INVESTIGATION OF EFFECT OF NANO CRYSTALLIZATION ON MECHANICAL AND SHAPE MEMORY EFFECT OF CuZnAI BASED SHAPE MEMORY ALLOY, SYNTHESIZED BY HIGH ENERGY BALL MILLING (HEBM)

Abstract

Present work is based on synthesis of shape memory nano-alloy by mechanical alloying using high energy ball mill. Shape memory alloys are very useful in various applications, especially in actuators, smart systems, and medical applications. These materials recognize their shape due to the pseudo-elastic property they exhibit. Pseudo-elastic transformation itself is not a naturally occurring phenomenon. Instead, it can be achieved by thermo mechanical training. Shape memory effect shown by alloys depends upon its constitution, method of preparation and thermo mechanical training. Method of preparation has been the most important aspect for shape memory alloys for achieving desired SME (shape memory effect). Earlier works done by most of the authors on shape memory effect were based on conventional method of synthesis i.e. casting and powder metallurgy routes. These methods do not emphasize on effect of nanocrystalline phase on shape memory effect and fatigue strength of devices based manufactured from shape memory effect. In the present work we have focused on synthesis of shape memory alloy by mechanical alloying using high energy ball mill. Several process parameters such as, ball to powder ratio, milling medium, and speed of ball mill are important factors which influence effectiveness of the milling process. By selecting proper ball to powder ratio, rotational speed and medium of milling, one can optimize the milling and alloying process. In our work we have chosen powder Cu, Zn and Al of known proportion of size 325 mesh and purity greater than 99.7%, ball to powder ratio of 3:1 and the planetary ball mill was maintained at 300 rpm. In first case we milled the powder up to 24 hours dry milling and taken out samples for XRD (X-Ray Diffraction), SEM (Scanning Electron. Microscopy), DSC (Differential Scanning Calorimetry) analysis in order to know about physical and morphological changes occurring with milling time and phase transformation changes can be analyzed by XRD and DTA/TGA (Differential Temperature Analysis/Temperature Gravimetric Analysis) test. We have found that when liquid nitrogen used at 24 hour milled state there has been drastic decrease of particle size, earlier up to 24 hour milling decrease in particle size has small and gradual. From 24 to 28 hour milling with liquid nitrogen, reduction in particle size was max, no further reduction of particle size was observed, in fact there has been increase of grain size observed due to agglomeration and grain growth due to evolved heat. In second set of sample we added liquid nitrogen at 16 hour stage and milled up to 20 hour and found that reduction of particle size was same as it was in-earlier sample. FE-SEM (Field Emission-Scanning Electron Microscopy) AND DSC analysis also conforms the results. Particle size reduction up to 7 nanometers was observed in optimize state. Micro hardness test shows increase of hardness and yield strength with decrease of particle size up to 15 nanometers and then reverse effect observed. Adequate yield strength and fatigue strength are most important properties of shape memory alloys in order to perform its role efficiently. To achieve desired property particle size control is most important aspect. Mechanical alloying by HEBM is the most competitive technique because we have complete control over morphological changes by controlling over milling parameters.