Supersaturated Solution Behavior Of W-Cu System And Its Effect On Microstructure And Properties

Alloying of immiscible elements can provide novel opportunities for the discovery and performance tuning of new materials.W-Cu is a typical binary immiscible system,and its bulk materials have both structural and functional properties,which has great application prospects in aerospace,electronic information,nuclear industry and military fields.However,their intrinsic properties of complete immiscibility in equilibrium and the large difference in melting points lead to poor mechanical and thermal properties of existing W-Cu materials.In the early stage,our research group developed a sol-spray drying and hydrogen reduction method based on non-equilibrium alloying strategy,and prepared multi-component fine-grained W-Cu nanocomposites by low-temperature sintering.Moreover,the supersaturated solid solution alloying characteristics of W and Cu,which endows the material with good mechanical properties and thermal properties,has been preliminarily observed using XRD and other techniques.However,the results of the previous study did not give intuitive evidence of supersaturated solid solution and the related mechanism of alloying and performance enhancement is still unknown,which greatly restricts its further development and application.To this end,this paper systematically investigates the alloying behavior of immiscible W-Cu systems and its effect on microstructure and properties through nano-thermodynamic calculations and experimental verification,in order to clarify its related mechanisms.The main findings are as follows:（1）Atomic-scale synthesis based on non-equilibrium alloying strategy gives the driving force for spontaneous alloying of W and Cu,and promotes the in-situ mutual dissolution of W and Cu.With the help of XRD,SEM,HRTEM,STEM-EDS,it was confirmed that the single-phase Cu（W）supersaturated solid solution composite powder with FCC structure was generated.The supersaturated solid solution W-Cu nanocomposite powder has good thermostability and can still maintain partial alloying characteristics after annealing at 300℃.In addition,based on the theoretical framework of nano-thermodynamics,the formation enthalpy is used as the criterion of spontaneous alloying,and it is found that the formation enthalpy decreases with the decrease of the original synthesis size,and the formation enthalpy decreases to negative value at about 10nm;and increases with the increase of shape factor and relaxation factor.（2）Bulk W-Cu nanocomposites is composed of fine,nearly spherical supersaturated solid solution particles and Cu phase.Atomic-scale STEM-EDS analysis showed that about 20%of the Cu atoms replaced the W atoms in the W particles in the form of substitution solid solution,forming W（Cu）supersaturated solid solution.The distribution of Cu within the supersaturated solid solution particles is heterogeneous,with concentration fluctuations at the atomic scale and a tendency to form clusters of 10 nm in size at the nanoscale.In addition,the phase interface between the supersaturated solid solution W particles and the Cu matrix is characterized to be a gentle gradient change.（3）The W-Cu nanocomposites with supersaturated solid solution characteristics exhibit excellent mechanical and thermal properties.At room temperature,the tensile strength of W-20 wt.%Cu was 806±19 MPa and the elongation was 9.8%±0.4%;the tensile strength of W-50 wt.%Cu was 610±15 MPa and the elongation was 17.9%±0.7%.The tensile fractures of both W-Cu nanocomposites exhibited ductile fracture characteristics.In addition,W-20 wt.%Cu still has a tensile strength of200±24 MPa and ductility of 11.4%±0.25%at 800°C.The thermal conductivities of the W-20 wt.%Cu and W-50 wt.%Cu nanocomposites were 220±10 W/（m·K）and 280±14 W/（m·K）,which were close to 94%and 85%of the theoretical values,respectively.In addition,the thermal expansion coefficients of W-20 wt.%Cu and W-50 wt.%Cu nanocomposites were（7.2±0.58）×10^-6/K and（13.5±0.4）×10^-6/K,respectively.Dislocations and polymorphic nano-twins are generated during the tensile deformation process of the W-Cu nanocomposites,and their excellent tensile strength derived from the particle strengthening,dynamic Hall-Pech effect induced by grain refinement,and interfacial strengthening via gradient diffusion;the better plasticity originated from the modulation of polymorphic nanotwins and the synergistic deformation of the particles and the matrix.（4）The W-Cu nanocomposites with supersaturated characteristics exhibited excellent plasticity processing ability.The size of the supersaturated solid solution W particles tended to increase with increasing rolling temperature and their distribution was at an angle of 45°to the rolling direction.There is no obvious texture in W particles at the five rolling temperatures.The microstructure after cold rolling at room temperature is characterized by inhomogeneous strain.As the rolling temperature increases,the tensile strength increases and then decreases,while the tensile ductility gradually increases.After rolling at 600°C,the strength and ductility of the W-50 wt.%Cu nanocomposites reached a good matching value.The hardness values of the samples after rolling at room temperature,200°C,400°C,600°C,and 800°C were 194.7±0.4 HV,208±3.1 HV,187.9±1.5 HV,188.4±3.2 HV,and 165.6±2.9 HV,respectively;with increasing rolling deformation,the size of the supersaturated solid solution W particles decreased and then increased.The tensile strengths of the samples treated with 25%,50%,and 75%rolling deformation were 546MPa,645 MPa,and 638 MPa,respectively;the total elongation were 7.0%,6.98%,and 7.95%,respectively;and the vickers hardnesses were182.17±1.2 HV,188.37±3.2 HV,and 190.63±2.0 HV,respectively.