3 Key Engineering Materials - Nanoindentation of Multi-wall CNT Reinforced Al Composites

Jin-Zhi Liao a, Jian-Jun Pang b, Ming-Jen Tan c

School of Mechanical and Aerospace Engineering, Nanyang Technological University,

50 Nanyang Avenue, Singapore 639798, Singapore

a liao0025@https://www.sodocs.net/doc/2f15659031.html,.sg,

b pang0079@https://www.sodocs.net/doc/2f15659031.html,.sg,

c mmjtan@https://www.sodocs.net/doc/2f15659031.html,.sg

Keywords: Aluminum composite; carbon nanotube; nanoindentation; mechanical properties

Abstract.This work used nanoindentation to characterize the local mechanical properties of the multi-wall carbon nanotube (MWCNT) reinforced aluminum (Al) composites. The Al-MWCNT (0.5, 1.0 and 2.0 wt.%) specimens were fabricated by spark plasma sintering (SPS) followed by hot extrusion. Different local regions of the as-extruded and tensile-fractured specimen over the longitudinal and transverse section were studied by nanoindentation. The nanoindentation results were compared with the conventional macro- and mircoscopic mechanical tests, and were found in good agreement. The values of hardness (H) and elastic modulus (E) obtained reached maximum at the 0.5 wt.% MWCNT adding Al samples. E was highest in the necking region then decreased with increasing distance from the localized deformed region; while H varied in different regions. In the same region, H and V were higher in the longitudinal than those in the transverse direction, due to the texture hardening and alignment of CNT.

1. Introduction

Since their discovery in 1991, immediately reorganization and great interest were attracted to carbon nanotubes (CNTs) due to their unique properties [1]. They have extraordinary mechanical properties, with Young’s modulus of 270 GPa - 1 TPa, strength of 11 - 200 GPa, good elongation at fracture (5-10 %) and good flexibility (bending repeatedly up to 90°) [2-5]. In the last decade, there has been a wide range of research work carried out using CNT as reinforcement in Al matrix [6-9], with the purpose of imparting the extraordinary high strength of CNT into the matrix. Previous work has devoted to fabricating Al-CNT composites then extracting their mechanical properties using conventional macroscopic mechanical tests, such as hardness and elastic modulus [6-9]. The conventional macroscopic mechanical tests (i.e. tensile and compressive test), however, are constrained by the result accuracy, the specimen size, and also unable to characterize the local mechanical properties of the specimen. For example, the measurement of strain may not be accurate due to the deformation from the grip zone in the conventional tensile test.

By comparison with the conventional mechanical tests, nanoindentation technique has many advantages. In indentation, a force as small as several n and displacement of several ?can be accurately measured [10]. With its high resolution load and depth sensing capabilities, nanoindentation is able to characterize the mechanical properties at specified locations in a broad range of materials systems with heterogeneous microstructures [11]. For example, nanoindentation has been used to investigate the mechanical behaviors near grain boundary [12] and on different crystallite planes [13]. Additionally it can avoid the sample size effect, since the sample size is comparable to the indent size.

Most the previous work on Al-CNT composites used conventional mechanical tests to characterize their mechanical properties. Less attention, however, was paid to their mechanical properties under nanoindentation. In this work nanoindentation load-displacement results, elastic moduli, and

hardness values of the as-extruded Al-CNT specimens were investigated. In addition, local

mechanical properties of the tensile-fractured specimens over longitudinal and transverse direction were studied.

2. Experimental procedures

The materials studied were Al metal matrix composites reinforced with 0 - 2.0 wt. % MWCNTs. The Al-MWCNT composites were fabricated by spark plasma sintering followed by hot extrusion (please see ref [7]).

For tensile tests, an I-shape type of cylindrical sample with a diameter of 5 mm and gauge length of 28 mm was used. The tensile test was under room temperature, with strain rate of 10-3/s. HV micro-hardness was also carried out. The detailed tests were elaborated elsewhere [7]. The after tension test fractured specimen was divided into five regions for nanoindentation test. The region was marked as section N1, N2, N3, N4 and N5 (Fig. 1). Five indents were made on each region for statistical analysis. Nanoindentation was made at both the longitudinal and transverse of each region. Nanoindentation tests were carried out on a MTS Nanoindenter XP with a diamond pyramidal-shaped Berkovich tip. Continuous Stiffness method (CSM) was used. In the nanoindentation test target strain rate is 0.05/s, up to a peak depth of 1500 nm, with holding time 20 s. Optical microscopy, scanning and transmission electron microscopy (SEM, TEM) were used to characterize the microstructure of the samples.

3. Results and Discussion

Nanoindentation hardness is calculated as the indentation load divided by the projected contact area of the indentation at peak load, then following Oliver & Pharr's analysis [14], and the elastic modulus is calculated by using the relation [14]:

2

2(1)

1(1)i r i

E E E υυ??=+ (1) where E and v are Young's modulus and Poisson's ratio for the specimen and E i and v i are the same parameters for the indenter. E r is reduced modulus, obtained from measurement data related only to the instrument. For the diamond indenter, E i = 1141 GPa, v i = 0.07. The Poisson ratio for Al is 0.33. The representative load-displacement curves of the nanoindentation made on the as-extruded pure Al and Al-CNT specimens are shown in Fig. 2a. It is apparent that the slope of the unloading curve is very steep, which indicates the good ductility of the specimens. The corresponding local mechanical properties, i.e. H and E, of these as-extruded specimens are shown in Fig. 2b. There is a significant mechanical enhancement in the Al-MWCNT composites properties at the content of 0.5 wt.% of MWCNTs. These results are in good agreement with conventional tensile and HV hardness test results, as shown in Fig. 3 and 4. The detailed macroscopic mechanical tests and explanation have been elaborated elsewhere [7]. By comparison with the HV and nanoindentation hardness, the Vickers hardness of the extruded samples is ~ 50% of that of the nano-hardness. This is because nanoindentation analysis utilizes the projected contact area at the peak load instead of the residual projected area [15].

Fig. 1. Nanoindentation test specimen. Five regions were divided. Nanoindentation test was carried out on both the longitudinal and transverse section.

T e

n s i l e s t r e s s (M P a )Tensile strain (%)

The H and V of the five regions of the tensile-fractured pure Al and Al-CNT specimens are shown in Fig. 5a. From Fig. 5a, there is a clear trend that E in the necking region is highest, then decreases as towards the undeformed region (N1 → N5). Whilst for H, its value varies in the five regions, but a general trend was observed that H decreases from the necking to grip region. The plastic deformation in the tensile process resulting in strain hardening, plays a significant contribution to the total strength of the fractured specimens. Strain hardening is highest in the localized necking region, then decrease towards the undeformed grip region, likewise values of H and E are highest in the necking region, then have a trend to decrease from localized deformed region (necking part) to the undeformed region.

By comparing the two sections of the same region tested, E of the longitudinal section (L) are higher than those of the transverse section (T), see Fig. 5a. Thus, the selective maximum force (F) applied on the different regions of the Al-0.5MWCNT specimen at peak depth is shown in Fig. 5b. It can be seen that F is much higher in L than that in the T section (Fig. 5b), which indicates higher mechanical strength in the L section.

As it is common in Al and other FCC extruded alloys, the texture comprises two well-defined fiber texture components, namely, the <111> and the <100>, where the fiber axis is that of the extrusion

H V

h a r

d

n e s

s

(k g

/m m

2)

CNTs contents (wt.%)

Fig. 3. Uniaxial tensile stress-strain

response of the as-extruded samples. Fig. 4. HV hardness of the as-extruded samples.

Fig. 2. Nanoindentation load-displacement of the (a) as-extuded pure Al and Al-CNT composites, and (b) their corresponing nanoindentation H and E. Based on the continuous stiffness method, constant depth 1500 nm.

F o r c e (m N )Displacement (nm)0.750.800.850.90

2.0

1.00Wt.% CNT

H

a r d n e

s

s

(G P a

)

E l

a

s t i c

m o d

u

l u

s (G P

a )

extrusion axis was observed (Fig. 6a). The extrusion axis is predominated by slip planes which have comparatively closer interatomic distance (close-packed). Closer interatomic distance leads to larger E. Thus E showed higher in the L section as a result of having high <111> <100> texture in the extrusion axis.

Additionally, the effect of alignment of reinforcement (CNT), in the matrix is another main factor attributed to the anisotropic mechanical properties in L and T sections. In the hot extrusion process, the CNTs embedded in the matrix were aligned along the extrusion axis, accompanyingly with the matrix flow. The alignment of CNT was examinated by TEM (Fig. 6b). From the SEM fractograph of the Al-0.5MWCNT specimen (Fig. 6c), the CNTs can be observed to be pulled out from the dimples. It is well known that the degree of fiber alignment is crucial in determining the mechanical properties. Aligned fibers lead to the maximum stiffness and strength in the direction of alignment.

Fig. 6. (a) Microstructure of the as-extruded Al-2.0 wt.% MWCNT composite, and (b) TEM image of the as-extruded Al-0.5 wt.% MWCNT composite showing alignment of MWCNTs along the extrusion direction (arrow indicates the extrusion direction), and (c) fractograph of the Al-0.5MWCNT specimen. CNTs were pulled out from the dimples.

20 μm (a) (b) Fig. 5. (a) Nanoindentation hardness and elastic modulus of the five regions of the tensile-fractured pure Al and Al-0.5CNT specimens, and (b) maximum force applied on the different regions of the Al-0.5CNT specimen when obtaining the peak depth. T: transverse; L: longitudinal.

C T

(c)

5 μm Grip part H a r d n e s s (G P a )E l a s t i c m o d u l u s (G P a )0.700.760.880.941.00 Necking Different regions of Al/0.5CNT fracture specimen

F o r c e (m N )

4. Summary

Nanoindentation test was applied to detect the local mechanical properties of Al-MWCNT specimens. Different local regions of the as-extruded and tensile-fractured specimen over the longitudinal and transverse section were studied.

In the as-extruded specimen the values of H an E obtained reached maximum at the 0.5 wt.% MWCNT adding Al samples, which corresponded well with those obtained from conventional macro- and mircoscopic tests previously reported.

E was highest in the necking region then decreased with increasing distance from the localized

deformed region. This variation is due to different degree of strain hardening.

H and E of the transverse direction were less than those of the longitudinal direction. This

anisotropy was due to texture hardening of the Al matrix and the alignment of CNT. References

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