Study On The Mechanisms Of The Effect Of Shear-strain On The Stability Of Ni Symmetrical Tilt Grain Boundaries

The structural instability of nanocrystalline metal materials is the outstanding bottleneck of its practical application.At present,in order to improve the stability of nanocrystalline metal materials,the inhibition of grain boundary migration is generally achieved by alloying method from both thermodynamics and kinetics.Although the alloying method can effectively improve the stability of nanocrystalline metals,the solute elements different from the matrix can inevitably affect the properties of the matrix itself.In addition,the alloying method is not suitable for pure nanocrystalline metals.Therefore,it is urgent to explore new ways to inhibit grain boundary migration and improve the stability of nanocrystalline metal materials without or less depending on alloy elements.For these reasons,on the one hand,the first principles method based on density functional theory is used to study the grain boundary migration behavior s of Ni ?3 [110](111)and ?11 [110](113)symmetric tilt grain boundaries under shear,and the effect mechanism of Mo segregation on grain boundary migration was clarified.On the other hand,the structural relaxation behavior of Ni ?17 [110](223)symmetric tilt grain boundary under shear loading was systematically investigated by molecular dynamics simulation,and the micro mechanism of the effects of impurity Ag atom and temperature on grain boundary relaxation was revealed.Our work can not only provide a useful reference for alloying methods to inhibit grain boundary migration,but also provide theoretical guidance for grain boundary control of nanocrystalline Ni without(less)depending on alloying elements.The main results are as follows :(1)The shear load parallel to the grain boundary plane can induce Ni ?3 [110](111)symmetrically tilt grain boundary to migrate along the vertical direction of the grain boundary plane.It is a coupling process including the relative sliding between grains on both sides of grain boundary,grain boundary structural transformation and grain boundary migration.The migration of Ni ?11 [110](113)symmetric tilt grain boundary induced by shear strain is also the same.(2)Mo segregation can effectively reduce the energy of Ni ?11 [110](113)symmetric tilt grain boundary,improve the thermal stability of the grain boundary,but reduce its migration energy barrier.However,for Ni ?3 [110](111)grain boundary,Mo segregation not only releases the excess grain boundary energy,but also enhances the grain boundary migration energy barrier,and makes the grain boundary migration need more shear strain.(3)Under the action of shear,the nucleation and evolution of Ni ?17 [110](223)grian boundary source Shockley partial dislocation ultimately result in the low-energystate transformation of the grain boundary structure units,and the grain boundary structure units are pinned by Hirth and Lomer-Cottrell dislocations.However,the Ag atom contained in the grain boundary increases the shear stress and strain required for the grain boundary relaxation,and the strain range for the grain boundary relaxation is expanded,indicating the inhibitory effect of Ag atom on the structural relaxation of Ni?17 [110](223)grain boundary.(4)As the temperature increases from 10 K to 250 K,the grain boundary relaxation of Ag-free Ni ?17 [110](223)grain boundary is easier;and the inhibition effect of Ag atom on the grain boundary relaxation weakens with the increase of temperature.But the stress and strain required for the grain boundary relaxation of Agcontaining grain boundary under shear is still greater than that of the Ag-free.