Martensitic Transformation Of Ni-Fe-Ga Magnetic Shape Memory Alloy

The microstructure,substructure and the twin relation,twinning elements of martensite in the Ni-Fe-Ga magnetic shape memory alloy have been investigated by optical microscope,SEM,XRD,DSC and TEM methods.Selected-area electron diffraction patterns of the 14M modulated martensite in a Ni-Fe-Ga alloy are investigated in detail by means of the profiles of the diffraction spots.The experiment results showed that the microstructure of Ni-18.52at.%Fe -25.85at.%Ga,Ni-18.10at.%Fe-24.88at.%Ga,Ni-20.27at.%Fe-25.14at.%Ga as-melted alloys are composed of martensitic phase andγphase at room temperature.But Ni-18.15at.%Fe-27.03at.%Ga and Ni-18.34at.%Fe-27.25at.%Ga alloys are nearly composed of martensitic phase.The crystal structure of the martensite phase is determined to be 14M.The crystal structure ofγphase was determined to be face-centred cubic.The martensitic phase transformation temperatures are higher than room temperature.The twin relations of A/C,A/B and A/D in the 14M martensite were determined to be (1 2 7)_A//(1 2 7)_C with[2 1 0]_A//[2 1 0]_C,(1 2 7)_A//(1 2 7)_B with[2 1 0]_A//[0 7 2]_B and(2 0 14)_A//(2 0 14)_D with[0 1 0]_A//[0 1 0]_D,respectively.They were named as(1 2 7) typeⅠtwin,[7 7 1]typeⅡtwin and(1 0 7) compound twin.The twinning interface of A/C and A/B is straight,while the A/D interface is wavy.The substructure of martensite plate is high-density stacking faults.The twin relations of A/D in the 12M martensite was determined to be(2 0 12)_A//(2 0 12)_D with[0 1 0]_A//[0 1 0]_D,named as(1 0 6) compound twin,while the A/D interface is curued gradually.In the present work,a new empirical formula has been proposed to express the relative intensity of electron diffraction spots.Selected-area electron diffraction patterns of the 14M modulated martensite in a Ni-Fe-Ga alloy were investigated in detail by means of the profiles of the diffraction spots.The result shows that the logarithm of the structure factor of crystal is proportional to the function of the change in the integral gray values of diffraction spots.