Fatigue coaxing experiments on a Zr-based bulk-metallic glass

Fatigue coaxing experiments on a Zr-based bulk-metallic glass

Available online at www.sciencedirect.com Scripta Materialia 62 (2010) 481–484 www.elsevier.com/locate/scriptamat Fatigue coaxing experiments on a Z...

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Available online at www.sciencedirect.com

Scripta Materialia 62 (2010) 481–484 www.elsevier.com/locate/scriptamat

Fatigue coaxing experiments on a Zr-based bulk-metallic glass Adel B. El-Shabasya,* and John J. Lewandowskib a

Department of Design and Production Engineering, Faculty of Engineering, Ain Shams University, 11517 Cairo, Egypt b Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, OH, USA Received 28 September 2009; revised 1 December 2009; accepted 8 December 2009 Available online 11 December 2009

Fatigue coaxing experiments were conducted on a Zr-based bulk-metallic glass using three-point bending at room temperature following the generation of an S/N curve under conventional testing methods at R = 0.1. Each new specimen was cyclically loaded initially just below the fatigue limit for different numbers of cycles prior to increasing the mean stress in increments of 15–25 MPa after each additional cycle increment until the sample failed. Possible implications and source(s) of the increased fatigue limit are discussed. Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Coaxing; High cycle fatigue; Step test; Zr-based bulk-metallic glass

The coaxing effect in fatigue refers to the improvement of the high cycle fatigue strength by the application of a gradually increasing stress amplitude, usually starting below the fatigue limit. An early demonstration of fatigue coaxing [1] on mild steel showed that cyclic loading just below the fatigue limit stress amplitude at 247 MPa for 250  106 cycles, followed by increasing stress amplitude increments of 3 MPa after every 10–18  106 cycles, increased the fatigue limit by 28%. Later experimental work by Sinclair [2] revealed that the magnitude of the coaxing effect was not consistent for all metal alloys, with no improvement obtained in an aluminum alloy and a 70/30 brass [2], suggesting that only materials capable of strain aging could be coaxed. Nisitani [3] reported a coaxing effect in the crack growth portion of the fatigue lifetime of steel specimens (S50C and S10C steels) and a 70/30 brass by cyclic loading just below their endurance limits to create non-propagating cracks and subsequently raising the stress amplitude in 5 MPa increments. This procedure reportedly led to a 5–10% increase in the endurance limits of the pre-cracked steels, and to a 5% increase in the fatigue strength of the pre-cracked brass. Coaxing effects on the fatigue behavior of other materials have been reported [4–8]. While relatively few studies of the fatigue behavior of Zr-based bulk-metallic glasses (BMGs) have been conducted [9–13], the authors are unaware of any

* Corresponding author. E-mail: [email protected]

studies to investigate whether a coaxing effect might exist in a Zr BMG. The zirconium-based BMG used in this investigation is commercially known as Vitreloy I, or Liquidmetal 1 (LM1). The composition was provided by the original material suppliers and was confirmed via wet chemistry as Zr41.8Ti12.9Ni9.5Cu12Be23.8, and is identical to the material tested previously [14–16]. The material was received in the form of plates with 2.7 mm thickness which had been double-disk-ground on both sides. The as-received plates were first sectioned into bars 9 mm in width so that the final dimensions of the samples were 2.7 mm  9 mm  50 mm. In order to avoid any crack initiation from sharp corners, all specimen corners were first rounded to about 0.7 mm radius using SiC grit papers. The bars were then initially polished with 600 grit paper to provide a consistent surface finish on each sample, followed by metallographic polishing to a 1 lm finish of all surfaces, including corners. The high cycle fatigue tests were conducted on the metallographically polished specimens using three-point bending (3 PB). All tests were conducted on MTS closed-loop servohydraulic equipment with MTS controllers and were cycled under load control using a sinusoidal waveform at a cyclic frequency of 20 Hz at room temperature with a load ratio, R = rmin/rmax, of 0.1 under tension–tension loading. The stress/life behavior was recorded and the maximum applied stress, r, at the tensile surface was calculated from beam theory. Fatigue lifetimes, Nf, were measured over a range of cyclic stress. Stress/life data are presented in terms of the stress

1359-6462/$ - see front matter Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2009.12.016


A. B. El-Shabasy, J. J. Lewandowski / Scripta Materialia 62 (2010) 481–484

amplitude, sa ¼ 12 ðsmax  smin Þ, normalized by the tensile strength, su, and plotted as a function of the number of cycles to failure, Nf, where one cycle is defined as a full stress reversal. High cycle fatigue (HCF) step (i.e. coaxing) tests were also performed at R = 0.1 on separate samples of the same material with the same test conditions. Each new specimen was initially subjected to cyclic loading just below the fatigue limit determined in the conventional tests. The mean stress was then increased in increments of 15–25 MPa after every cycle increment until the sample failed. In order to reduce the extreme duration of some of the tests which exceeded 38 days, three different cycle increments were used: 6  106 cycles, 1  106 cycles, or 0.5  106 cycles. The mirror finish on the sample surfaces enabled visual examination to detect any cracks after each loading step in the coaxing experiments. Fracture surfaces were examined using optical microscopy as well as scanning electron microscopy (SEM) on a Philips XL-30E SEM operated at 15 kV. The normalized stress amplitude, sa/su, is plotted in Figure 1 as a function of the number of cycles to failure, Nf, for the Zr-based BMG samples tested at a single stress level. In addition, the high cycle fatigue results from previous work on similar materials tested in either 4 PB [11] or 3 PB [12,13] are plotted for comparison. The fatigue limit value is somewhat higher in the present work (i.e. ranging from sa/su = 0.04–0.05) compared to early tests conducted in 4 PB (i.e. ranging from sa/su = 0.02–0.03) [11], likely due to the smaller sampling volume in the present 3 PB tests. However, more recent work on similar materials tested in 3 PB possess higher fatigue limit (e.g. ranging from sa/su = 0.1–0.21) and dependent on the processing details [12,13]. Because of the apparent variability in materials and test techniques utilized previously that may contribute to variability in fatigue behavior, Figure 2 replots only the conventional S/N data (i.e. Fig. 1) along with the step tests (i.e. coaxed) conducted on the present materials. The present Zr-based BMG appears to exhibit a relatively strong coaxing effect under the present test conditions. Coaxing appears to increase the mean stress at failure to 350–575 MPa, at least double the value of the fatigue limit obtained from conventional R = 0.1 3 PB fatigue tests in the present study, as well as that reported previously [11]. Furthermore, sa/su ranges from 0.15 to 0.25 for the coaxed samples shown in Figure

Figure 1. Stress/life results normalized by tensile strength including data from literature [11,12]. Arrows indicate that the samples were not broken.

Figure 2. Mean stress vs. number of cycles for BMG from present single fatigue tests compared to fatigue coaxing step tests. Solid arrow indicates that the sample was not broken.

2, exceeding all of the previously reported values [11–13]. Careful optical examination of the fractured samples revealed that failure originated away from the corner edges of the polished beams, with the extent of stable fatigue region increasing with increasing number of cycles to failure. Detailed SEM analysis of the fracture surface revealed a very distinct transition from stable fatiguecrack propagation to overload fracture, consistent with much previous work [9–14]. The high cycle fatigue life of the present BMG exhibits many similarities to previously published work [9– 13], although the fatigue limits are somewhat different between the various investigations. At least part of these differences are due to the differences in test geometry and sampling volume, as tests conducted under 3 PB, 4 PB, smooth tension, and notched tension provide very different sampling volumes for regions under peak stress, in addition to the possibility of different stress states and stress gradients between notched and smooth samples, and the presence/absence of any residual stresses from processing. Reported fatigue limits can be relatively low with respect to the strength (e.g. 0.02–0.05), depending on the material system and test techniques utilized. In addition to the differences in material and test technique, the material cleanliness may also be important. Recent work [17] on Fe-based BMGs has demonstrated an effect of both changes in chemistry and the presence/ absence of inclusions/porosity on both the magnitude of the notch toughness as well as the Weibull slope for multiple samples. Inclusion-initiated failure significantly decreased the magnitude of toughness while decreasing the Weibull slope. In the present fatigue tests conducted under 3 PB, the edges were carefully polished to a large radius, in addition to polishing the tensile surface. All samples exhibited fracture initiation at regions removed from the edges where stress concentrations and/or residual stresses may affect the results, as suggested in previous works [11–13]. The 3 PB fatigue geometry provides a peak tensile stress along the tensile surface of the sample along a line, in contrast to 4 PB which presents the peak tensile stress in an area on the tensile surface between the inner loading points. This difference in sampling volume could also affect the magnitude of the fatigue limit if defect-dominated initiation events are important.

A. B. El-Shabasy, J. J. Lewandowski / Scripta Materialia 62 (2010) 481–484

Smooth tension fatigue samples provide a nominally constant tensile stress throughout the volume of the gage length. The coaxing experiments were conducted on samples of identical chemistry, loading configuration, and sample geometry/preparation to the conventional tests on this identical material, summarized in Figure 1. Three of the coaxed samples exhibited significant increases to the fatigue limit, as shown in Figure 2. Four additional samples subsequently tested in an identical manner similarly exhibited increases to the mean stress at the fatigue limit, although more modest in extent, ranging from 125 to 215 MPa. Fracture surface examination of the coaxed samples revealed that samples exhibiting the greatest coaxing effect possessed fracture surface features very different from conventionally tested S/N fatigue samples summarized in Figure 1, as well as different from those coaxed samples that exhibited more modest changes to the fatigue limit, as discussed below. The presence of a coaxing effect requires local changes to the material in the regions subjected to tensile stress [1–8]. Interestingly, recent work [18] on cyclical loading of a BMG via cyclic nano-indentation testing has revealed cyclic hardening in local regions. While the mechanisms of such nano-hardening are still under discussion, one effect of this phenomenon is to inhibit the initiation of shear bands at low stresses. Early shear band initiation provides one source of fatigue damage/ initiation, while porosity, inclusions, and devitrified regions similarly may compromise the damage tolerance. The present results which appear to illustrate a beneficial effect of coaxing on this BMG could have arisen due to the globally elastic stresses producing local deactivation of potential flaw nuclei. These potential flaw nuclei could be associated either with shear transformation zones at the smallest scale, while local hardening around other potential defect sites (inclusions, pores, etc.) would inhibit shear band initiation and fracture from these regions. As indicated above, examination of the fatigue initiation sites in the “coaxed” samples exhibiting large beneficial effects was significantly different from that of the samples fractured under conventional fatigue conditions, or those exhibiting lesser effects of the “coaxing” treatments. Figure 3a shows the representative appearance of the fatigue initiation site near the tensile surface in various “coaxed” samples summarized in Figure 2, while Figure 3b shows representative initiation regions


in non-coaxed samples. The differences in appearance are striking and suggest some local changes to the mechanical behavior of the material in the vicinity of the fatigue initiation and initial crack growth, despite the presence of some inclusions in the coaxed samples. The fracture surface appearance and increase in the fatigue limit of the coaxed samples are consistent with that of a higher strength and/or more relaxed glass. Interestingly, recent work [13] has shown that structural relaxation via annealing can improve the fatigue limit of a Zr-based BMG. In order to assess this possibility in the present materials, additional samples of identical size and chemistry were annealed at either 330 °C or 350 °C and then tested in conventional 3 PB fatigue at R = 0.1 at room temperature. The relaxed structure created by annealing is known to inhibit shear banding and change the elastic constants in a manner that has been shown to reduce the toughness of the relaxed glass in comparison to an as-cast sample [15,19]. The effects of these kinds of annealing treatments have been previously characterized by XRD and DSC in the present materials [15,19] as well as similar materials [13]. Figure 4 shows the S/N plots of the presently annealed samples in comparison to that of the as-cast conventional S/N behavior on the present materials previously shown in Figure 1. Also included is other recent work [13] that clearly shows that annealing to relax the glass can improve the S/N behavior, presumably via the inhibition of shear band

Figure 4. Stress/life results for as-cast and annealed samples normalized by tensile strength, including data from literature [11,13]. Arrows indicate that the samples were not broken.

Figure 3. (a) Typical appearance of fracture initiation region near the tensile surface of “coaxed” samples exhibiting large increases to the fatigue limit in Figure 2. (b) Typical appearance of fracture initiation in non-coaxed samples.


A. B. El-Shabasy, J. J. Lewandowski / Scripta Materialia 62 (2010) 481–484

dominated initiation events, even in the presence of inclusions. However, other recent work [20] has shown that annealing to the point of devitrification can produce severe degradation to the S/N fatigue behavior due to the preferential nucleation of cracks at such heterogeneities combined with the lower fracture toughness [15,19] of such materials. The fracture surface appearance of the annealed/relaxed glass in the region of initiation showed similar features to that of the coaxed samples shown in Figure 4. However, the annealed/relaxed glass samples do possess lower toughness [13,15,19], hence the fatigue-crack growth regime is quite different in extent to both the ascast and coaxed samples. Estimates of the toughness obtained from the fracture surface observations of the extent of fatigue and the maximum cyclic load at failure revealed toughnesses in the range 19–26 MPa m1/2 for the as-cast and coaxed samples, and only 4.2–10 MPa m1/2 for the annealed samples tested presently. The calculated toughness of the as-cast materials are similar to the large database that is developing on material with this identical chemistry tested under fatigue pre-cracked conditions [14–16]. The coaxed samples exhibit similar values for toughness as the as-cast materials, consistent with the potential benefits of coaxing in the fatigue initiation stage in such tests. While the annealed (but not crystallized) samples exhibited improved S/N behavior, the fracture toughness calculated from the samples fractured in the S/N experiments was significantly reduced by the annealing at both 330 and 350 °C, consistent with much previous work [13,15,19]. This observation is consistent with a delay in the nucleation of fatigue cracks in the annealed (but not crystallized) samples, similar to that proposed in recent work [13] on annealed (but not crystallized) samples. The relaxed glass is known to exhibit reduced shear banding and this may provide some benefit to the fatigue initiation process in the absence of large inclusions, porosity, and/or devitrified regions. While the above observations of improved S/N behavior appear to be due to some beneficial effects of coaxing in the as-cast materials, the statistical nature of fatigue and the importance of defects in reducing the fatigue life and Weibull modulus of the fatigue lifetimes cannot be ignored. It is possible that the higher fatigue life of the coaxed samples is simply due to sampling “cleaner” regions of the BMG, facilitated by the reduced fatigue sampling volume produced in 3 PB testing. Much additional work is needed to confirm the presence of, and possible sources of, this apparent coaxing effect in this and similar BMGs. This may be particularly important in small-scale metallic glass structures (e.g. MEMs, NEMs devices) as suggested by the recent nano-indentation studies [18] that appear to demonstrate local cyclic hardening. To conclude, the 3 PB S/N fatigue behavior of a Zr BMG was compared to the behavior of identical materials given fatigue coaxing as well as those annealed to relax, but not crystallize, the BMG. Identical samples

given a fatigue coaxing cycle often exhibited significant increases to the fatigue limit, with corresponding changes to the fracture surface morphology in the fatigue initiation regions. Annealed (but not crystallized) samples were tested and also showed some benefits to the S/N behavior as shown previously [13], suggesting that the prevention of fatigue-crack nucleation via shear banding may provide some benefits to the S/N behavior, whether this arises due to fatigue coaxing or via annealing. The detriments of annealing to the toughness will produce catastrophic fracture at smaller critical crack lengths and/or stresses, however. Additional work is needed in order to determine the source(s) of the improved fatigue limit in the coaxed samples in addition to determining the generality of such observations on other BMG systems, sample sizes, and fatigue test conditions. The authors thank NSF-OISE-0710957 for partial support of this work. Partial support of ONRN00014-07-1-0839 via the DARPA SAM program is also gratefully acknowledged. [1] H.J. Gough, The Fatigue of Metals, Scott, Greenwood and Son, London, 1924, p. 108. [2] G. Sinclair, Proc. ASTM 52 (1952) 743. [3] H. Nisitani, Z. Yamaguchi, Trans. Jpn. Soc. Mech. Eng. 45 (1979) 260. [4] S. Ishihara, A.J. McEvily, Scr. Mater. 40 (1999) 617. [5] B.A. Lerch, S.L. Draper, J.M. Pereira, Metall. Mater. Trans. A 33A (2002) 3871. [6] T. Nicholas, Fatigue Fract. Eng. Mater. Struct. 25 (2002) 861. [7] L. Xi, Z. Songlin, Int. J. Fatigue 31 (2009) 341. [8] Y. Murakami, Y. Tazunoki, T. Endo, Metall. Trans. A 15A (1984) 2029. [9] G.Y. Wang, P.K. Liaw, A. Peker, B. Yang, M.L. Benson, W.H. Peter, L. Huang, M. Freels, R.A. Buchanan, C.T. Liu, C.R. Brooks, Intermetallics 13 (2005) 429. [10] B.C. Menzel, R.H. Dauskardt, Acta Mater. 54 (2006) 935. [11] C.J. Gilbert, J.M. Lippmann, R.O. Ritchie, Scr. Mater. 38 (1998) 537. [12] M.E. Launey, R. Busch, J.J. Kruzic, Scr. Mater. 54 (2006) 483. [13] M.E. Launey, R. Busch, J.J. Kruzic, Acta Mater. 56 (2008) 500. [14] P. Lowhaphandu, J.J. Lewandowski, Scr. Mater. 38 (1998) 1811. [15] J.J. Lewandowski, Mater. Trans. JIM 42 (2001) 633. [16] H.A. Hassan, Laszlo Kecskes, J.J. Lewandowski, Metall. Mater. Trans. A 39 (2008) 2077. [17] A. Shamimi Nouri, X.J. Gu, S.J. Poon, G.J. Shiflet, J.J. Lewandowski, Philos. Mag. Lett. 88 (2008) 853. [18] C.E. Packard, L.M. Witmer, C.A. Schuh, Appl. Phys. Lett. 92 (2008) 171911. [19] J.J. Lewandowski, W.H. Wang, A.L. Greer, Philos. Mag. Lett. 85 (2005) 77. [20] G.Y. Wang, P.K. Liaw, Y. Yokoyama, M. Freels, R.A. Buchanan, A. Inoue, C.R. Brooks, J. Mater. Res. 22 (2007) 493.