Investigation Of Abnormal Annealing Hardening Mechanisms And Thermal Stability Of Nano-/Ultrafine-Structured Nickel Alloys

Annealing often causes softening in conventional large-grained materials.However,annealing has been found to cause hardening in many nanocrystalline metals and alloys.This abnormal hardening by annealing has been explained by such factors as solute segregation at grain boundaries,grain boundary relaxation,grain boundary pinning by second phase particles,et al.To shed light on which mechanism dominates annealing hardening,we have prepared a homogenous single-phase nanocrystalline Ni（Fe）alloy with approx.1 at%Fe by electrodeposition.This alloy is a solid solution of Fe in Ni without any precipitated phase.Thus,the hardening caused by second phase particles pinning can be excluded.We indeed found that microhardness of the alloy increased slightly just before the grains started to grow during annealing.In addition,long time annealing at the hardening temperature resulted in the decrease of hardness due to grain growth.Both lattice parameter and atom probe tomography studies suggest that the annealing hardening is caused by the segregation of solute and impurity atoms on grain boundaries in our nanocrystalline Ni（Fe）alloy.Nanocrystalline and ultra-fine-grained metals and alloys show superior strength as compared to their conventional coarse-grained counterparts.These materials also exhibit attractive electrical,magnetic,and corrosive properties owing to their ultra-fine or nano-sized grains.These superior properties make them desired materials for structural engineering applications.However,the poor thermal stability of nanocrystalline and ultra-fine-grained metals and alloys,which is related to the high stored energy associated with the high density of grain boundaries,seriously restricts their practical application.The grains start to grow even at low and room temperature.It is important to investigate the thermal stability of nanocrystalline and untra-fine-grained metals fabricated and applicated at high temperature and explore new approaches to improve their thermal stability.The corresponding study should be of scientific and practical importance.In this study a nanocrystaline Ni₉₉Fe₁ alloy is designed and produced.High purity Ni（99.98%）is used as base metal and 1 at.%Fe is added as solute to make the alloy.The objective of the present study is to explore the effects of solid solute Fe on the thermal stability of the nanocrystalline Ni₉₉Fe₁ alloy.The thermal stability of the nanocrystalline Ni₉₉Fe₁ alloy is investigated over a wide range of annealing temperature from 66℃to 1000℃.It is found that the microhardness exhibit a step drop with increasing annealing temperature.The microhardness remains almost constant or drops slightly in the wide annealing temperature range of≤216℃,256⁶26℃and 646¹000℃.However,the microhardness drops sharply in the narrow annealing temperature range of 216²56℃and 626⁶46℃.The microhardness drops about40%and 30%in the narrow temperature range of 40℃and 20℃respectively.After investigating the microstructure and thermodynamic data,it has been found that the addition of Fe plays a vital role on the unique step change of microhardness with annealing temperature.The main conclusions are:（1）In the low annealing temperature,Fe and impurity atoms dissolved in Ni base segregate to grain boundaries gradually,which inhibits grain growth.After most or all of the Fe and impurity atoms segregate to grain boundaries,the grains grow abnormally with increasing annealing temperature because there isn’t any more atoms segregated to grain boundaries.Fe and impurity atoms segrate at grain boundaries dissolve back to Ni lattice while the grains is growing.（2）With increasing annealing temperature continuously after the grains growing up,Fe and impurity atoms dissolve back to Ni lattice and finally segregate to grain boundaries again which inhibites grain growth.The microhardness dropped slightly.（3）When the annealing temperature is increased to 626℃,the high temperature allows to form Ni₃Fe particles thermodynamicly.Fe and Ni atoms at grain boundaries form Ni₃Fe,therefore the grains grow and microhardness drops sharply again because there isn’t Fe atoms segregated at grain boundaries inhibitting grain growth.（4）With increasing annealing temperature continuously,grain grow rapidly.The nanoprecipitates of Ni₃Fe act as barriers for dislocation motion,increasing the shear stress required for dislocation motion,therefore the microhardness doesn’t drop but keeps stable.This investigation makes clearly that step change of microhardness of Ni₉₉Fe₁ alloy is attributed to Fe and impurity atoms segregated to grain boundaries at low and medium annealing temperatures and Ni₃Fe particles act as barrier for dislocation motion at high annealing temperature.In this study,it also indicates that atoms segregation is more effective for stabilizing nanocrystalline Ni（Fe）alloy than particles pinning.In this study nanocrystaline Ni and Ni alloys with different types and concentration of solutes are designed.Thermal stability of the alloys is predicted based on their Gibbs free energy and grain boundary energy changed by solute segregation.Thermal stability of Ni and Ni alloys are also experimentally studied by their microhardness changes with annealing temperature.Ultra-fine-grained pure Ni and Ni₉₉Fe₁,Ni₉₉Cr₁,Ni₉₉V₁,Ni₉₇Cr₃alloys are designed and produced.The experimental results agree well with the theoretical predictions which testify the reliability of the thermodynamic model.Compared to ultra-fine-grained pure Ni,thermal stability of Ni₉₉Fe₁ is improved from140℃to 220℃.Ni₉₉Cr₁ is improved to 240℃and Ni₉₉V₁ is improved to 300℃.Thermal stability of Ni₉₇Cr₃ is improved to as high as 320℃.This study indicates that thermal stability of the alloys with lower Gibbs free energy and grain boundary energy after segregation is better.With the addition of Cr,the thermal stability of Ni₉₇Cr₃ is much better than Ni₉₉Cr₁.This study should help design UFG Ni alloys with improved thermal stability for practical applications.