High-strength, light-weight intermetallic compounds which are ductile at high temperatures are of great technological interest. Purely experimental searches for improved intermetallic materials are time consuming and expensive. Theoretical studies can shorten the experimental work by focusing on candidate compounds which have several of the desired properties. Although current abinitio density functional calculations are not adequate to calculate material properties at high temperatures, it is possible to compute the equation of state and elastic moduli of ordered compounds with several atoms in a unit cell. Known correlations between equilibrium and high temperature properties can then be used to point the way for experiments.

We have demonstrated the power of this approach by applying the linearized augmented plane wave (LAPW) method to calculate the equation of state and elastic moduli for several simple metals (Al, Ca, and Ir), binary cubic intermetallics (SbY, AlCo, AlNi, AlRu, and RuZr), and binary L1o intermetallics (AlTi and IrNb). Most of the calculated equilibrium lattice constants are within 3% of the experimentally observed lattice constants. Although the available experimental information about the elastic moduli is limited by the lack of single crystal data for most of these materials, we are in excellent agreement (within 10%) with the available experimental data, except for the shear modulus of IrNb. We also use known correlations between the elastic moduli and the melting temperature to predict melting temperatures of the intermetallics. In general the agreement there is good agreement between theory and experiment, indicating that we can qualitatively predict melting temperatures.

The solute-dislocation (S-D) interaction energies for various kinds of transition-metal(TM) and B-subgroup solutes in nickel are calculated using a simple tight-binding electronic theory with lattice relaxations taken into account. As a result, the S-D interaction energy for TM solutes is almost identical with that for B-subgroup solutes when compared at an equal level of size-misfit strain( SMS) in Ni. This is neither more nor less than the answer from the classical elastic theory. However, in the solutions of the present calculation there are some points which can not be derived from the continuum elastic theory. Experiments show that Ni3Al is, differently from Ni, more strengthened by TM solutes than by B-subgroup ones. It is discussed that this may be due to the extra contribution of TM solutes to the S-D interaction energy in Ni3Al. As another possibility that TM solutes may contribute differently from B-subgroup ones to the macroscopic property of Ni, the (100) surface segregation of solute atoms in Ni is discussed.

Cobalt-modified NiAl alloys are being studied for their potential for room temperature ductility and toughness. An alloy of Ni - 29.3 a/o Al - 36.7 a/o Co has shown improved toughness and ductility with respect to binary NiAl alloys due in part to a stress-induced martensitic transformation. Furthermore, the cobalt additions have altered the slip behavior to {110}<111> type from {110} <001> for binary NiAl alloys. Hafnium was added to improve the alloy's relatively poor creep resistance and high temperature strength. Hf was found to be insoluble in the NiAlCo alloy and formed precipitates with a hexagonal structure. The Hfmodified alloy had improved high temperature strength. In addition, the Hf apparently changed the creep mechanism resulting in poorer creep resistance at low temperatures, but improved creep resistance at higher stresses and temperatures.

The glide behaviour of superdislocations at a coherent twin boundary in ordered Cu3Au was examined in a transmission electron microscope for the case when the superdislocations are sessile in the boundary. Possible schemes for dissociation of a superdislocation in the boundary were analysed geometrically. The leading superpartial of each superdislocation dissociated into a superpartial in the matrix and a residual Shockley partial in the boundary of glissile type. The trailing superpartial remained undissociated in the boundary. The superpartial in the matrix glided on a cube plane, and a ribbon of APB connected to the boundary was left in its trail. The cube slip occurs as a result of (i) a maximal resolved shear stress for the observed slip system and (ii) the geometric criteria for slip applied to all possible slip systems in the matrix. The Schmid factors for the slip systems in the matrix could be calculated by assuming a uniform tensile axis in the foil. The tensile axis was deduced from the observed slip systems in the twin.

Deformation characteristics of Ti3A1 single crystals has been examined as a function of orientation and temperature. This paper mainly describes the result of compression along the c axis. In this orientation, the yield stress increases from 300 to 1100K, decreasing rapidly thereafter. The slip system activated are {1121}<1126> and dislocations observed are nearly of edge character. The anomaly is discussed in comparison with the behavior of other intermetallics and pure metals. An important role this pyramidal slip plays in polycrystalline ductility of Ti3Al is shown in the light of the briefly described results of the prism slip.

Ordered intermetallics based on aluminides and silicides constitute a unique class of metallic materials possessing promising high-temperature properties. However, brittle fracture and poor ductility have limited their use as engineering materials in most cases. During the past ten years extensive research has been conducted on ordered intermetallics. As a result, significant progress has been made in identifying various causes of brittle fracture, and their relative importance in different ordered alloys. In some cases this understanding has helped achieve dramatic improvements in ductility. We review here three different classes of brittle fracture in ordered intermetallics and discuss the results in terms of model alloy systems chosen from each class. Ni3A1 and NiAl are discussed as prototypical ordered alloys prone to intrinsic intergranular brittleness. They are used to review our current understanding of intrinsically weak grain boundaries and the mechanisms by which boron is thought to suppress intergranular fracture. Next, FeAl and Fe3A1 are discussed as examples of ordered intermetallics that are susceptible to environmental embrittlement at ambient temperatures. Recent discoveries in these two alloy systems are reviewed with special emphasis on some of the rather interesting but subtle effects of test environment. Finally, A13X type intermetallics (A13 Sc, Al3Ti-base, and Al3 Zr-base alloys) are discussed as examples of ordered alloys that have high symmetry (L12 structure), are relatively soft, yet cleave transgranularly with very little ductility. In all these cases, experimental results are compared with theoretical calculations.

The relation between intergranular fracture and grain boundary character was studied in Ni3Al using the electron backscattering pattern (EBSP) technique. Using specimens of small grain size, we are able to index the orientation of a large number of grains, thus improving the statistical quality of the data compared to previous results. Furthermore, we are able to characterize the boundary planes using the EBSP method because of the special specimen geometry used. It was found that Σ3 boundaries do have a substantially lower propensity for cracking as compared to other boundaries, but there is no correlation between grain boundary Σ value and the propensity for cracking for low Σ and low angle boundaries. These results are in general agreement with recent theoretical predictions of grain boundary structure.

The Hammer-Anvil technique is employed for the rapid solidification of Ni3Al in the present work. The effect of off-stoichiometry on the change in the microstructure in the quenched alloy is examined for compositions within the L12 phase field at equilibrium. It is shown that non-equilibrium phases of various morphologies appear with fine grains of Ll2 phase, which have never been reported for the compound rapidly solidified by the roll techniques. With increasing aluminum concentration and with increase in cooling rate, the latter being judged by the difference in the thickness of the quenched foil, the fraction of the non-equilibrium phases appearing is found to increase giving more complex microstructure.

The effects of stress state on the fracture behavior of Ni3Al, Ni3Al + B, and NiAl were determined using either notched, or fatigue-precracked bend bars tested to failure at room temperature, in addition to testing specimens in tension under superposed hydrostatic pressure. Although Ni3AI is observed to fail in a macroscopically brittle intergranular manner in tension tests conducted at room temperature, the fracture toughnesses (i.e. K IC presently obtained on Ni3Al exceeded 20 MPa1m and R-curve behavior was exhibited. In-situ monitoring of the fracture experiments was utilized to aid in interpreting the source(s) of the high toughness in Ni3Al, while SEM fractography was utilized to determine the operative fracture modes. The superposition of hydrostatic pressure during tensile testing of NiAl specimens was observed to produce increased ductility without changing the fracture mode.

A combination of minor solutes was added to an Fe3AI-type base alloy (Fe-28%AI6%Cr-0.05%B) to produce carbide precipitates. These solutes reacted to produce coarse Ti-rich MC and Cr- and Mo-rich M6C precipitates during fabrication at 650–1000°C. X-ray diffraction of hot-worked material revealed a significant amount of the higher temperature B2 phase is retained at room temperature, indicating that the D0s.↔,B2 phase transformation was retarded. During subsequent heat-treatments, the coarse carbides partially dissolved during annealing at 1100–1200°C, and no grain coarsening was observed at temperatures up to 1200°C. Specimens annealed for lh at 1250°C and quenched exhibited fine carbide precipitates both along some of the dislocations and at grain boundaries. The annealed and quenched specimens also contained many fine dislocation loops which coarsened and evolved into networks during further aging at 750°C for 1-10h. Tensile testing at 600°C showed improvements in strength and ductility of annealed and aged material relative to the hot-worked condition. Correlation of microstructural behavior with tensile properties changes supports the idea that microstructural control can be used to develop better high-temperature strength in the B2 phase. However, more data is needed on the combinations of alloying and heattreating which produce fine, stable matrix carbide dispersions in iron-aluminides.

The microstructures and mechanical properties of an Fe-modified L12 alloy containing 7.5 at.% Fe have been investigated. This alloy has been determined to be essentially single phase following a homogenization heat treatment (HHT) at 1100°C for 100h, with a very small volume fraction of precipitates having been observed to form along dislocations. Conversely, in the case of the as-cast (AC) condition, the alloy has been determined to contain band-like precipitates, which have also formed along dislocations. In addition, a high density of very thin plate-like precipitates have formed parallel to {001} planes of the L12 matrix. Although these plate-like precipitates appear to exhibit lattice tetragonality, their crystal structure cannot be explained by assuming D022 and D023 structures. Five different <110>-type dislocations havfe been activated within a small region of a matrix grain during deformation at 1100°C, some of which cross slipped from {111} to {001} planes. Specimens in both AC and HHT conditions were deformed in compression to approximately 0.5% without fracture at both 22 and 1 100°C, the yield stress for the HIT condition having been determined to be 192 MPa at 22°C and 98 MPa at 1100°C.

Based on the alloying method to improve the intergranular cohesion of LI2 alloys, it was shown that the Ni3 Si alloy was ductilized by the addition of Ti element and the control of stoicliometry. Metallographic and structural observations in the alloys along Ni3Si-Ni 3 Ti section were performed on optical microscopy, X-ray diffraction, TEM and ALCHEMI. It was shown that Ti element with a solubility of about 11 at.% substituted for the Si sites and the Ni3 (Si,Ti) alloys were highly ordered. The mechanical properties and fracture behavior of Ni3 (Si,Ti) polycrystals were investigated in terms of composition, the environmental effect, test temperature and the doping of B, C and Be. At ambient temperatures, higher ductility accompanied by a high portion of transgranular fractures was observed in the alloys with higher Ti and Ni contents, doped with B and C, and tested in vacuum. Hydrogen and oxygen from environment strongly affected the ductility and fracture of these alloys at ambient and elevated temperatures, respectively. The yield stress increased with increasing Ti content.

Additions of the interstitial elements oxygen and carbon to the A15 structure V3Au intermetallic compound resulted in the formation of the L'2 perovskite structure. The composition of the L'12 phase is about 17.5 at.% oxygen. Analysis of x-ray diffraction line intensities indicated that the interstitial stabilized phase was L'12 perovskite structure rather than the L12, Cu3Au structure type. The microhardness of the L'12 phase is about 20% lower than that of the A15 phase and its fracture toughness is 2 to 3.5 times greater. The mechanism for the improved fracture toughness in the L'12 phase is the subject of continued research.

This paper summarizes the results of our work aimed at overcoming the intrinsic grainboundary weakness of NiAI by microalloying with boron. In previous work we have shown that 300 wppm boron is very effective in suppressing intergranular fracture in NiAI [1]. It does this by segregating strongly to the grain boundaries and strengthening them. Despite this dramatic effect on the fracture mode, however, boron is unable to improve ductility because it is a potent solid solution strengthener, more than doubling the yield strength relative to that of undoped NiA1. The present work attempts to decrease this deleterious hardening effect by lowering the bulk concentration of boron in NiA1. Our results show that if the boron concentration in the bulk is lowered to 30 wppm, the yield strength of boron-doped NiA1 is only about 30% higher than that of undoped NiAI. In addition, there is enough boron at the grain boundaries of this alloy to suppress intergranular fracture. Under these conditions, boron-doped NiAI has a tensile ductility of 2%, which is essentially identical to that of undoped NiA1. This result, namely that the strengthening of grain boundaries by boron does not by itself improve ductility, indicates that although grain boundaries might well be the weakest links in NiAI, cleavage planes are not much stronger. In other words, even though boron additions serve to strengthen the grain boundaries and suppress intergranular fracture, ductility is not improved, because the next brittle fracture mode, namely transgranular cleavage, takes over before significant plastic deformation can occur.

Ternary TiAl-base alloys containing Cr have been studied, using rapidsolidification processed materials. The mechanical behavior has been characterized and related to other results on microstructure, site occupancy, phase stability, and deformation mode. It was found that Cr additions enhance the plasticity of duplex γ+a2 alloys, but not single-phase alloys. The ductilization effect of Cr is partially due to its ability to occupy Al lattice sites and modify the Ti-Al bond. It is also partially due to its ability to promote twin formation, by modifying the Al partitioning and therefore the a2/γ volume ratio in transformed regions.

Alloy phase stability is a critical parameter in the design and implementation of nickel-base superalloys. To achieve the high temperature properties required of single crystal superalloys for application in gas turbine machinery, these alloys must have a stable gamma plus gamma prime microstructure for long periods of time at elevated temperatures. Significant deviation from this stable two phase microstructure, due to the precipitation of other phases, results in the loss of critical alloy properties which can have a deleterious impact on engine performance.

Empirical methods based on the electron vacancy concept, commonly employed to predict and prevent the formation of undesirable topologically close packed (TCP) phases such as sigma or mu in polycrystalline nickelbase superalloys, are also used with single crystal superalloys. These undesirable phases result in the loss of alloy properties primarily as a result of the depletion of refractory strengthening elements from the superalloy matrix. The consequence of the formation of undesirable TCP phases on alloy properties and subsequent single crystal component behavior is reviewed.

Aeronautical turbine disks are very critical parts in an aeroengine. Disk burst results in the loss of the engine and sometimes that of the aircraft. Their service lifetime being several thousand hours, it is necessary that the mechanical properties are not altered significantly during service.

The upper shelf fracture toughness of ultra high strength steels is dependent on both the microstructure, which is determined by composition and heat treatment, and on the inclusions present in the steel. The inclusions In ultra high strength steels are typically oxides and sulfides [1]. In most ultra high strength steels the sulfides are manganese sulfides, although depending on the composition of the steel and the melt practice used, other sulfides are found, such as chromium sulfide, calcium sulfide and lanthanum oxy-sulfide [2]. If the inclusions can be regarded as pre-existing voids then the inclusion volume fraction and spacing appear to be sufficient to characterize the inclusion population from the standpoint of fracture toughness [3,4]. The purpose of this paper is to discuss results which show sulfur can be gettered as particles which are much more resistant to void nucleation than manganese sulfides and that this increased resistance to void nucleation can result in vastly improved upper shelf fracture toughness. In particular, when HY180 steel contains manganese sulfides the fracture toughness is about 250 MPa but when the sulfur is gettered as particles containing titanium, carbon and sulfur the fracture toughness of HY180 steel will approach 550 MPa . These particles, believed to be titanium carbosulfides, are much more resistant to void nucleation than manganese sulfides and this increased resistance to void nucleation appears to be the reason for the improved fracture toughness.

The phases responsible for high temperature strength and ductility of Al-Fe based alloys, specifically Al-Fe-Mo alloys, have yet to be unambiguously identified. The phases appear to vary slightly under different experimental and processing conditions. This poses some queries concerning the reproducibility of mechanical properties of these alloys. In this paper an attempt is made to address these points and focus on the structure-property relationships of Al-Fe-Mo and Al-Fe-V rapidly solidified powder alloys.

Microstructurally-induced changes in the local stress state (triaxial constraint) and their effect on fracture-toughness behavior are examined at ambient and cryogenic temperatures in an Al-Li-Cu-Zr alloy, processed in the form of 12.7 mm-thick "naturally laminated" plate containing aligned-weak interfaces and 1.6 mm-thin unlaminated sheet. It is shown that marked improvements in long-transverse (L-T) toughness can be achieved in the plate material at cryogenic temperatures by promoting through-thickness delamination along these interfaces, which relaxes local constraint and promotes a fracture-mode transition from global plane strain to local plane stress. Conversely, in thin sheet material, the absence of such interface delamination leads to a reduction in toughness with decrease in temperature, consistent with the greater degree of crack-tip constraint.