Simulation Investigations On The Interface And Alloying Effects Of Ni/Ni₃Al Nanowires

Due to the good creep, fracture resistance, antioxidation and corrosionresistances, Ni-based single crystal superalloy with a typical two-phase structure ofNi/Ni3Al, is a key material to produce the blade of aero-engines and industrial gasturbines. In the dissertation, molecular dynamics (MD), together with the modifiedanalytic embedded-atom method (MAEAM), is used to explore the effects of thealloying elements, interface and their influence on the deformation mechanisims ofNi/Ni3Al nanowire (NW), which can provide theoretical guidances for the designs andapplications of advanced high-temperature alloy materials.In the dissertation, the size effect of Ni, Al and V NWs are firstly studied. Theresults obtained indicate that the mechanical and thermodynamic properties decreaselinearly with the size decreasing as result of the existence of the free surface. We findout that the melting mechanism of Ni, Al and V NWs also changes with the sizedecreasing and the corresponding critical size of these NWs is about0.476nm,0.526nm and0.625nm, respectively. For the small-sized NWs, the melting behaviorresults mainly from the anharmonic effect of atomic vibration, on the contrary, for thelarge-sized NWs, the free surface plays a predominant role in the surface premeltingand melting transition of NWs.We calculate the apparent interfacial energy (AIE) of the bulk and NWs. The AIEvalues of the bulk are negative; in contrast, those of NWs are positive. And the AIEmagnitudes of the bulk and NWs decrease linearly with the decreasing thickness ofthe interfacial transition region (ITR). For the bulk materials, the critical thickness ofITR is about1.7nm by examining the thickness dependence of the average energy ofatom. We discuss the γ’-phase volume (γ’-VF) effect on the apparent interfacial energyof ITR with the critical thickness. The results show that the value of AIE is the lowestas γ’-VF ranges from50%to70%, which indicates that the bonding strength of theNi/Ni3Al interface with these γ’-VFs is the most excellent. For the Ni/Ni3Al NWs with50%γ’-VF, we estimate that the critical thickness of ITR is about6.20nm bymonitoring the size dependence of the differentia between the average energy of ITRand its limited value. Successively, the ductile-brittle fracture transition mechanism ofNW is in detail discussed. We discover that the critical size of discriminating betweenthe ductile-brittle transition behaviors is in the range of (2.3～3.5) nm. The site preference of the alloying elements (Re, Ru, Co and Ta) in the Ni3Alalloy is studied, which shows that the Co atom prefers to substitute the Ni site and itssubstituting formation energy is positive, conversely, the Re, Ru and Ta atoms preferto occupy the Al site and the substituting formation energies are less than zero. Westudy the effect of the alloying elements, together with the substituting manners, onthe microstructure and the mechanical properties. When these alloying elements (Re,Ru and Ta) substitute the1st,3rd,1stand3rdnearest neighboring Ni atoms or all matrixatoms within the4thnearest neighboring distance, the mechanical property of Ni3Alalloy is significantly enhanced. In contrast, as the alloying elements replace the2nd,4th,2ndand4thnearest neighboring Al atoms, the strengthening effect of the solutedoping on the mechanical property is obscure. In addition, no matter what types of thedoping manners, the strengthening effect of the Co element is not obvious. Thedetailed analysis is focused on the strengthening mechanism of the alloying elementson the mechanical property, which indicate that there are two main factors, namely thesize of the solute atom and the solute-solvent interaction, to control the strengtheningeffect. According to the solute (Re, Ru, Co, V and Ta) cluster having an effect on themechanical properties of Ni/Ni3Al NWs, we find out that the evolution tendencies ofthe bulk modulus varying with the incremental size are apparently different fromthose of the bulk counterpart. The mechanical properties of Ni/Ni3Al NWs with Realloying cluster increase apparently with the cluster size increasing. The sizes of theRu and Co alloying clusters have a weak effect on the improvement in the mechanicalproperties. And for the V and Ta cluster, their addition can reduce the mechanicalproperties, the trends of which are more and more prominent as the cluster sizeenlarges.The size effect on the mechanical properties of Ni/Ni3Al NW is studied with agiven external load. The obtained results indicate that the yield strength and theelastic modulus increase exponentially and the yield strain descreses exponentiallywith the increasing size. And three of them are gradually close to the limit value asthe wire size reaches a critical one. Then, we investigate the deformation mechanismof Ni/Ni3Al NW and find out that the fualt slip is the decisive factor. The temperaturedependence of the stress-strain process of Ni/Ni3Al NW is in detail discussed, whichindicates that the deformation of NW is mainly ascribed to the anharmonic vibrationand dislocation slip. The dominant function of the above factors is different with thetemperature increasing. Under the condition of a loading strain, we study theinfluence of the Re, Ru, Co, Ta and V clusters on the mechanical propert ies of Ni/Ni3Al nanowire. The doping solute clusters except the Co cluster can soften themechanical properties of NW.Finally, taking Ni/Ni3Al NW with the Re, Ru and Ta cluster locating in theγ’-phase as an example, we analyze the temperature dependence of the mechanicalproperties of nanowire and detailedly discuss the deformation behavior and itsinherent mechanism of wire. For NW with the Re cluster, the deformation results fromthe dislocation slip at the lower temperatures. Because the cluster impedes thedislocation motion, the slip phenomenon occur around the Re cluster and then thenecking generate here. The similar deformation mechanism has been also found inNWs with the Ru cluster at the lower temperatures. At the higher temperatures, thedeformation of NW has yet originated from the dislocation slip. But the Re cluster hasno influence on the dislocation motion for the stronger anharmonic effect. However,the deformation mechanism of NWs with the Ru cluster is in disagreement. Becausethe Ru cluster with a perfect lattice can effectively hold back the dislocation spread,similar to that of NW with the Ru cluster at the lower temperatures. The causes ofdeformation of NWs with the Ta cluster are different. The deformation behavior isalmost confined to the cluster vicinity. Therefore, the deformation mechanism of NWwith the Ta cluster originates mainly from the solute size and the anharmonic effect.