This paper will provide an overview of the current status of the development of ordered alloys for high temperature structural applications. The physical and mechanical properties of ordered alloys will be reviewed with particular emphasis on aluminides and (FeNiCo)3V alloys. Alloy theory, slip systems, yielding, strain hardening, fatigue, wear resistance and processing are among the subjects to be covered. Current research programs in the U.S. and abroad will be outlined, and the kinds of scientific and application-oriented research needed in the future will be discussed.

The task of any alloy ordering theory simply put is to predict from a small amount of experimental data the ordering phase transitions, their temperature, composition and crystal structure in some selected alloy system. Ideally, the experimental information needed would be not more than the atomic numbers of the alloy components involved. This paper attempts to assess the progress that has been made towards that objective, the various techniques that have been used to provide solutions and the probable avenues of most rapid progress in the next ten years. The major areas that will be reviewed are Ising model approaches, many body interaction approaches, computer simulation techniques, and first principle calculations. It will be argued that we are on the verge of some quite exciting breakthroughs in the field of alloy phase prediction.

Many stable or metastable intermetallic phases useful to the alloy designer have crystal structures which are ordered superstructures of a parent disordered phase. A highly reliable statistical mechanical method (CVM) has now been developed for calculating such superstructure phase equilibria derived from say, the fcc parent lattice. To obtain phase diagrams, one needs certain physical parameters, such as effective pair interaction ratios. It is possible, in principle, to extract these parameters from band structure calculations in the coherent potential approximation (CPA), particularly from recently developed cluster-CPA techniques. If sufficient accuracy can be achieved, truly first-principles phase diagram calculations may soon become feasible.

The nature and the role of the pair interatomic interaction of the noble and transition metal ordering alloys are considered. The determining role in the electron susceptibility in the behaviour of the interatomic interaction is shown. The important role of the Fermi surface local geometry in the definition of superstructure periods is established. Illustrating calculations are made. It is shown that in the noble and transition metal alloys with complicated Fermi surface the oscillations of the pair interatomic interaction are of essentially nonfriedel character.

Many ordered β-phase alloys of the B2 or D03 types exhibit unusual diffraction patterns and images when examined in the transmission electron microscope. These effects include “extra” diffraction spots, diffuse scattering, satellites, “tweed microstructures, spinodal decomposition and the formation of incommensurate phases. Examples of these phenomena will be presented and discussed. Although most of the β-alloys also undergo a martensitic transformation at some lower temperature it will be argued that the above effects are not “premartensitic” or precursory in nature. However many of them can be related to certain properties of the β-phase such as elastic anisotropy, a soft phonon mode and electronically driven instabilities.

The isothermal time-temperature dependence of the transformation from SRO to LRO as measured by resistivity changes is given. Problems of defining an initial state from which the transformation occurs are reviewed with emphasis on the microdomain structure from which the LRO structure develops. Questions of homogeneous transformation relative to heterogeneous transformation by clear nucleation and growth are addressed in terms of experimental evidence and theoretical predictions. Strong evidence for a nucleation and growth mechanism, at least above 750° C, is presented.

The antiphase boundary (APB) structure and the disordering process have been studied by transmission electron microscopy (TEM) in a long-rangeordered alloy with Ll2 structure and composition (Ni70Fe30)3(V58Al40Ti2). With an increase in temperature from a single-phase field into a two-phase field, disordering occurs both homogeneously within ordered domains and heterogeneously along APBs. Disordering continues by a ripening process with the disordered islands shrinking while the disordered bands on the prior APBs grow. The APBs are partially aligned on {100} while the material is single phase. As disordering proceeds, the degree of alignment of the disordered bands on {100} increases until complete alignment results. A correlation of the APB structure with room temperature mechanical properties indicates no dramatic change at the transition to the two-phase structure.

The sequence of disordering transformation processes in the A3B type alloy series (Ni70Fe30)3(V98-xAlxTi2), currently under development for high-temperature structural applications, was studied by differential scanning calorimetry (DSC), x-ray diffraction, optical microscopy, and transmission electron microscopy (TEM). Results of DSC show that in all alloys there are two endothermic stages of phase transformation from the ordered to the disordered state. With increasing x, the disordering transition temperature, Tc, reaches a maximum ∼1000°C at x ≈ 50, and then decreases. Interrupted heating, followed by water quenching, was used to characterize the crystal structure and the microstructure of the intermediate phases. For the x + 20 alloy, TEM observations showed ordered regions of DO22 phase in a matrix of disordered fcc (Al) phase at intermediate temperatures. The ordered domains transformed morphologically into cuboid like regions at higher temperatures. From a combined study by all the techniques, we conclude that in alloys with x between 0 and 20, the sequence of phase transformations upon heating is: DO22 → DO22 + Al → Al, whereas in alloys with x > 40, the major sequence is Ll2 + B2 → Ll2 + Al → Al.

A microstructural study of the melt spun alloys Ni75 Al25. Ni72.5 Al22.5Cr5. and Ni67.5A120Cr12.5 was carried out. Variations in grain morpfology; phase separation and ordering structure were observed. The results are discussed in terms of alloy stoichiometry, Cr effect on phase reaction, microsegregation of Al and the cooling rate.

Hard sphere models were used to determine densest configurations in symmetrical [100] and [110] tilt boundaries in compounds with the Ll2 structure. The minimum allowed interatomic distances used in these models were estimated from interatomic potentials and the structures of the intermetallic phases in the binary system. The structural unit model is used to analyze the possible ground states for ordering.

Two different cases were analyzed corresponding to compounds with “soft” potentials (i.e. Cu3 Au) and “hard” potentials (i.e. Ni3Al). For the Cu3Au type the grain boundary structures obtained were similar to those reported by other investigators for pure fcc metals. Several boundaries were found to be a “two phase” structure, differing in composition and ordering state. This leads to a certain degree of clustering in the boundaries. The contribution of clustering to the grain boundary energy is calculated in a point approximation based on the first coordination shell.

For compounds of the Ni3Al type the structures that are densest were found to be generally diffetent from the low energy configurations of boundaries in, pure fcc metals and Cu3 Au. These configurations preserve order, but are much less dense. The possibility of grain boundary “phases” that are not present in other fcc materials may constitute an explanation for the extreme GB weakness observed in Ni3Al and other Ll2 compounds with high ordering energy.

In the approximate interval 850–1150C, the Laves phase TiCr2 exhibits several composition dependent structures, ranging from two layer hexagonal (2H), denoted C14, to face-centered cubic, denoted C15, for Ti-deficient and Ti-excess material, respectively. Intermediate stacking variants such a dihexagonal (4H), denoted C36, may also exist. The Laves phase band in a Ti-Cr microdiffusion couple is examined in cross-section TEM at ambient temperature, after a 1000 C anneal. The band consists of very faulty hexagonal material, at least in part reflecting rapid shear transformation on cooling; in addition the cubic phase is present, in some instances within the same grain as the hexagonal phase. The cubic/hexagonal interfaces are approximately parallel rather than perpendicular to the bulk diffusion direction. Due to a difference in Ti and Cr fluxes, there is a gradual transformation of cubic to hexagonal by a shear mechanism. A model for the mobility of the hexagonal/cubic interface in a concentration gradient is proposed.

The isothermal ordering kinetics of stoichiometric Ni2Cr was investigated in specimens aged at 500°C and containing 3 different phosphorous impurity concentrations; 0.004, 0.032, and 0.142 atomic percent P. In every case, higher P specimens were found to transform more rapidly than lower P content specimens, with as much as an order of magnitude difference in transformation times. Low and medium P specimens transformed via a continuous ordering process. In contrast, the highest P specimens transformed by an unusual process in which colonies of ordered domains were formed which consumed grains through the advance of an order-disorder interface. Colonies tens of microns in size were typically observed while individual ordered domains were only a few tens of nm in size.

The influence of a number ternary additions in Fe3Al upon the DO3 domain structures and DO3 → B2 transformation temperature, Tc, has been investigated. It was found that additions in (atomic percent) of 5 Ti, 5 Cr, 6 Mn, 12 Mn, 10 Ni, 3 Mo, 6 Mo, 3 Si, and 5 Si raise Tc. In the Fe-20 Al-5 Ti alloy not only is Tc raised significantly (≧ 750°C), but the DO3 antiphase domain boundaries with fault vector become highly crystallographic and are parallel to {100} planes.

Short-range order (SRO) and long-range order (LRO) reactions in Ni Mo as a function of room temperature deformation ahd thermal treatments betwean −269 and 1300°C are Reported. Resistivity results show that SRO is dispersed by heating near 1300°C and by cold working to 68% reduction of area (RA). The resistivity decreases to a minimum at 68% RA after which an increase is attributed to an increase in defect density.

The resistivity as a function of variations in the time-temperature history is reported. Cooling at 60°C/min; results in the maximum “frozen in” SRO with resistivity of 144 μohm cm at 25°C; th8 resistivity of the LRO state obtained on slow cooling is 48 μohm cm. At −269°C, the resistivities are 139 and 8 μohm cm for the SRO and LRO states respectively. Correlations relate kinetics and microstructures to resistivity changes during heating, cooling and isothermal SRO-LRO transformations. Preliminary results of reheating fully ordered samples to near 868°C and quenching are reported. Results are discussed with relevance to existing theories.

Although the B2-type intermetallic compounds, NiAl, FeAl, NiTi and FeTi all maintained a high degree of order during irradiation, metastable phase formation was observed. Dislocation loops and network developed in NiAl and FeAl during irradiation and a nickel rich, fcc phase, most likely disordered Ni3Al, formed in NiAl after high doses. The build-up of constitutional vacancies (excess vacancies on Ni sites) and subsequent elemental partitioning of Ni and Al can explain the formation of a nickel rich phase in NiAl. No metastable phases formed in FeAl. Dislocation loop structures did not develop in NiTi or FeTi, rather a transformation to an amorphous phase occurred at relatively low doses. The amorphous transformation results when the free energy of the crystal lattice containing a high defect concentration exceeds the free energy of the amorphous configuration.

The flow stress of many L12 ordered alloys has a very unusual temperature dependence: the flow stress increases with increasing temperature. This unusual behavior is related to the nature of dislocation dissociation and core structure. The flow stress increase is the result of thermally activated cross slip of [101] screw dislocations to the (010) plane which is accompanied by a transformation of the dislocation core from a glissile to a sessile form. Thus dislocations which are mobile on (111) planes become immobile after cross-slip into (010) planes. The dependence of the flow stress on temperature, orientation and sense of the applied uniaxial stress will be discussed in the light of this cross slip model for Ni3Al, Ni3Ga and for γ/γ′ nickel base superalloys.

The response of Ni3Al to cyclic plastic strains (plastic strain controlled fatigue) will also be shown to be in accord with the cross slip model. The mean stress in such a test becomes compressive or tensile, depending on the orientation of the sample, even though the net plastic strain is zero after each cycle.

The strengthening of Ni3Al by ternary additions will also be discussed. It will be shown that ordinary solid solution strengthening models are not applicable but that the cross slip model can also be applied.

Finally, it will be shown that dislocation core simulation studies predict that there should also be a class of L12 ordered alloys that show a “normal” flow stress-temperature behavior, i.e., the flow stress increases at low temperatures. The results of our studies on Pt3Al will be used to illustrate this behavior.

Three points usually come to mind when considering the effects of grain size on the mechanical properties of polycrystals: strengthening at low temperatures through grain refinement, strengthening at high temperatures through grain coarsening, and lowering of the ductile to brittle transition temperature in materials such as ferritic steels and zinc through grain refinement. Ordered alloys, as we shall see, exhibit the same effects.

In many LI2 ordered alloys the flow stress increases with increasing temperature and is in this “anomalous” regime strongly dependent on orientation and sense of the applied stress. These dependences can be predicted from the nature of the dissociation and core structure of the 1/2<101> screw superpartials in these alloys. Computer modelling shows that two different configurations, a glissile one on {lll} planes and a sessile one on {010} planes, exist and both are described here in detail. The anomalous increase of the flow stress may then be explained by an increasing amount of core transformations from the glissile to sessile forms as the temperature increases. The theoretical model for the immobilization of screw dislocations by this mechanism is then discussed and its validity illustrated by comparison with experimental results on Ni3 (Al, Ta) single crystals.

A rapid increase of the yield stress with increasing temperature, often observed in L12 ordered alloys, is commonly called the “anomalous flow behavior”. This phenomenon is believed to result from the thermally activated transformation of the core of 1/2<110> screw dislocations from a glissile form to a sessile form at high temperatures. It is shown here that another class of L12 alloys exists in which these two forms of the screw dislocation core are not available. These are the alloys in which the APB on {111} planes is not stable and the atomistic studies of screw dislocations in such alloys show that their cores are always sessile. The yield stress of these alloys then increases with decreasing temperature and no increase at high temperatures occurs. Such behavior has been observed, for example, in Pt3Al. This “normal” behavior is analogous to that of b.c.c. metals and a theory of the temperature dependence of the yield stress has been developed along the same lines as in the case of b.c.c. metals. Comparison of this theory with measurements on Pt3Al single crystals shows a good agreement.

The elevated temperature mechanical properties of rapidly solidified, boron doped Ni3Al have been investigated. Melt spun ribbon, plasma deposits and HIP'ped powder were tested with the same general results. It was found that the boron doped, rapidly solidified alloy exhibits an increasing flow stress with increasing temperature similar to that reported for conventionally cast Ni3Al. In addition, the rapidly solidified alloys exhibited a severe ductility minimum in the range 500–750C. The ductility minimum is relatively insensitive to both boron and aluminum concentration, but the embrittlement onset temperature changes with processing and testing parameters. In all cases, the onset of the ductility drop coincides with the onset of the decrease of the flow stress. It is postulated that this behavior is due to a change in the flow mechanism at elevated temperature to one that is associated with grain boundary stress concentration.