Studies on NiMnGa ferromagnetic Shape memory alloys and their characterization

Abstract

newline Ni-Mn based Heusler shape memory alloys are emergent materials for the technical newlineapplications. The possible applications areas of these alloys are actuators, sensing devices, newlinemagnetic refrigeration and energy harvesting systems as they exhibit properties such as shape newlinememory effects and magnetocaloric effects. The aim of this work is to develop the formation newlinemechanism of Ni-Mn based Heusler alloys by different techniques and experimentally newlineinvestigate their structural, magnetic and mechanical properties. Also, this work is an effort to newlinebroaden the current knowledge by contributing to the literature of Ni-Mn based Heusler alloys, newlinefocusing on synthesis and characterization studies. In recent years there has been considerable newlineinterest in Ni-Mn-Sn and Ni-Mn-Al Heusler alloys as these alloys show shape memory newlineproperties. Interest in these materials has been growing because of their low cost, good ductility newlineand high martensitic transformation temperature over mostly studied Ni-Mn-Ga Heusler shape newlinememory alloys. newlineIn this work, Ni-Mn-Sn and Ni-Mn-Al Heusler alloys were synthesized by Vacuum Arc newlineMelting (VAM) and Mechanical Alloying (MA) techniques separately under an argon newlineatmosphere and investigated for their microstructural, thermal, magnetic and mechanical newlineproperties. The microstructural studies of arc melted bulk alloys and mechanically alloyed ball newlinemilled powder samples were performed using X-ray Diffraction (XRD) and Scanning Electron newlineMicroscope (SEM) coupled with Energy-Dispersive X-ray spectroscopy (EDX). Thermal and newlinemagnetic behaviors were analyzed using Differential Scanning Calorimetry (DSC) and newlineVibrating Sample Magnetometer (VSM). Mechanical properties of bulk alloys were newlinedetermined using tribometer. Nanoindentation test was performed using nanoindenter to newlinedetermine elastic modulus and hardness of mechanically alloyed dense pellet specimens newlineprepared by consolidation and sintering using Oliver-Pharr method. newlineThe results of EDX analysis revealed that the alloys synthesized by VAM and MA techniques newlineconfirmed the desired Heusler stoichiometric composition (X2YZ) by atomic percent without newlinecontamination. By microstructural studies, prepared alloys were identified as cubic L21 Heusler newlinestructure with space group Fm-3m (number 225) and F-43m (number 216) according to newlinecorresponding JCPDS (Joint Committee on Powder Diffraction Standards) cards. Upon heat newlinetreatment, the arc melted bulk alloys were identified as orthorhombic martensite structure. newlineAlso, the formation of nanocrystalline Ni2MnSn and Ni2MnAl Heusler alloys were obtained newlineafter 15hr of ball milling through the balance between particle coalescence and fracture newlineaccompanied by uniform grain refinement. The optimum milling time required for the newlineformation of alloys by MA resulted through the proper selection of milling parameters. The newlinecrystallite size and lattice strain of prepared Ni-Mn-Al alloys found to be less than Ni-Mn-Sn newlinealloys. Martensitic transformation temperatures of the prepared alloys were revealed by newlinethermal studies. Also, during thermal stability observations, the subsequent heating of the ball newlinemilled powder particles at different milling time in DSC showed crystallization of Ni2MnSn newlineand Ni2MnAl alloy. Magnetic behaviour of as-synthesized and heat-treated Ni-Mn-Sn bulk newlinesamples suggest that the alloy seems to be useful for the development of magnetocaloric newlinematerials. Magnetic parameters determined from hysteresis curve of the prepared alloys newlinerevealed ferromagnetic behaviour. Ni-Mn-Sn bulk alloy showed more wear rate and frictional newlineforce than Ni-Mn-Al bulk alloys. The coefficient of friction of Ni-Mn-Sn found to be less than newlineNi-Mn-Al. The average elastic modulus and hardness of Ni-Mn-Al alloy found more than NiMn-Sn alloy. The density measurement results showed that higher sintering temperatures are newlinerequired to achieve the homogeneity and reduced porosity of pellet specimens. newlineThus, the results of successfully synthesized Ni-Mn based Heusler alloys using VAM and MA newlinetechniques and experimentally investigated properties of these alloys suggest that they might newlinebe possible candidates for the development of gallium free shape memory alloy applications newlinesuch as actuators, sensors and magnetic cooling devices.