Preparation And Properties Of Microalloyed Supra-nano Ni-B Alloy

Supra-nano metals(< 10 nm)have been extensively investigated due to their unique structure and comprehensive properties recently.However,most of the research work is theoretical analysis or computer simulation,and the experimental research work is little.This is because the supra-nano metals usually have poor structural stability,and there is a trend of grain growth through interface migration and other ways.The intrinsic instability of the supra-nano structure not only greatly limits the preparation and the study of their intrinsic structure-performance relationships,but also severely restricts the application and development of supra-nano metals.It's known that the stability of nanostructure can be improved by alloying.However,high alloying makes the development of materials more and more dependent on resources and makes materials more and more difficult.Therefore,how to prepare microalloyed supra-nano metals and investigate their intrinsic structure-property relationship is a problem that must be solved before the application of supra-nano materials.In this paper,by introducing a small amount of alloy element B with strong segregation ability to stabilize nanostructure,a series of supra-nano Ni-B alloys with different B cntent were prepared and their thermal stability and mechanical behaviors were studied.Firstly,the Ni-B alloys were fabricated by pulse electrodeposition and the structural characterization was determined by using X-ray diffractometer(XRD)and transmission electron microscope(TEM),which illustrates all prepared supra-nano Ni-B alloys are of single-phase fcc structure.The higher the trimethylamine borane(TMAB)content in the plating solution,the higher the B content of the Ni-B alloy and the smaller the grain size.And the lattice parameters increase first and then decrease with increasing of B content.Furthermore,the nanocrystalline Ni and supra-nano Ni-B alloys were annealed at different temperatures,and the microstructure evolution of the samples was characterized by TEM and XRD.The grain boundary(GB)energy of the as deposited and annealed samples was quantitatively analyzed by DSC.The results show that the grain size of as-deposited Ni-B alloys change from no obvious change to rapid growth with the increase of annealing temperature.There is a slow exothermic process with a large temperature range and a concentrated exothermic process on the corresponding DSC curves.The slow exothermic process is due to the relaxation of non-equilibrium GB and the segregation of B to GBs with the increase of temperature in the process of thermal analysis,which results in the slow release of GB energy;the concentrated exothermic process is the concentrated release of energy caused by the rapid growth of grain.The calculated GB energy of as-annealed supra-nano Ni-B alloy is lower than that of as-deposited samples due to GB relaxation.At last,the mechanical behaviors of supra-nano Ni-B alloy were studied by microhardness and tensile tests,and the structure-property relationship as well as the corresponding micro deformation mechanism of supra-nano Ni-B alloy were explored.The results indicate that a transition from normal Hall-Petch to abnormal Hall-Petch relationship in the as-deposited Ni-B alloys at a grain size of about 7 nm.Obvious annealing-hardening phenomenon was discovered in supra-nano Ni-B samples and the maximum hardness increment of Ni-1B is 47.7%.The strain rate sensitivity index of supra-nano Ni-1B alloy is smaller than that of nanograined Ni,indicating that the addition of trace B element can help stabilize nanostructure.Uniaxial tensile test results indicated that the as-deposited and low-temperature annealed supra-nano Ni-1B alloys exhibited outstanding strength-plasticity match.In summary,the introduction of trace B element with strong segregation tendency can not only greatly refine the grains,but also effectively stabilize the nanocrystalline structure.It is expected to provide theoretical basis and technical support for designing and preparing nanocrystalline metals with excellent properties such as ultra-high hardness / strength and structural stability.