The Negative Thermal Expansion And Magnetic-field-driven Martensitic Transformation In Hexagonal Mn(Co,Ni)Ge Based Alloys

The MnMX?M=Co?Ni,X=Ge?Si?alloys with Ni₂In-type hexagonal structure have become potential magnetic multifunctional materials in recent years due to their rich structural and magnetic properties.As a member of MnMX alloys,the magnetostructural phase transitions of Mn?Co,Ni?Ge-based alloys can be completed in a very narrow temperature range,leading to a narrow negative thermal expansion temperature range,which is not conducive for practical applications.At the same time,the martensitic transformation of these alloys is very insensitive to the magnetic field,which results in the fact that the field can hardly drive the martensitic transformation.As a result,the magnetic properties of alloys,such as magnetostriction and magnetoresistance,are very weak,which greatly limit the research and application fields of these alloys.In order to broaden the temperature range of the magnetostructural phase transitions and to increase the sensitivity of martensitic transformation to the magnetic field,the stress distribution in the MnCoGe and MnNiGe-based alloys was adjusted by ball milling and secondary annealing,respectively.The results are systematically reported in this dissertation.The magnetostructural phase transitions and negative thermal expansion effect in hexagonal Mn_0.965Co_1.035Ge alloys.Defects and residual were introduced into Mn_0.965Co_1.035Ge alloy by high energy ball milling.At the same time,the stress distribution in alloys was changed,and the residual strain distribution was not uniform,which stabilized the high temperature hexagonal Ni₂In phase,resulting in the loss of martensitic transformation in a high proportion of austenite phase.Therefore,the temperature range of austenite phase coexisting with martensite phase is enlarged,and the temperature range of first-order magnetostructural phase transitions is widened.As a result the corresponding negative thermal expansion temperature interval is widened.Excellent negative thermal expansion properties were obtained in BM-4h sample.The linear thermal expansion coefficient?_L^-41.3 ppm/K was observed in the temperature range of negative thermal expansion?35?T=221 K?140-360 K?.The physical mechanism of negative thermal expansion in alloys is discussed by using the synchrotron radiation technology combined with the magnetic measurement.The magnetic-field-driven martensitic transformation and magnetocaloric effect in hexagonal Mn_1-xFe_xNiGe alloys.By substituting little ferromagnetic element Fe for Mn,the first-order magnetostructural coupled phase transitions from ferromagnetic martensite to paramagnetic austenite was obtained in Mn_1-xFe_xNiGe alloys.On this basis,the stress distribution in the sample is changed via secondary annealing treatment,which make the residual strain to redistribute and release.This not only improves the transition sharpness,but regulates the phase transition in a wide temperature range.The magnetic-field-driven martensitic transformation is realized in these alloys,which results in excellent magnetocaloric properties.In the Mn_0.84Fe_0.16NiGe alloy,the maximum value of?S_M reaches 30.6 Jkg^-1K^-1 under the magnetic field change of 0-5 T,and the refrigeration capacity RC reaches up to 224.4Jkg^-1K^-1 after secondary-annealing for 10 hours at 973 K.The results reported in this dissertation lay the experimental and theoretical foundation for the practical application of these two kinds of alloys.