Processing and Deformation Behavior of Gamma TiAl Alloys with Fine-Grained Equiaxed Microstructures

This paper describes how a submicrocrystalline structure can be produced by isothermal deformation of cast and powder metallurgy -TiAl-based alloys at temperatures 1000°C and below using relatively inexpensive nickel-based superalloy die tooling. A detailed analysis of the effects of initial microstructure, chemical compositions, conditions of heat treatment and hot working on the formation of a homogeneous fine-grained microstructure in -TiAl-based alloys is presented.     

Dynamic strain aging and hydrogen-induced softening in alpha titanium

Compression tests were carried out on samples of commercial-purity titanium charged with up to 4.7 at. pct hydrogen. Strain rates of 10⁻³ to 1 s⁻¹ were employed and testing was limited to the α phase field at temperatures of 773 to 973 K. The dependences of the flow stress on strain, strain rate, and temperature were determined. A plateau or bulge appeared in the temperature and strain-rate dependences of the flow stress, and the work-hardening rate also showed peaks. Serrations were observed on some of the stress-strain curves. All these features indicated that dynamic strain aging (DSA) was occurring. Analysis of the results (together with data from other authors) indicates that there are three ranges of DSA behavior in this material within the experimentally investigated temperature range; these appear to be associated with the diffusion of iron, carbon, and oxygen, respectively. Alloying with hydrogen decreases the magnitude of the DSA attributable to these elements and displaces the phenomenon to higher temperatures and/or to lower strain rates. The dependence on strain rate and temperature of the relative softening attributable to hydrogen addition was determined. The results indicate that hydrogen-induced softening is related to the occurrence of DSA in this temperature range. Possible explanations for this relationship are discussed. O.N. SENKOV, on leave from the Institute of Solid State Physics, Russian Academy of Sciences, Moscow Region 142432, Russia     

Fatigue and Fracture Behavior of a Ca-Based Bulk-Metallic Glass

The compression and fatigue behavior of a Ca₆₅Mg₁₅Zn₂₀ bulk-metallic glass (BMG) was studied in air at room temperature. During the preparation of cubical samples of the Ca₆₅Mg₁₅Zn₂₀ for compression and fatigue investigations, small spherical cavities were found. Under both monotonic and cyclic compression loadings of the samples, fractures initiated at these cavities and propagated in a direction generally parallel to the loading axis. Finite-element analysis (FEA) was used to model the fracture behavior. The FEA of a centrally located spherical void showed that under compression loading, large tensile stresses evolved in the cavities. The orientation of the maximum principal stress (P1) was found to be normal to the direction of crack propagation, which is consistent with the experimental finding. Stresses in deeply embedded adjacent voids and those in superficial voids were also studied. The influence of the void location in the cubical sample on the fracture behavior was quantitatively discussed.     

Microstructure and properties of cast ingots of Al-Zn-Mg-Cu alloys modified with Sc and Zr

The effect of combined additions of Sc and Zr on the microstructure and tensile properties of the direct chill (DC) cast ingots of developmental Al-Zn-Mg-Cu alloys has been evaluated in this work. The properties in both the longitudinal and transverse directions were determined in as-cast and cast-plus-heat-treated conditions, at room temperature (RT) and cryogenic temperature (CT). Extensive microstructural evaluation was carried out using optical microscopy and scanning electron microscopy, including orientation image microscopy (OIM) by the electron backscatter diffraction (EBSD) technique. The Sc-containing developmental cast alloys showed the tensile properties, which are much better than the properties of commercial cast Al alloys and are similar or even superior to the properties of 7075-T6 alloy forgings. The microstructural evolution, the strengthening mechanisms, the optimum content of the dispersoid-forming elements, and the processing-structure-property correlations are discussed.     

Recent advances in the thermohydrogen processing of titanium alloys

In this article, the status of the methods and applications of the thermohydrogen processing of Titanium alloys is reviewed. Increased understanding of the mechanisms by which such processing is enhanced and the microstructure refined should lead to industrial applications.     

Microstructural control in Ti-Al for enhanced mechanical properties

A study was initiated to understand the effect of grain refinement on mechanical properties of TiAl-based alloys at room and elevated temperatures. The following was found in that study. Utilizing a processing approach that includes homogenizing annealing or deformation in the α-phase field, followed by multi-step isothermal forging in the γ+ α₂ phase range at relatively low strain rates and a continuously decreasing temperature, homogeneous submicro-crystalline structures were produced in titanium aluminides, facilitating further processing such as superplastic forming at temperatures below 1,000°C. Also described in this paper is a fully lamellar structure with a colony size as small as 15 μm, produced in gamma TiAl-based alloys containing borides by the fast-heating of an initially submicro-crystalline structure in the alpha-phase range followed by air cooling. For more information, contact F.H. Froes, Institute for Materials and Advanced Processes, University of Idaho, Mines Building, Room 321, Moscow, Idaho 83844-3026; (208) 885-7989; fax (208) 885-4009; e-mail imap@uidaho.edu.     

Relative x-ray reflectometry of discrete layered structures

The relative x-ray reflectometry (RXR) technique has been used for the first time for determining the parameters of a discrete layered structure. The scheme of x-ray optics used for these measurements on two wavelengths is described, the conditions of applicability of the proposed method are formulated, and the algorithm of data processing is described and justified. The results of RXR measurements for a test structure prepared by magnetron sputtering of tantalum onto a silicon substrate via a mask with periodically arranged windows are presented. The proposed RXR scheme can be used in semiconductor technology for the monitoring of parameters of device structures containing nanolayers.     

Energy-dispersive X-ray-absorption edge spectrometry with spectrum filtration by X-ray mirrors

A highly sensitive energy-dispersive scheme with a semiconductor spectrometer is proposed for determining impurity concentrations from the X-ray-absorption edge spectra. The optimization of spectrum in a band studied is ensured by sequential reflections of transmitted radiation from X-ray mirrors and by variation of the grazing angle of the analyzed beam. Results of measurements of the X-ray-absorption edge spectra of Fe in dispersed salt samples and W impurity in beryllium plate are presented. It is shown that the problem of deconvolution can be numerically solved by smoothing X-ray-absorption fine-structure oscillations with power functions.