Dispersion Strengthened Intermetallics by Mechanical Alloying: Creep Results and Dislocation Mechanisms
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DISPERSION STRENGTHENED INTERMETALLICS BY MECHANICAL ALLOYING: CREEP RESULTS AND DISLOCATION MECHANISMS
E. Arzt, E. G~hring and P. Grahle, Max-Planck-Institut fiir Metallforschung and Institut fUr Metallkunde Stuttgart, Germany
Abstract In order to increase the creep strength, small dispersoids were introduced into the intermetallic compound NiAl by mechanical alloying. Under favorable conditions, the resulting specific creep rates approach those of superalloys. To accompany these experimental effects, a new model has been developed which predicts tendencies for the success of dispersion strengthening in ordered alloys. It is shown that the dissociation distance of the partial dislocations, relative to the dispersoid particle size, has an important effect on the strengthening achievable. Introduction The creep strength of monolithic intermetallics such as aluminides is typically by far inferior to that of advanced superalloys [1]. Recent developments have sought to improve the high temperature strength of NiAl by preparing particulate composite materials containing second phase particles of more than 10 vol.% [2]. The probably first study on oxide dispersion strengthened (ODS) intermetallic alloys [3] showed the producibility, in principles of ODSNiAl and ODS-FeAl based on a processing technique that is now called Mechanical Alloying (MA). The main advantage of ODS alloys lies in the retention of useful creep strength up to high homologous temperatures, where other strengthening mechanisms are no longer effective [4]. Model approaches of creep in dispersion strengthened, disordered alloys show that the choice of the dispersoid parameters has a strong influence on the efficacy of dispersion strengthening [4, 5]. However, no real theoretical understanding of creep in dispersion strengthened ordered alloys exists [6] and thus, the promises and limitations of dispersion strengthened intermetallics are unclear. The present paper adresses new experimental and theoretical aspects of this promising new class of high-temperature materials. Experimental Processing of dispersion strengthened and composite materials ODS NiAl materials were prepared from prealloyed, gas-atomised Ni3AI and Al-24 at.% Ni powders supplied by Homogeneous Metals, Inc.. Mechanical Alloying was carried out with 1 to 2 vol.% Y203 dispersoid powder for 50 hours in a high energy ball mill at the Metallgesellschaft AG Frankfurt. Milled powders were vacuum canned in stainless steel and hot extruded at 1200°C [7]. Further details can be found in table 1. The preparation of NiAl-NiAINb composite materials is described elsewhere in these proceedings [8]; it resulted in globular second phase particles, randomly distributed in the NiAI matrix, with a mean diameter of 18 gm. As an alternative to the cryomilling process [2], a NiAl alloy containing about 10 Vol.% AIN was prepared by a gas-metal absorption reaction: atomised NiAl powder was milled for lh under argon to an average particle size of about 7 gm; the as milled powder was heat treated at 1200'C in a n
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