Using a modified Knoop Indentation Technique to estimate the cavitation erosion resistance of NiTi

Materials Characterization 52 (2004) 129 – 134

Using a modified Knoop Indentation Technique to estimate the cavitation erosion resistance of NiTi F.T. Cheng a,*, P. Shi a,b, H.C. Man c a

Department of Applied Physics, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, PR China b Department of Materials and Chemical Engineering, Liaoning Institute of Technology, PR China c Department of Industrial and Systems Engineering, The Hong Kong Polytechnic University, Hong Kong, PR China Received 6 November 2003; accepted 24 April 2004

Abstract An Ni-rich (50.8 at.% Ni) NiTi alloy was heat treated with different aging temperatures to obtain specimens possessing different cavitation erosion resistance. A modified Knoop Indentation Technique that combines indentation measurements and thermal recovery has been used to determine the total-recovery/deformation ratio for the heat-treated NiTi specimens. This ratio, which reflects both superelasticity and pseudoplasticity, has been found to correlate well with the cavitation erosion resistance for NiTi. The technique thus serves as a simple method for estimating the performance of NiTi against cavitation erosion. D 2004 Elsevier Inc. All rights reserved. Keywords: Cavitation erosion; Knoop Indentation; Hardness; Elastic properties; Metals and alloys; NiTi

1. Introduction NiTi is widely used in sensors and actuators, as pipe couplers and as medical implants [1,2]. The popularity of NiTi in these applications mainly originates from its unique properties, like shape memory effect and superelasticity. The applications of these superior and peculiar properties of NiTi are, however, far from being exhausted. These properties, among others, have also led to excellent tribological properties for NiTi [3] and, in particular, extremely high cavitation erosion resistance [4,5].

* Corresponding author. Tel.: +86-852-2766-5691; fax: +86852-2333-7629. E-mail address: [email protected] (F.T. Cheng). 1044-5803/$ - see front matter D 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2004.04.006

The functional and mechanical properties of NiTi are very sensitive to its alloy composition and also to thermal or thermomechanical treatment. For example, the cavitation erosion resistance of an Ni-rich NiTi alloy with proper aging treatment could be increased by about eight times relative to that of the solutiontreated specimen [6]. Owing to the extremely high cavitation erosion resistance, the evaluation of the effect of alloy composition or thermomechanical treatment could be very time consuming. Thus, the search for a simple method to predict or estimate the cavitation erosion resistance of NiTi is of practical importance because it would greatly reduce the amount of work in preliminary test runs. The feasibility of such a method relies on the establishment of a relationship between cavitation erosion resistance and some readily measurable quantities.

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Cavitation refers to the generation and collapse of bubbles in a flowing or vibrating liquid due to local pressure fluctuations. When bubbles collapse, shock waves and microjets are generated, thus exerting intense stress pulses on nearby solid surfaces. Repetitions of these stress pulses eventually lead to fracture and erosion loss from the surface, and cavitation erosion is a common problem in hydraulic machinery and other liquid-handling equipment or parts. There is no universal quantitative relationship between cavitation erosion resistance Re and conventional mechanical properties [7], although Re is related to hardness for some specific classes of materials [8]. For NiTi, however, Re does not correlate with hardness obtained in indentation tests [9– 11]. On the other hand, important mechanical properties could be obtained from the load – displacement curve by using instrumented indentation [11 – 13]. It has been shown in a previous study that the cavitation erosion resistance of NiTi correlates with some specific quantities derivable from instrumented nanoindentation measurements [11]. While such a correlation is useful, the nanoindentation system is expensive and may not be readily available. Thus, the present study aims at searching for a simple alternative that may be used in the prediction or estimation of the cavitation erosion resistance for NiTi.

2. Experimental details Specimens of 12
12 mm were spark cut from an NiTi (50.8 at.% Ni) plate of thickness 2 mm. The specimens were solution treated at 1000 jC for 1 h (ST) followed by ice quenching (IQ), and then aged at one of several selected temperatures for 3 h, followed by IQ. The designations of the specimens are shown in Table 1. Details of the effect of heat treatment on the behavior of NiTi are not the concern of the present paper and have been reported elsewhere [6]. After heat treatment, the specimens were polished with 1-Am diamond paste before testing. The heat-treated specimens were subjected to cavitation erosion tests in deionized water at 23 jC, conforming to ASTM Standard G32-92 [14], using a cavitation facility (Sonicator XL 2020); details of the experimental setup and conditions are described elsewhere [6]. The cavitation erosion rate was measured

Table 1 Specimen designation, heat treatment, mean erosion rate, and cavitation erosion resistance of various NiTi specimens Specimen

Heat treatment

MER (Am/h)

Re (h/Am)

Re*

NiTi-ST NiTi-200

STa + IQb ST + IQ + 200 aging + IQ ST + IQ + 300 aging + IQ ST + IQ + 400 aging + IQ ST + IQ + 500 aging + IQ ST + IQ + 600 aging + IQ

jC

0.115 0.0795

8.68 12.57

1.00 1.45

jC

0.0812

12.32

1.42

jC

0.0312

32.09

3.70

jC

0.0133

75.13

8.66

jC

0.0760

13.16

1.52

NiTi-300 NiTi-400 NiTi-500 NiTi-600 a b

ST: solutionizing treatment. IQ: ice quenching.

by the mean erosion rate (MER), and the cavitation erosion resistance Re, by the reciprocal of MER, according to: 10DW qADt

ð1Þ

Re ðh=AmÞ ¼ ðMERÞ1

ð2Þ

MER ðAm=hÞ ¼

where DW is the weight loss in mg, Dt is the time interval in h, A is the eroded area in cm2, and q is the density of the surface layer in g cm  3. In the calculation of MER, it is assumed that the depth of damage is uniform across the affected area. Knoop Indentation Tests of the NiTi specimens were performed at room temperature at a load of 200 g and a loading time of 15 s using a microhardness tester (Buehler Micromet II). The lengths of the major diagonal 2a Vand minor diagonal 2bVof the indents at five locations were recorded for each specimen. The specimens were then heated in a water bath to above 80 jC to achieve shape recovery, if any. As reported in Ref. [6], the values of the phase transformation temperature Af (finish temperature of the transformation from martensite to austenite) were below 60 jC for all the specimens. Thus, heating to above 80 jC ensured that all the specimens were transformed to the austenitic phase and that shape recovery was complete. The new lengths of the diagonals, 2aW and 2b W, were then recorded. These lengths are shown in Fig. 1,

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131

Fig. 1. Schematic diagram (not to scale) to show the lengths of the major and minor diagonals of the Knoop indent under load (2a, 2b), after elastic recovery (2a V, 2b V), and after thermal recovery (2a W, 2b W). Note that 2a c 2a Vc 2a W and 2b = 2a/7.11.

together with the relative dimensions of the indent under load, 2a and 2b, as defined by the Knoop indenter geometry.

3. Results and discussion In the present study, NiTi specimens were heat treated at different aging temperatures to vary the mechanical properties of the specimens, including the cavitation erosion resistance. Graphs showing the cumulative weight loss as a function of time in the cavitation erosion test are given in Fig. 2 for a total test time of 40 h. The mean rate of erosion (MER) and the cavitation erosion resistance (Re) calculated from Eqs. (1) and (2) are shown in Table 1 for various heat-

treated specimens. For easy comparison, the normalized cavitation erosion resistance Re* relative to specimen NiTi-ST is also given. The Knoop Indentation Method, besides being an alternative to the Vickers Indentation Method, is usually employed in probing surfaces or layers with varying hardness, such as in gradient or anisotropic materials, because of the unequal diagonals. The Knoop Indentation Technique has also been ingeniously employed by some authors in determining the hardness-to-elastic-modulus ratio, H/E, for linearelastic/plastic materials [15 – 17]. For superelastic/ plastic materials like NiTi, owing to the nonlinear and complex nature of the stress –strain relationship, no correlation between the Knoop indentation and H/ E has been established. Nevertheless, with modifica-

Fig. 2. Cumulative weight loss as a function of time in a cavitation erosion test.

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F.T. Cheng et al. / Materials Characterization 52 (2004) 129–134

tion to include the shape memory effect, the Knoop Indentation Technique has been employed in the present study of NiTi without reference to H/E. The ratio of the major to minor diagonal (2a/2b) of the Knoop indent under load is equal to 7.11, as defined by the Knoop indenter geometry. Upon unloading, elastic recovery reduces the length of the minor diagonal of the indent to 2bV, while the length of the major diagonal remains almost unchanged (i.e., 2aVc 2a) [15]. Similarly, thermal recovery further reduces 2b V to 2b W, while 2a Wc 2a. The average values for 2a (c 2a V), 2b ( = 2a/7.11), 2b V, and 2b W are shown in Table 2, together with the elasticrecovery/deformation ratio Fe, defined as (b  b V)/b, the thermal-recovery/deformation ratio Fs, defined as (bV bW)/b, and the total-recovery/deformation ratio Ft, defined as (b  b W)/b. The Knoop hardness H calculated from a V is also tabulated. It is interesting to note that, for NiTi-ST, Fs vanishes, indicating absence of thermal recovery. This is consistent with the fact that NiTi-ST is totally austenitic at room temperature (Ms =  70 jC, [6]). The relationships between the normalized cavitation erosion resistance Re* and various parameters H, Fe, Fs, and Ft are shown in Fig. 3. It can be seen that Re* does not correlate with the hardness, consistent with the results of previous studies [9– 11]. On the other hand, the correlation between Re* and Fe, Fs, and Ft was, in turn, tested using the same relationship employed in a previous study [11]: Re* ¼

k ½F0  Fn

ð3Þ

with the value of F0 chosen to maximize the correlation coefficient r of the log – log linear plot of Eq. (3). The corresponding values of the constants k and n, together with those of F0 and r, are given in Fig.

3(b) –(d). The asymptotic behavior of Eq. (3) as F approaches F0 is an artifact arising from the short test time relative to the high resistance of NiTi [11]. The better correlation of Re* with Ft than with Fe or Fs, as is obvious from Fig. 3(b)– (d), and also from the significantly higher r for Ft, clearly indicates that both elastic and thermal recovery are related to cavitation erosion resistance. For NiTi, elastic recovery mainly originates from the superelasticity of the austenite phase, and thermal recovery originates from the shape memory effect of the martensite phase. The austenite phase contributes to a high Re* by being able to accommodate large deformation with a small plastic (slip) strain (i.e., by superelasticity), while the martensite phase contributes via accommodating large deformation by the reorientation of the martensite variants (i.e., by pseudo- or superplasticity). Both processes lead to a small unrecoverable strain, hence resulting in less severe damage and a high cavitation erosion resistance. Thus, it is not unexpected that Re* correlates with the ratio Ft, which reflects both superelasticity and pseudoplasticity. The better correlation of Re* with Ft than with Fe or Fs alone confirms that the deformation behaviors of both austenite and martensite play a role in resisting erosive attack. According to the results in Ref. [11], Re* correlates with W/du via a relationship of the same form given by Eq. (3), where W and du are the work done in nanoindentation and the unrecoverable plastic deformation, respectively. Thus, the ratio Ft is comparable to the parameter W/du. The linear relationship between Ft and W/du shown in Fig. 4 supports this point. The Knoop Indentation Technique is, of course, a quicker and simpler method than the nanoindentation approach is, although the latter is more precise and informative. Recently, it has been reported that the cavitation erosion resistance of some ferrous shape

Table 2 Dimensions (in Am) of the Knoop indentation and the recovery/deformation ratios Fe, Fs, and Ft Specimen

2a c 2a V

2b = 2a/7.11

2b V

2b W

Fe=(b  b V)/b

Fs=(b V b W)/b

Ft=(b  b W)/b

H

NiTi-ST NiTi-200 NiTi-300 NiTi-400 NiTi-500 NiTi-600

73.7 84.2 73.8 80.2 84.3 95.2

10.4 (0.15) 11.8 (0.17) 10.4 (0.14) 11.3 (0.15) 11.9 (0.13) 13.4 (0.17)

7.9 (1.2) 8.5 (1.3) 7.1 (1.4) 7.3 (1.1) 6.1 (1.2) 10.5 (1.4)

7.9 6.9 5.7 4.9 3.9 9.5

0.24 0.28 0.32 0.35 0.49 0.22

0.00 0.14 0.13 0.22 0.18 0.07

0.24 0.42 0.45 0.57 0.67 0.29

524 401 523 442 400 314

(1.1) (1.2) (1.0) (1.1) (0.9) (1.2)

Standard deviation is given in parentheses.

(1.2) (1.4) (1.2) (1.0) (1.0) (1.3)

(0.021) (0.027) (0.025) (0.018) (0.025) (0.021)

(0.007) (0.026) (0.027) (0.030) (0.024) (0.012)

(0.020) (0.032) (0.022) (0.020) (0.027) (0.024)

(17) (11) (15) (12) (8) (8)

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Fig. 3. Correlation of the normalized cavitation erosion resistance Re* with (a) the Knoop hardness H, (b) the elastic-recovery/deformation ratio Fe, (c) the thermal-recovery/deformation ration Fs, and (d) the total-recovery/deformation ratio Ft.

Fig. 4. Correlation of the total-recovery/deformation ratio Ft with the nanoindentation parameter W/du.

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memory alloys and stainless steels correlates with the recoverable and residual penetration depths using the Rockwell test, which probes macrohardness [18]. As no thermal recovery was measured, it was assumed that the samples tested were purely austenitic. On the other hand, the method employed in the present study is applicable to samples in which both austenite and martensite might coexist. The present study, being motivated by the works of others [15 – 17] on using the Knoop indentation to determine the ratio H/E, has modified the Knoop Indentation Technique to study superelastic/plastic materials like NiTi without involving this ratio. In fact, the ratio H/E for NiTi specimens obtained from nanoindentation tests did not correlate with Re* [11]. For ordinary materials, the ratio H/E is an important parameter in indentation fracture toughness [19]. For shape memory alloys like NiTi, the total-recovery/ deformation ratio Ft plays a similar role.

4. Conclusions A study to search for a simple method to estimate the cavitation erosion resistance of NiTi using a modified Knoop Indentation Technique has been undertaken. The following conclusions are drawn: 1. Knoop indentation measurements, coupled with thermal recovery, have resulted in the establishment of a figure of merit, the total-recovery/ deformation ratio Ft. This ratio was found to correlate well with the cavitation erosion resistance for various heat-treated NiTi specimens and thus could be used in predicting or estimating the performance of NiTi against cavitation erosion. 2. The ratio Ft correlates with the cavitation erosion resistance because it reflects both superelasticity and pseudoplasticity, both of which being contributive in resisting cavitation attack.

Acknowledgements The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. PolyU 5252/03E). Support from

the infrastructure of the Hong Kong Polytechnic University is also acknowledged.

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