Temperature effect on nanotwinned Ni under nanoindentation using molecular dynamic simulation

Temperature effect on atomic deformation of nanotwinned Ni (nt-Ni) under localized nanoindentation is investigated in comparison with nanocrystalline Ni (nc-Ni) through molecular simulation.The nt-Ni exhibits enhanced critical load and hardness compared to nc-Ni,where perfect,stair-rod and Shockley dislocations are activated at (111),(111) and (111) slip planes in nt-Ni compared to only SSockley dislocation nucleation at (111) and (111) slip planes of nc-Ni.The nt-Ni exhibits a less significant indentation size effect in comparison with nc-Ni due to the dislocation slips hindrance of the twin boundary.The atomic deformation associated with the indentation size effect is investigated during dislocation transmission.Different from the decreasing partial slips parallel to the indenter surface in nc-Ni with increasing temperature,the temperaturedependent atomic deformation of nt-Ni is closely related to the twin boundary:from the partial slips parallel to the twin boundary (~10 K),to increased confined layer slips and decreased twin migration(300 K–600 K),to decreased confined layer slips and increased dislocation interaction of dislocation pinning and dissociation (900 K–1200 K).Dislocation density and atomic structure types through quantitative analysis are implemented to further reveal the above-mentioned dislocation motion and atomic structure alteration.Our study is helpful for understanding the temperature-dependent plasticity of twin boundary in nanotwinned materials.

Investigation of point defect evolution and Voronoi cluster analysis for magnesium during nanoindentation

The present study investigates the effect of nanoindentation on single-crystal magnesium specimens using the embedded-atom method potential in molecular dynamics simulation.Analyses are done under dynamic loading where the load-bearing capacity and change in the structural configuration are studied on the basal(Z-direction)and two prismatic planes(X-and Y-directions)with varying indenter velocities.The investigation of structural evolution is done using atomic displacement analyses to measure the net magnitude of displacement,atomic strain analyses to evaluate the shear strain developed in the process,and Wigner-Seitz defect analyses to calculate the total vacancies at varied timesteps.Furthermore,Voronoi analyses are done when indented on the basal plane to identify the cluster distribution at different planar depths of the specimen.From the analyses,it has been observed that the load-bearing capacity of the specimen varies with the indentation velocity and the direction of indentation on the specimen.Additionally,it is seen that the observed shear and total atomic displacement in the Z-direction is the least in comparison to the other two axes.The partial dislocation 1/3<-12-10>is seen to be majorly present and the population of dislocation loops is more abundant for lower indenter velocities.Furthermore,clusters<0,4,4,6>and<0,6,0,8>are the major indices developed during nanoindentation on the basal plane where they exhibit symmetrical distribution as observed from the Z-direction.

Material Removal Characteristics of Single-Crystal 4H-SiC Based on Varied-Load Nanoscratch Tests

Single-crystal silicon carbide(SiC)has been widely applied in the military and civil fields because of its excellent physical and chemical properties.However,as is typical in hard-to-machine materials,the good mechanical properties result in surface defects and subsurface damage during precision or ultraprecision machining.In this study,single-and double-varied-load nanoscratch tests were systematically performed on single-crystal 4H-SiC using a nanoindenter system with a Berkovich indenter.The material removal characteristics and cracks under different planes,indenter directions,normal loading rates,and scratch intervals were analyzed using SEM,FIB,and a 3D profilometer,and the mechanisms of material removal and crack propagation were studied.The results showed that the Si-plane of the single-crystal 4H-SiC and edge forward indenter direction are most suitable for material removal and machining.The normal loading rate had little effect on the scratch depth,but a lower loading rate increased the ductile region and critical depth of transition.Additionally,the crack interaction and fluctuation of the depth-distance curves of the second scratch weakened with an increase in the scratch interval,the status of scratches and chips changed,and the comprehensive effects of the propagation and interaction of the three cracks resulted in material fractures and chip accumulation.The calculated and experimental values of the median crack depth also showed good consistency and relativity.Therefore,this study provides an important reference for the high-efficiency and precision machining of single-crystal SiC to ensure high accuracy and a long service life.

The Influence of Crystallographic Orientation and Grain Boundary on Nanoindentation Behavior of Inconel 718 Superalloy Based on Crystal Plasticity Theory

The crystal plasticity finite element method(CPFEM)is widely used to explore the microscopic mechanical behavior of materials and understand the deformation mechanism at the grain-level.However,few CPFEM simulation studies have been carried out to analyze the nanoindentation deformation mechanism of polycrystalline materials at the microscale level.In this study,a three-dimensional CPFEM-based nanoindentation simulation is performed on an Inconel 718 polycrystalline material to examine the influence of different crystallographic parameters on nanoindentation behavior.A representative volume element model is developed to calibrate the crystal plastic constitutive parameters by comparing the stress-strain data with the experimental results.The indentation force-displacement curves,stress distributions,and pile-up patterns are obtained by CPFEM simulation.The results show that the crystallographic orientation and grain boundary have little influence on the force-displacement curves of the nanoindentation,but significantly influence the local stress distributions and shape of the pile-up patterns.As the difference in crystallographic orientation between grains increases,changes in the pile-up patterns and stress distributions caused by this effect become more significant.In addition,the simulation results reveal that the existence of grain boundaries affects the continuity of the stress distribution.The obstruction on the continuity of stress distribution increases as the grain boundary angle increases.This research demonstrates that the proposed CPFEM model can well describe the microscopic compressive deformation behaviors of Inconel 718 under nanoindentation.

In situ TEM observations and molecular dynamics simulations of deformation defect activities in Mg via nanoindentation

In this work, we performed in situ nanoindentation in TEM to capture the real-time dislocation and twinning activities in pure Mg during loading and unloading. We demonstrated that the screw component of dislocations glides continuously, while the edge components rapidly become sessile during loading. The twin tip propagation is intermittent, whereas the twin boundary migration is more continuous. During unloading, we observed the elastic strain relaxation causes both dislocation retraction and detwinning. Moreover,we note that the plastic zone comprised of dislocations in Mg is well-defined, which contrasts with the diffused plastic zones observed in face-centered cubic metals under the nanoindentation impressions. Additionally, molecular dynamics simulations were performed to study the formation and evolution of deformation-induced crystallographic defects at the early stages of indentation. We observed that,in addition to dislocations, the I1stacking fault bounded with a <1/2c+p> Frank loop can be generated from the plastic zone ahead of the indenter, and potentially serve as a nucleation source for abundant dislocations observed experimentally. These new findings are anticipated to provide new knowledge on the deformation mechanisms of Mg, which are difficult to obtain through conventional ex situ approaches. These observations may serve as a baseline for simulation work that investigate the dynamics of dislocation slip and twinning in Mg and alloys.

Stress distribution variations during nanoindentation failure of hard coatings on silicon substrates

Regarding quality inspection of technologically important nanocomposite hard coatings based on Ti,B,Si,C,and N and bioceramics such as hydroxyapatite that are used in small-scale high-precision devices and bio-implants,it is essential to study the failure mechanisms associated with nanoindentation,such as fracture,delamination,and chipping.The stress imposed by the indenter can affect the fracture morphology and the interfacial fracture energy,depending on indenter shape,substrate type,crystallographic properties,pre-existing flaws,internal microcracks,and pre-strain.Reported here are finite-element-based fracture studies that provide insights into the different cracking mechanisms related to the aforementioned failure process,showing that the fracture morphology is affected by the interaction of different cracking events.The interfacial fracture energy,toughness,and residual stress are calculated using existing models with minor adjustments,and it is found that increasing the indenter sharpness improves the shear stress distribution,making the coating more prone to separation.Depending on the prevailing type of stress,the stress distribution beneath the depression results in either crack formation or a dislocation pile-up leading to strain hardening.Different forms of resistances resulting from the indentation process are found to affect the tip–sample conduction,and because of its stronger induced plasticity than that of a Berkovich indenter tip,a sharper cube-corner tip produces more resistance.

Understand anisotropy dependence of damage evolution and material removal during nanoscratch of MgF_(2) single crystals

To understand the anisotropy dependence of the damage evolution and material removal during the machining process of MgF_(2) single crystals,nanoscratch tests of MgF_(2) single crystals with different crystal planes and directions were systematically performed,and surface morphologies of the scratched grooves under different conditions were analyzed.The experimental results indicated that anisotropy considerably affected the damage evolution in the machining process of MgF_(2) single crystals.A stress field model induced by the scratch was developed by considering the anisotropy,which indicated that during the loading process,median cracks induced by the tensile stress initiated and propagated at the front of the indenter.Lateral cracks induced by tensile stress initiated and propagated on the subsurface during the unloading process.In addition,surface radial cracks induced by the tensile stress were easily generated during the unloading process.The stress change led to the deflection of the propagation direction of lateral cracks.Therefore,the lateral cracks propagated to the workpiece surface,resulting in brittle removal in the form of chunk chips.The plastic deformation parameter indicated that the more the slip systems were activated,the more easily the plastic deformation occurred.The cleavage fracture parameter indicated that the cracks propagated along the activated cleavage planes,and the brittle chunk removal was owing to the subsurface cleavage cracks propagating to the crystal surface.Under the same processing parameters,the scratch of the(001)crystal plane along the[100]crystal-orientation was found to be the most conducive to achieving plastic machining of MgF_(2) single crystals.The theoretical results agreed well with the experimental results,which will not only enhance the understanding of the anisotropy dependence of the damage evolution and removal process during the machining of MgF_(2) crystals,but also provide a theoretical foundation for achieving the high-efficiency and low-damage processing of anisotropic single crystals.

Factors impacting nanoindentation testing results of the cuticle of dung beetle Copris ochus Motschulsky

The cuticle of dung beetle is a layered composite material in micro- or nano-scale. Dung beetle can fly, walk and dig. It can shovel and compact dung of mammals into balls. It use foreleg to walk, midleg and hindleg to hold and impel dung ball. Its two foreleges as digging legs are developed. The factors impacting the nanoindentation testing results of the femur cuticle of forelegs of dung beetle Copris ochus Motschulsky were examined. The nanomechanical test instrument used for the tests was Hysitron nanomechanical system. The results shown that the holding time and loading time are important factors im- pacting the accuracy of such indentation properties as reduced modulus (Er) and the harness ( H ) of the femur cuticle of the forelegs of dung beetle Copris ochus Motschulsky in nanoscale. There exists a threshold holding time of 20 s for the reduced modulus of the femur cuticle. The tests of nanoindentation creep property and the regression analysis of relationship between the depth increment at the maximum load and the time further confirmed the correction of the above threshold holding time. There exist visco-elastic-plastic behaviour and creep phenomenon in the femur cuticle during indenting. Its creep property during the holding procedure at maximum load can be regressed by a general logarithmic equation. The equation fitted by the testing data is ? h = 54.83452 ln(0.00723t +1.00486), where, ? h is the depth increment at the maximum load and t is the time.

Fracture Toughness Properties of Three Different Biomaterials Measured by Nanoindentation

The fracture toughness of hard biomaterials, such as nacre, bovine hoof wall and beetle cuticle, is associated with fibrous or lamellar structures that deflect or stop growing cracks. Their hardness and reduced modulus were measured by using a nanoindenter in this paper. Micro/nanoscale cracks were generated by nanoindentation using a Berkovich tip. Nanoindentation of nacre and bovine hoof wall resulted in pile-up around the indent. It was found that the fracture toughness （Kc） of bovine hoof wall is the maximum, the second is nacre, and the elytra cuticle of dung beetle is the least one.

The microscopic mechanical performance for nonuniform welded joint of nickel-based alloy with nanoindentation

To quantify the nonuniform micromechanical performance of welded joint,the load-displacement curves by nanoindentation test were introduced to examine different zones including base metal,coarse grained heat affected zone,partially melted zone,weld metal near the fusion boundary and weld metal center.The results showed that the strengthening effect of weld metal was more obvious than that of heat affected zone for nickel based welded joint and especially in coarse grained heat affected zone,the hardening resulted from overheating was not apparent.Nickel based weld metal with high content of alloying elements which were often segregated at interdendritic regions or precipitated in grain interior under nonequilibrium solidification contributed to the characteristics that differ from conventional low alloy steel welded joint.

Determination of Elastoplastic Mechanical Properties of the Weld and Heat Affected Zone Metals in Tailor-Welded Blanks by Nanoindentation Test

The elastoplastic mechanical properties of the weld and heat affected zone metals have comparatively major impact on the forming process of tailor-welded blanks. A few scholars investigated the elastoplastic mechanical properties of the weld and heat affected zone, but they only simply assumed that it was a uniform distribution elastoplastic material different from the base materials. Four types of tailor-welded blanks which consist of ST12 and 304 stainless steel plates are selected as the research objects, the elastoplastic mechanical properties of the tailor-welded blanks weld and heat affected zone metals are obtained based on the nanoindentation tests, and the Erichsen cupping tests are conducted by combining numerical simulation with physical experiment. The nanoindentation tests results demonstrate that the elastoplastic mechanical properties of the weld and heat affected zone metals are not only different from the base materials, but also varying between the weld metals and the heat affected zone metals. Comparing the Erichsen cupping test resulted from numerical with that from experimental method, it is found that the numerical value of Erichsen cupping test which consider the elastoplastic mechanical properties of the weld and heat affected zone metals have a good agreement with the experimental result, and the relative error is only 4.8%. The proposed research provides good solutions for the inhomogeneous elastoplastic mechanical properties of the tailor-welded blanks weld and heat affected zone metals, and improves the control performance of tailor-welded blanks forming accuracy.

Hierarchical structure observation and nanoindentation size effect characterization for a limnetic shell

In the present research, hierarchical structure observation and mechanical property characterization for a type of biomaterial are carried out. The investigated bioma- terial is Hyriopsis cumingii, a typical limnetic shell, which consists of two different structural layers, a prismatic ＂pillar＂ structure and a nacreous ＂brick and mortar＂ structure. The prismatic layer looks like a ＂pillar forest＂ with variationsection pillars sized on the order of several tens of microns. The nacreous material looks like a ＂brick wall＂ with bricks sized on the order of several microns. Both pillars and bricks are composed of nanoparticles. The mechanical properties of the hierarchical biomaterial are measured by using the nanoindentation test. Hardness and modulus are measured for both the nacre layer and the prismatic layer, respectively. The nanoindentation size effects for the hierarchical structural materials are investigated experimentally. The results show that the prismatic nanostructured material has a higher stiffness and hardness than the nacre nanostructured material. In addition, the nanoindentation size effects for the hierarchical structural materials are described theoretically, by using the trans-scale mechanics theory considering both strain gradient effect and the surface/interface effect. The modeling results are consistent with experimental ones.

Molecular dynamics study of the mechanical characteristics of Ni/Cu bilayer using nanoindentation

In the present work, a three-dimensional molecular dynamics simulation is carried out to perform the nanoindentation experiment on Ni single crystal. The substrate indenter system is modeled using hybrid interatomic potentials including the many-body potential embedded atom method （EAM）, and two-body morse potential. To simulate the in- dentation process, a spherical indenter （diameter = 80A, 1A=0.1 nm） is chosen. The results show that the mechanical behaviour of a monolithic Ni is not affected by crystalline orientation. To elucidate the effect of a heterogeneous interface, three bilayer interface systems are constructed, namely Ni（100）/Cu（111）, Ni（110）/Cu（111）, and Ni（111）/Cu（111）. The simulations along these systems clearly describe that mechanical behaviour directly depends on the lattice mismatch. The interface with the smaller mismatch between the specified crystal planes is proved to be harder and vice versa. To describe the relationship between film thickness and interface effect, we choose various values of film thickness ranging from 20 A to 50 A to perform the nanoindentation experiment. It is observed that the interface is significant only for the relatively small thickness of film and the separation between interface and the indenter tip. It is shown that with the increase in film thickness, the mechanical behaviour of the film shifts more toward that of monolithic material.

Nanomechanical behaviors of (110) and (111) CdZnTe crystals investigated by nanoindentation

The nanomechanical behaviors of （110） and （111 ） CdZnTe crystals were investigated by nanoindentation. It was found that the indenter tip was adhered by the removed materials in scanning testing area although the scanning force on the tested surface was very small （1000 nN）, which would affect the testing result of nanoindentation, so the indenter was clean before nanoindentation test. The experimemtal results showed that the hardness and Young＇s modulus decreased with the increase of indentation loads on the same plane. Because of the anisotropy of the CdZnTe crystal, the average hardness of （110） plane is 35% lower than that of （111） plane, and there are about 30% difference of the hardness along different crystallographic directions on the same plane. The hardness in 0° and 120° testing directions was the same due to the threefold symmetry of a Berkovich indenter. And the anisotropy affected the surface quality during machining of CdZnTe crystal.

NANOINDENTATION OF THIN-FILM-SUBSTRATE SYSTEM DETERMINATION OF FILM HARDNESS AND YOUNG'S MODULUS

In the present paper,the hardness and Young's modulus of film-substrate systems are determined by means of nanoindentation experiments and modified models.Aluminum film and two kinds of substrates,i.e.glass and silicon,are studied.Nanoindentation XP Ⅱ and continuous stiffness mode are used during the experiments.In order to avoid the influence of the Oliver and Pharr method used in the experiments,the experiment data are analyzed with the constant Young's modulus assumption and the equal hardness assumption.The volume fraction model(CZ model)proposed by Fabes et al.(1992)is used and modified to analyze the measured hardness.The method proposed by Doerner and Nix(DN formula)(1986)is modified to analyze the measured Young's modulus.Two kinds of modified empirical formula are used to predict the present experiment results and those in the literature,which include the results of two kinds of systems,i.e.,a soft film on a hard substrate and a hard film on a soft substrate.In the modified CZ model,the indentation influence angle,(?), is considered as a relevant physical parameter,which embodies the effects of the indenter tip radius, pile-up or sink-in phenomena and deformation of film and substrate.

Creep characteristics of coal and rock investigated by nanoindentation

In coal mining industry,with the depth growing of coal mines,the creep behaviours of coal and rock can extensively affect the mining safety,coalbed methane recovery and geo-sequestration.To acquire a better insight into their creep characteristics,an efficient and robust researching technique,nanoindentation,was applied to investigate the creep performances of coal and rock samples obtained from two coal mines in the east of China.Creep characteristics were reflected by evaluating the curves of creep depth versus creep time of nanoindentation tests during the load-holding period at the peak load of 30 mN.These curves can be divided into two stages:transient stage and steady stage;and the time of load-holding period of 5 s,which is the dividing point between two stages,can efficiently avoid the influence of creep displacement on the unloading curves.The exponential function can perfectly fit creep curves and Kelvin model can be used to calculate the rheological parameters of coal and rock samples.Calculated results yield values for the creep modulus and viscosity terms of coal and rock.This study also settled a particular emphasis on the selection of the positions of indentations to obtain the rheological properties of mineralogical constituents in heterogonous coal and rock samples and their elastic aftereffect.

MECHANICAL DEFLECTION OF POLYSILICON MICROCANTILEVER BEAMS USING NANOINDENTATION

The validity of a novel, direct and convenient method for micromechanical property measurements by beam bending using a nanoindenter is demonstrated. This method combines a very high load resolution with a nanometric precision in the determination of the microcantilever beam deflection. The method is described clearly. In the deflection of microbeams, the influence of the indenter tip pushing into the top of the microbeams and the curvature across its width must be considered. The measurements were made on single-layer, micro-thick, several kinds of width and length polysilicon beams that were fabricated using conventional integrated circuit (IC) fabrication techniques. The elastic of a polysilicon microcantilever beam will vary linearly with the force and the deformation is thought to be elastic. Furthermore, it suggests that Young modulus of the beam can be determined from the slope of this linear relation. From the load deflection data acquired during bending the mechanical properties of the thin films were determined. Measured Young modulus is 137 GPa with approximately a ±2.9%～±6.3% difference in Young modulus.

Investigation of mechanical properties of bedded shale by nanoindentation tests: A case study on Lower Silurian Longmaxi Formation of Youyang area in southeast Chongqing, China

The mechanical properties such as Young's modulus, hardness and fracture toughness of Lower Silurian Longmaxi shale samples from Youyang area in southeast Chongqing, China were investigated using dot matrix nanoindentation measurements. With the help of field emission scanning electron microscope(FESEM) and energy dispersive X-ray fluorescence spectroscopy(EDS), the indentation morphology and mineral composition in indentation area were quantitatively analyzed. According to mechanical strength classification, a micromechanical model with three components was introduced and the Mori-Tanaka model was used to upscale mechanical parameters from nano-scale to centimeter-size scale, which were further compared with uniaxial compression results. The experimental results show that there is a positive linear correlation between Young's modulus and hardness and between the Young's modulus and the fracture toughness under nano-scale; the Young's modulus, hardness and fracture toughness perpendicular to the bedding are slightly lower than those parallel with the bedding. According to data statistics, the mechanical properties at the nano-scale follow Weibull distribution feature and the dispersion degree of hardness results is the highest, which is mainly due to shale anisotropy and nanoindentation projection uncertainty. Comparing the results from nanoindentation test, with those from upscaling model and uniaxial compression test shows that the mechanical parameters at the nano-scale are higher than those from upscaling model and uniaxial compression test, which proves mechanical parameters at different scales have differences. It's because the larger the core, the more pores and internal weakness it contains, the less accurate the interpreted results of mechanical parameters will be.

Influence of sample preparation on nanoindentation results of twinninginduced plasticity steel

Nanoindentation is an attractive characterization technique,as it not only measures the local properties of a material but also facilitates understanding of deformation mechanisms at submicron scales.However,because of the complex stress-strain field and the small scale of the deformation under the nanoindenter,the results can be easily influenced by artifacts induced during sample preparation.In this work,a systematic study was conducted to better understand the influence of sample preparation methods on the nanoindentation results of ductile metals.All experiments were conducted on a steel(Fe-22Mn-0.65C,wt%)with twinning-induced plasticity(TWIP),which was selected for its large grain size and sensitivity to different surface preparation methods.By grouping the results obtained from each nanoindent,chemical polishing was found to be the best sample preparation method with respect to the resulting mechanical properties of the material.In contrast,the presence of a deformation layer left by mechanical polishing and surface damage induced by focused ion beam(FIB)scanning were confirmed by the dislocation-nucleation-induced pop-in events of nanoindentation.

Nanoindentation Size Effect on Type 316 Stainless Steel

Nanoindentation size effect was investigated under very low loads on type 316 stainless steel. Nanoindentation measurements were carried out on the samples surfaces with a Berkovich pyramidal diamond indenter applying loads in the range of 25-1000μN. Simultaneously, AFM images of the sample surface were recorded before and after indentation process .For type 316 stainless steel, the indentation size effect was found. The results were discussed in the terms of the model of geometrically necessary dislocations proposed to interpret the indentation size effect.It can be seen that the square of the nanohardness, H 2, vs the inverse of indentation depth, 1/h, is linearly dependent on the indented depth in the range of 25-150nm,which is a good qualitative agreement with the predictions of the model. However, for shallow indents, the slope of the line severely changes.Some possible mechanisms for this change were proposed.