| Plastic deformation mechanism and microstructure evolution are one of the very important in the field of nanostructure metallic materials, which have not came to consensus yet. In conventional coarse-grained materials, the strength/hardness increases with the decrease of grain size, which is called Hall-Petch relationship. However, some experimental phenomena are not consistent with Hall-Petch relationship in nanocrystalline materials; this indicates the deformation mechanism and microstructure in nanocrystalline materials are not the same as conventional coarse-grained materials. For conventional coarse-grained materials, the dislocations originate from grain boundaries or grain interiors and their activities (dislocations propagate,pile up and interact to form dislocation networks) realize plastic deformation. However, the dislocations in nanocrystalline materials are not stable in grain interiors and they tend to bow out, which resulted from the little scale-effect. Therefore, there are very few dislocations or no in nanocrystalline grains. Many deformation mechanisms have been related to nanocrystalline materials, such as dislocations emitted from grain boundaries shear the grain and are absorbed by the opposite grain boundary; grain boundaries emit partial dislocations, leaving behind stacking faults or twins; grain rotation; grain boundary slipping; phase transformation; and vacancies/vacancy-clusters migration. However, the deformation of nanocrystalline materials won't be realized by only one mechanism. So further investigation is needed to understand which mechanism dominates the deformation under what kind of the applied conditionThe nanocrystalline nickel with average grain size about 20nm is cold rolled at room temperature in this paper, the microstructure evolution and deformation mechanism were analyzed by means of X-ray diffraction, micro-hardness test, annealing, transmission eletron microscope (TEM) and high resolution TEM (HRTEM). Three stages are found in the cold-rolled deformation of nanocrystalline nickel:(1)strain (thickness reduction)ε< 20%. In this stage, the deformation is dominated by grain rotation, which can induce dislocation density reduction so that caused work-hardening. (2) 20%<ε<30%. In this stage, grain rotation mechanism takes lower effect and GBs emit dislocations/partial dislocations with low density. Stress concentration may leads to thin twining segment which can hinder dislocation slipping, so work-hardening also can be captured. (3)ε>30%. In this stage, grain rotation stopped. Twining is found by HRTEM observation. This indicates that deformation twining dominates plastic deformation. The twining is formed via partial dislocation that nucleated and emitted from GB. In addition, three different twining boundaries(TBs) are captured:(1) If the share stress is sufficient enough and continue, a straight TB can be formed. (2)If the shear stress is not sufficient enough(stress withdraw or a unfavorable orientation), the partial dislocations that emitted from GBs will remain at the different locations of the adjacent plan, then a step TB can be formed. (3) When twinning grows, the Lomer-Cottrell lock is formed by TB generating dislocation and GB dislocation reacting with TB. Both the two factors influence the straight TB so that a distortion TB can be formed. When grain rotates, GB eliminating causes grain coarsening that attributes to stress concentration. XRD profile is analyzed to calculate the grain size with different strain. TEM results show that some of grains reach to 60-70nm after deformation. Further investigation shows that grain coarsening is the main reason for work-softening whenε>30%. A strain-induced face-centered-cubic (fcc)→body-centered-cubic (bcc) phase transformation occurs whenεexceeds 30%. The orientation relationship of y (fcc)and a(bcc) is (11-1)γ//(10-1)αwhich is consistent with the Kurdjumov-Sachs (K-S) orientation relationship. It can be regarded as strain induced martensite phase transformation. The average lattice constant of bcc structure is a=0.2921nm. A phenomenon of reversible phase transformation which is from bcc to fcc appears after annealing time over 90 min. Annealing process causes GBs relaxation which is difficult to emit dislocations. Some other defects such as twin boundary can hinder dislocation slipping as well. So both of the two factors lead to increased hardness during annealing. A new model of vacancy formation energy in nanocrystalline is established. The calculation base on this model shows that the vacancy formation energy decreases with grain size reduction when grain size is less than 40nm and it is as equal as coarse grain while grain size is more than 40nm. Additionally, the vacancy formation energy decreases linearly with the increasing figure factor. At last the vacancy is regarded as a rigid ball, and the formula that to calculates the multi-vacancy formation energy is gotten and discussed. The formation and cohesive energy of the multi-vacancy in bcc transition metals are calculated.
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