The equilibrium state of metallic alloys is crystalline, macroscopic samples usually consist of many individual crystallites. Amorphous alloys can only be prepared by nonequilibrium methods. Very rapid solidification of the melt by melt spinning and mechanical alloying. Metallic glasses are of fundamental as well as technical interest.
Metallic glass research was increasingly active from about 1975 to 1985. By then only the hard questions remained unanswered - many of them still are. Industrial application in pick-up coils and power transformers became a reality.
The most interesting recent developments of metallic glass research are the preparation of Al and Mg based metallic glasses by Inoue at Sendai and the very stable Be containing glasses developed at W. Johnson's group at CalTech. Amorphization by mechanical alloying was demonstrated by several laboratories, first by A. Ye. Ermakov in the S.U. and C.C. Koch at North Carolina State U.
I was duing research on metallic glasses during the above Busy period. I was particularly interested in the structure, electron band structure, stability, and magnetic properties of metallic glasses. Some of the papers representing this work and related subjects are listed below.
1. L. Takacs, M. C. Cadeville, and I. Vincze, "Mossbauer Study of the Intermetallic Compounds (Fe1-xCox)2B and (Fe1-xCox)B," J. Phys. F: Metal Phys. 5 (1975) 800-811.
This is the paper from my M.S thesis. The main purpose of the investigations was to compare the concentration and temperature dependence of the magnetic moments of iron and cobalt. The question was whether these compounds behave similar to largely itinerant magnetic metals or closer to nonmetallic compounds like oxides. The results indicated an interesting mixture of features. A temperature and concentration dependent variation of the easy magnetization direction was also discovered. When iron-boron based alloys became the most important model system for metallic glasses, this paper became an important reference.
2. L. Takacs, "Mossbauer Investigation of the Magnetic Anisotropy and Electronic Structure of a Metallic Glass," Solid State Commun. 21 (1977) 611-613.
This is my first paper on metallic glasses and also the first Mossbauer study of the iron-boron system. The results show that the hyperfine field and isomer shift of the Fe80B20 glasss interpolate quite well between the data for pure iron, Fe2B and FeB compounds, in spite of the fundamental structural difference. In fact, later research on noncrystalline materials proved that the band structure and related properties are much less dependent on translational symmetry (crystallinity) than believed earlier.
3. L. Takacs, "Electronic Structure of Transition Metal-Metalloid Crystalline and Glassy Alloys," phys. stat. sol. (a) 56 (1979) 371-377.
The concentration dependence of the magnetization in metallic glasses show a Slater-Pauling curve type behavior, but the curves are shifted as if the metalloid elements - boron, carbon, phosphorous - would "donate" electrons into the 3d band of the transition metals. This paper is the last one using this model, carefully pointing out the limitations of the model - namely that it applies to strong ferromagnetic materials with one full 3d sub-band only. There is also a discussion on the localized moment - itinerant magnet nature of these materials.
4. L. Takacs and C. Hargitai, "Hyperfine Fields and Local Environments in Iron-Boron Metallic Glasses," J. Phys. F: Metal Phys. 13 (1983) 183-190.
This paper deals with the issue of how to understand the hyperfine field distribution of metallic glasses. Its assumption is that the hyperfine field at any iron site is primarily dependent on the number of metalloid neighbours. For sites with few or no metalloid neighbours, the volume of the site is also important. The paper derives hyperfine field distributions using a computer generated random close packing of hard spheres structural model and an empirical relationship between local environment and hyperfine field.
5. L. Takacs, E. J. Hiltunen and J. A. Lehto, "Crystallization Kinetics Studied by EDXD and Mossbauer Spectroscopy," in Rapidly Quenched Metals, eds. S. Steeb and H. Varlimont (Elsevier Science Publishers B. V., 1985) pp. 275-278.
Metallic glasses are metastable, their transformation to more stable crystalline states is of obvious interest. This paper is the best of several with my Finnish friends, comparing the crystallization kinetics of metallic glasses as measured by energy dispersive X-ray diffraction and Mossbauer spectroscopy. Both methods have technical difficulties when used in situ at high temperature. The problems range from measuring the intensity of overlapping diffraction peaks to making corrections for the temperature dependence of the recoil free fraction. Careful experimental work and data evaluation resulted in consistant Avrami exponents from the two methods.
6. L. Takacs, "Statistical Geometry of Some Dense Random Packing of Hard Spheres Model Structures I, Description of Local Order," J. Non-Cryst. Solids 81 (1986) 1-11.
7. L. Takacs, "Statistical Geometry of Some Dense Random Packing of Hard Spheres Model Structures II, Concentration and Radius Ratio Dependence," J. Non-Cryst. Solids 81 (1986) 13-28.
A pair of papers on the characterization of computer built amorphous model structures. The main issues are how much of the structure and stability of metallic glasses can be explained by purely geometric factors and how to characterize a nonperiodic arrangement of atoms beyond the pair correlation funtions.
8. X. S. Chang, C. Hohenemser, and L. Takacs, "Mossbauer Study of Disordered, Quenched CoxPd1-x," Phys. Rev. B 40 (1989) 29-35.
Palladium is not magnetic by itself. However, its alloys are magnetic with quite little additions of magnetic 3d metals, first forming giant moments, then regular ferromagnetic alloys. This paper deals with the magnetic structure and the magnetic phase transformation of the alloy at the Curie temperature.