Synthesis of Metastable Structures in Copper Base Alloy Systems by Mechanical Alloying

Abstract

Mechanical Alloying (MA) is a versatile technique, for developing a wide variety of conventional as well as novel materials, with unique microstructures having unusual properties. Mechanical Alloying involves milling of powders in a desired weight ratio in an ambient or inert atmosphere, during which, repeated welding, fracturing and rewelding of powder results in a homogeneous mixing of powders with fine structunes. Apart from its ability to synthesise conventional alloy systems, Mechanical Alloying can also be used to produce alloys of immiscible systems, intermetallics, nanocrystalline phases, ceramics and composites. In addition, MA has been used directly in the synthesis of metals from compounds such as oxides by milling in a reducing atmosphere. Alloys of the immiscible Copper-Iron system have potential applications in automotive, electrical and electronic industries. The main difficulty in the processing of copper-iron alloys is the immiscibility of the elements in each other. Therefore these alloys have to be processed by nonequilibrium techniques. Several techniques such as vapour deposition; ion mixing and Mechanical Alloying (MA) have been used to prepare copper -iron alloys. Among the various techniques. Mechanical Alloying is a very versatile technique for synthesising equilibrium phases, metastable phases such as quasicrystalline and nanocrystalline phases, composites and the like. Mechanical Alloying of several metallic and non -metallic systems has been done. Mechanical Alloying is being used commercially for manufacturing several commercially important alloys and composites. In this work, Mechanical Alloying has been chosen for preparing copper - iron alloys because ofits commercial application potential. Chapter 1 Chapter 1 gives an introduction to Mechanical Alloying, its advantages over conventional and other new material processing techniques such as Rapid Solidification Processing (RSP) .The processing capabilities of MA are also explained. Chapter 2 A detailed literature review on the formation of several metastable phases by MA and in particular on studies on MA in Cu-Fe system have been presented in chapter 2. Mechanical Alloying of Cu-Fe system has been investigated by several workers, covering IV the entire range of compositions from copper rich end to the iron rich end of the phase diagram. Various aspects of MA of copper - iron system such as formation of solid solutions, formation of metastable supersaturated nanocrystalline phases, crystal structure and compositions of the metastable phases have been investigated using X-ray diffraction, optical microscopy Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Differential Scanning Calorimetry (DSC) and Energy Disperse Xray Analysis (EDAX). Also the thermal stability of the alloys formed on MA during annealing have been investigated. Magnetic property studies using Mossbauer spectroscopy, Extended X-ray Absorption Fine Spectroscopy (EXAFS), as well as measurement of Giant Magento Resistance (GMR) have also been done. XRD investigations have confirmed the formation of solid solutions having a FCC structure (when the copper content exceeds 70 at %), and a BCC structure for Copper less than 30 at %, or a mixture of both FCC and BCC phases (Copper 30 to 70 at %). Grain sizes calculated from XRD patterns have shown the formation of nanocrystalline grains. Optical microscopy and Scanning Electron Microscopy (SEM) studies have confirmed the particle deformation and fracture resulting in finer particle sizes as milling progressed. Transmission Electron Microscopy (TEM) studies has shown the evidence for the formation of a lamellar structure typical of Mechanical Alloying. TEM studies on thermally annealed samples have shown evidence for a reverse martensitic type of transformation during annealing. Magnetic measurements using Mossbauer Spectroscopy have shown evidence for the formation of solid solutions during MA, confirming the XRD results. Extended Fine Absorption Spectroscopy (EXAFS) investigations to study the near neighbour environment in the milled powders have confirmed the alloying of copper and iron powders. Giant Magneto Resistance (GMR) studies on MA Cu- Fe samples have shown a sharp increase in the magnetic resistance of the alloyed samples which decreased on subsequent annealing. Quasicrystalline phases have been observed in Copper -iron alloys prepared by ion mixing and solid state inters diffusion (SSI). Though extensive work on MA of Copper- Iron system has been done, the formation of quasicrystalline phases have not been reported. The present work has been taken up to investigate the possibility of solid state crystalline lattice instability, inducing formation of Quasicrystalline or amorphous phases, by continued milling under favourable milling conditions. Interest was particularly focussed on understanding the role of defect generation rate-millingmilling intensity), types of defects and their mobility (milling temperature) under the chosen milling conditions in governing the formation ofmetastable phases on MA. The objective of the present work was also to study the effect of the nanocrystalline state formed on MA on the phase stability of a wide range of systems including Cu-Fe with a positive enthalpy of mixing and to compare with systems showing a negative heat of mixing like Cu-Ti, Cu-Al etc. The thermal stability of the phases synthesised by MA was also of interest, considering the varying tendencies for alloying in these two classes ofsystems. Chapter 3 The scope of the present investigations is presented in Chapter 3. These include investigation of the influence of defect density on the formation of equilibrium phases, formation of metastable phases on MA, factors influencing the formation of such metastable phases as well as thermal stability ofthe phases formed on MA. Chapter 4 Chapter 4 discusses in detail the various experimental procedures adopted in the study. In the present work MA of 65 Copper - 35 iron powders were carried out using both a vibratory ball mill and an attritor mill. Milling was carried out for varying time durations including 2 to 100 hours for ball milling and 15 min to lOh in the case of attritor. The milled powders were subjected to investigation by particle size analyser, optical microscopy, Scanning Electron Microscopy (SEM), x-ray Diffraction and Transmission Electron Microscopy (TEM) to study the: i) Evolution of microstructure during milling (particles shape and size change) ii) Formation ofmetastable phases iii) Thermal stability of metastable phases formed iv) Detailed microstructural characterisation of the phases formed v) hot forging of MA powders and evaluation of properties. V!