| The conventional alloys are designed generally by considering one primary element as the main group of ingredients and by adding other alloy elements to improve certain properties.The emergence of high-entropy alloys,which contain a range from 5 to 13 kinds of equal or near equal atomic elements,breaks the traditional alloy design concept.In high-entropy alloys,the atomic percentage of all components is between 5%and 35%.The high-entropy alloys possess high-entropy effect,severe lattice-distortion effect,sluggish diffusion effect and cocktail effect.Under the role of those effects,high-entropy alloys have many excellent properties such as high strength,high hardness,high corrosion resistance and high wear resistance.Therefore,it is enormously significant on scientific research and engineering application to develop and design new kinds of high-entropy alloys have.Currently,investigations on high-entropy alloys mostly focus on the aspects of phase composition,properties and component design.However,the theory of plastic deformation of high-entropy alloys appeals few attention.Therefore,innovative design of Ni-Ti-Fe-Al-Cu alloys is realized independently by means of changing the contents of Ni and Ti element to produce new kind of high-entropy alloys and Ni-Ti based alloys.Moreover,the microstructure,mechanical properties,and plastic deformation mechanism under high temperature are respectively analyzed through a variety of experiment methods.In the series of NixTixFe100/3-2x/3Al100/3-2x/3Cu100/3-2x/3 alloys（namely Ni20,Ni26,Ni32,Ni35,Ni41 and Ni47 where x respectively stands for 20,26,32,35,41 and 47）,the microstructure morphology of all high-entropy alloys refers to dendrite crystal where segregation of component elements are unequally distributed among the dendrites.Ni-Ti based alloys mostly possess equiaxial structure and primary elements are equally distributed.Generally,high-entropy alloys consist of simple solid solution structures.Face-centered cubic phase and body-centered cubic phase constitute Ni20 alloy.With the increase of Ni and Ti components,body-centered cubic phase gradually replace face-centered cubic phase,so Ni35alloy only contains body-centered cubic phase.The matrix of Ni41 of Ni-Ti based alloy is Fe0.2Ni4.8Ti5 and precipitation phases are NiTi,AlTi3 and Cu2Ti.The corresponding matrix of Ni47 of Ni-Ti based alloy is NiTi and precipitation phases are Fe0.2Ni4.8Ti5 and AlCu2Ti.Under the condition of compression which is conducted under room temperature and1×10-3/s strain rate,among the high-entropy alloys,the compressive strength of Ni26 alloy is the highest,which amounts to about 1385MPa,whereas Ni20 alloy possesses the best plasticity,and the biggest compressive strain could reach 10.23%.Among the Ni-Ti based alloys,Ni41 alloy has the highest compressive strength of 2023MPa,and Ni47 alloy has the biggest compressive strain to 35.77%.Alloys are characterized by brittle fracture,so river shape pattern can be observed in some compression fracture morphology which stands for cleavage fracture characterization.Under the condition of high temperature compression which is conducted under 1×10-3/s strain rate,where compression temperatures are 850℃and1000℃,Ni20 alloy shows the biggest compression strain to 50%.Consequently,Ni20 alloy possesses the best plasticity at high temperature.Based on high temperature dislocation slip theory,the second phase precipitation theory and the consequences of high temperature deformation experiment of Ni20 alloy,plastic deformation mechanism under high temperature is comprehensively obtained.Under the effect of high temperature compressive stress,slip system is activated and starts to move.Dislocation slips along the specific slip plane and slip direction.With the increase of deformation,dislocation density increases and dislocation tangles and blocks with each other,which leads to dislocation hardening and second phase hardening owing to solid precipitation.Under the effect of these hardening mechanisms,flow stress increase.When dislocation density reaches a critical value,the area with higher dislocation density provides sufficient deformation energy to induce the precipitation of the second phase and this,in turn,causes the decreasing of dislocation density.Dislocation,which blocks near the second phase,finally crosses and climbs the second phase and continues to slip.Consequently,dislocation multiplication and dislocation annihilation reach a dynamic equilibrium and deformation enters the stage of steady deformation. |
