Phase Transition,negative Thermal Expansion And Incommensurate Magnetic Structure In Ni₂In-type Mn?Co,Ni?Ge Alloys Studied By Neutron Powder Diffraction

The MM'X alloys with hexagonal Ni₂In-type structure exhibit aboundant structural and magnetic properties,and derive various physical behaviors such as negative thermal expansion effects,magnetocaloric effects,piezomagnetic effects,magnetoelastic effects and many other physical effects.Thus,the MM'X alloy is one of the most important research focus in multifunctional materials.In this dissertation,I investigated MnCoGe-based and MnNiGe-based alloys,which are the members of MM'X alloys family.By means of neutron diffraction,X-ray diffraction,TEM,SEM,magnetic measurements,strain measurements and other measurements,negative thermal expansion and the correlation with the specific magnetic structure were studied in these alloys.Finally,adjustable negative thermal expansion,i.e.ultra-low thermal expansion and giant negative thermal expansion,was realized.Besides,I worked out the complex incommensurate magnetic structure in MnFeNiGe alloy.The main researches are shown as follow:1.As for the MnCoGe_0.99In_0.01 component of MnCoGe-based alloys,previous researches have shown that it exhibits giant negative thermal expansion effect.In this dissertation,I changed the particle size of the sample by ball milling and studied the effect of particle size on the negative thermal expansion of the material.High-resolution transmission electron microscopy?TEM?images manifested that as the particle size decreasing,a large amount of amorphous structures began to appear in the sample,and big grains gradually broke up into microcrystals that are filled with defects.For the small particle sample P5?0.3¹?m?of the MnCoGe_0.99In_0.01 alloy,with a particle size of 0.3¹?m,the proportion of the amorphous phase reaches 40%,and the average grain size is only about 8nm.These amorphous structures exhibit positive thermal expansion behavior and compensate for the negative expansion properties of the material.The refinements of neutron powder diffraction and variable temperature X-ray diffraction revealed that as the particle size decreasing,more and more austenite in microcrystalline phase lost martensitic transformation.For P5?0.3¹?m?,the fraction of austenite losing martensitic transformation in the microcrystalline phase is 55.2%.In addition,the refinement data also indicated that the residual martensitic transformation became smooth with the particle size decreasing.The strain test results revealed that the continuous control of the negative thermal expansion behavior of the MnCoGe_0.99In_0.01 alloy powder sample was achieved by changing the size of the sample particles,and controlling the degree of crystallization and phase transformation.Besides,relying on the self-compensating effect of the amorphous phase,the ultra-low positive thermal expansion was realized in the bonded P5?0.3–1?m?samples,withcoefficient as small as+6.8×10^-7/K,from 200 K to 310 K.2.As for the structural transition of MnNiGe-based alloys,previous studies have shown that Fe-doped MnNiGe alloys undergo a martensitic structural transition,with a negative thermal expansion of unit cell volume as large as?V/V?2.68%,equivalent to8933 ppm linear negative thermal expansion.I considered the anisotropy of martensitic transformation,which means the c-axis of the hexagonal phase shrinks by 12.3%during heating,while the b-axis extends by 9.40%.And I studied the influence of texture on the negative thermal expansion of MnFeNiGe alloy.Finally,the texture was introduced on the surface of the bonded samples of Mn FeNi Ge alloy powder,and I realized giant24445ppm linear negative thermal expansion.I collected X-ray diffraction data on the surface of the bonded sample and analyzed the texture coefficients.It was found that the texture coefficient?>1 for hexagonal?110?plane,where the impact of the c-axis is far more than the b-axis.It means that there is a strong preferred orientation for hexagonal?110?plane on the surface of the bonded samples.However,the texture coefficient?<1 for?102?and?202?plane,where the impact of the c-axis is far less than the b-axis.As a result,the influence of the c-axis of the hexagonal phase on the negative thermal expansion properties of the alloy bonded sample surface is enhanced,and the influence of the b axis is weakened.Thus,I achieved giant negative thermal expansion on the surface of the bonded samples.3.As for the magnetic transition of MnNiGe-based alloys,there is an abnormal magnetic transition in martensite phase of Fe-doped MnNiGe alloys.I explored the mechanism of the abnormal magnetic transition,and successfully worked out the magnetic structure of the martensite phase.I also found two reasons causing the abnormal magnetic transition.Rietveld refinements of the neutron diffraction data were performed,and results revealed an incommensurate magnetic structure.Moreover,the incommensurate magnetic structure underwent a change during the abnormal magnetic transition.Through further analysis,I found out that the incommensurate magnetic structure is Cone spiral type,and the change of the incommensurate magnetic structure come from the change of cone angle of the Cone spiral type incommensurate magnetic structure.Besides,after analyzing the data of crystal structure,I found that there was also a change in the Mn-Mn band length during the abnormal magnetic transition.All these mean that the changes of cone angle of Cone spiral type incommensurate magnetic structure and Mn-Mn bands are both the primary reasons,resulting in the abnormal magnetic transition.