Mechanical Properties And Strain Hardening Of Gradient Nanostructure Nickel

The main purpose for the synthesis of nanocrystalline (NC) and ultra-fine grain (UFG)originates from the expectation to achieve high strength and ductility simultaneously.Otherwise, the strengthening mechanism of NC/UFG is characterized by trade-off. This limitsthe structural application of NC/UFG. Therefore, the efficient way to enhance uniformelongation is to improve strain hardening rate and inhibit deformation localization. So far,Many strategies of improving strain hardening and uniform elongation have been reported, forexample, introducing nano-twin, precipitated phase/second phase, bi-modal/multi-modal andphase transition/twin deformation. Currently, producing gradient nanograined structure is anew approach to improve ductility but lack of deep understanding of yield and strainhardening mechanism. Therefore, It is scientifically significant to study mechanical behavior,strain hardening behavior and related microstructure mechanism of the multi-scale structure.The pure nickel(＞99.6%) was chosen as the experimental material. Two nano-grainstructure was obtained by severe plastic deformation (SPD), surface mechanical attritiontreatment (SMAT) was employed to prepare the gradient structure; Equal-channel anglepressing (ECAP) and proper heat treatment was conducted to produce multi-scale structure.The tensile mechanical properties have been characterized through quasi-static tensile tests,hardness tests and repeated stress relaxation experiment. The microstructure mechanism have been analyses by optical microscope (OM), scanning electron microscope (SEM) andtransmission electron microscopy (TEM). The main conclusions are summarized as follows：1. GNS-Ni were prepared by SMAT. Quasi-static tensile tests show that the yield strength ofGNS-Ni depends on the distribution of Vickers hardness. Yield strength of GNS-Ni isbetween200-450MPa,3-6times higher than CG (70MPa); Uniform elongation of GNS-Ni isbetween13%-35%compared with CG(45%)Four kind of yield behavior were discovered in gradient nickel:1) continuous yield:stress rises rapidly after yield, behaves continuous strain hardening,2)transient hardening:after yielding,stress rises slowly, a stagnation of the hardening rate is frequentlyobserved;3)yield point phenomenon: stress drops after yielding first then followed bycontinuous strain hardening;4)yield drop: stress drops rapidly after yielding.Vickers hardness tests show gradient distribution of Vickers hardness in GND-Ni beforetensile. The hardness of core and surface rise after tensile, the core is higher than surface ofΔ Hv, the analysis shows a good combination of high strength and high ductility require thesurface of high strength and the core of hardening ability.Repeated stress relaxation tests show a continuous drop of mobile dislocation density inCG with strain increases and decrease first and then increase of mobile dislocation density inGNS-Ni. Combined with analysis based on classical dislocation theory reveal that the mobiledislocation in surface layers is able to accumulate successively, resulting in strain hardening,suppressing the localized plastic deformation in surface layers and thus improving theuniform elongations.2. The UFG nickel was prepared by ECAP1Pass. TEM shows UFG nickel behave lamellarstructure characteristic and lamellar spacing is between300-500nm. The UFG Ni was heatedby proper temperature and time.Quasi-static tensile tests show that the yield strength of UFG nickel is570MPa which isnearly6times higher than CG which is100MPa. It exhibits high yield strength the same asUFG’s and5%uniform elongation after427℃54h treatment. A excellent combination ofstrength (320MPa) and ductility (17%) was obtained after477℃8h treatment.TEM and EBSD show that partial recrystallization occurred and the island-like UFGregion still survived with a uniform distribution. The sample after477℃8h treatmentexhibits larger grain size (＞10μm) and volume fraction. The corresponding volume fraction of recrystallized grain after427℃54annealing treatment was about10%. The excellentmechanical properties are attributed to strain partitioning during tensile deformation.