Strain-rate dependence of mechanical behavior and deformation mechanisms in bimodal nanostructured Ni under micro-scratch testing

Accepted Manuscript

Strain-rate dependence of mechanical behavior and deformation mechanisms in bimodal nanostructured Ni under micro-scratch testing Zhu Rongtao , Wang Xian , Li Chaoyong , Hu Bintao , Li Yanfeng , Zhang Xinxi PII: DOI: Reference:

S0167-6636(17)30827-X 10.1016/j.mechmat.2018.03.005 MECMAT 2853

To appear in:

Mechanics of Materials

Received date: Revised date: Accepted date:

2 December 2017 2 February 2018 20 March 2018

Please cite this article as: Zhu Rongtao , Wang Xian , Li Chaoyong , Hu Bintao , Li Yanfeng , Zhang Xinxi , Strain-rate dependence of mechanical behavior and deformation mechanisms in bimodal nanostructured Ni under micro-scratch testing, Mechanics of Materials (2018), doi: 10.1016/j.mechmat.2018.03.005

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HIGHLIGHTS 

In this paper, the micro-scratch technique that is a simple, convenient and reliable method was selected to provide continuous scratch hardness value over a wider range of strain rate (0.03 s-1~30 s-1) in a full dense, high purity and well-characterized electrodeposited NS Ni with bimodal grain size distribution.

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First, the strain rate varying with scratch speed was investigated. Second,the mechanical behaviors of the bimodal NS Ni sample were investigated carefully. Further, the strain-rate

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sensitivity exponents of the bimodal NS Ni were obtained by linear fitting under different scratch speed. Finally, the microscopic deformation mechanism in the bimodal NS Ni sample

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during scratch plastic deformation was discussed in details under different strain rates.

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Strain-rate dependence of mechanical behavior and deformation mechanisms in bimodal nanostructured Ni under micro-scratch testing Zhu Rongtaoa,*, Wang Xiana, Li Chaoyonga, Hu Bintaoa, Li Yanfenga, Zhang Xinxia a

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School of Chemical Engineering and Technology, China University of Mining and Technology,

Xuzhou, Jiangsu, 221116, China

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Abstract

Bimodal nanostructure is a strategy to obtain a good combination of strength and uniform ductility for nanostructured (NS) metals. To inspect strain-rate dependence of mechanical behavior and deformation mechanisms in NS Ni with bimodal grain size distribution, the micro-scratch

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testing technique was selected to assess the average strain-rate sensitivity exponent over a wide range of strain rate based on continuous data of scratch hardness and plastic strain rate, the

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relationship between scratch plastic deformation mechanisms and microstructure of the bimodal NS Ni under different strain rate range were discussed. First, the scratch behaviors of the bimodal

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NS Ni were characterized in details, some critical parameters under the scratch test were investigated, such as scratch ditch depths and widths, scratch strain-rate, scratch hardness. Then,

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the scratch hardness varying with plastic strain rate under different scratch speeds was confirmed,

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and the average strain-rate sensitivity exponents of the bimodal NS Ni were calculated through the linear fitting. The results suggested that the values of strain-rate sensitivity of the NS Ni are much greater than the value of coarse-grained counterparts, and increase with increasing scratch speeds. Finally, the deformation mechanisms of scratch plastic deformation in bimodal NS Ni was inspected according to the strain-rate sensitivity exponents and TEM morphologies at end of scratch ditches under different scratch speeds. From the results, the deformation mechanism of the bimodal NS Ni sample changes gradually from the dislocation gliding and diffusion to grain boundary dislocation pile-up with increasing strain rate under micro-scratch testing. Meanwhile, 2

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we thought that the enhancement in ductility of the bimodal NS metals is dependent with the strain rate hardening behavior induced by the dislocation activity. Keywords Bimodal nanostructure Ni; Micro-scratch testing; Strain-rate dependence; Mechanical behavior; Deformation mechanism

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*Corresponding author. Tel.: +86516 83884442 E-mail address: [email protected] 1. Introduction

Nanostructured (NS) metal materials, considered as poly-crystal metals or alloys with mean

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grain size less than 100 nm, have become a research focus for its special physical and mechanical properties [1, 2]. Compared with conventional coarse-grained metals, the NS metals exhibit higher strength and hardness, but have a decrease in ductility due to high propensity to deformation instability [3, 4]. Many of them fail in the elastic regime without observed plastic deformation and

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others exhibit no more than 3% elongation to failure [2]. This dismal ductility seriously limits the application of such metals as engineering materials. Hence, extensive studies have been

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undertaken to try to find some approaches and strategies to reach a good combination of strength and uniform ductility for NS metal materials, including preparation of high-quality samples,

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introduction of bimodal grain size distribution and use of deformation twins [5-8]. Amongst, the

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second strategy is increasingly adopted because it is simple and feasible [6, 7], and the main idea of this strategy is to preserve uniform deformation by invoking strain hardening or strain rate

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hardening mechanisms to suppress the instabilities from which the NS metal materials tend to suffer. The strain hardening and strain rate hardening mechanisms, however, in bimodal NS metal materials are not well understood, specifically under a wider strain rate range. Admittedly, the plastic deformation mechanism of the NS metal materials is closely related with deformation rate, so strain-rate sensitivity is a key parameter to evaluate the deformation mechanism in NS metal crystal materials [9], especially in NS metals with bimodal grain size distribution because the synergistic action between bigger and smaller grains is sensitive to the deformation rates [7]. Even so, there still exist some disputes about the correlation of strength and 3

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strain rate in NS metal materials. Some researchers show that the strength of NC metal increases with the increasing of strain rate, and strain-rate sensitivity increases with decreasing of grain size [10-12]; In contrast, the decreasing strength of NS metal materials with the increase of strain rate were found in some literatures [13, 14]. Meanwhile, other scholars believe that the strain-rate sensitivity of NS metal shows different change laws in different strain-rate range [15-18].

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Obviously, the strain-rate dependence of mechanical behavior in NS metal is inconclusive, even the change trend of strain-rate sensitivity is contradictory, which have a significant influence on evaluating the deformation mechanisms in NS metals. We consider that this phenomenon is caused by some reasons as following: (a) different preparation technology of NC metal materials, the different

preparation

techniques

usually

results

in

different

internal

structure

and

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processing-induced defects [19-22]; (b) In different strain rate tests, such as tensile test and impact experiment under high strain rate, the design of the sample, loading method, detection method of displacement and strain, and the precision of testing data are different [23, 24]; (c) When using different testing instruments, testing technique and loading method, the size of sample usually

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different from each other which affect the mechanical response of NC materials [25]. Furthermore, most studies about strain-rate dependence addressed only the NS metals with narrow grain size

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distribution, such as ultrafine-grained metals (the size of the grains are above 100 nm but below 1 ｕm) and truly nanocrystalline metals (grain size is less than 100 nm), rather than the NS metals

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with bimodal grain size distribution in which submicron grains distribute uniformly in subgrain

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matrix (such as low-angle grain boundaries) below 100 nm. Therefore, it is crucial to investigate the hardening mechanism in bimodal NS metals and the

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effect of strain rate on the ductility of bimodal NS metals using the same experimental technique to reduce the influence of the above factors on the mechanical behavior of NS metals with bimodal grain size distribution. Thus, micro-scratch testing is a preferable method which can focus on the same sample to examine the mechanical response of materials under different scratch rate and progressive loading. In the micro-scratch test, with the increase of scratch loading, the depth and width of the scratch are constantly changing, and continuous strain rate and scratch hardness can be obtained during the same scratch process. Through mathematic calculation, the strain-rate sensitivity data in the same sample under different strain rate range can be acquired, and the strain 4

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hardening mechanism in bimodal NS metals under wider strain rate range can be evaluated carefully. In this paper, the micro-scratch technology was selected to analysis the scratch mechanical behaviors in electrodeposited bimodal NS Ni with high purity and high dense first. Then, the strain-rate dependence of scratch hardness was assessed according to the relationship between scratch hardness and strain rate under the wider strain rate range obtained from the

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micro-scratch testing. Finally, the microcosmic mechanisms of strain hardening and strain rate hardening in bimodal NS Ni sample under different strain rate range were discussed in details.

2. Experimental method

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2.1 Sample preparation

The commercially available eletrodeposited NS Ni with high dense and high purity (99 %) was purchased from Integran Technologies Inc. (Canada). The same batch of the samples was used as

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reported in Ref [26]. The as-received NS Ni was quoted to have a nominal grain size of about 20 nm and a thickness of about 200 µm. Then the as-received sample was heat treated in a vacuum to obtain microstructure with

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oven (better than 1 × 10-3 Pa) for 90 min at a temperature of 200

bimodal grain size distribution [27]. After heat treatment, the sheet sample was polished on both side

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using SiC paper with different particle size (3、1、0.25 µm) respectively to remove surface layers until the thickness of the sample reach about 150 µm, then the rectangular sample (25 mm×15 mm)

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were electro-discharge machined from the polished sheet sample for the micro-scratch test. Finally the rectangular sample was glued onto a toughened glass using an standard cyanoacrylate glue,

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and the sample was put aside for at least 24 h after gluing onto the glass before performing the micro-scratch test. 2.2 Micro-scratch and tensile testing The micro-scratch test was carried out on the CSM micro-scratch test system (model: RST-S-EE-0000). This system is composed of the diamond indenter, mechanical loading and feedback system, light microscope photographing system. Rockwell W-209 diamond with 200µm tip radius was used as blunt micro-contact geometry. In this paper, the loading method of 5

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progressive load is used in the micro-scratch test, and the loading process can be split into three steps: the first step can be called pre-scan, in which the indenter scan the sample starting surface with a constant and small load (about 1N) along the setting scratch pattern to eliminate the influence of surface morphology on penetration depth; after the pre-scan, the indenter go back to the initial position of the pattern to perform the scratch with a progressive normal loading

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(perpendicular to the loading surface) along the same pattern and this step is loaded scratch scan, during this step, the penetration depth and friction coefficient can be obtained by mechanical feedback system; finally the indenter return the initial position again after the loaded scratch scan to measure the profile of the scratch ditch with a constant and small load (about 1N) along the same pattern and this step is called as post-scan, after the post-scan, the residual depth of the

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scratch ditch can be obtained under the progressive loading. In the micro-scratch experiment, the progressive loading scratch test was performed by using an overall scratch length of 2 mm and scratch speed of 0.6、6、30、150 mm/min separately. The scratch test at each speed was repeated at least three times, and the spacing of each scratch on the same sample is not less than 3 mm, after

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the test, taking the average of 3 test data as the experimental results. To accurately obtain the mechanical properties of the plastic deformation of the scratches, should avoid the maximum

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depth of the scratches exceed the radius of the indenter in the testing process, peak of the

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progressive loading program should be less than 30 N. During the micro-scratch testing, the penetration depth of each scratch can be determined by

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subtracting the pre-scan depth from the loaded scratch scan depth, however, the depth of residual scratches by plastic deformation is generated by elastic response of loading scratch scan depth,

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therefore, evaluating the plastic deformation behavior of NC Ni sample in the micro-scratch test must utilize the post-scan residual depth of scratch. At the same time, the real-time digital image of the scratch ditches (40 times magnification, ×40) can be used to determine the width of the residual scratches ditches. For comparison, tensile testing was conducted using a Zwick BZ2.5/TS1S universal test machine at fixed strain rates that ranged from

to

s-1 (displacement control mode).

The same geometry of the tensile samples was used as reported in Ref [26]. The tensile samples 6

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are mounted using detachable clamps with serrated grip surface. The strain-rate tests are carried out by moving the crosshead of the linear actuator an interval of displacement at different speeds, i.e. different intervals of displacement time. The normal tensile loading is recorded as the crosshead position is measured as a function of time using a displacement transducer. Thus, the yield stress of the tensile samples can be determined using the intercept of a plot of engineering

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stress versus engineering strain with the 0.2% total strain offset parallel to the elastic slope. 2.3 Characterization of the samples

The micrographs of scratch ditches at different scratch speeds was observed by JEOL 6400 field-emission scanning electron microscope (FESEM) operating at 15 kv to analyze the scratch

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profile of the bimodal NS Ni how to change with scratch speed and load. At the same time, the fracture morphologies of the scratch and tensile samples were also investigated using the FESEM at the same condition to inspect the deformation modes of the samples. To investigate the grain size distribution and the local impurity in the heat-treated samples, the changes of micro-structure

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in electrodeposited NC Ni before and after scratch testing was characterized using JEM-3010 transmission electron microscope (TEM) operating at 300 kv and equipped with an Oxford energy

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dispersive X-ray (EDX) detector, the scratch plastic deformation mechanism in the bimodal NS Ni sample was inspected by comparing the TEM morphology before and after micro-scratch test.

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Meanwhile, the TEM samples preparation after the micro-scratch testing was described as following. First, a discs sample with diameter of 3 mm for TEM investigation was extracted from

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the zone of scratch ditch tip and the discs were polished on the bottom side (opposite the top side with scratch ditch) to thin by about 60 µm. Then, the samples were prepared by double-jet

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electro-polishing again with electrolytic bath of 25% nitric acid and 75% methanol at the temperature below -30 ℃ and 50 V. Finally, the thin discs sample was put in the TEM operating at 300 kV to observe the microstructure morphology.

3. Results and discussion 3.1 Theoretical model of strain-rate sensitivity for micro-scratch test 7

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The strain-rate sensitivity for NS metal materials could provide information about its plastic deformation mechanism and deformation mode. In general, the strain-rate sensitivity can be obtained by linear fitting the log-log scale plot of flow stress versus strain rate under tensile test, the theoretical formula can be expressed as [9]:

m  (ln  ) / (ln ) ,

(1)

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here m is strain-rate sensitivity, σ is flow stress,  is strain rate. In micro-scratch experiment, the scratch hardness should be substituted into the equation above to replace the plastic deformation flow stress [25], so the strain-rate sensitivity of scratch hardness in the micro-scratch

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test can be written as:

m  (ln H s ) / (ln )

,

(2)

where the scratch hardness can be determined by scratch width (ѡ) and normal loading (N)

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[28], it is given as

,

(3)

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H s  k  N / w2

here the parameter k is a geometric constant that is specific to the geometric shape of

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indenter tip, when the radius of the spherical indenter tip is greater than the depth of scratch ditch in micro-scratch test, the parameter k equals 8/π, the value is derived for a projection of the

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leading half of the hemispherical tip. Meanwhile, an empirical formula for scratch strain-rate during the micro-scratch test as a function of scratch speed (v) and scratch ditch width (w), it can

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be expressed as [29]:

  C

v w,

(4)

here C is a material constant, for spherical blunt contact, the constant C could be considered proportional to relative strain r/R' , where r is the effective contact radius between the indenter and sample surface, R' is the geometrical radius at the end of the scratch tip, it is introduced to account for the relative strain imposed due to the indenter geometry. 8

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Figure 1 shows the schematic drawing of contact relationship between indenter and sample surface during micro-scratch test, from this figure, the effective contact radius r between indenter and sample surface is given as below:

r  R2  (R  dr )2 ,

(5)

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where R is the radius of scratch indenter, d r is the residual depth of scratch ditch, from the equation (4), the reducing strain rate with the progressive normal load under each constant scratch speed can be determined easily, because scratch ditch width increases with the increasing normal load. Meanwhile, a wider range of strain rate can be obtained if continuous different scratch

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speeds are given during the micro-scratch test. Therefore, different strain-rate sensitivity exponents could be confirmed by the linear fit of the log-log scale variation of strain rate versus scratch hardness under different scratch speeds.

The onset of plastic flow that occurs during micro-scratch testing provides a measurement to

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quantify strength where the strength is approximately one-third the hardness. Therefore, the scratch hardness values can be converted to an equivalent stress for direct comparison with the

√

(6)

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tensile test results using the formulations

(7)

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.

Here,

is shear stress computed from the von Mises criteria and is equal to one-third the

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scratch hardness.

3.2 Micro-structure characterization for electrodeposited bimodal NS Ni Fig.2 gives the bright field TEM morphology of the heat-treated electrodeposited NS Ni sample and the corresponding selected area electron diffraction (SAD) pattern of grain morphology. From Fig. 2a, it shows obvious bimodal structure in Ni sample after heat treatment at 200

, the size of

some grains separated by high-angle grain boundaries are above 100 nm, but well below 1 µm, which

can be called ultrafine grains. The ultrafine grains seem to be filled in the nanocrystalline matrix. 9

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From the statistical size distribution observed in the TEM, the actual size of ultrafine grains averages around 650 nm in the heat-treated sample, and the volume fraction of the ultrafine grains is about 40%. Fig. 2b shows the SAD pattern of [001] crystal zone for a single ultrafine grain in Ni sample, which is well agree with the face-centered-cubic (FCC) coarse-grained Ni. To further study the microstructure of the bimodal sample, Fig. 2c and 2d gives the TEM micrograph and

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corresponding SAD pattern of the nanocrystalline matrix. It is shown that the grain size of the matrix is tiny (average grain size is about 20 nm obtained by statistical analysis), and most are equal-axis grains separated by small-angle grain boundaries. Meanwhile, it can be seen from the SAD pattern (diffraction ring and diffraction spot) that the NS matrix is polycrystalline, and there are many small particles scattered in the selected area. The results of indexing of the rings are

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shown in Table 1 by calibrating the SAD pattern, we can get that the corresponding interplanar spacing of each diffraction ring is well match with the interplanar spacing of standard FCC Ni. Moreover, some impurities in electrodeposited NS Ni usually originate from the impurities in the anode material and from the chemicals used for the electrolytic bath, since the samples were

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prepared by electrodeposited method in this paper. Sulphur, for instance, originates from bath additives, such as natrium-saccharin (Na-C7H4NO3S). However, EDX of the heat-treated sample did not pick up marked sulfur segregation that usually occurs upon thermal annealing along

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at 200

the grain boundaries, suggesting that the sulfur distributed in the electrodeposited NS Ni is at very

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low concentration or below the EDX detection limit.

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It should be noted that the sample with bimodal grain size distribution are often still categorized as NS metal, though the sizes of the ultrafine grains are above 100 nm, because they have

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extensive sub-grain structures, such as low-angle boundaries and domain structure, below 100 nm, which contribute substantially to their properties or even dominate their deformation behaviors. 3.3 Scratch mechanical properties of bimodal NS Ni To assess the deformation mechanisms in the bimodal NS Ni sample, the scratch mechanical properties of the sample were investigated in details first. Fig.3 gives the variation of the depth of residual scratches ditch (scratch distance is 2 mm) in bimodal NS Ni sample with the normal load at different scratch speeds (0.6 mm/min、6mm/min、30mm/min、150mm/min). It can be seen that 10

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the residual scratch depth increases with increasing normal load at different scratch speed, and the maximum value of the residual scratches depth at each scratches is not greater than the radius of the indenter. It also can be seen from the Fig.3 that the depth of residual scratch ditches decrease as the rate of scratch increases. Moreover, the values of residual scratch depth in the larger normal load range under different scratch speeds become dispersive while the values under the smaller

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load range have smaller variance. Fig.4 shows the optical images of scratch ditch in the bimodal NS Ni at the initial stage (scratch distance < 1mm) of scratch distance under different scratch speeds. Obviously, at this stage, each scratch ditch widen with increasing normal load, whereas the width of the scratch ditches decrease

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with the increase of the scratch speed. Fig 5 gives the SEM micrographs at end of scratch ditches under different scratch speeds (0.6 mm/min、6mm/min、30mm/min、150mm/min). The same trend as the initial stage of scratch ditches can be found. However, plastic accumulations grow gradually on both sides of scratch and the front of the indenter with increasing scratch distance. So the plastic accumulation is obvious under the larger scratch distance at each scratch speed, while the

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scratch surface at smaller scratch distance is relatively smooth with few plastic accumulations as

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shown in Fig.4. This plastic accumulation has a significant influence on determining of residual scratch width. As described in section 2.2, the residual scratch width can be determined by the

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optical image of scratch ditch taken by the scratch system, thus the residual width obtained from the optical images should subtract the plastic accumulation from the overall scratch ditch width to

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reduce the its influence on scratch width. The statistical residual scratch width developing with the increase of normal load under

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different scratch speeds (0.6 mm/min、6 mm/min、30 mm/min、150 mm/min) was given in Fig.6. It can be seen that the width of each scratch ditch increases rapidly with increasing normal load at smaller load (scratch distance < 1 mm), but as the load increase, the increase rate of the scratch ditch width become slow (the slope of the curve decreases gradually), this trend have a good agreement with the images in Fig.4 and Fig. 5. Meanwhile, the residual scratch width at different speed become narrow with increasing scratch speed, which is also agree with the appearance observed in Fig.4 and Fig. 5. For example, the width of terminal scratch ditch is about 286µm 11

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when the scratch speed is 0.6 mm/min, and the width of terminal scratch ditch decrease to 195µm under the scratch speed of 150 mm/min. Fig.7 shows the relationship between scratch strain rate and normal load of indenter in bimodal NS sample calculated by the Eq. (4) under different scratch speeds (0.6 mm/min、6 mm/min、30 mm/min、150 mm/min). It can be seen that the strain rate decrease with the increase of normal

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load at different scratch speeds, and continuous value of strain rate can be obtained at each scratch-speed, when scratch speed increase from 6 mm/min to 150 mm/min, the strain rate almost increase from 0.03 s-1 to 30 s-1 accordingly, which grows about three orders of magnitude. Therefore, the micro-scratch testing technology can be used to evaluate the deformation

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mechanism and mechanical behavior of metal materials over a wider strain rate range.

Fig.8 shows the functional relationship between scratch hardness and scratch distance (2 mm) under different scratch speeds (0.6 mm/min、6 mm/min、30 mm/min、150 mm/min). From the Fig.8, the scratch hardness shows an obvious rate-dependence behavior, and become stronger with

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the increase of scratch speed. For bimodal NS Ni sample, when the scratch speed is 0.6 mm/min、6 mm/min、30 mm/min and 150 mm/min, the maximum of scratch hardness that occur at onset of

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plastic flow during scratch hardness testing is about 1094 MPa、1180 MPa、1479 MPa and 1943 MPa respectively. Meanwhile, scratch hardness decrease quickly with the increase of scratch

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distance at each scratch speed when the scratch distance is less than 1 mm. As shown in Fig.7, the strain rate is inversely proportional to the scratch distance (or normal load), so the scratch

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hardness under each scratch speed increases with increasing strain rate sharply over the range of scratch distance from 0 to 1 mm. However, the increasing trend of the scratch hardness as the

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scratch strain rate increases is inconspicuous when the scratch distance larger than 1 mm, and the hardness exhibits a weak rate-dependence behavior over a range of scratch distance from 1 to 2 mm, though the hardness increases with the increasing strain rate at this range of scratch distance. In order to verify the practicability of the micro-scratch technique on assessing the rate sensitivity of NS metals, conventional tensile tests were conducted under different strain rates that ranged from

to

s-1. Fig. 9 gives the plots of engineering stress versus engineering 2

strain using three different strain rates ( 12

s-1) which cover the strain-rate range

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obtained from the scratch speed of 0.6 mm/min as shown in Fig. 7. Similarly, the rate-dependent strength was also observed from the stress-strain curves for the NS bimodal tensile samples, and the tensile flow strength increases with increasing strain rate which have a similar tendency with micro-scratch testing. Meanwhile, the stress-strain curves in Fig. 9 indicate that the yield strength values determined using the intercept of a plot of engineering stress versus engineering strain with the 0.2% total strain offset parallel to the elastic slope for NS bimodal Ni samples are: 391 MPa at s-1, 461 MPa at strain rate of

2 -1

s-1,

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strain rate of

s and 625 MPa at strain rate of

respectively. Obviously, an increase in yield strength was obaerved for tensile samples with increasing strain rate.

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The onset value of scratch hardness (maximum value) at the scratch speed of 0.6 mm/min as listed in Fig. 8 can be converted into equivalent strength values using Eqs. (6) and (7). This hardness-based strength value (𝜀̇

. 2 s-1) is about 631 MPa, which do indeed appears to be

quantitatively consistent with the tensile yield strength at strain rate of

s-1 within the scale

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of the error for the strength measurements between the two test methods. 3.4 The strain-rate sensitivity of scratch hardness in bimodal NS Ni

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As mentioned above, the strain-rate sensitivity is a key parameter in evaluating the deformation mode of metal crystalline materials. Fig.10 shows the relation curve between scratch hardness and

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strain rate at logarithmic axis under different scratch speed (0.6 mm/min、6 mm/min、30 mm/min、

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150 mm/min) based on the mechanical response of scratch hardness and strain rate in bimodal NS Ni sample during the scratch testing process. From the definition of strain-rate sensitivity

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exponent, the value of strain-rate sensitivity at different scratch speed can be obtained from the slope of the log curve of scratch hardness and strain rate through linear fitting. The average strain-rate sensitivity exponents and standard errors of the bimodal NS Ni sample at different strain-rate ranges were shown in Table 2. Obviously, the strain rate sensitivity exponent, as evaluated from a power-law relationship between scratch hardness and scratch strain-rate, increases with increasing strain rate, though the values are basically the same at the range of strain rate from 0.03 s-1 to 1 s-1. Meanwhile, there is clear continuity for micro-scratch rate-dependent exponents across the broad range of strain rates, i.e. from 13

2

s-1 to

s-1, within the

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scale of the error for the strength measurement in the micro-scratch test method. This successive mechanical behavior in the micro-scratch testing proves the practicability of the micro-scratch technique again. Moreover, the values in bimodal NS Ni over the whole strain rate (from 0.03 s-1 to 30 s-1) are much larger than that of coarse-grained Ni [15], but slightly less than that of truly nanocrystalline FCC metals, in which the strain rate sensitivity exponent can reach 0.3~1 [30-33].

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The change in strain-rate sensitivity exponent suggests that the deformation mechanisms in bimodal NS Ni sample during micro-scratch test are different from the conventional coarse-grained and truly nanocrystalline counterparts.

3.5 Deformation mechanisms in bimodal NS Ni under micro-scratch testing

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It is well known that the grain boundary volume fraction increases sharply with the decrease of grain size, and the grain boundary plays an important role in the plastic deformation of NC material [34]. For NS metals, the higher strain rate sensitivity exponent is usually attributable to the grain boundary mediated mechanisms, such as grain boundary dislocation [35, 36], grain

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boundary sliding [37], grain rotation and coalesce [38]. In our experiment, for bimodal NS Ni sample during the scratch plastic deformation process, the strain-rate sensitivity exponent in

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region of strain-rate from 10 s-1 to 30 s-1 is very close to the value in NS metals. Therefore, one would consider that the microcosmic mechanism in the bimodal NS Ni sample is mainly

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dominated by grain boundary under higher strain rate (10 s-1~30 s-1) during the micro-scratch testing. However, as shown in Table 2, the difference of the value of strain-rate sensitivity

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exponent in bimodal NS Ni sample between smaller strain-rate range (0.03 s-1~1 s-1) and higher strain-rate range (10 s-1~30 s-1) is obvious, thus the operative deformation mechanism for bimodal

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NS Ni sample maybe change with strain rate at different range. To examine how operative deformation mechanism changes with the strain rate for bimodal NS

Ni sample during scratch plastic deformation, Fig.11 gives the TEM micrographs of bimodal NS Ni at end of the scratch ditch under the scratch speed of 0.6 mm/min and150 mm/min respectively. As shown in Fig.11a, the grains in nanocrystalline matrix seem to be elongated when the scratch speed is 0.6 mm/min, which covers a range of strain rate from 0.03s-1 to 0.12s-1, and some dislocation pile-up occurs in the ultrafine grains under this range of strain rate. Thus, we can 14

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deduce that dislocation glide mechanism and diffusion controlled mechanism is the main carrier of plastic deformation under the smaller strain rate range [39], and this mechanisms are completely different with that in untreated sample (commercially available nanostructured Ni with uniform grain size distribution) as reported in Ref. [26]. First, the dislocations are triggered in low-angle grain boundaries in nanocrystalline zones and ultrafine grain interior simultaneously, then part of

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the dislocations glide through the nanocrystalline grains and annihilate at another grain boundary. However, a number of dislocation storages can be found in ultrafine grains due to their larger dimension. Meanwhile, atomic diffusions in the nanocrystalline zones perform continuously with the dislocation gliding. Finally, the grain elongation occurs assisted by the combination of the grain boundary dislocation and atomic diffusion in nanocrystalline zones under the smaller strain

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rate range. Similarly, for the larger strain rate range of the bimodal NS Ni sample, the dislocations also accumulates in low-angle grain boundary in nanocrystalline zone and ultrafine grain interior, as shown in Fig. 11b, but the diffusion controlled mechanism has no enough time to operate and the dislocation also has no enough time to annihilate during recovery under higher strain rate

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loading. Thus, dislocation density under higher strain rate range is more than that under the smaller strain rate range. So the different mechanical responses such as strain rate sensitivity,

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between smaller and higher strain rate range, can attribute to the different activity modes of

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dislocation under different strain rate range. The strain rate hardening behaviors of the bimodal NS Ni samples under tensile and

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micro-scratch testing can also provide evidences for the dislocation pile-ups mechanism in bimodal NS samples, since these materials do evidence dislocation generation and storage under

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plastic deformation as shown in Ref. [15]. In other word, though the activity modes of the dislocation activity under different strain rate range have a difference, the strain rate hardening behavior in the bimodal NS Ni sample is attributable in a wider strain rate range (0.03 s-1 to 30 s-1) to the dislocation activities. According to Hart’s criterion, the higher strain-rate sensitivity of a material would delay the plastic instability and prolong the uniform tensile deformation [7], so the bimodal NS Ni sample exhibits a better uniform tensile ductility as shown in the reference [5-8]. To further verify the deformation mechanisms of dislocation activity, Fig. 12 gives the fracture 15

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s-1 and micro-scratch sample at scratch

morphologies of tensile sample at strain rate of

speed of 0.6 mm/min which covers a strain rate range from 0.035- 0.12 s-1. Obviously, the fracture surface of the tensile sample annealed at 200

shows ductile features with dimple sizes several

times larger than the grain size of nanocrystalline matrix, the ductile dimple feature is a representative feature attributed to the dislocation pile-up during plastic deformation in metal

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materials. At the same time, the fracture surface of the scratch ditch in the same sample under the micro-scratch testing also exhibits a ductile fracture feature, such as plastic pile-up belts and built-up edges as shown in Fig. 12b and Fig. 5, which also can attribute to combine effect between the dislocation accumulations and shearing action during the scratch test process. However, the deformation mechanisms in bimodal NS Ni sample change gradually from the dislocation gliding

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and diffusion to grain boundary dislocation pile-up with increasing strain rate under micro-scratch testing, as shown in TEM morphologies in Fig.11, although the dislocation activity are dominant during the wider strain rate range. Meanwhile, this deformation mechanism generates the strain rate hardening behavior in bimodal NS Ni sample, which could prevent the instability like as

4. Conclusions

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necking and shear banding to give extensive uniform elongation.

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Strain-rate sensitivity exponent is a key parameter in evaluating deformation mechanism of NS metals. In this paper, the micro-scratch technique that is a simple, convenient and reliable method

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was selected to provide continuous scratch hardness value over a wider range of strain rate (0.03 s-1~30 s-1) in a full dense, high purity and well-characterized electrodeposited NS Ni with bimodal

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grain size distribution. First, the strain rate varying with scratch speed was investigated, the strain rate increases from 0.03 s-1 to 30 s-1 approximately as the scratch speed covers a range from 0.6 mm/min to 150 mm/min in this experiment, thus the micro-scratch technique can be used to evaluate the mechanical behavior of NC Ni under a wider range of strain rate. Second, the mechanical behaviors of the bimodal NS Ni sample were investigated carefully. The results show that the bimodal NS Ni exhibits an obvious rate-dependence behavior and its maximum of scratch hardness reaches to 1943 MPa at the scratch rate of 150 mm/min. Further, the strain-rate 16

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sensitivity exponents of the bimodal NS Ni were obtained by linear fitting under different scratch speed. Generally, the values of the strain rate sensitivity exponent are all within the range of 0.23-0.34, which is much larger than that of conventional coarse-grained counterparts, but slightly less than that of truly nanocrystalline FCC metals. This phenomenon shows that the plastic deformation mechanism in the bimodal NS Ni is different from that in coarse-grained and

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nanocrystalline metals. Finally, the microscopic deformation mechanism in the bimodal NS Ni sample during scratch plastic deformation was discussed in details under different strain rates. From the results, the deformation mechanism of the bimodal NS Ni sample changes gradually from the dislocation gliding and diffusion to grain boundary dislocation pile-up with increasing strain rate under micro-scratch testing. Meanwhile, we thought that the enhancement in ductility of

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the bimodal NS metals is dependent with the strain rate hardening behavior induced by the dislocation activity.

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Acknowledgments

This work was supported by the Fundamental Research Funds for the Central Universities

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References

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(China University of Mining and Technology) (2017XKQY011).

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in materials science 37 , 289-401.

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Figures:

Fig. 1. Schematic drawing of contact relationship between indenter and sample surface during

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micro-scratch testing

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Fig. 2. (a) Bright field TEM morphology of the heat-treated electrodeposited NS Ni with bimodal grain size distribution, (b) SAD pattern of [001] crystal zone for a single ultrafine grain in bimodal

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NS Ni sample, (c) Bright field TEM micrograph of nanocrystalline matrix in bimodal NS Ni sample and (d) SAD pattern of nanocrystalline matrix 55

v=0.6mm/min v=6mm/min v=30mm/min v=150mm/min

45 40

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35 30 20 15 10

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/ Depthof residual scratch ditchm

50

5

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0

0

2

4

6

8

10 12 14 16 18 20 22 24 26

Normal force / N

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Fig. 3. Depths of residual scratch ditch varying with the normal force in bimodal NS Ni sample under different scratch speeds

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Fig. 4. Optical photos of scratch ditch at initial stage (scratch distance < 1mm) under

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micro-scratch testing, (a) 0.6 mm/min, (b) 6 mm/min, (c) 30 mm/min, (d) 150 mm/min

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Fig. 5. SEM morphology of bimodal NS Ni sample at the end of the scratch ditch for different

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scratch speeds, (a) 0.6 mm/min, (b) 6 mm/min, (c) 30 mm/min, (d) 150 mm/min

v=0.6 mm/min v=6 mm/min v=30 mm/min v=150 mm/min

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/ Width of resudual scratch ditchm

300 250 200 150 100

50

0

2

4

6

8

10 12 14 16 18 20 22 24 26 Normal Force / N

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Fig. 6. Widths of residual scratch ditch varying with the normal force in bimodal NS Ni sample under different scratch velocities 0.14

1.2

a

0.12

b 1.0

0.06 0.04

0

2

4

6

8

0

10 12 14 16 18 20 22 24 26 Normal force / N

2

4

6

8

10 12 14 16 18 20 22 24 26

Normal force / N

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c

6

d

30 28

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5

v=30 mm/min

-1

Strain rate / s

-1

0.6

0.4

7

Strain rate / s

0.8

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v=0.6mm/min

0.08

0.02

v=6 mm/min

-1

Strain rate / s

-1

Strain rate / s

0.10

4 3

24

v=150 mm/min

22 20 18 16 14

4

6

8

10 12 14 16 18 20 22 24 26 Normal force / N

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2

12

2

4

6

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10 12 14 16 18 20 22 24 26 Normal force / N

Fig. 7. Strain-rate varying with the increasing normal force in bimodal NS Ni sample under

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different scratch velocities, (a) 0.6 mm/min, (b) 6 mm/min, (c) 30 mm/min, (d) 150 mm/min

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2200 v=0.6 mm/min v=6 mm/min v=30 mm/min v=150mm/min

1800 1600 1400 1200

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Scratch hardness / MPa

2000

1000 800

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0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25 Scratch distance / mm

Fig. 8. The scratch hardness as a function of scratch distance in bimodal NS Ni sample under the

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different scratch velocities

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800

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1000

600

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Engineering Stress / MPa

1200

400

Strian rate = 10-3 s-1 Strain rate =10-2 s-1 Strain rate = 10-1 s-1

200 0

0.00

0.01

0.02

0.03 0.04 0.05 Engineering Strain

0.06

0.07

0.08

Fig. 9. Engineering stress versus strain plots of bimodal NS Ni samples are plotted at nominal strain rate of 10-3, 10-2, 10-1 s-1, respectively

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v=0.6 mm/min v=6 mm/min v=30 mm/min v=150 mm/min Linear fit at v=0.6 mm/min Linear fit at v=6 mm/min Linear fit at v=30 mm/min Linear fit at v=150 mm/min

7.6 7.5 7.4 7.3 7.2 7.1 7.0 6.9 6.8 6.7 6.6

-4

-3

-2

-1

0

1

2

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Ln(strain rate) / Ln(s-1)

Equation Adj. R-Square

y = a + b*x 0.96923

ln (hardness) ln (hardness) ln (hardness) ln (hardness) ln (hardness) ln (hardness) ln (hardness) ln (hardness)

Intercept Slope Intercept Slope Intercept Slope Intercept Slope

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Ln(scratch hardness) / Ln(MPa)

7.7

3

4

Fig. 10. Relationships between scratch hardness and strain rate under different scratch speeds in a

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logarithm format for bimodal NS Ni

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Fig. 11. TEM morphology of bimodal NS Ni at the end of the scratch ditch for different scratch speeds, (a) 0.6 mm/min, (b)150 mm/min

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Fig. 12. SEM fracture morphology of bimodal NS Ni for (a) tensile sample at strain rate of 10-1 s-1,

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(b) micro-scratch sample at scratch speeds of 0.6 mm/min

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Tables:

Table 1. Comparison of interplanar distances between NC Ni and standard pure FCC Ni Table 2. Average strain-rate sensitivity exponents of bimodal NS Ni sample under micro-scratch testing at different scratch strain rate through linear fitting

Number of rings

1

2

3

4

5

6

7

Crystal face

{111}

{200}

{220}

{311}

{222}

{400}

{331}

0.2025

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Table 1. Comparison of interplanar distances between NC Ni and standard pure FCC Ni

Interplanar

NS Ni

distance 0.2041

0.1245

0.1067

0.1025

0.0889

0.0815

0.1768

0.1250

0.1066

0.1020

0.0884

0.0811

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Standard pure Ni

0.1771

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Table 2. Average strain-rate sensitivity exponents of bimodal NS Ni sample under micro-scratch

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testing at different scratch strain rate through linear fitting

Strain rate range

Strain-rate sensitivity

Standard

(mm/min)

-1

(s )

exponent

error

0.6

0.035~0.125

0.235

0.0071

6

0.373~1.153

0.240

0.0076

2.258~6.597

0.298

0.0071

12.771~30.195

0.338

0.0123

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Scratch velocity

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150

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30

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