This paper reports on the effects of carbon, nitrogen, boron, and oxygen on the microstructure and properties of gamma-TiAl alloys. It has been found that nitrogen and boron are capable of altering the solidification structure, whereas the others are not. The additions of >1 at.% B or 0.5 at.% N changed the cast structure from columnar to equiaxed with fine grain sizes. As a result, the strength has been significantly increased. Additional results on phase and structural stabilities will be presented.

The partial phase diagram of the Fe-Mn-Si system at 900°C was obtained by electron microprobe measurement. In addition, the effect of carbon on these equilibrium were also studied. Depending on these, a series of Fe-Mn-Si-C alloys with the matrix of Fe3Si were designed. It was found that they all have some plasticity and can satisfy the engineering requirement. Moreover, after appropriate heat treatment, all these alloys have good corrosion resistance in sulphuric acid with various concentrations.

Mechanical alloying (MA) has been used to synthesize MoSi2 from elemental Mo and Si powders. SiC and A12O3 dispersoids were also incorporated as a means of increasing high temperature strength. After synthesis, the powders were consolidated using vacuum hot pressing. It is shown that the size and volume fraction of porosity increases anomalously with increasing hot pressing temperature over the range 1200 to 1700°C. The strengths of the A12O3 composite are, in most instances, considerably higher than the monolithic material produced under similar conditions.

Fe-35Al alloys containing various amounts of ZrB2 have been prepared by melt spinning and the microstructure and its stability examined. The mechanical properties are evaluated both on as cast materials as well as after high temperature heat treatments. The ZrB2 additions lead to a dramatic increase in hardness and strength. In addition, small amounts of ZrB2 lead to significant increases in ductility. Because the microstructure is fairly stable, these improvements in properties are maintained even after high temperature exposures.

Alloys with up to about 1%ZrB2 have the dispersoid particles arranged in an imperfect cellular microstructure after rapid solidification. The cell walls contain many fine particles and are efficient barriers against dislocation propagation. Strain occurs as dislocations escape through gaps in the imperfect cell walls at a stress level that is controlled by the size of these gaps. Ductility is improved since the deformation that results from such controlled strain progression through the material is much more uniform and large stress and strain concentrations are avoided.

Porosity formation during solidification in equiaxed microstructure of multicomponent nickel aluminide IC396M is examined. Two types of porosities are identified and related to the other scales of microstructure that evolve during solidification. A correlation is established between the relevent processing parameters and the microstructure variables.

Thin films of Ni3Al formed by co-evaporation onto insulating substrates form a single phase fcc disordered lattice upon rapid solidification following excimer laserinduced melting with an interface velocity of ~4 m/s. Transmission Electron Microscopy (TEM) and x-ray diffraction (XRD) analyses exhibit no superlattice diffraction at room temperature. Resistivity measurements, indicating that the disordered phase has a higher resistivity and much smaller temperature coefficient at room temperature than the stable ordered (L12) phase, permit us to monitor phase changes and ordering on a fast time-scale. Subsequent annealing recovers long-range order, with resistivity measurements indicating that reordering begins just below 300°C.

Nb-Al and Nb-Al-X(X is Cr,Ti or Zr) ternary alloys ribbons, with compositions around the A15 (Nb3Al) structure, a candidate intermetallic as an advanced high temperature material, were rapidly solidified by an arc melt spin process. The rapid solidification structure of these tri-niobium aluminide alloys and the relationship between formed phases and compositions of ribbons are investigated.

Polysynthetically twinned (PST) crystals of Ti-rich TiAl have been grown and specimens cut from these crystals have been rolled at room temperature and subsequently annealed at 900°C and 1000°C. When the shear deformation parallel to the lamellar boundaries occurs during rolling, PST crystals of Ti-rich TiAl can be rolled to about 50% reduction in thickness at room temperature.

The recovery in microhardness occurs in two stages; the first stage associated with the decrease in dislocation density and the second stage connected with the annealing-out of deformation induced twins. The recrystallization mode depends on the amount of reduction. Up to 20% reduction, the lamellar structure is preserved even after the full recovery in hardness. When the amount of reduction exceeds 40%, a structure composed of equiaxed grains of TiAl is obtained after recrystallization. A mechanism of recrystallization of cold-rolled PST crystals, which may explain the dependence of recrystallization mode on the amount of reduction, is proposed.

Crystallographic and chemical analysis of the microstructures obtained by annealing a heavily deformed Ti-24Al-11 Nb alloy have been performed. The influence of the niobium distribution within the different phases present and the variation of degree of long range order caused by this distribution have been related to the deformation mechanisms leading to strain hardening at the early stages of deformation. In particular, slip band formation and twinning have been observed to occur within the (α2 phase according to the different degree of order measured.

The elimination of microsegregation and subsequent breakdown of the lamellar structure in near-gamma titanium aluminides was investigated through a series of high temperature homogenization heat treatments and isothermal hot compression tests. The homogenization heat treatment studies revealed that microsegregation could be eliminated after approximately five hours or one and one-half hours at temperatures either 15 C or 40 C, respectively, above the transus in the single phase alpha field. Subsequent attempts to breakdown the coarse prior alpha grain, lamellar structure were successful at low strain rates (∼ 10−3 s−1) and temperatures low in the alpha + gamma phase field, but led to wedge cracking and gross fracture at high strain rates (∼ 1 s−1) high in the alpha + gamma phase field. Subsequent annealing treatments to globularize the hot-worked structure in order to obtain a uniform, fine, and stable grain structure were also identified.

Forward extrusion of a gamma-TiAl alloy (Ti-49.5A1-2.5Nb-l.lMn) was investigated, with special reference to the role of preheat temperature, ram velocity, and can material. Cylindrical billets were sealed in evacuated cans, soaked at extrusion temperatures ranging from 1050 to 1350°C for 2 hr and then extruded through streamlined dies held at 260°C. Microstructural features of the extruded billets which evolved under steady-state conditions were characterized. In conjunction with these observations, a coupled (thermal-deformation) finite-element analysis procedure was used to predict the local distributions of strain, strain rate, temperature, and stress present during extrusion. The microstructures of the extruded products are compared with those obtained in samples subjected to isothermal constant-strain-rate compression tests.

The fracture behavior of a cast gamma-titanium-aluminide alloy (Ti-49.5A1–2.5Nb-l. 1Mn) during deformation processing was investigated using hot-tension testing and isothermal-forging experiments. In uniaxial tension, fracture ductility increased with an increase in the test temperature and a decrease in the imposed strain rate. During open-die forging of this alloy under isothermal conditions, tensile stresses generated due to frictional effects and the associated inhomogeneous plastic deformation led to free-surface fracture via intergranular cracking. The failure observations under these two deformation modes were correlated successfully using a tensile-damage criterion.

The production, characterization, and consolidation of Fe3Al-based iron-aluminide powders are described. The consolidated powders were fabricated into sheet. The sheet was tensile tested from room temperature to 800°C and creep tested in the temperature range of 450 to 704°C. Mechanical properties were compared for nitrogen and argon atomized powders. Mechanical properties were also compared for argon atomized powders of Fe3Al alloys containing 2 and 5% Cr.

The melting of Fe3Al-based alloys at the Oak Ridge National Laboratory (ORNL) and commercial vendors is described. The melting processes evaluated include arc melting, air-induction melting (AIM), vacuum-induction melting (VIM), vacuum-arc remelting (VAR), and electroslag remelting (ESR). The quality of the ingots studied are based on internal soundness and the surface finish obtained. The ingots were analyzed for impurity levels observed in the alloys by various melting processes. Recommendations are made for viable processes for commercial melting of these alloys.

Monolithic Ni 3Al and composite Ni3Al/ceramic whisker bodies were fabricated by combined Self Propagating High Temperature Synthesis and in situ containerless Hot Isostatic Pressing (SHS/HIP), starting from cold pressed blends of nickel, aluminum, and boron powders, with and without additions of alumina whiskers. The resultant bodies were dense and had good ductility and mechanical strength at room temperature.

Weldability is a key issue for successful applications of ductile Ni3Al in structural components. Here we report the results of an investigation of gas tungsten arc welding behaviors of continuously cast thin sheet and cast ingot of a Ni3Al alloy, known as IC-50, containing boron and zirconium. Cracking in heat-affected zone (HAZ) was prevalent in both types of specimens. Both in cast sheet and ingot, liquation cracking was observed to occur along the grain boundaries in HAZ. Microstructural studies showed that continuously cast thin sheet consisted of columnar grains along the thickness. Easy cracking along the boundaries of columnar grains made cast sheets more susceptible to hot cracking than ingots. The possibility of segregation of boron and zirconium, causing liquation cracking, will be discussed.

This study focuses on the sequential formation of aluminide phases during annealing of titanium and aluminum thin film bilayers. The formation of titanium-rich intermetallic phases at higher annealing temperatures is emphasized. Using Rutherford Backscattering Spectrometry (RBS) analysis, and x-ray diffraction, phases formed as a function of temperature have been identified. The phases Al3Ti through Ti3Al were observed over the temperature range 450–750°C, where reaction with the SiO2 substrate occurred. All phases were present as discreet layers within the samples with several layered phases coexisting at the higher temperatures.

Six super α2 titanium aluminide pancakes were produced by rapid omnidirectional compaction of prealloyed Ti-25Al-l0Nb-3V-lMo (at%) [Ti- 14.1Al-19.5Nb-3.2V-2Mo (wt%)] powder. Various solution and aging heat treatments were investigated to yield the best combination of tensile and stress rupture properties. The β transus temperature was found to be about 1,090 °C (1,990 °F). The alloy was heat treated with four different schedules to develop various structure combinations of α2 and β, and followed by a series of microstructural analyses and mechanical property determinations. The highest ultimate tensile strength and yield strength attained at room temperature were 1,174.9 and 977.7 MPa (170.4 and 141.8 ksi), respectively, for a specimen heat treated at 1,140 °C (2,084 °F) for 1 hr and aged at 816 °C (1,500 °F) for 4 hr + air cooling. However, the elongation was less than 2%. Under the same heat treatment condition, the ultimate tensile strength and yield strength measured at 427 °C (800 °F) were 1,139.1 and 711.6 MPa (165.2 and 103.2 ksi), respectively, with an elongation of 6.7%. The alloy also showed good stress rupture resistance. Compared with a similar hot isostatically pressed material, the current alloy shows superior mechanical properties due to a finer microstructure.

The oxidation behavior of a number of gamma-TiAl alloys with ternary additions has been studied for extended periods during rapid thermal cycling to temperatures of up to 1000°C in flowing air. Under these conditions, binary alloys, such as Ti-48a/oAl and Ti-52a/oAl form flaking scales with sub-scale embrittlement, although the initial oxidation rate decreases with increasing aluminum content. Ternary Ti-Al-Cr and Ti-Al-Ta alloys with up to 4 a/o Cr or Ta also show a tendency to form spalling scales at these temperatures. On the other hand, the addition of niobium or tungsten results in a dramatic improvement in oxidation resistance at 850 °C, although at 1000°C even these alloys begin to spall after an induction period of several hundred hours.

In this study, an acid-chloride electrolyte at pH = 4 (H2SO4) and containing 200 ppm Cl- was used to define the effects of Cr concentration (0–6 at.%) and Mo additions (0–2 at.%) on the aqueous corrosion behavior of iron aluminides containing 28 at.% Al. For the Fe-28Al composition, cyclic-anodic-polarization testing indicated passivation, but with a relatively low breakdown potential for pitting corrosion, and a protection potential lower than the open-circuit corrosion potential. Cr additions alone proved beneficial by continuously increasing the pitting potential. However, even at the highest Cr level, 6%, the protection potential was still lower than the corrosion potential, indicating that pitting could initiate after an incubation period. Mo additions were found to raise the protection potential, such that at 1 and 2% Mo levels (4 % Cr), it was higher than the corrosion potential, indicating significantly improved resistance to the initiation of localized corrosion. Immersion testing showed that the latter compositions remained passivated with no localized corrosion for a period of four months, at which point the tests were terminated. The overall results indicated that for satisfactory resistance to chlorideinduced localized corrosion, both higher Cr levels (4–6 at.%) and Mo additions (1–2 at.%) are desirable.