Amorphous Phase Formation In Mechanically Alloyed Fe-based Systems.

Bulk metallic glasses have interesting combination of physical, chemical, mechanical, and magnetic properties which make them attractive for a variety of applications. Consequently there has been a lot of interest in understanding the structure and properties of these materials. More varied applications can be sought if one understands the reasons for glass formation and the methods to control them. The glass-forming ability (GFA) of alloys can be substantially increased by a proper selection of alloying elements and the chemical composition of the alloy. High GFA will enable in obtaining large section thickness of amorphous alloys. Ability to produce glassy alloys in larger section thicknesses enables exploitation of these advanced materials for a variety of different applications. The technique of mechanical alloying (MA) is a powerful non-equilibrium processing technique and is known to produce glassy (or amorphous) alloys in several alloy systems. Metallic amorphous alloys have been produced by MA starting from either blended elemental metal powders or pre-alloyed powders. Subsequently, these amorphous alloy powders could be consolidated to full density in the temperature range between the glass transition and crystallization temperatures, where the amorphous phase has a very low viscosity. This Dissertation focuses on identifying the various Fe-based multicomponent alloy systems that can be amorphized using the MA technique, studying the GFA of alloys with emphasis on improving it, and also on analyzing the effect of extended milling time on the constitution of the amorphous alloy powder produced at earlier times. The Dissertation contains seven chapters, where the lead chapter deals with the background, history and introduction to bulk metallic glasses. The following four chapters are the published/to be published work, where the criterion for predicting glass formation, effect of Niobium addition on glass-forming ability (GFA), lattice contraction on amorphization, effect of Carbon addition on GFA, and observation of mechanical crystallization in Fe-based systems have been discussed. The subsequent chapter briefly mentions about the consolidation of amorphous powders and presents results of hot pressing and spark plasma sintering on one of the alloy systems. The final chapter summarizes the Dissertation and suggests some prospective research work that can be taken up in future. The Dissertation emphasizes the glass-forming ability, i.e., the ease with which amorphization can occur. In this work the milling time required for amorphization was the indicator/measure of GFA. Although the ultimate aim of this work was to consolidate the Fe-based amorphous alloy powders into bulk so as to undertake mechanical characterization, however, it was first necessary to study the glass forming aspect in the different alloy systems. By doing this a stage has been reached, where different options are available with respect to amorphous phase-forming compositions and the knowledge to improve glass-forming ability via the mechanical alloying technique. This will be ultimately useful in the powder compaction process into various shapes and sizes at optimum pressure and temperature. The study on mechanical crystallization indicates, or in a way defines, a limit to the process of amorphization, and it was also demonstrated that this phenomenon is more common in occurrence than and not as restricted as it was earlier reported to be

Lattice contraction during amorphization by mechanical alloying

Amorphization has been achieved in blended elemental Fe-based multicomponent alloy powders by mechanical alloying. The effect of Nb addition to the Fe(42)Ni(28)Zr(10-x)Nb(x)B(20) alloy in the composition range of 1-6 at. % Nb has been investigated and it was shown that the glass-forming ability (GFA) of the alloys, defined as the milling time required to produce an amorphous phase, improved with Nb addition. The improvement was not regular; the highest GFA was achieved at an Nb level of 2 at. %. Associated with the amorphization process, lattice contraction was noted. The processes of occurrence of the amorphous phase in this alloy system, maximum GFA in the alloy with 2 at. % Nb, and lattice contraction were explained on the basis of the atomic strain model developed first for binary alloys and extended later to ternary and multicomponent alloys, and the change in coordination number with the size ratio of the constituent atoms

Effect of carbon addition on the glass-forming ability of mechanically alloyed Fe-based alloys

The effect of carbon addition on the glass-forming ability (GFA) of mechanically alloyed Fe-based Fe(42)M(28)Zr(10)B(20) (M=Ni, Al, or Ge) amorphous alloy systems was investigated. It was shown that when B was partially replaced by 10 at. % C in the Fe-Ni-Zr-B and Fe-AI-Zr-B alloy systems, the GFA of the systems had increased significantly, as determined by the reduced milling time required for amorphization. However, when carbon was added to the Fe-Ge-Zr-B alloy system, the GFA was decreased drastically and no amorphization was observed. The role of carbon on the GFA of alloy systems has been discussed from the thermodynamic and kinetic points of view

Mechanical crystallization of Fe-based amorphous alloys

Mechanical alloying of a number of blended elemental powders of Fe-based alloy systems containing four or five components was undertaken to determine if amorphous phases could be produced and also to compare the glass-forming ability achieved by mechanical alloying and that obtained by solidification-processing methods. Amorphous phase formation was achieved in all the alloy systems investigated, the time for the amorphous phase formation being a function of the glass-forming ability of the alloy system investigated. However, in some alloy systems it was noted that on milling, beyond the time required for the formation of the amorphous phase, the amorphous phase started to crystallize, a phenomenon designated as mechanical crystallization. The present paper specifically discusses the results of mechanical crystallization obtained in the Fe42Ge28Zr10B20 and Fe42Ni28Zr10C10B10 alloy systems as representatives of the typical quaternary and quinary (five-component) systems, respectively. In the case of the quaternary system, mechanical crystallization led to the formation of a supersaturated solid solution of all the solute elements in Fe, while in the quinary system, a mixture of the solid solution and intermetallic phases has formed. The possible reasons for mechanical crystallization and the reasons for the differences in the behavior of the quaternary and quinary systems are discussed

Glass Formation In Mechanically Alloyed Fe-Based Systems

Rapid solidification processing of metallic melts has been traditionally employed to synthesize metallic glasses in several alloy systems. However, in recent years, solid-state processing methods, and more specifically, mechanical alloying, have become popular methods to synthesize glassy phases in metallic alloy systems. Although a large number of criteria have been developed to identify alloy compositions that can be solidified into the glassy state, very few attempts have been made to predict the glass-forming ability by solid-state processing methods. To evaluate if some clear criteria could be developed to predict glass formation by solid-state processing methods and to understand the mechanism of glass formation, mechanical alloying of powder blends was conducted on several Fe-based alloy systems. Three different aspects of glass formation are specifically discussed in this paper. One is the development of a criterion for identifying glass-forming systems from phase diagram features, the second is the process of mechanical crystallization (formation of a crystalline phase on continued milling of the amorphous powders obtained by mechanical alloying), and the third is the novel phenomenon of lattice contraction during amorphization. It was shown that the conditions under which a glassy phase is formed by mechanical alloying are different from the solidification methods. © 2009 World Scientific Publishing Company