Dislocation Exhaustion By Diffusional Phase Transformation And Ultra-hardening Mechanism In Nanograined Metals

Grain refinement is one of the prevailing strengthening methods.However,the extension of the grain-size strengthening to the size range below 10-15 nm is restricted either by the challenges in materials generation or by the inverse Hall-Petch effect.In the grain size range where the Hall-Petch law works,the plastic deformation is determined by the dislocation mechanism.Under mechanical loading,dislocation sources are activated and the glide of lattice dislocations provides the carriers for plastic strain.But when the grain size is further reduced to the so-called extremely-fine nanograined regime(10-15 nm),it is suggested that the grain boundary(GB)processes,e.g.,GB sliding and GB migration or coupling,become dominant in plasticity,which contributes to the observed inverse Hall-Petch effect.Accessing the strength limit of NMs drives one to explore pathways to tailor the behavior of GBs and dislocations,instead of focusing on the grain size alone.This work mainly focuses on the relationship between dislocations and strength in nanograined(NG)materials.A diffusional phase transformation at GBs,which is triggered by low-temperature annealing,exhausts the residual lattice dislocations and gets a “dislocation-free” polycrystal.These structural changes could result in the formation of dislocation-exhausted grains and in a transition of plasticity from being dominated by the glide of pre-existing lattice dislocations to being controlled by dislocation nucleation at GBs,which induces an ultra-hardening effect.The main research results are as follows:(1)The NG single BCC phase Fe-Ni alloy was prepared by using the thermal evaporation inert gas condensation(IGC)method.The strengthening mechanism of the NG Fe-Ni alloys was studied by combining the unique structure evolution and hardness.The intergranular Ni enrichment and lattice dislocations are observed in the NG Fe-Ni alloys,and the pre-existing dislocations will dominate plastic deformation under low stress.The triggering of diffusional phase transformation(BCC→FCC)at GBs at 300 ℃ will consume lattice dislocations significantly without grain growth.Meanwhile,the hardness could increase from 5.1 GPa to10.8 GPa.Due to the consumption of dislocations by diffusional phase transformation,the plastic deformation mechanism is essentially controlled by the activation of dislocation sources at GBs instead of pre-existing lattice dislocations,which leads to an ultra-hardening effect.(2)To confirm the applicability of the diffusional phase transformation strengthening mechanism in NG materials,an ω single-phased NG Ti-Fe alloy with rich diffusional phase transformation(ω→α?/α→β)pathway was prepared by using the laser-IGC method.The strengthening of NG Ti-Fe alloy was realized by adjusting the microstructure during the annealing process.The diffusion of Fe promotes the ω→α?/α at the early stage and reduces the microstrain,and the hardness reaches the highest value when the fraction of the second phase is 20%.Amazingly seen the hardness increases for the second time with the phase transformation α→β in spite of the growth of grain size.Meanwhile,the microstrain reduces to an extremely low value.It is well known that dislocation could be the source of microstrain,it indicating the importance of dislocation consumption by diffusional phase transformation in the strengthening of NG materials,as well as the universality of diffusional phase transformation in the strengthening of NG materials.(3)The NG Al-Mg-Li alloy with a low melting point shows GB phase precipitations and low dislocation density at room temperature,which makes it exhibit excellent mechanical properties at room temperature.In addition,abnormal structural evolution occurred during the heating process: grain boundary segregation and precipitation of ordered phase → Guinier Preston Zone(GP Zone),which is different from the precipitation sequence in the corresponding coarse grain Al-Mg-Li alloy(GP zone → ordered phase),resulting in the NG AlMg-Li alloy can still maintain high strength in the case of grain growth.In addition,by introducing dislocations in the NG Al-Mg-Li alloy can lower the stress required for further plastic deformation,which explains the effect of residual dislocations on mechanical properties in NG materials.