Seetharama C. Deevi

Advances in processing of Ni3Al-based intermetallics and applications

Abstract The intermetallic-based alloys for structural applications have been an active field of research around the world for the last 20 years. Several major breakthroughs have occurred in this field during this time period. These breakthroughs include: (1) the dramatic effects of boron on ductility improvement for Ni 3 Al at ambient and high temperatures, (2) effect of chromium addition for intermediate temperature ductility improvement of Ni 3 Al, and (3) identification of an environmental effect from hydrogen generated by the reduction of moisture in air by aluminum in the aluminides. The knowledge of the compositional effects has led to the development of Ni 3 Al-based alloys, which allowed them to be taken from laboratory-size melts to commercial applications. This paper will describe the advances in melting practice, casting practices, solidification modeling as it applies to static and centrifugal castings and weld repairs, and welding of castings. This paper will also describe various applications of Ni 3 Al-based alloys and their current status of commercialization.

Vacancy formation and effects in FeAl

Abstract Iron aluminide, FeAl, has been widely studied because of its excellent high temperature oxidation and corrosion properties. At high temperatures, FeAl generates a large number of thermal vacancies, and the vacancy concentration increases with increasing aluminum content. The mechanical properties depend on the vacancy concentration, so a thorough understanding of the generation and annihilation of vacancies is necessary. Heat treatment, temperature, and time control the vacancy concentration. Increasing vacancy concentration will increase the hardness and decrease the ductility of FeAl. The yield strength anomaly of FeAl, increasing yield strength with increasing temperature is believed to be due to vacancy hardening, and the decrease in strength above the peak temperature is attributed to the creep of FeAl. Ternary alloying elements, except boron, have little effect on the hardening at high temperature and can increase the hardness after long time anneals at low temperature. This review and analysis presents a summary of the current literature available on FeAl vacancies and their affects, including hardness, the yield strength anomaly, and the effect of ternary alloying elements.

Creep and fatigue properties of high temperature silicides and their composites

Abstract A review of creep and fatigue behavior of high temperature silicides and their composites is presented along with new results pertaining to titanium silicides and functionally graded materials. The emphasis was placed on the molybdenum disilicides and their composites in comparison to other silicides and high temperature ceramics. It was shown that the grain size effects on creep are significant in these materials. The effects were present not only in the Newtonian creep but persists even in the power-law creep, with grain size exponents of the order of 4.2. Factors that influenced the grain size effects were examined. This anomalous behavior, which appeared to be exhibited by some stoichiometric intermetallics, could not be explained by the currently known theories of grain boundary creep. Effects of alloying and reinforcement on creep rates were also discussed. The creep rates in other high temperature silicides were compared. It was shown that molybdenum disilicide with silicon nitride has a potential for high temperature applications in terms of its creep and oxidation resistance, although its creep resistance appears to be much less than that of an advanced Si3N4, such as AS800. Published results on fatigue crack growth in MoSi2 and its composites were analyzed using the recently developed Unified Approach for Fatigue.

Powder processing of FeAl sheets by roll compaction

Abstract Powder metallurgical processing techniques were employed to manufacture 0.2 mm thickness of iron aluminide sheets based on Fe-40 at% Al using water atomized powder of an FeAl alloy containing C, B, Mo and Zr as alloying elements. Powders of FeAl mixed with polymeric binders were roll compacted using two counter rotating rolls to a thickness of approximately 0.66 mm. Roll compacted sheets were then debindered in nitrogen using a two stage debindering cycle over a period of several hours to effectively decompose the binder. Debindered sheets were vacuum sintered prior to cold rolling them for densification in several different stages. The properties of the FeAl sheets are dependent on the processing temperature, Al content of the final sheet, oxygen content, and the microstructure. Sheets exhibit fine microstructure with a dispersion of Al2O3 particles throughout the matrix. Variation of electrical resistivity, specific heat, thermal expansion, and thermal diffusivity with variation of temperature is presented, and compared with 304 steel.

Exo-MeltTM process for melting and casting intermetallics

Abstract Melting of Ni 3 Al-based intermetallic alloys was successfully accomplished by the Exo-Melt TM process by modifying the furnace-loading sequence to initiate the highly exothermic NiAl reaction to form superheated NiAl liquid initially. Recorded images of the melting process showed the occurrence of exothermic diffusional reactions between liquid aluminum and solid nickel melt stock. Combustion synthesis of NiAl and Ni 3 Al is also described and compared with the Exo-Melt TM process. In contrast to the Exo-Melt TM process, the conventional melting practice of adding aluminum to molten nickel increased the temperature of the bath from 1600 to 2300 °C, and a vapor cloud formed due to the oxidation of alloying elements. The Exo-Melt TM process results in power savings of 47% over conventional melting practices, and the process has been used to melt and cast a total of 8000 kg of Ni 3 Al-based alloy. Reaction mechanisms involved in the Exo-Melt TM process and conventional melting processes are described and compared with the reaction mechanisms operating in combustion synthesis.

Direct Magnetic Patterning due to the Generation of Ferromagnetism by Selective Ion Irradiation of Paramagnetic FeAl Alloys

Sub-100-nm magnetic dots embedded in a non-magnetic matrix are controllably generated by selective ion irradiation of paramagnetic Fe(60)Al(40) (atomic %) alloys, taking advantage of the disorder-induced magnetism in this material. The process is demonstrated by sequential focused ion beam irradiation and by in-parallel broad-beam ion irradiation through lithographed masks. Due to the low fluences used, this method results in practically no alteration of the surface roughness. The dots exhibit a range of magnetic properties depending on the size and shape of the structures, with the smallest dots (<100 nm) having square hysteresis loops with coercivities in excess of micro(0)H(C) = 50 mT. Importantly, the patterning can be fully removed by annealing. The combination of properties induced by the direct magnetic patterning is appealing for a wide range of applications, such as patterned media, magnetic separators, or sensors.

Processing maps for hot working of a P/M iron aluminide alloy

The hot working behavior of a Fe-24 wt.% Al iron aluminide alloy processed by the powder metallurgy route has been studied in the temperature range 750-1150°C and strain rate range

Electronic structure and transport properties of Fe–Al alloys

0.001-100 \hspace{2mm}s^{-1}

Isothermal oxidation behavior of a sheet alloy of Fe–40at.%Al at temperatures between 1073 and 1473 K

by establishing processing maps at different strains in the range 0.1-0.5. The features in the processing maps have changed with strain suggesting that the mechanisms of hot deformation are evolving with strain. Early in the deformation (strain of 0.1), the map exhibited a single domain with a peak efficiency of power dissipation of about 44% occurring at about 1100°C and a strain rate of about

Influence of temperature and strain rate on the flow stress of an FeAl alloy

0.03 \hspace{2mm} s^{-1}