more deleterious. In the case of H2O, the micromechanism involves reaction with the intermetallic to form an oxide (or hydroxide) and simultaneous generation of atomic hydrogen which then enters the metal and causes embrittlement. In the case of H2, on the other hand, atomic hydrogen is generated as a result of the dissociation of physisorbed hydrogen molecules on the intermetallic surfaces. Consistent with the proposed embrittlement mechanism, ductility is found to increase with decreasing amounts of H2O (or H2) in the test environment, increasing strain rate, and decreasing (or increasing) temperature. Environmental embrittlement in Ni3Al (and other L12 alloys) occurs predominantly intergranularly, whereas in FeAl (and other B2 alloys) it can also occur transgranularly—presumably because diffusion of hydrogen is fast enough through the bulk in the more open B2 structure but only so along grain boundaries in the L12 structure. Microalloying with B, which segregates strongly to the grain boundaries, can overcome environmental embrittlement in L12 alloys, but not in B2 alloys; in the latter, alloying additions probably have to be added at significantly higher (macroalloy) levels to affect the bulk properties. In neither alloy is environmental embrittlement the sole source of brittleness: depending on the alloy stoichiometry, and grain boundary character, a given grain boundary may be intrinsically weaker (or stronger) than the bulk, thereby influencing overall ductility.

A minute amount of boron doping in polycrystalline Ni3Al can suppress embrit-tlement due to environmental moisture. However, B is ineffective in suppressing hydrogen embrittlement due to cathodic charging. A mechanism is proposed to explain this seemingly contradictory dichotomy. Grain boundaries in B-free Ni3Al contain crack-like microcavities, to the tips of which atomic hydrogen, generated by the reaction of moisture with Al, can diffuse and cause embrittlement. In B-doped Ni3Al interstitial B atoms interact with Ni atoms and reduce the strength of directional bonding between Ni and Al atoms, such that the atoms can relax easily to close up the microcavities, thereby reducing environmental embrittlement. In the presence of a large amount of hydrogen, introduced by cathodic charging, microcracks can be nucleated in B-doped Ni3Al by hydrogen-enhanced dislocation activity, and then hydrogen embrittlement can proceed by enhanced plasticity at the crack tips.

Tensile tests were carried out on B-doped and undoped Ni3Al having different hydrogen contents in order to examine whether the amount of impurity hydrogen affects the ductility of Ni3Al. Specimens were melted either in a high vacuum of ∼10−3Pa or argon, isothermally forged and finally annealed for 15hr at 430°C in an ultra high vacuum of ∼10−3Pa or in argon, respectively. Measurement of hydrogen gas evolved from the specimen during the annealing at 430°C in an ultra high vacuum of ∼10−7Pa confirmed that vacuum treated specimen had actually smaller hydrogen content than the argon treated one. The ductility of vacuum treated specimens both B-doped and undoped was found to be larger than that of argon treated ones, which means a detrimental influence of hydrogen. Hydrogen evolution behavior during the test on B-dopcd specimens in an ultra high vacuum of ∼10−8Pa revealed that the amount of hydrogen gas evolved at the moment of fracture was smaller in vacuum treated specimens than in argon treated one. Impurity hydrogen atoms were considered to move and enhance the formation and growth of voids, accelerating transgranular fracture.

The effect of additions of chromium, manganese and iron on environmental embrittlement of the Ni3(Si,Ti) alloy is investigated by room temperature tensile tests in various kind of atmospheres (vacuum, air, distilled water and H2 gas) and at various strain rates. The observed tensile elongation and the associated fracture mode are very much dependent on atmospheres and strain rates. It is shown that additions of these transition elements to the Ni3(Si,Ti) alloy are effective in reducing the embrittlement particularly in air and distilled water, and their magnitude of reducing the embrittlement decreased in the order, chromium, manganese and iron. However, additions of these transition elements are only slightly effective in reducing the embrittlement in H2 gas. The beneficial effect of additions of these transition elements on environmental embrittlement of the Ni3(Si,Ti) alloy is discussed, based on some possible mechanisms.

Three Fe-Al alloys, FAP-Y, FA-129, and Fe-35a%Al, containing 16, 28, and 35a%Al, respectively, have been subjected to fatigue crack growth testing in moist air, in oxygen, and in gaseous hydrogen. In each case hydrogen and air were embrittling. Crack growth rates increased significantly as frequency decreased. Fatigue crack growth results have been compared with those for other structural iron-base alloys. Surprisingly, FAP-Y displays the highest crack growth rate of any alloy examined, except at very low levels of stress intensity range. The mechanisms for embrittlement by hydrogen and by moisture in air are discussed.