Environmental Effects on Mechanical Disordering of Ni 3 Al-Base Alloy
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ENVIRONMENTAL EFFECTS ON MECHANICAL DISORDERING OF Ni3AI-BASE ALLOY
S. GIALANELLA*, M. GUELLA*, F. MARINO', M.D. BARO'0 0 , J. MALAGELADA0 0 , S. SURINACH°°, R.W. CAHN** *Dipto di Ingegneria dei Materiali, UniversitA di Trento, 38050, Mesiano, Trento, Italy °Dipto di Scienza dei Materiali e Ingegneria Chimica, Pol. di Torino, 10129, Torino, Italy **Dept. de Fisica, Universitat Aut. de Barcelona, 08193, Bellaterra, Barcelona, Spain "**Dept.of Mat. Sci. and Metallurgy, Univ. of Cambridge, CB23QZ, Cambridge, U.K. ABSTRACT We present some results regarding the influence of grinding atmosphere on the phenomena occurring during mechanical milling of a Ni 3 Al-base alloy. We have disordered this permanently ordered alloy by ball-milling in a vibratory mill, using alternatively argon and hydrogen atmospheres. We observed a slower reduction of the long-range order parameter during experiments carried out under a hydrogen atmosphere, regardless of the used grinding media. We discuss our findings in terms of the effects, such as in grain boundary cohesive strength or localize ductility enhancement, that hydrogen may induce in such alloy. The microstructural parameters of the powders at different stages of disordering have been investigated using calorimetric tests, x-ray diffraction analysis and microscopic observations. INTRODUCTION The main driving force for the study and development of advanced ordered alloys and intermetallics is the potential that this class of materials offers for high temperature applications. The extreme service conditions faced in aerospace engines, electrical power plants turbines, etc., require good mechanical strength, surface stability and creep resistance at elevated temperatures and under environmentally aggressive atmospheres. Apart from the obvious consequences that such environments may have on the oxidation [1] and corrosion [2] resistance of intermetallics, they have also been studied in view of their effects on the mechanical properties of these materials. For some of them testing atmospheres can cause dramatic changes in the observed results [3]. So, for example, according to the cited study, at room temperature oxygen reduces dislocation mobility in '-TiAI and, consequently, ductility. Analogous effect was observed in Ni 3 AI compounds deformed at high temperatures: oxygen grain boundary diffusion leads to the formation of brittle regions which facilitate intergranular crack growth. Hydrogen too has been shown to produce embrittlement in a number of intermetallics, through different mechanisms: by interacting with dislocations and therefore hindering their motion under applied stresses by reducing grain boundary strength and increasing stress concentration. Hydrogen may be cathodically charged into the alloys or adsorbed from the gaseous products of the reaction between water vapour and the alloy. The latter phenomenon is particularly active in the case of Al alloys and leads to the contemporary formation of aluminum oxide. Apart from these detrimental effects, hydrogen has to be cited
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