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ENGINEERING PHYSICS II ADVANCED ENGINEERING MATERIALS UNIT – V ADVANCED ENGINEERING MATERIALS 5. 1 INTRODUCTION New engineering materials such as metallic glasses, shape memory alloys, nano phase materials, non linear materials etc. are the advanced materials, which are the integral part of our life. All these materials are composite materials which are prepared by grouping one type of material with the other. In this chapter we are going to discuss about the new engineering materials along with their properties and the wide range of applications in this modern engineering world. 5. 2 METALLIC GLASSES (MET GLASSES) Metallic glasses are new type of materials which share the properties of both metals and glasses. In general, they are strong, ductile, malleable, opaque and brittle. They also have good magnetic properties and corrosion resistance. They are also called as amorphous metals. Properties of metals + Properties of glasses = Properties of metallic glasses 5.3 FORMATION OF METALLIC GLASSES Metals are made into glassy state by increasing their rate of cooling to a very high level. At that state the atoms are unable to arrange in a proper manner and thus form an new amorphous state. These new type of materials which are formed by the rapid cooling technique are called metallic glasses. Glass Transition Temperature The temperature at which the metals in the molten form transforms into glasses (i.e. from liquid to solid) is known as glass transition temperature. It was found that the glass transition temperature for metallic alloys varies from 200 C to 3000 C. 5. 4 TECHNIQUES FOR PREPARATION There are several techniques available for the production of metallic glasses. 1. Melt spinning system Molten alloy is made to impinge on a fast rotating roller to form metallic glasses. 2. Twin roller system Molten alloy is made to pass through the two rollers rotating in opposite directions to form metallic glasses. 3. Metal extracting system Fast moving roller sweeps off molten droplet into a strip to form metallic glasses. 4. Sputtering The sputtering gas is ionized and the atoms are made to fly towards the substrate to form metallic glasses. 99 ENGINEERING PHYSICS II ADVANCED ENGINEERING MATERIALS 5.5 PREPARATION OF METALLIC GLASSES Principle Quenching is a technique used to form metallic glasses. Quenching means rapid cooling Atoms move freely in the liquid state. When the liquid is quenched (rapidly cooled) it results in an irregular pattern, which results in the formation of metallic glasses. Technique The process involved in the formation of metallic glasses is melt spinning technique. MELT SPINNING TECHNIQUE Experimental setup The setup consists of a refractory tube with fine nozzle at the bottom. The refractory tube is placed over the rotating roller made up of copper. An induction heater is wounded over the refractory tube in order to heat the alloy inside the refractory tube. Preparation The alloy is put into the refractory tube and the induction heater is switched ON. This heats the alloy and hence the molten alloy is ejected through the nozzle of the refractory tube onto the rotating roller and is made to cool suddenly. The ejection rate may be increased by increasing the gas pressure inside the refractory tube. Thus due to rapid quenching a glassy alloy ribbon called metallic glass is formed over the rotating roller. Hence this technique is used to develop materials that require extremely high cooling rates in order to form metallic glasses. The cooling rates achievable by melt-spinning method are in the order of 104–107 Kelvin per second. 5.6 TYPES OF METALLIC GLASSES 1. Metal - metal metallic glasses They are formed by combination of metals. 100 ENGINEERING PHYSICS II ADVANCED ENGINEERING MATERIALS Example: (i) Ni - Nb (Nickel & Niobium) (ii) Mg - Zn (Magnesium & zinc) (iii) Cu - Zr (Copper & Zirconium) (iv) Hf - V (Hafnium & Vanadium) 2.Metal - metalloid metallic glasses They are formed by combination of metals and metalloids. Example: Metals like Fe, Co, Ni and metalloid such as B, Si, C, P. 5.7 PROPERTIES OF METALLIC GLASSES (i) Structural properties They have tetrahedral closely packed structure rather than hexagonal closely packed structure They do not have any crystal defects (ii) Mechanical properties They are strong in nature They have high corrosion resistance They posses malleability and ductility (iii) Magnetic properties They can be easily magnetized and demagnetized They have narrow hysteresis loop (iv) Electrical properties High electrical resistance Electrical resistance will not vary with temperature Low eddy current losses 101