In-situ Neutron Diffraction Studies On Phase Transformation And Functional Behaviors Of Ni(Mn)-Fe-Ga Ferromagnetic Shape Memory Alloys

Intelligent material is one kind of important functional material which can sense such as temperature,force,electric,magnetic and other external environment and generates the effect of driving(displacement,etc.).It mainly includes three types of materials,such as shape memory,piezoelectric and magnetostrictive.Intelligent materials play an important role in the national economy and national defense key components and core systems.The research level has a great influence on the overall technological level and modern national defense.Therefore,intelligent materials have been included in the national guideline on long-term program for science and technology development.Ferromagnetic shape memory alloys(FSMAs)are novel intelligent materials which exhibit excellent physical properties and great potential such as superior magnetic induced strain(stress),high reaction speed.It becomes the most attractive topics in the current international metal materials field.Neutron diffraction is a powerful materials characterization tool to study the evolution of crystal structure and microstructure in bulk materials under various(temperature,magnetic field and stress)environments,which is due to the high resolution and high penetration depth of neutron.Therefore,the neutron diffraction technique enables deciphering the site occupancies in the crystal structure of FSMAs system,owing to the differentiable scattering contrasts of transition metals elements such as Ni,Fe,Mn,Ga and Co neighboring elements.In the current work,neutron diffraction technique is used to neutron diffraction to study the relationship of the evolution of crystal structure and other functional behaviors under the multi fields coupling condition of Ni-Fe-Ga-Co and Fe-Mn-Ga two types of FSMAs.All of the in-depth studies are helpful for understanding the physical mechanisms of magnetic/stress induced martensitic transformation and superelasticity.The current work provides the direct experimental evidence for the further development of crystallographic physical models for functional behaviors in magnetic-field-driven functional materials under various environments and plays an important role in guiding the future development of magnetic functional materials.Utilizing the advantage of the neutron diffraction to distinguish the nearby elements,one can obtain the atom occupancy and lattice parameters information on off-stoichiometric Ni49.3Fe18Ga27Co5.7 alloy.It is shown that the alloy has a face-centered cubic(Fm3m)L21 Heusler structure austenite phase when the temperature is above 240 K.The crystal structure changes into the tetragonal I4/mmm structure martensite phase when the temperature is below 240 K in cooling.And no other phase transformation occurs during the temperature cooling to 20 K process.The phase transformation from tetragonal martensite phase to austenite phase in heating process occurs at around 250 K,indicating the reversible phase transformation of the shape memory effect.In-situneutron compression experiments were performed on Ni49.3Fe18Ga27Co5.7 alloy at room temperature.The martenstic transformation occurred when the applied stress reached over 170 MPa.The volume fraction of martensite is linear growth during the compressive loading process,and it has an opposite trend in the unloading process.The martensitic volume fraction displays a linear decrease process in the unloading stage.Therefore,room temperature superelasticity is caused by stress induced martensitic transformation.The critical stress of the stress induced martensitic transformation decreased as increasing the number of loading cycles.It is attributed to the generation of the deformation twinning in austenite phase during loading process.The irreversible of the deformation twinning leading to the residual stress retained in the alloy after cyclic unloading.The uniaxial compression deformation on Ni49.3Fe18Ga27Co5.7 alloy at 573 K was observed.The alloy exhibited a recoverable strain-3.55%when the applied stress reached 600 MPa.The novel high temperature superelasticity is the result of the stacking faults in the crystallographic structure which is caused by the excessive Co doped in the alloy.The stress induced confined martensitic transformation occur when the applied stress is higher than the critical stress of the phase transformation.The confined martensite is easy to nucleate and hard to grow at high temperature.In comparison,the traditional high temperature superelasticity is caused by the stress induced martensitic transformation,in which the formation mechanism is the improvement of the martensitic transformation temperature.Therefore,there are essential differences between the two kinds of high temperature superelasticity in physical mechanism.Fe(46-x)Mn26Ga(28+x)(x=0,1,2)alloys which were homogenized is face-centered(Fm3m)Heusler austenite structure and display the paramagnetism at room temperature,the alloys perform the ferromagnetism when the temperature below the Curie temperature.The alloys have ferromagnetic y phase at room temperature after annealing at 1073 K.There are ferromagnetic Fe-rich and Ga-rich phases existence at room temperature after annealing at 773 K.