1997 TMS Annual Meeting: Monday Session

CHEMISTRY AND PHYSICS OF NANOSTRUCTURES AND RELATED NONEQUILIBRIUM MATERIALS: Session II: Phase Transformations

Session Chairperson: John Morral, Univ. of Connecticut, Dept. of Metallurgy, Storrs, CT 06269

2:00 pm INVITED

ALUMINIUM-BASED NANOPHASE COMPOSITES BY DEVITRIFICATION: A.L. Greer, University of Cambridge, Department of Materials Science & Metallurgy, Pembroke Street, Cambridge CB2 3QZ, UK

Al-TM-Ln alloys (TM... transition metal; Ln...lanthanide) can be rapidly quenched into a fully amorphous state and then partially devitrified to give a nm-scale microstructure of aluminium crystallites uniformly dispersed in an amorphous matrix. This is a highly unusual microstructure for an aluminium alloy, indicating a high density of independent nucleation events in the glass. This study focuses on the development and stability of microstructure in a series of Al-Ni-Y alloys. The effects of various single- and two-stage heat treatments are explored. The emphasis is on the particular features associated with the unusual nm-scale among these is the overlap between coarsening and further transformation arising from capillarity effects on the crystallites.

2:30 pm INVITED

KINETICS OF NANOPHASE CRYSTALLIZATION IN Al-Fe-Gd ALLOYS: A.A. Csontos, G.J. Shiflet, Materials Science and Engineering, University of Virginia, Charlottesville, VA 22903

This presentation will provide results an the formation and subsequent growth of nanocrystalline aluminum phases in an amorphous matrix. A prototype for this family of metallic glass alloys that can be transformed into nanocrystalline material is the Al90Fe5Gd5 system. Detailed measurements of nanocrystalline isothermal growth from an amorphous Al90Fe5Gd5 matrix were made from 150 to 500°C. Coupled with growth of the nanocrystals, measurements of Gd and Fe segregation between the nanocrystal and the matrix were secured in a TEM with a field emission gun. Both Fe and Gd are preferentially rejected into the remaining matrix as the aluminum-rich nanocrystal grows. After reaching a specific size, which varies with temperature, growth slows down and funkier changes are slight until subsequent nucleation and growth of compound phases. such as Al4Gd occur. The relative stability of the formation of nanocrystals from an amorphous matrix will be addressed. Research supported by the University of Virginia Academic Enhancement Program.

3:00 pm INVITED

DIFFUSION FIELD IMPINGEMENT DURING PRIMARY CRYSTALLIZATION OF ALUMINUM NANOCRYSTALS: D.R. Allen, J.C. Foley, J.H. Perepezko, Materials Science and Engineering, University of Wisconsin - Madison, 1509 University Avenue, Madison, WI 53706

Aluminum-rich glasses containing about 85 at% Al and a combination of transition and rare earth element additions have yielded microstructures of Al nanocrystals in an amorphous matrix with nanocrystal volume fractions approaching 20% and excellent mechanical properties. A high density of nanocrystals (>1020 m-3) develops during the primary crystallization reaction but growth is limited. A new kinetics analysis shows that diffusion field the nanocrystals. The kinetics model has been applied to DSC exotherms that correspond to primary fcc nanocrystal formation. A thermodynamic model of the fcc-liquid phase support heat evolution rate calculations used in the model. The results indicate that modification of the nucleant density should be the primary focus in limiting nanocrystal growth due to reduced length scales.

3:20 pm

MICROSTRUCTURAL AND THERMAL ANALYSES OF CRYSTALLIZATION IN ULTRAFINE AMORPHOUS TITANIA PARTICLES: J.-S. Yin, L. He, G.L. Griffin*, E. Ma, Mechanical Engineering, *Chemical Engineering, Louisiana State University, Baton Rouge, LA 70803

Ultrafine amorphous titania particle aggregates, with a mean particle size of 145 nm, were prepared using a hydrolysis technique in an aerosol reactor. Their crystallization bahavior has been studied using transmission electron microscopy, differential scanning calorimetry, X-ray diffraction, and scanning electron microscopy. The amorphous powder crystallized into anatase with a crystallization enthalpy of 27 kJ/mol and an apparent activation energy of 2.0 eV. The anatase phase nucleated preferentially in contact regions between neighbouring particles. This crystallization mode is interpreted as a consequence of the presence of appreciable local pressure (stress) which was predicted by model calculations and observed under TEM. An analysis suggests that the pressure effect reduces the relative stability of the amorphous phase by enhancing the thermodynamic driving force for nucleation and possibly also the crystallization kinetics. The nucleation and growth behavior observed has important implications when these amorphous particles are used as precusors to form nanocrystalline titania. The results are also discussed in comparison with the crystallization behavior reported previously for other ultrafine-structured oxides.

3:40 pm BREAK

3:55 pm INVITED

PRESSURE INDUCED CRYSTAL-TO-AMORPHOUS TRANSFORMATIONS: R.B. Schwarz, P.J. Yvon*, Center for Materials Science, Los Alamos National Laboratory, Los Alamos, NM 87545, *Present address: SRMA, Centre d'Etudes Saclay 91191 Gif/Yvette, France

Pressure-induced crystal-to-glass transformations have been observed in tetrahedrally coordinated elements (e.g. Ge, Si), ionic and molecular crystals (alpha-SiO2, FeSiO4, AlPO4, Fe PO4, SnBr4), and hydroxides (ice, Ca(OH)2, Co(OH)2). In these materials, the crystal transforms polymorphically to a higher density glassy phase. Crystal-to-glass transformation have also been observed to occur through pressure-induced reactions between mixtures of elements such as germanium and aluminum. This paper reviews the formation of amorphous phases in alloys and elemental mixtures and discusses the possibility of obtaining bulk amounts of amorphous phases.

4:25 pm

STABLE VS. METASTABLE PHASE EQUILIBRIA IN FACETED/NON-FACETED METALLIC GLASS SYSTEMS: T.M. Adams, M.J. Kaufman, Materials Science and Engineering, University of Florida, Gainesville, FL 32611

Since metallic glasses can be used as precursors for nanocrystalline structures, it is important to understand the relationship between the transformation characteristics of the stable faceting phases and the metastable ordered phases (MOP's) in faceted/non-faceted systems. Such an understanding of the competitive nucleation and growth kinetics should allow better control of the transformation structures. Following some past work on Al-Ge alloys, it has been proposed that, in general, for faceted/non-faceted systems, no equilibria exist between the MOP's and the stable faceted phases. In order to support this assertion of generality, the Hf-Be and Al-Ge systems are being investigated. Crystallization of amorphous melt spun ribbons and co-evaporated thin films is effected by standard furnace anneals and in-situ electron beam heating. Once annealed, the resulting microstructures are characterized using XRD and TEM. In addition, in-situ hot-stage TEM is used to examine the relationship between the MOP and the stable faceting phase while the transformations are occurring. All of this work is being done in the vicinity of the stoichiometric composition of the most stable MOP (Hf-50Be and Al-50Ge). Basic modelling efforts of the metastable phase equilibria are also under way using THERMOCALC with estimated heats of formation for the MOP's determined through DSC/DTA and empirical formulations.

4:45 pm

STRUCTURAL TRANSITIONS IN TI/AL NANOLAYERED THIN FILMS: R. Banerjee, X.D. Zhang and H. L. Fraser, Materials Science and Engineering, Ohio State University, Columbus, OH 43210; M. Asta, A.A. Quong, Computational Materials Science, Sandia National Laboratories, Livermore, CA, R. Ahuja, Multi Arc Scientific Coatings, Troy, MI

Nanolayered materials often exhibit unusual structural features which are significantly different from those of their bulk counterparts. Such structural transitions could lead to novel properties of the material motivating research directed towards engineering the structure at the nanoscale. Laminated thin films based on Ti, Al and Ti-aluminides have potential application as coatings for components used in high temperature aerospace applications. A series of structural transitions were observed in Ti/Al multilayered thin films on reducing the layer thickness of the Ti and Al layers1. An hcp-fcc transition was found to occur in the Ti layers on reduction of the layer thickness to 5 nm. Al too exhibited an fcc-hcp transition on reducing the layer thickness to 2.5 nm. Interestingly, a 2.5 nm Ti layer had an hcp structure. An atempt was made to explain these transformations in the stacking sequence of the Ti and Al layers using a model initially proposed by Redfield and Zangwill. Subsequently, first principles electronic structure calculations are in progress to determine the effect of bulk, interfacial and thin film strain energies on the structural stability of the multilayers. Initial results suggest that strain energy may be playing a pivotal role in determining the structure.

5:05 pm

NANOSTRUCTURES AND PROPERTIES IN RAPIDLY SOLIDIFIED Ti(50)Ni(50-X)Cu(X) ALLOYS: V.G. Pushin , S.B. Volkova, N.M. Matveeva*, A.S. Chistjakov, Institute of Metal Physics, Ural Division of Russian Academy of Sciences, S. Kovalevskoi 18, 620219 Ekaterinburg, Russia; *Baikov Institute of Metallurgy, Russian Academy of Sciences,Leninskij prospect 49, 117911 Moscow, Russia

Rapidly solidified Ti-Ni-Cu alloys prepared by melt spinning were studied. It is found that under super rapid cooling rates alloys with 25-40 at% Cu are formed in initial amorphous state, with 15-25 at% Cu in amorphous-crystalline state, with 15 at% Cu don't become amorphous under spinning. Crystallization heat treatment of amorphous alloys leads to the formation of nano-scale TiNi-based solid solution, which undergo martensitic B2->B19 martensitic transition in cooling. Temperatures of martensitic transformations for RS-alloys have been established to be lower, than for same alloys prepared under traditional cooling rates, because of nano-scale grain size of B2-phase crystalizing from amorphous structure. The martensite in a nano- and micro grains may have different morphology and orientations. Nanophase (B2-TiNiCu) and nanocomposite (B2+B11(TiCu)) structures formed under crystallization by means of laser treatment, heating effect of electrical current and high temperature treatment were investigated. The ribbons with such structures in optimum state have good elastic and shape memory properties.