Formation And Deformation Mechanism Of Nanotwins In Nickel-based Superalloys By Molecular Dynamics Simulations

With the continuous improvement in the thrust-to-weight ratio of aeroengines,the aviation industry has always aimed to improve the comprehensive performance of Ni-based wrought superalloys used for aero-engine turbine discs.Currently,the nominal composition of state-of-the-art aerospace superalloys includes as many as ten elements.It faces challenging of“easy strengthening and difficult deformation”to further improve the properties of Ni-based wrought superalloys via alloying strengthening.Fortunately,for Ni-based wrought superalloys,the theoretical basis of the role of alloying elements and the relationship between microstructure and properties are relatively mature.The alloy composition can be optimized based on the theory of material plainification and existing Ni-based wrought superalloys,combined with high-throughput calculations,obtaining a low-energy stable interface through interface regulation,which provides an important method for the development of high-performance Ni-based wrought superalloys.The Ni,Cr,Co,and Al elemental contents in high-performance Ni-based wrought superalloys account for more than 80%of the total elements,and twin boundaries are common low-energy stable interfaces.The lower the stacking fault energy(γ_ISF),the easier it is to form a large number of twins.Therefore,in this paper,molecular dynamics simulations are used to study the effect of alloying elements onγ_ISF in Ni-based wrought superalloys,and further reveals the formation mechanism,stability,and deformation mechanism of nanotwins in Ni-based wrought superalloys at the atomic scale.It provides effective ideas and theoretical guidance for realizing high-performance of Ni-based wrought superalloys by regulating the twin interface to replace the alloying.First,theγ_ISF of Ni-based alloys formed from Cr,Co,and Al with the Ni matrix were investigated based on the composition of existing high-performance Ni-based wrought superalloys.The results show that the order of reduction ability is Co<Cr<Al forγ_ISF,and that theγ_ISF of binary Ni-based alloys changes linearly with the alloying element content.Theγ_ISF of Ni–Cr–Co alloys and the contents of Cr and Co satisfy the binary linear combination relationship ofγ_ISF=122.89±0.47-0.66（at.%,Cr）-0.65（at.%,Co）.Theγ_ISF of Ni–Cr–Al,Ni–Co–Al,and Ni–Cr–Co–Al alloys fluctuated up or down with increasing in alloying element content.A novel Ni-based wrought superalloy strengthened by twins andγ’phases was designed based on the extreme value ofγISF fluctuation.The composition of the alloy was 0.1C–19Cr–19Co–1.42W–5Al–2.9Ti–1.5Nb–Bal.Ni（at.%）.Second,the relationship betweenγ_ISF and the deformation mechanism during the tensile process of Ni and its alloys with differentγISF were simulated based on the contents of the Cr,Co,and Al alloy elements in the novel Ni-based wrought superalloy.The results show that the monocrystalline Ni₅₇Cr₁₉Co₁₉Al₅ alloy with lowγ_ISF makes it easy to form parallel twin lamella or deformation twin when the model with few atoms is deformed.As the cross-sectional side length of the model was 30 times greater than the lattice constant,the flow stress,phase structures,and dislocation density in the plastic flow stage displayed stable fluctuations with the strain variation.For stretched monocrystalline Ni and its alloys with the same atomic number and stable flow,lowerγ_ISF results in a larger area of the stacking fault plane during plastic deformation.Third,the deformation behavior of uniformly nanotwinned Ni₅₇Cr₁₉Co₁₉Al₅ alloys at different temperatures were simulated to reveal the deformation behavior of the nanotwins and the formation mechanism of secondary twins in the novel Ni-based wrought superalloy.The results show that the twin lamellar thickness and mean flow stress satisfied the Hall–Petch relationship.The twin boundaries exhibited good mechanical stability at low and medium temperatures.Twin boundaries are unstable at high temperatures.Secondary twins mainly form at the steps where the twin boundaries intersect with the stacking faults and at the stacking faults where they intersect with each other.The formation of secondary twins is a result of stacking fault sequence expansion.The detwinning of secondary twins results from the shrinkage of twin boundaries along a direction perpendicular to the twin boundaries towards the interior of the twin.Fourth,based on the polycrystalline structural characteristics of the novel Ni-based wrought superalloy,the effects of grain size and twin lamellar thickness on the deformation behavior of the nano-polycrystalline Ni₅₇Cr₁₉Co₁₉Al₅ alloy were simulated.The results show that the elastic modulus and tensile strength of the polycrystalline Ni₅₇Cr₁₉Co₁₉Al₅ alloy exhibited obvious grain-size dependence.As the grain size exceeded 16 nm,the elastic modulus remained unchanged.The tensile strength and grain size satisfy the Hall–Petch relationship,and the dislocation motion dominates the plastic deformation.For the grain size lower than 16 nm,the elastic modulus decreased as the grain size decreased.The tensile strength has an inverse Hall–Petch relationship with grain size.The grain boundary slip and grain torsion mechanisms dominated plastic deformation in the nano-polycrystalline Ni₅₇Cr₁₉Co₁₉Al₅ alloy without twins.In the nano-polycrystalline Ni₅₇Cr₁₉Co₁₉Al₅ alloy with twins,the twin boundaries inhibited the torsion of grains,and grain boundary slip dominated plastic deformation.Finally,based on the good strength-plastic integration of the gradient nanostructured metals,the deformation behavior of the gradient nanostructured Ni₅₇Cr₁₉Co₁₉Al₅ alloys were simulated.The results show that in the gradient nanotwinned Ni₅₇Cr₁₉Co₁₉Al₅ alloy and gradient nano-polycrystalline Ni₅₇Cr₁₉Co₁₉Al₅alloy,the larger the twin lamellar thickness or grain size,the greater the ability to accommodate dislocations,so that the material has good plasticity.The smaller the twin lamellar thickness or grain size,the more the nucleation and propagation of dislocations are hindered,so that the material has good strength.The twin boundaries inhibit the movement of dislocations while accommodating many dislocations.Therefore,the gradient nano-polycrystalline Ni₅₇Cr₁₉Co₁₉Al₅ alloy with twins has better mechanical properties than the polycrystalline Ni₅₇Cr₁₉Co₁₉Al₅ without twins.