| Disordered alloys are a new type of alloy materials that break through the traditional strategies for alloy design and bring vitality to their research.In broad sence,disordered alloys include structural disorder and chemical disorder.The former one,also known as amorphous alloys or metallic glasses,are alloys in which the atoms are randomly arranged and have not long-range order but only short-range order.While the latter one can also be called chemically complex alloys or multi-principal element alloys,which are firstly defined as single-phase random solid solutions composed of multiple metallic elements in near equi-atomic ratios.Their disordered nature has led to many unexpected excellent properties and has greatly broadened the scope of research in metallic materials.In this paper,studies on the mechanism of plastic deformation as well as the origin of noncoplanar spin structure have been carried out for metallic glasses and chemically complex alloys respectively.The main work is summarized as follows.The most direct effect of structural disorder on the material is the alteration of the deformation mechanism.The absense of crystal structure makes the material no longer able to absorb plastic deformation through dislocations and slips of crystal plane,but undergoes highly localized shear deformation instead.Using the Griffith’s theory of fracture as reference,the yielding process before shear band formation was analyzed by the conservation of energy,and the energy conversion during the growth of plastic flow unit(PFU)was calculated by Eshelby’s inclusion theory.It was found that the intrinsic plastic strain of the PFU at the yield point is exactly equal to the macroscopic shear strain,and the contributions of the entropy increment as well as the elastic constraint effect of the glass matrix to the activation energy barrier of PFU are also equal to each other.These provide a quantitative explanation for the microscopic mechanism of plastic deformation of metallic glasses.Nanoindentation is an important tool to characterize the mechanical properties of metallic glasses at nanoscales.However,only the modulus and hardness of the samples can be obtained from the loading curves,the information about the plastic yield strength is yet inaccessible.By analyzing the stick-slip dynamics under nanoindentation conditions and the critical serrated flow of shear bands,the theoretical loadings for the first pop-in was derived,which was further verified by experimental results from various metallic glass systems.The comparison with the shear band nucleation theory revealed that the first pop-in event may occur after the plastic yielding,which corresponds to the onset of the deviation from elastic solution for the loading curves.This model not only explained the root cause for the variation of the loading at the first pop-in event under various conditions,but also proposed a method and the corresponding theoretical basis for calculating the yield strength based on the nanoindentation data.As the simplest and purest model of complex systems,the physical methods and ideas generated by the research of spin glass have guided many disciplines.However,limited by the doping range of magnetic atoms,spin glass materials themselves do not have practical applications due to their low phase transition temperature.Based on the essential features of spin glass states,a bulk chemically complex alloy system composed of multiple magnetic elements was designed.Structural characterizations such as XRD and TEM have demonstrated that the alloy system can maintain a single-phase random solid solution structure within a wide range of compositional space.Magnetic characterzation experiments have shown that the transition temperature can be continouously regulated up to room temperature.This system can be utilized as an ideal medium for the study of spin glass states and provides a new strategy for the design of high-temperature spin glass materials.Chiral spin structures such as skyrmions are considered as the next generation information storage media.And spin chirality can be detected by the topological Hall effect.Through PVD method,the above chemically complex alloys were fabricated into thin films and the presence of high density of chiral magnetic structures was perceived by transport and magnetic measurements.Further L-TEM and SANS experiments demonstrated that the high-intensity topological Hall effect is caused by the atomic-size chiral spin structure,which is favored by applied magnetic field.And the mechanism of spin chirality is also verified by first-principles calculations.The preparation and characterization of chemically complex alloy films can contribute to a deep understanding of their noncoplanar magnetism and promote their practical application in spintronic devices.In this paper,the mechanical,electrical and magnetic properties of structural and chemical disordered alloy materials are investigated by experimental,theoretical and computational methods,the theoretical models of plastic deformation at different length scales and the mechanism of frustration-induced noncoplanar magnetism are proposed,which are of great significance for the exploration and study of underlying phsics for the disordered alloy materials. |
