| The chemical composition close to stoichiometric Ni2MnGa magnetic shape memory alloy is a new intelligent material sensitive to temperature field,force field and magnetic field,which shows multi-functional characteristics such as shape memory effect,superelasticity,magnetostrictive torsion and acoustic emission under multiple fields.These features provide an option for designing and developing multifunction devices.However,the bulk alloy is difficult to process due to the intrinsically brittle,and the preparation of single crystal is time-consuming and costly,which restricts the development and industrial application of shape memory alloy.In this thesis,Ni-Mn-Ga(Co,Cu)microwires were prepared by glass coating method,and the mechanical properties were greatly improved by rapid solidification and grain refinement.On this basis,the microstructure,superelasticity,martensitic transformation,Wiedemann effect and magnetic-induced twist of microwires were studied systematically to discover new laws and new phenomena.The results provide experimental and theoretical basis for optimizing alloy properties and multifunctional design.(1)Preliminary study on superelasticity,shape memory effect and stress-induced martensitic transformation of microwires.The bulk alloys were fabricated into microwires by a glass-coated method,and the diameter was 15-400 ?m.It was found that the austenitic phase Ni53Mn24Ga23(at.%)microwires at room temperature can obtain up to 14%recoverable strain and?90%superelastic strain in cyclic tensile test,showing excellent mechanical stability.Large strains and superelasticity benefit from stress-induced martensitic transformation.The critical stress of the transition gradually decreases with the increase of the number of cycles to stabilize,with a"training" effect.This improvement in mechanical properties is due to the large free surface of the microwires,which reduces the grain boundary constraints and improves the ability to coordinate deformation between the grains.(2)Study of multistep superelastic and multistep phase transition behavior of microwires under stress-temperature coupling systematically.On the basis of improving the mechanical properties,the structure,microstructure and mechanical properties of Ni-Mn-Ga and Ni-Mn-Ga-Co-Cu polycrystalline shape memory microwires were studied systematically under the stress-temperature coupling was studied.The martensitic and intermediate martensitic transformation behavior is studied intensively.It was found that the Ni-Mn-Ga and Ni-Mn-Ga-Co-Cu alloy microwires with austenite phase and martensite phase at room temperature exhibited multistep behavior on the stress-strain curve when stretched at different temperatures.Characterization by in-situ TEM and XRD confirmed stress-induced multistep phase transition behavior(the presence of intermediate martensitic transformation).Ni-Mn-Ga shape memory micro wires with<001>preferred orientation exhibits four step or multi-step martensitic transformation sequences in the temperature range of 213-300 K,ie.P?nM(n=2,5,7)and 5M ?7M ?NM.Subsequent replacement of Ni and Mn by Co and Cu elements broadens and increases the phase transition temperature window.The multistep superelastic behavior also occurs in the tensile stress-strain curve at 263-533 K.Then the critical stress-temperature phase diagram under the force-temperature coupling is established according to the critical stress and temperature change.The multistep rotation of the shape memory alloy wire becomes beneficial to the bamboo-like morphology,and the superelasticity is close to the single crystal,which can greatly reduce the application cost of the microwires in the micro device.This result is of great significance for the theoretical study and practical application of martensitic transformation.(3)Premartensitic transformation behavior of austenitic phase microwires is illustrated.The mechanical properties of shape memory alloys are closely related to the evolution of microstructure.At room temperature,there are satellite spots around the main diffraction point of the austenite phase Ni-Mn-Ga microwires,and the alloy has a superlattice structure.The satellite spot corresponding to the "tweed structure" on the TEM is a premartensitic phase transition feature.The analysis of single domain and multidomain structures by HRTEM was found to consist of ordered atomic stacking.When the temperature is lower than Ms,the various domain structures were transformed into a five-layered martensite structure having streak characteristics.It was found through experiments that the martensitic transformation order was P?PM?M during the cooling process.The synergistic transformation of magnetism and structure caused by low temperature is verified at the atomic scale,which helps to understand the martensitic transformation process and advanced shape memory alloy.(4)Twist behavior of magnetic shape memory microwire:Weidmann effect,stress-induced twist and magneto-induced twist.The functional properties of torsional behavior of austenite phase and martensite phase microwires at room temperature were studied.Fisrtly,by using the optical lever method,the Weidmann twist of the microwires under the circumferential and axial magnetic fields was studied,and the torsion angle of 234"/cm was obtained in the?10 Oe circumferential magnetic field.Secondly,the stress-induced phase transition characteristics of the microwires were found.The microwire also exhibited a twisting behavior:force-induced torsion,and the measured twist angle was ?/l=1250"/cm.Finally,a rotating magnetic field is added on the basis of stress-induced torsion.The magnetically induced twist behavior of microwires under two modes under stress-magnetic coupling was studied systematically.The coupling relationship between the torsion angle and the magnitude,direction and static force of the magnetic field was determined for the first time.(5)Acoustic emission(AE)characteristics of shape memory microwires.It was found that the microwire was audible when it was stretched,and then the dynamic signal analyzer system was used to study the acoustic emission spectrum and cut-off frequency characteristics of different diameter microwires during martensitic transformation.The acoustic emission spectrum analysis shows that the AE frequency is concentrated in a certain frequency range,reflecting the characteristics of the atomic collective motion of the microwire during the tensile stress induced phase transition process.At the same time,it was found that the finer grains of the shape memory alloy microwires,the larger cut-off frequency value during the acoustic emission.In this thesis,the dependence of Ni-Mn-Ga(Co,Cu)shape memory microwire on the microstructure evolution and performance under the action of force field,temperature field and magnetic field is studied,which provides theoretical guidance for alloy design and exploration of high performance alloy.And the meaning of the event.At the same time,exploring the twisting behavior and acoustic emission characteristics of microwires provides technical support for engineering applications of alloys. |
PDF Full Text Request