Ni-44A1, Ni-50Al and NiAl-0.3 at.% Hf single crystals have been studied in compression to understand the effects that alloying additions and deviation from stoichiometry can have on the mechanical response of NiAl-based single crystals. While all three single crystals deform through a<111> slip at lower temperatures, the active slip systems differ at higher temperatures. Climb of a<010> dislocations contributes to deformation in Ni-50AI single crystals beyond the slip transition temperature, while Ni-44Al and NiAl-0.3Hf crystals deform through a<101> glide. But several microstructural differences have been observed in the mode of deformation between Ni-44Al and NiAl-0.3Hf crystals. In addition, significant strengthening is exhibited in the Hf-doped crystals at higher temperatures. The post-deformation microstructure is also observed to be sensitive to both strain and strain rate. A possible explanation is offered for some of the observed differences in deformation behavior between the three alloys.

Titanium silicide Ti5Si3 whose melting temperature is 2130°C bears the potential for very hightemperature application. This paper reports our results on the alloying behaviour of Nb or Cr addition to this compound. Total energy calculation shows that the substitution of Ti by Nb or Cr atoms in Ti5Si3 crystal stiffens the bonding between the atoms. In experiment, two means of alloying are considered: stoichiometric and off-stoichiometric alloying. Stoichiometric alloying in Ti5Si3 results in compounds consisting of single Ti5Si3 phase while off-stoichiometric alloying yields hypereutectic microstructure with Ti5Si3 being the primary phase. The Ti5Si3 phase in both cases dissolves certain amount of Nb or Cr alloying element and its composition agrees with the stoichiometric composition of (Ti,Nb) 5Si3 or (Ti,Cr) 5Si3 The moduli of the stoichiometric alloys increase with the increase of alloying element addition, indicating an enhancement in Ti5Si3crystal.

Most evaluations of the fracture and fatigue-crack propagation properties of γ + α2 titanium aluminide alloys to date have been performed using standard "large-crack" samples, e.g., compact-tension specimens containing crack sizes which are on the order of tens of millimeters, i.e., large compared to microstructural dimensions. However, these alloys have been targeted for applications, such as blades in gas-turbine engines, where relevant crack sizes are much smaller (<500 µm) and where the small-crack fatigue threshold may be the most relevant design parameter. In this study, we compare and contrast the cyclic crack-growth behavior of both large (a > 5 mm) and small (c ∼ 25–300 µm) cracks in a γ-TiAl based alloy, of composition Ti-47A1–2Nb-2Cr-0.2B (at%), specifically for duplex (average grain size ∼17 µm) and refined lamellar (average colony size ∼150 µm) microstructures. It is found that, whereas the lamellar microstructure displays far superior fracture toughness and fatigue-crack growth resistance in the presence of large cracks, in small-crack testing the duplex microstructure exhibits a better combination of properties. The reasons for such contrasting behavior are examined in terms of the intrinsic and extrinsic (i.e., crack bridging) contributions to cyclic crack advance.

To investigate the deformation behavior of the C15 HfV2+Nb Laves phase, compression tests were conducted on the C15/bcc two-phase alloys at very low temperatures down to 4.2 K. Transmission electron microscopy revealed that substantial mechanical twinning takes place in the C15 Laves phase at room temperature and 77 K. A deep minimum in the flow stress, with a drop of nearly 500 MPa, appears at around 77 K for the C15/bcc two-phase alloy. Since no such anomaly is seen in the bcc phase, we believe that the cause of this can be attributed to mechanical twinning in the C15 Laves phase. A high density of ultra-fine twin bands is the characteristic feature of twinning in the C15 matrix. We also investigated the phase relations between the C15 HfV2 Nb and bcc (V, Nb) solid solution in the Hf-V-Nb ternary system in an attempt to understand why we were unsuccessful in our attempts to grow single crystals of the Laves phase.

Dispersion-strengthened nickel aluminide NiAl was synthesized by reactive mechanical alloying from elemental powders. The microstructure and creep properties of these materials was compared for two cases: (i) NiAl-Mo, where dispersoids are in the form of a ductile, metallic phase and (ii) NiAl- Mo2C, where the dispersoids are a brittle, ceramic phase. Also, the effect of annealing treatments which coarsened both matrix grains and dispersoids was investigated. Inhibited diffusional creep is identified as the rate-controlling creep mechanism.

Superplastic behavior has been found in Fe3A1 and FeAl alloys with grain sizes of 100–600μm. The large-grained Fe3Al and FeAl alloys exhibit all deformation characteristics of conventional fine size superplastic alloys. However, superplastic behavior was found in large-grained iron aluminides without the usual pre-requisites for superplasticity of a fine grain size and grain boundary sliding. The metallographic examinations have shown that the average grain size of large-grained iron aluminides decreased during superplastic deformation. Transmission electron microscopy (TEM) observations have shown that there were a great number of subgrain boundaries which formed a network and among which the proportion of low and high angle boundaries increased with increased strain. The observed superplastic phenomenon is explained by continuous recovery and recrystallization. During superplastic deformation, an unstable subgrain network forms and these subboundaries absorb gliding dislocations and transform into low and high angle grain boundaries. A dislocation glide and climb process accommodated by subboundary sliding, migration and rotation, allows the superplastic flow to proceed

Mechanical properties of E21 (Mn, Fe)3AlC-base alloys were investigated. The E21 crystal structure is closely related to L12, and thereby E21 compounds are expected to exhibit superior properties similar to those of the L12 Ni3AL. Nominal compositions are fixed to be 60mol%(Mn, Fe)-20mol%A1–20mol%C. Alloys were prepared by mechanical alloying and hot pressing. ICP chemical analysis, X-ray diffraction analysis, scanning electron microscopy and electron probe microanalysis were carried out for alloy characterization. Mechanical properties were evaluated by Vickers hardness tests at room temperature (RT) and compression tests from RT to 1273K. Strain rate dip tests were also carried out. It is found that most alloys are composed of two phases of E21 as the primary phase and graphite as the precipitates, and that the volume fraction of graphite increases with increasing Fe content. Hardness and 0.2% flow stress at RT are raised with increasing Fe content. At RT, 0.2% flow stress and fracture strength of Fe3A1C alloy used are 2.9GPa and 3.4GPa, respectively. An alloy containing 40%Mn-20%Fe shows weak positive temperature dependence of strength at 700–800K, similar to the observation in some Co3AIC alloys. Moreover, work-hardening coefficient of all alloys shows strong positive temperature dependence below 700K. These results suggest the occurrence of K-W related mechanism for plastic deformation in these alloys.

To examine the effect of thermal expansion mismatch on cracking in two-phase Cr-Cr2X (X=Hf, Nb, Ta, Zr) alloys, we calculate the elastic constants and associated elastic anharmonicity by the local-density-functional approach from which the coefficients of thermal expansion (CTEs) of Cr and Cr2X are determined. The calculation shows that the CTE of Cr at high temperatures is notably larger than those of Cr2X. If the difference in CTE between Cr and Cr 2X is a primary source of crack initiation, our results fail to explain the experimental observation that, among Cr-Cr2X alloy systems, the ingot cracking occurs mainly in Cr-Cr2Nb. We suggest that for the cracking to occur, thermal mismatch stresses are retained by a hard and supersaturated Cr matrix (e.g., due to the relatively high solubility of Nb in Cr in the case of Cr-Cr2Nb). On the other hand, a soft Cr matrix can accommodate the thermal misfit dislocations plastically even when the CTE difference between Cr and Cr2X is large (e.g., in the case of Cr-Cr2Zr).

In the present study, the local elastic and plastic properties of the γ- TiAl and the Laves phases have been investigated in a series of γ + Laves alloys using room-temperature compression, Vickers microhardness, and nanoindentation with an emphasis on elucidating the local property changes in γ + Laves alloys deformed at room temperature. This study shows that nanoindentation can be used to provide useful information on plastic flow in multiphase intermetallic alloys.

Several buttons of each of three binary Ti-Al alloys containing nominally 70, 71 and 72 at.% Al were prepared from elemental Ti and Al by plasma arc-melting. One button of each composition was then either solution heat-treated, subjected to a two-step heat treatment, or hot forged. Each composition and material condition was then characterized using SEM/EDS and microhardness testing. The results show that while all three compositions have a predominantly two-phase, coarse microstructure in the as-cast condition, it is possible to produce an essentially single phase material through an appropriate solution heat treatment for the two lower Al content alloys. A fine two-phase microstructure can be achieved through an additional, lower temperature heat treatment step. The microhardness results show that the solution heat treatment reduced the hardness for all three compositions compared to the as-cast condition while hot forging as-cast samples increased hardness. The 70% Al alloy has the highest hardness for all four material conditions studied while the 71% Al alloy has the lowest hardness in three of the four material conditions.

The Zr-Cr-Mn system is used to explore the effect of a lowered SFE on the room temperature mechanical properties of a Laves phase using elements of similar atomic size. The ternary Zr-Cr-Mn diagram in the region from 0 to 12 at. % Mn is first determined and it is shown that Mn substitutes only for Cr in the Laves phase. TEM analysis of the density of stacking fault energy related defects such as annealing twins indicates that Mn substitution for Cr in ZrCr2 lowers the SFE of the cubic Laves phase. Mechanical testing of the two phase alloys is used to explore the effects of Mn content and the volume fraction of each phase on the ductility and fracture behavior in compression. It is found that the mechanical properties are well-described by a model incorporating solid solution strengthening in a ductile-brittle two phase alloy.

Plates of three gamma titanium aluminide alloys have been investment cast with a wide variety of casting conditions designed to influence cooling rates. These alloys include Ti-48Al-2Cr-2Nb, Ti- 47Al-2Cr-2Nb+0.5at%B and Ti-45Al-2Cr-2Nb+0.9at%B. Cooling rates have been estimated with the use of thermal data from casting experiments, along with the UES ProCAST simulation package. Variations in cooling rate significantly influenced the microstructure and tensile properties of all three alloys.

The diffraction contrast from dissociated ½<112] superdislocations in γ-TiAl intermetallic alloy cannot always be analyzed using conventional rules of diffraction contrast. In particular, the configuration involving three similar Shockley partials on adjacent planes has often been ruled out due to the absence of fringes indicating the presence of stacking faults. In order to determine the dissociated configuration, weak-beam transmission electron microscope observations of edge-oriented ½<112] superdislocations have been correlated with computer simulated images. Dissociation of these superdislocations into three similar ⅙<112] partial dislocations bounding a superlattice extrinsic and intrinsic stacking fault pair has been consequently determined from these analyses. It has been found that diffraction contrast alone cannot distinguish between the various configurations that lead to the formation of the fault pair, but the formation of an antiphase boundary or complex stacking fault linked dissociation or locking by stair rod dislocations can be ruled out.

The kinetics of low temperature deformation were investigated in several different polycrystalline RuAl alloys with the use of strain rate change experiments at 77 K and 298 K. Compositions investigated include RuAl, RuAl+0.5%B, Ru51.5 A48.5, Ru52 Al48, RU53 A147+0.5%B, Ru54.5 Al45.5, and Ru52 Al43 Sc5. Flow stresses did not vary substantially with temperature between 77 K and 298 K. Rate sensitivities were low compared to other B2 compounds and similar in all compositions investigated. Analyses of dislocation substructures after low strain deformation were conducted. The deformation kinetics and substructural observations suggest a higher intrinsic deformability for RuAI alloys with respect to the other high temperature B2 aluminides.

Dislocation activity in two-phase commercial TiAl alloys occurs most readily in the γ-TiAl phase, and measurements of the CRSS of single crystalline γ-TiAl provides a solid foundation for understanding the mechanical performance of these alloys. Single crystals of γ-TiAl with greater than 54.5 at%Al have been grown using the optical float zone crystal growing technique, but single crystals with lower Al content, closer to that of commercial alloys, have not. In the present study, polycrystalline ingots of Ti-52 at%Al have been heat treated to form very large grains, and microsample tensile specimens, which have a nominal gage cross-section of 250μm × 300μm, a gage length of 250μm and an overall length of 3 mm, have been machined from within single grains. Microsample high-temperature stress-strain curves for Ti-55.5 at%Al single crystals oriented along the [001] and [010] are presented. Tensile tests results for Ti-52 at%Al tested at 1073K along the [237], [173] and [344] orientations will also be discussed.

The influence of crystal orientation on the high temperature creep behavior of Ni3(Al Ti Ta) was investigated by tensile creep testing under a constant load at a temperature of 1123K. The single crystals were oriented close to [001], [011], [111], [557] and [012]. The results show an increasing stationary creep rate from [111] over [011] to [001]. The evolution of the microstructure during creep deformation was studied, using transmission electron microscopy (TEM). Only few systems are active in the primary regime, whereas several slip systems operate in the secondary stage. Habit planes of dislocation pairs separated by an antiphase boundary were examined and compared with anisotropic elastic calculations, explaining the good creep response of [111] oriented single crystals.

Uniaxially aligned polysynthetically twinned two-phase TiAl/Ti3Al material is produced by induction zone melting and deformed in uniaxial compression. Above 1000 K the strain rate sensitivity is independent of the lamellar orientation and increases strongly with increasing deformation temperature. Results for the strain rate sensitivity parameters are somewhat lower than those obtained for γ-TiAl single- and polycrystals. If the lamellae are oriented parallel or perpendicular to the deformation axis, the flow stress decreases with increasing strain. After plastic deformation the dislocation density in the lamellae is remarkably low indicating recovery processes. At the lamellar interfaces misfit dislocations and periodic arrangements of dislocations with Burgers vectors inclined to the lamellar boundaries are found. In contrast to deformation at lower temperatures, deformation twinning is rare.

Electron energy loss spectroscopy and density functional theory have been used to show that there is a covalent component to the bonding in NiAl, CoAl and FeAl, between the transition metal atom and Al. There is no charge transfer and no ionic component to the bonding in NiAl and probably not in CoAl and FeAI. The bonding in non-stoichiometric NiAl is studied. Preliminary results are given for a Σ3 boundary in NiAl.

A study of the occurrence of ordinary slip in single crystals of Ti 54.5 at% Al with various orientations at different temperatures shows that the critical resolved shear stress is approximately the same for ¼⟨110] slip on {111} and {110} planes near the peak of the yield stress anomaly. However the shapes of the glide loops are quite different, suggesting that the order of relative mobilities of screw and edge dislocations is reversed in the two cases. The reason for this and its possible effect on the mechanism responsible for the yield stress anomaly of ½⟨110] {111} slip are discussed. Experiments on the thermal reversibility of the yield stress when either ordinary- or super- dislocation slip systems are operating at both temperatures have shown that the yield stress is reversible for the latter but not reversible in the former case.

Deformation mechanisms of fully lamellar TiAl with a refined microstructure (γ lamellae: 100 ∼ 300 nm thick, α2 lamellae: 10 ∼50 nm thick) crept at 760°C have been investigated. As a result of a fine structure, the motion and multiplication of lattice dislocations within both γ and α2 lamellae are limited at low creep stresses (< 400 MPa). Therefore, the glide and climb of lattice dislocations are insignificant to creep deformation. The cooperative motion of interfacial dislocations on γ/α2 and -γ/γ interfaces (i.e. interface sliding) is proposed to be the dominant deformation mechanism at low stresses. Lattice dislocations impinged on lamellar interfaces are found to be the major obstacles impeding the motion of interfacial dislocations. The number of impinged lattice dislocations increases as the applied stress increases and, subsequently, causes the pileup of interfacial dislocations along the interfaces. Accordingly, deformation twinning activated by the pileup of interfacial dislocations is proposed to be the dominant deformation mechanism at high stresses (> 400 MPa).