H. Clemens

Thermal-cycling creep of γ-TiAl-based alloys

In two-phase TiAl-based alloys, the coexisting 2 and phases exhibit a thermal expansion mismatch, so that increased creep rates during thermal cycling may be expected. Creep deformation of two g-TiAl-based alloys was investigated during thermal cycles between 900 and 300 or 350C with applied tensile stresses of 32.5 or 37.0 MPa. Measured thermal cycling creep rates were compared with isothermal creep rates calculated at the eAective average temperature. No creep enhancement was measured upon thermal cycling within uncertainties of 1.610 ˇ3 % strain per cycle. # 2000 Elsevier Science Ltd. All rights reserved.

Microstructure of die-cast alloys Mg-Zn-Al(-Ca): A study by electron microscopy and small-angle neutron scattering

Abstract The microstructure of a magnesium die-cast alloy with 8 wt.% Zn and 5 wt.% Al (ZA85) and of two alloy modifications with 0.3 and 0.9 wt.% Ca (ZACa8503 and ZACa8509) are compared. Optical, scanning electron and transmission electron microscopy (TEM) were used to study the microstructure in as-cast and annealed conditions. Small-angle neutron scattering (SANS) measurements were carried out to characterize the early stages of the precipitation process. The die-cast microstructure of ZA85, ZACa8503 and ZACa8509 consists of dendritic α-Mg grains and coarse intermetallic grain-boundary phases. In the Ca-free ZA85 alloy, grain boundaries are decorated with a quasi-crystalline phase with icosahedral point-group symmetry (I phase), a metastable modification of the equilibrium τ phase (Mg32(Al, Zn)49). At the grain boundaries of the Ca-containing ZA85 alloys, small amounts of the stable cubic τ phase Mg32(Al, Zn, Ca)49 as well as Al2(Ca, Zn) phase, were detected in addition to the I phase. In the as-cast c...

Microstructure and mechanical properties of Si and YN doped powder metallurgical tantalum

Abstract Tantalum has a wide range of applications due to its high melting temperature, its corrosion resistance and its mechanical and electrical properties. One of these applications are fine wires which are needed for Ta-capacitors. During fabrication of such components, the wires are exposed to high temperatures and, thus, grain growth combined with embrittlement becomes a major problem. In the present work, the effects of doping with small amounts of Si and YN on microstructure and mechanical properties of annealed Ta-wires have been investigated. Doped samples show a higher hardness and strength than samples of pure powder metallurgical (PM) Ta; however, the grain growth kinetics are very similar. Samples doped with Si and samples doped with both Si and YN exhibit almost identical properties.

Mechanical Anisotropy in Sheets of γ-TiAl Alloys

At room temperature sheets of {gamma}-TiAl exhibit a higher yield stress in the rolling direction than in the transverse direction. Around 700 C the opposite behavior is observed. The texture mainly consists of a modified cube component. The tetragonal c-axis is aligned in the sheet plane transversely to the rolling direction. Taken into account this special texture and the single crystal yield surface of {gamma}-TiAl the authors conclude that around 700 C the CRSS of super-dislocations is higher than the CRSS of ordinary dislocations. At RT the relation changes to the opposite.

Evolution of texture in alloy 80A during initial ingot breakdown

Abstract This paper presents details on the evolution of texture in Alloy 80A during the initial ingot break-down process by means of repeated hot-working (cogging). The ingot was produced by vacuum arc re-melting showing equiaxed and columnar grains. Compression tests were performed with a Gleeble system covering the range of temperatures, strains, and strain rates experienced during processing on an industrial scale. Texture analysis (neutron and electron back-scatter diffraction) was applied to all specimens before and after hot deformation to reflect the thermo-mechanical history. The ingot exhibited a well pronounced -fiber texture of the initial microstructure. Low deformation ratios did not change the type of texture, but the degree of the preferred orientation. High strains partially result in the destruction of the fiber texture and lead to the appearance of additional texture components, such as cube- and Goss-components. With continued deformation, recovery and re-crystallization become pr...