TiAl Components Research Articles

Influence of water vapour on the oxidation behaviour of titanium aluminides

The influence of water vapour on the oxidation of a technical γ-TiAl based alloy at temperatures of 700 and 750 °C is discussed. The present paper focuses on the oxidation behaviour in dry air and air containing 10 vol.% H 2O with respect to oxidation kinetics and microstructural investigations of the oxide scales and the subsurface zones. The presence of water vapour led to a significant acceleration in the oxidation kinetics as well as to a change in the crystal morphology and microstructure of the oxide scales formed. The results indicate that the kinetics of the reaction of water molecules with the rutile phase and a change in its defect structure are important parameters in the mechanism. The subsurface zone formed in water vapour-containing atmospheres consisted (in addition to the cubic Al-depleted layer) of a lamellar layer of Al 2O 3 and Ti due to either internal oxidation or decomposition of the cubic phase. Both, the oxide scale and the depletion layer are important regarding the mechanical behaviour of a TiAl component at high temperatures which was also investigated.

Notch strengthening in titanium aluminides under monotonic loading

An important concern for titanium aluminides is their limited ductility and its consequences for TiAl components containing stress concentrators. In recent experiments, evidence of notch strengthening has been found in one TiAl alloy under monotonic loading. The goal of this study is to fully explore this issue through tests on two cast alloys, one with a microstructure consisting of predominantly equiaxed gamma grains and the other having a fully lamellar microstructure. Tests involve monotonic tensile loading of notched specimens at room temperature under conditions of plane stress, where the notch radii are large relative to grain size. For each material, results from the testing of three notched specimen geometries are presented and finite element models are used to interpret the test results. This includes using the numerical models to apply Weibull statistical methods to predict notch strengthening in the specimens. It is shown that notch strengthening is clearly seen in both alloys tested and thus is likely to be a characteristic of TiAl alloys in general; however, strengthening is not as great as would be predicted by Weibull statistical methods as applied to brittle materials.

Modeling and measurement of the notched strength of gamma titanium aluminides under monotonic loading

A substantial concern with gamma titanium aluminides is their limited ductility and, in particular, the consequences of limited ductility at stress concentrators. In this study, the notched strength of a cast Ti-47.9Al-2.0Cr-2.0Nb alloy is considered under monotonic tensile loading at room temperature. Efforts are further focused on the alloy’s behavior under conditions of plane stress and on cases where notch radii are large relative to grain size. Finite element predictions for notched tensile specimens and the Neuber design criterion are used to quantify relationships between tensile ductility, the ability to reduce local stress concentrations through plastic flow, and ultimate failure loads in notched components. Results from the testing of two modeled configurations are presented, and the numerical models are used to interpret the test results. Two major issues are addressed in this work. The first is how much plastic deformation is needed to blunt stress concentrations in gamma TiAl components under monotonic loading. For values of elastic stress concentrations commonly encountered in components, it is demonstrated that a total strain at failure as low as 1 pct (corresponding to a plastic strain at failure of 0.8 pct) is sufficient to essentially achieve the maximum reduction in stress concentration due to plastic flow. Second, the relationship between continuum-theory predictions of notched component failure (made using uniaxial tensile test strains at failure) and failure loads observed in notched specimens is explored. It is shown that, on average, continuum-theory predictions of notched strength based on unnotched specimen strains at failure act as conservative lower bounds on actual results. It is suspected that the notch strengthening observed in gamma TiAl is due to smaller volumes of highly strained material (a size effect) in the notched specimens.

BEYOND PARALLEL PROCESS: RELATIONAL PERSPECTIVES ON FIELD INSTRUCTION

This article explores the implications of contemporary rela- tional perspectives for transforming our understanding and use of the concept of parallel process in supervisory relationships in social work field instruction. We review the history of the concept of parallel process, summarize current rela- tional views in psychodynamic supervision, and critique the social work litera- ture in light of the paradigmatic shift to a relational, constructivist viewpoint. We present two vignettes illustrating how relational sensibilities enrich our un- derstanding and increase the effectiveness of the field instruction relationship. We relocate parallel process, which up to now has been a primary, organizing framework in psychodynamic supervision, in a larger relational matrix, where it is one element in a complex, interactive process and serves as a marker for fur- ther exploration in reflective social work practice. Field instruction is the centerpiece of social work education, provid- ing an experiential overlay to theoretical underpinnings of practice. The practicum is the arena in which the student integrates theoretical know- ledge with actual case material, acquiring skills in assessment, employ- ing models of treatment, and testing intervention strategies. An essen- tial component of field instruction is the relationship between the field instructor and the student, since the supervisor models professional be- havior, explores therapeutic impasses, delineates ethical boundaries, and lays the groundwork upon which the student builds a professional self. Intrinsic to the supervisory relation is the parallel process, an un- conscious reenacting by the student with the supervisor of the issues

Development of permanent-mold cast TiAl automotive valves

Abstract This paper presents the results of an EMTEC funded project to develop low-cost TiAl automotive valves. The alloy studied was Ti-47Al-2Nb-1.75Cr (at %). Over 800 valves were cast, using several variations of the permanent-mold process, in a multi-cavity steel mold. Applying pressure during solidification improved the casting fill. However, none of the permanent-mold casting methods produced pore free as-cast valves. The as-cast microstructures of valves produced by permanent-mold casting were much finer than investment castings of similar section sizes. Of the permanent-mold casting methods, the injection cast method exhibited the finest as-cast structure showing a potential for a new high output method for producing fine-grained TiAl components. Room temperature tensile properties of the permanent-mold material were superior to those of investment castings with a similar microstructure. Two sets of valves were road tested for a total of 50000 km with average 2% fuel savings and no valve damage. There is a need to overcome few challenges before this technology can be implemented in the automotive industry.

Three Mazatec Wise Ones and their Books

■ Most accounts of psychedelic mushroom use in Mexico's Sierra Maza teca describe the practice as a prehispanic survival degraded by tourism. This article explores how - through chants, self-descriptions and life stories - Mazatec curers who deal with 'outsiders' use the discourse surrounding mushrooms to construct a mimetic model of power and the relationship between inside and outside that highlights the importance of the border and the position of the mediator. The themes of shamanic travel and the book make both sense and power out of magical mushroom mediation and involve the outsider as an essen tial component of Mazatec identity.

The fatigue resistance of TiAl-based alloys

TiAl-based alloys are a class of intermetallic alloys that show potential as a high-temperature structural material. Many applications considered for TiAl-based alloys require certain damage-tolerance characteristics. In this article, the fatigue resistance of two-phase TiAl alloys is reviewed to assess the current understanding of the fatiguedamage processes in this class of emerging materials. Salient features of fatigue-crack growth and stress-life curves are summarized to identify the challenges these fatigue characteristics present in the life assessment of TiAl components.

Effect of prior cooling rate on the grain size of fully-lamellar TiAl-base alloy developed by tempering/quenching

The near-gamma titanium aluminides are attractive for aerospace and automobile applications, because of their low density, good high-temperature strength and acceptable oxidation resistance. Investment casting appears to be the lowest cost alternative for the manufacture of near-net-shape TiAl components; but, the unavoidable remnant of coarse ingot grains in the final product deteriorates the advance of cast alloy. Recently, the authors found that the fine lamellar structure can be developed in cast TiAl-base alloy by a quenching/tempering process. By recrystallization of the quenched massive {gamma} structure at temperature above the {alpha} transus, a fine fully-lamellar structure with a grain size of 150 {micro}m can be obtained. For the engineering application of this process, it must be considered that, in some TiAl-base alloys, microcracks might be introduced during water quenching, and for thick-wall TiAl components, the cooling-rate in center region is lower than that in the surface. In this context, this paper aims to study the effect of prior cooling rate on the tempered lamellar grain size in a cast Ti48Al2W0.5Si alloy.