Thermal Stability And Fatigue Behavior Of Gradient-structured Ni

Thermal stability and fatigue property are two keys to the service performance of metals in engineering,and both of them have a strong dependency on the grain size.Gradient structure possesses a wide span of the grain size,and can be employed to exploring size-dependent grain growth behavior.With regard to the cyclic deformation of gradient-structured metals under stress-controlled mode,the mutual constraints among multi-scale grains contribute to suppression of strain localization,which is in favor of improving comprehensive fatigue properties(fatigue property under stresscontrolled mode and for crack growth).The revelation of cyclic deformation mechanism,thus,can provide guidelines for the optimization of gradient structure and the discovery of optimal gradient(fatigue property under stress-controlled mode);and the clarification of the influence of gradient structure on fatigue crack growth behavior can enlighten the architecture of gradient structure for enhancing crack growth resistance of the grains with various sizes(fatigue property for crack growth).In this work,the synthesis of the gradient-structured nickel(Ni)that is of 2.5 mm thick was enabled via direct electrodeposition,and the accurate control over the current density and additive concentration allows for the tailoring of gradient profile with the grain size spanning from 4 ?m to 40 nm.Taking an advantage of our gradient-structured Ni,that is,the span of grain size is over three orders of magnitude,we demonstrated a method for screening accurately and rapidly the critical size of grain growth.That is,the critical size of grain growth dc can be readily determined at any given temperature,provided that the change in hardness distribution profile induced by annealing were determined.As an example,for pure Ni annealed at 503 K for 3 h,the critical hardness was identified as 3.82 GPa by highthroughput characterization the hardness profile before and after annealing.On the basis of Hall-Petch relation,the critical size of grain growth(dc)of 95 nm was determined at one stroke.This critical size was double checked using transmission electron microscope in a direct fashion.and is consistent.Meanwhile,different critical sizes of grain growth can be obtained at ease by varying the annealing condition(e.g.,temperature and time).The kinetic description for the critical size(dc)that depends on annealing temperature(T)and time(t)can be expressed in equation:do=k tn,where k=k0 exp(-Q/RT)),k0 and R are constants.Moreover,the time exponent n and activation energy Q for grain growth are 0.46 and 107.4 kJ/mol,respectively.In addition,the critical size of grain growth is independent of the gradient structure,showing that our gradient-structured Ni provides a feasible and reliable methodology to study the critical size of grain growth.To what effect the gradient structure has on fatigue property of metals under stresscontrolled mode has been systematically investigated in this work.It was found that fatigue strength keeps rising from 450 MPa to 700 MPa with an increase in tensile strength,while fatigue ratio first increases from 0.58 to 0.76 and then decreases to 0.5,where the optimal fatigue strength and fatigue ratio are 650 MPa and 0.76 respectively.The relationship between the average height of slip bands along with the normalized distance x was observed and analyzed.When a change in derivative of cross-sectional hardness profile,slip bands were concentrated at the such zone on lateral surface.This localized zone experienced severe cyclic deformation and corresponds to the maximum slip band average height,then tends to be weakness zone for crack initiation.When the derivative of cross-sectional hardness profile kept constant and was 1(hardness:2.30 GPa?3.40 GPa),the distribution of slip bands on lateral surface is more dispersed,and the average heights of slip bands at various positions are comparable.This suggests that both coarse and fine grains are capable of accommodating cyclic deformation,and thus strain localization was effectively suppressed,resulting in the optimal fatigue propertiesTo what influences the gradient structure and gradient orientation have on fatigue crack growth behavior of the grains with different sizes have been revealed in this work,using three-point bending testing at a constant stress intensity factor amplitude ?K.Here,for fatigue crack grows from coarse-grained(CG)to nanograined(NG)structure,the corresponding orientation was termed CG?NG,and vice versa,termed NG?CG.When the grain size is less than 50 nm,the crack growth rate shows a marked reduction in CG?NG gradient.That was because that the crack deflection and rough fracture surface formed in the CG region that is behind crack tip induce the closure effect for the crack growing in subsequent NG region advancing crack tip.As a result,the effective driving force for crack growth was lowered.When the structure that is comprised of grains with a size ranging from 50 nm to 1 ?m,a reduced growth rate in the NG?CG gradient was observed.As the crack grows in the gradient,the scale of plastic zone ahead of crack tip increases with increasing grain size,and the crack becomes blunted.This crack blunting contributes to reduce the stress concentration.Especially for grains with a size ranging from 50 nm to 1?m,the size of the plastic zone varies greatly.Additionally,the gradient structure with a gentle distribution profile contributes to lower growth rate,especially for the structure having a grain size larger than 1?m.This can be attributed to the fact that the gradient structure can be tailored to optimize the stress distribution ahead of crack tip,and to potentially inhibit strain localization,thus allows for enhancement in fatigue crack growth resistance.