Temperature orthorhombic alloys

Fig. 15. Oxidation kinetics for Ti-22 Al-23Nb orthorhombic alloys in air at temperatures in the range 500-900 °C (left) and the effect of Nb content on the parabolic rate constant for Ti-25 at% Al alloys at 800°C (right).

Fig. 24. Schematic diagram of the scale and interstitial affected zone (IAZ) which forms on orthorhombic alloys in the temperature range 500-800 °C (left) and the time dependence of the IAZ thickness (right).

Fig. 25. Room temperature ductility (measured in 3-point bending) of orthorhombic alloys after 100 hours exposure in air at various temperatures.

Fig. 31. Schematic diagram of the oxidation morphology and cracking observed in orthorhombic alloy/SiC composites (left) and the oxidation rates of the composites over the temperature range 500-900 °C (right).

The iron-carbon solid alloy which results from the solidification of non blastfurnace metal is saturated with carbon at the metal-slag temperature of about 2000 K, which is subsequendy refined by the oxidation of carbon to produce steel containing less than 1 wt% carbon, die level depending on the application. The first solid phases to separate from liquid steel at the eutectic temperature, 1408 K, are the (f.c.c) y-phase Austenite together with cementite, Fe3C, which has an orthorhombic sttiicture, and not die dieniiodynamically stable carbon phase which is to be expected from die equilibrium diagram. Cementite is thermodynamically unstable with respect to decomposition to h on and carbon from room temperature up to 1130 K... 

Nd-Pd-Sb. Marazza et al. (1980) established the Caln2 type structure with a = 0.4580, c = 0.7716 for NdPdSb compound by using X-ray powder diffraction and metal-lographic analyses. For the sample preparation and the purity of starting components, see Y-Pd-Sb system. The crystallographic characteristics were confirmed from powder diffraction of arc melted and annealed at 1073 K alloys with a = 0.4577, c = 0.7676 (Zygmunt and Szytula, 1995). Mehta et al. (1995) reported an orthorhombic structure for NdPdSb at room temperature SG Pmma, a = 0.45833, b = 0.77189, c = 0.7937. 

The transformations above room temperature (monoclinic-orthorhombic, orthorhombic-tetragonal I) are more easily understood. One notes the expected decrease in transition temperature with increasing sodium content, as is found in many alloy phase diagrams. Attempts to observe transitions to a cubic phase at higher temperatures failed, owing to sublimation of samples of very low sodium content (x < 0.02) or decomposition at higher sodium concentrations. 

Metallurgically, uranium metal may exist in three allotropic forms orthorhombic, tetragonal, or body-centered cubic (EPA 1991), and may be alloyed with other metals to alter its structural and physical properties to suit the application. Like aluminum metal powder, uranium metal powder is autopyrophoric and can burn spontaneously at room temperature in the presence of air, oxygen, and water. In the same manner, the surface of bulk metal, when first exposed to the atmosphere, rapidly oxidizes and produces a thin surface layer of UO2 which resists oxygen penetration and protects the inner metal from oxidation. 

Ti-6A1-4V is an alpha-beta alloy that can be modified extensively by both thermal and thermomechanical processing to produce a large variety of microstructures and hence a wide spectrum of mechanical properties. The beta-transus temperature is approximately 1000 °C (1830 °F) and is a function of interstitial content (Ref 1). Samples of Ti-6A1-4V cooled at relatively slow rates from elevated temperatures contain mainly the alpha and beta phases as a result of diffusional transformations, while those cooled rapidly may also contain martensitic phases such as the cc (hep structure) or the a" (orthorhombic structure) phases. 

NdFeSi was claimed to be single-phase with an orthorhombic TiNiSi-type of structure. The lattice parameters (a = 11.18, b = 6.89 and c = 5.32), however, do not correspond to a TiNiSi-type unit cell. The existence of a compound NdFeSi was confirmed by Bodak et al. (1970) but at variance with Mayer and Felner (1973) a PbFCl-type of structure [P4/nmm, a = 4.057(3), c = 6.893(5)] was obtained from arc-melted alloys heat treated at 800 ° C for 3 months (low-temperature phase ). Due to the high temperature of preparation and homogenization the TiNiSi-type phase as reported by Mayer and Felner (1973) is likely to represent a high-temperature modification. 

Shikhmanter et al. (1983b) have carried out TEM experiments in order to study the crystallization behavior of some R-Au (R=Gd, Tb, Dy and Er) vapor-deposited amorphous films (120 run thick). Crystallization takes places in the temperature range of 463-513 K, and further heating by an additional 50 K leads to the formation of the RAu alloys (CsCl strueture type). Further annealing at 533 K induces an allotropic transformation such as CsCl type (cubic structure) —+ CrB type (orthorhombic structure). The former is metastable, while the latter is, as in the bulk, more stable at low temperatures. The presence of R2O3 crystallites can act as catalyst for the transformation. It is concluded that conditions amenable to heterogeneous nucleation will appear on the R-Au films at higher temperatures than in the R-Cu films (413—423 K) or R-Ag films (388-398 K). 

As with the a2-based alloys, orthorhombic-based alloys also suffer from oxygen embrittlement during elevated-temperature... 

Figure 6-27. Unpublished data of Brindley (1996) (reported in Brady et al. (1996a)) showing that although the oxidation-resistance of the orthorhombic-based alloy Ti-22Al-20Nb-2Ta-lMo at.% is superior to that of Ti-48Al-2Cr-2Nb at.% at 800°C in air, it still suffers from extensive hardening during elevated-temperature exposure in air. (a) Cyclic oxidation data (b) Knoop microhardness data (25 g/15 s).

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