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Fatigue properties of Galfenol steels
International Journal of Fatigue 128 (2019) 105177
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International Journal of Fatigue journal homepage: www.elsevier.com/locate/ijfatigue
Fatigue properties of Galfenol steels David I. Holsworth, David L. DuQuesnay
T
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Royal Military College of Canada, Canada
A R T I C LE I N FO
A B S T R A C T
Keywords: Galfenol Gallium Fatigue Porosity Functional material
The fatigue behaviour of three Galfenol steels, with gallium contents of 12.4, 14.9 and 18.4 at.% was investigated. Specimens were machined from as-received 25.4 mm diameter Bridgman rods and were tested under constant amplitude R = −1 cyclical loading. Results were highly scattered with one standard deviation on life being of one order of magnitude. The as-received material was also very porous, with macropores as large as 500 μm. Fatigue results of each of the three alloys were approximately the same. It is suggested that the inherent porosity imparted on the as-extruded rods is the main source of the scatter.
1. Introduction 1.1. Background Magnetostriction. Galfenol is a relatively new functional Fe-based alloy best known for its unusual ability to be strained by application of applied magnetic fields. Such strains, most commonly referred to as magnetostrictive strains (λ ), are caused by the reorientation of magnetic domains within the Galfenol. Because domain reorientation may only occur up to 90 degrees, λ is limited. Such limits in strain are called saturated strain (λs ). Seeing as λs are small, on the order of 10−5 to 10−4, they are typically described in parts per million (ppm). Evolution of Magnetostrictive materials. First documentation of the magnetostrictive effect was reported by Joule in 1847 where it was determined that steel could be strained up to 30 ppm [1] by applied magnetic fields. First applications of magnetostrictive metals was seen only a century later, during WWII, when nickel-based alloys were used in newly invented sensing applications, sonar technology [2]. These Ni alloys possess λs nearly twice that of ordinary steel alloys. Since WWII, technological advances have led to the discovery of new materials with magnetostrictive potentials orders of magnitude larger than those observed in Ni-based alloys. Piezoceramics such as lead zirconate titanate (PZT) and Terfenol-D (Tb.27Dy.73Fe1.95)1 are examples of such advanced materials with λs as high as 2000 ppm [3]. . 1.2. Evolution of Galfenol Although PZTs and Terfenol-D possess λs suitable for the design of highly responsive sensing and actuating devices, these materials are
unfortunately inherently brittle and expensive to manufacture in the single crystal form. As such, Galfenol, a steel-like material discovered by research efforts at the U.S. Navy’s Naval Ordnance Laboratory (N.O.L.), has been found to be a potentially promising material for use in the design of instrumentation destined for use in explosive or rugged environments [4]. It was found by Clark et al. that single crystals of Galfenol, Fe-Ga alloys containing up to ∼20 at.% gallium content could produce magnetostrictive potentials as high as 400 ppm [5]. Given that this alloy also possesses mechanical properties and machineability comparable to that of steel, Galfenol has been found to be a good prospective replacement for more fragile magnetostrictive materials. Table 1 lists some magnetostrictive alloys along with approximate values of key mechanical properties. For Galfenol to be an economically viable replacement for more expensive and fragile alternatives, it needs to be manufactured in bulk, in the polycrystalline form. The major challenge with producing Galfenol in bulk remains the production of uniform grain orientation, a requirement for utilization of magnetostrictive properties. The three most used manufacturing methods for producing preferential grain orientation in bulk Galfenol are wire extrusions [8], sequential hot rolling processes [9] and directional solidification (DS) techniques such as Bridgman or Stockbarger extrusions [10]. The Zone Melt Crystal Growth (ZMCG) method developed by Summers et al. consists effectively of a modified Bridgman rod extrusion technique shown to produce λs as high as 220 ±25 ppm in Galfenol with Fe81.6 Ga18.4 [11]. These levels are over 75% of λs obtained in single crystals with the same composition. Given ZMCG has been shown to produce the largest λs in bulk Galfenol, it has been selected as the processing method for the samples obtained in this project.
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Corresponding author. E-mail address: [email protected] (D.L. DuQuesnay). 1 Subscript numbers indicate alloy compositions in atomic percent (at.%.). https://doi.org/10.1016/j.ijfatigue.2019.06.037 Received 3 May 2019; Received in revised form 3 June 2019; Accepted 25 June 2019 Available online 29 June 2019 0142-1123/ Crown Copyright © 2019 Published by Elsevier Ltd. All rights reserved.
International Journal of Fatigue 128 (2019) 105177
D.I. Holsworth and D.L. DuQuesnay
Table 1 Comparison of approximate material properties for Galfenol and other magnetostrictive materials [3,6,7]. Material Magnetostriction (ppm) Modulus of elasticity (GPa) Tensile strength (MPa) Ductile/Brittle
Terfenol-D
Galfenol
Ni alloys
Steel
2000 50 30 Brittle
400 200 500 Ductile
50 207 60–300 Ductile
30 200 400–1200 Ductile
1.3. Mechanical properties of Galfenol Complementary work on the mechanical behaviour of Galfenol was conducted by Kellogg [12] and Brooks et al. [13]. It was found that binary Galfenol with no carbon additions undergoes ductile-to-brittle (DTB) transition at gallium contents as low as 5 at.%. It was also found that increases in gallium content resulted in linear decreases in stiffness and tensile strength. Carbon additions to Galfenol steels was found to enhance intergranular cohesion hereby improving mechanical properties drastically. DTB was increased to 15.5–18.4 at.% and hardness was increased by 20%. With a composition of Fe81.6Ga18.4 , polycrystalline Galfenol steel produced by ZMCG was shown to possess tensile strengths of 370 MPa, a Young’s Modulus ranging from 72.4–86.3 GPa and ductility of 0.81–1.2% [13].
Fig. 1. Comparison of a as-received Galfenol sample and a completed fatigue specimen. Length of the cylindrical specimens was 53 mm and the specimen diameters were 4.5 and 8.7 mm, as depicted.
increase strength and ductility of binary Galfenol steel [20]. Clark et al. showed that λs was maximized in binary Galfenol with ∼19 at.% Ga [21] and it was also found that Fe100 − xGax for x ⩾ ∼ 20 proved too brittle for bulk production [9]. As such and because it was desired to broaden the spectrum of gallium contents tested, the third alloy tested possessed much lower gallium contents with a binary alloy composition of Fe87.4Ga12.4 . Specimens with this alloy composition were labelled as belonging to the M Series.
1.4. Motivations and objectives Galfenol steels have been considered for the design of more rugged and durable sensing and actuating devices because of their enhanced mechanical properties [14]. Given its strength and toughness, Galfenol has also been considered for design of new engineering applications, including energy harvesting and active vibration control [15–17]. Components in such applications experience cyclic loading. Therefore, the fatigue behaviour of Galfenol needs to be examined. As such, the two objectives of this work are to investigate the fatigue behaviour of three DS polycrystalline Galfenol steels and to determine the effect of gallium content on fatigue properties of the investigated alloys.
2.2. Specimen preparation and preliminary tests As-received rods were quartered along their length and each quartered segment was then further machined to produce a total of eight specimens per rod (see Fig. 1). Given the brittle nature of the as-received material, approximately 10% of the specimens fractured and were wasted during machining. Once machined, the gauge sections of the specimens were prepared in accordance with standards detailed in the Fatigue Handbook [22] and ASTM E466 [23]. Based on work published by Menezes et al., it is expected that the 400 grit emery paper used to polish the gauge sections produced an arithmetic mean roughness value (Ra ) of 0.3 μm [24]. Before fatigue testing, two hardness tests were conducted on each grip end of each specimen. Results of these tests are presented in Section 3.
2. Materials and methodology 2.1. Materials As-received Galfenol samples for this project were procured from ETREMA Inc. [3]. The DS cylindrical rods grown by ZMCG were 24 mm in diameter and 125 mm in length. Because carbon additions are known to increase strength and hardness of Galfenol [3] as well as enhance λs [18], all Galfenol samples for this project were Galfenol steels possessing 0.15 at.% carbon content. For the sake of brevity, carbon content will not be indicated when describing alloy contents in this paper. The first Galfenol steel alloy tested was a binary Galfenol steel possessing the same alloy composition as the polycrystalline Galfenol steel tested by Brooks et al. [13], Fe81.6Ga18.4 . Specimens machined from this alloy were labelled as belonging to the F series and data points belonging to this series are shown as red square boxes in all figures in this paper. As stated in Section 1.2, this alloy composition had been shown to possess the highest levels of λs for bulk polycrystalline Galfenol. The second alloy tested was Fe82.8Ga14.9Cr2.2 and specimens with this alloy composition are labelled as belonging to the C series. Data points from this series are presented as green circles in all figures. As with conventional stainless steel alloys, the chromium additions in Galfenol have been shown to enhance corrosion resistance, a highly desired characteristic for naval applications. Mechanical characterization of similar stainless Galfenol steels had previously been investigated by Nolting et al. [19]. In addition to enhancing corrosion resistance, it has also been shown that tertiary alloying elements such as Cr tend to
2.3. Testing Equipment. The equipment used in this project consisted of a servohydraulic loading frame and a computer to control and monitor load cycle amplitudes and cycle frequency. The electrohydraulic frame was equipped with a load cell and a linear variable displacement transducer (LVDT) used to monitor the displacement of the lower grip. A computer software was used to control the loading patterns which would produce the desired stress amplitudes (σa ), count the number of applied cycles as well as to monitor and minimize deviations and errors on the applied loads. Test details. Tests were conducted in laboratory air at an average temperature of 25 °C. Fatigue testing was performed in constant amplitude fully reversed load control, with applied stress derived by dividing the applied load by the initial gauge section area. Initial loading frequency was 10 Hz for the first few hundred cycles of loading and was then raised to 20 Hz for most specimens. Steady-state testing frequency was achieved at around 1000 cycles. The maximum testing frequency of 20 Hz was chosen so as to limit loading errors imparted on the specimens to ±3.5 MPa. Specimens tested at stresses above 400 MPa were tested at 10–15 Hz in order to also keep loading errors below ±3.5 MPa. 2
International Journal of Fatigue 128 (2019) 105177
D.I. Holsworth and D.L. DuQuesnay
Table 2 Average surface hardness of the tested Galfenol steels. Series
# specimens tested
Rockwell B hardness∗
56 19 60
99.7 ± 2.6 93.0 ± 3.6 90.2 ± 3.4
F C M ∗
± one standard deviation.
Fig. 2. Visible surface-connected porosity on a machined Galfenol rod.
Yield stress. Given that material costs were significant, effectively all machined specimens were used for fatigue testing. Although standard tensile strength testing practices could not be followed, the load–displacement path of the first cycle was captured for specimens tested near the yield stress (σy ). For such specimens, loading was done slowly for one fully-reversed cycle. After one full cycle, cyclical loading was started and controlled by software following standard fatigue testing practices. Because use of extensometers within the gauge section of the specimens was deemed to potentially cause surface damage, the stress–strain relationship in the first cycle could not be captured.
Fig. 4. Stress-displacement paths for specimens from each of the three alloys tested. Making the gross assumption that the majority of the specimen elongation occurs within the gauge length would produce an estimate that a displacement d = 0.1 mm equates to approximately 0.2% strain.
3. Results and discussions 3.1. Mechanical properties Hardness. The average of the measured hardness calculated for each alloy is tabulated in Table 2. Yield stress. Audible pings or metallic crunching sounds were heard for many of the tests, especially in the first few cycles for specimens loaded near the material’s estimated σy . These pinging sounds were also heard when gripping the specimens within the mechanical frame grips. Examples of stress-displacement paths for specimens from each alloy tested are depicted in Fig. 4. These results indicate that, as expected, σy increased with added Ga content. Since increases in gallium content have also been shown to increase embrittlement in Galfenol [13], σa for specimens from the more brittle F series were limited to the first audible signs of yielding, around 500 MPa. The C series was found to yield at 475 MPa ±20 and the softer M series, at 375 MPa ±20 MPa. These values are in line with what was presented earlier, in Section 1.3. A few F series specimens were tested in monotonic tension to failure with an axial extensometer mounted to measure strain. It is interesting to note that for F and M series, both binary Galfenol steels, material flow beyond the point of yielding was accompanied with sequential crunching or pinging sounds. Kellogg et al. observed similar sounds and discontinuous stress–strain behaviour [9]. It was suggested in this study that this behaviour was caused by deformation twinning. The pinging observed in this study coincides with the discontinuities in the stress-displacement paths captured in Fig. 4 and the stress–strain curve for an F series specimen shown in Fig. 5. This observation was not made for the C series Galfenol steel containing tertiary Cr additions. If this discontinuous behaviour during material flow is linked with propagation of damage at inter-grain boundaries, results would suggest that Cr additions enhance inter-grain cohesion. This reinforces recent observations from Nolting and Summers [20] which demonstrate that tertiary alloying additions such as Cr not only enhance strength and ductility of binary Galfenol steels but also help transition the predominant fracture mechanism from inter-granular to inra-granular fracture mechanisms. Further evaluation of the effects of Cr additions to inter-grain cohesion and fracture mechanisms was not pursued as it was not within the scope of this study.
2.4. Post-processing As can be seen in Fig. 2, some of as-received material possessed significant surface-connected porosity. Further observation of micrographic samples showed that porosity was also present within and throughout the as-received material. After fatigue failure, all fracture surfaces were examined and micrographed under magnification with an optical microscope. Fracture surfaces of specimens with abnormally long or short lives were further investigated with the aid of Scanning Electron Microscopy (SEM). Specimen fracture surfaces were prepared as depicted in Fig. 3b. for observation by SEM. Microstructure texture and porosity were further investigated on metallographic samples produced from sectioned halves of failed fatigue specimens, as depicted in Fig. 3a. Approximately 5% of the polished surface area of the metallogrpahic samples was captured by micrograph for analysis.
Fig. 3. Specimen sectioning for post-processing where the shadowed area indicates the examination surface. a) Preparation of grip for metallographic observation. The halved grips provided an observation surface area measuring approximately 8 mm × 20 mm, 160 mm2. b) Sectioning of fracture surfaces for investigation by microscope or SEM. Note that some of the long-lasting specimens which endured long fatigue lives, with Nf > 106 , were removed from testing before failure and entire gauge sections could be sectioned for observation. 3
International Journal of Fatigue 128 (2019) 105177
D.I. Holsworth and D.L. DuQuesnay
Fig. 5. Typical stress–strain path for F series specimens strained well beyond the point of yielding. The relative stress depicted is the ratio of applied stress over yield stress (σ / σy ).
Fig. 6. Stress-life (S-Nf ) data for all three as-received Galfenol steels with R = −1. Labelled solid lines and accompanying shadowed areas indicate mean S-N curves ± 2 standard deviations on log(Nf ). The error on σa is ± 3.5 MPa, smaller than the data point symbols. ∗ indicates the presence of two data points and arrows indicate run-on data points.
Fig. 7. Relative Stress-life (S-Nf ) data for all three as-received Galfenol steels with R = −1. labelled solid lines and accompanying shadowed areas indicate mean S-N curves ± 2 standard deviations on log(Nf ). The error on σa is ± 3.5 MPa, smaller than the data point symbols. ∗ indicates the presence of two data points and arrows indicate run-on data points.
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International Journal of Fatigue 128 (2019) 105177
D.I. Holsworth and D.L. DuQuesnay
Fig. 8. Mixed mode inter and intragranular fracture surfaces of F and C series Galfenol.
Fig. 9. Suspected sources of failure in short lived fatigue specimens.
Fig. 10. Micrograph of a particularly large surface connected pore. Fig. 11. Micrograph of a metallographic sample from specimen F46.
3.2. Fatigue results
Nf as the dependant variable. It was also assumed that the data was scattered in accordance with a log-normal Gaussian distribution. As can be seen in Fig. 6, there was significant scatter in the test results. The largest scatter on Nf was observed on the C series Galfenol with one standard deviation on life being approximately one order of magnitude (log (Mean) ±100.97 ). Given that the scatter bands for the M
Fig. 6 includes the fatigue test results obtained for fully reversed loading cycles (R = −1) on all three Galfenol steels. The test was conducted in load control and as such, even if there was up to 3.5 MPa error on the stress amplitude (σa ), the applied stress was effectively a controlled variable. The mean curve fits were therefore established with 5
International Journal of Fatigue 128 (2019) 105177
D.I. Holsworth and D.L. DuQuesnay
Fig. 12. Measured surface area % porosity of metallographic specimens from F and C series alloys. Hollow bars represent measurements made on one micrograph, 1.44 mm2 of the total sectioned surface area. The dark bars represent the average surface area porosity measured for each fatigue specimen, over the six micrographs (over 6.862 mm2 of surface area).
Fig. 13. Pore size distribution on F and C Series Galfenol over all of the micrographed surface area, 27.45 mm2.
were intergranular. This suggests that weak grain boundaries contributed to hastened crack propagation. For the F series Galfenol which is stronger and stiffer than the other tested alloys, fracture was predominantly intragranular. As can be seen in Fig. 8, crack propagation in the F series was limited to approximately 100 μm at the time of fracture while the more ductile C series specimens produced critical crack lengths closer to 500 μm. Fractographs were also useful in identifying potential crack initiation sites, as was the case for specimen C2 in Fig. 8a. In many instances, material artefacts such as macropores and grain boundary irregularities known to promote premature crack initiation were identified. This was especially true in the most short-lived specimens tested, as evidenced by the fractographs in Figs. 9 and 10. Other examples of surface-connected porosity were also observed on the surface of as-received samples (Fig. 2) and many of the micrographs obtained from metallographic samples, such as the micrograph in Fig. 10. Porosity. In an effort to further describe and quantify the degree of porosity in the as-received material, micrographs of polished metallographic samples were captured. A minimum of six micrographs each covering 1.14 mm2 were obtained per polished metallographic sample. A minimum of four fatigue specimens for the F and C series Galfenol were used to produce metallographic samples. Micrographs were then analyzed with the aid of image analysis software to quantify shape, size and number of pores. An example of how predominant the inherent porosity could be is depicted in Fig. 11. Evidence of porosity was captured in effectively all samples, regardless of the alloy. Fig. 12 summarizes the total percentage of surface area that was deemed to be porous in each micrograph and the pore size distribution by alloy is presented in Fig. 13. . ASTM standard E446 indicates that pore size description in cast steels is based on qualitative assessments [31]. Some further define
and F series fall within the larger C series scatter band, it is suggested that gallium content ranging from 12.4–18.4 at.% does not affect the fatigue behaviour of DS polycrystalline Galfenol steels. This being said, when analyzing the results as a function of relative stress amplitude (σa / σy ), the M series outperforms the higher Ga content Galfenol steels by nearly one order of magnitude on life as can be seen in Fig. 7. The scatter in these results was higher than what is typically expected for a steel-like alloy. Given that it has been well documented that porosity in steel is linked with reductions in mechanical properties [25] and fatigue behaviour [26–30] and that surface-connected pores were visible on parts of the as-received samples, it was believed that inherent porosity was the main contributor to the data scatter. 3.3. Post-processing of specimens Upon failure, specimen fracture surfaces were examined under microscope with up to x75 magnification. Specimens with especially long and short fatigue lives were further examined with SEM. Polished but unetched metallographic samples were also obtained from these same specimens so as to obtain information on material porosity and texture, as detailed in Section 2.4. Fracture surfaces. Fatigue striations and clamshell or thumbnail formations typically visible in failed fatigue specimens were only observed on the fracture surfaces of a few of the longer lasting specimens. One such rare example was captured on the fracture surface of specimen C2, relatively one of the longest lasting specimens from the C series, as depicted in Fig. 6 (σa = 350 MPa and Nf = 700 618 cycles). Thumbnail markings from the fatigue crack propagation can be seen at the bottom of the fractograph in Fig. 8a. Failure was very brittle in nature for the majority of the specimens. Although some specimens demonstrated intergranular failure, all specimens failed in mixed mode with the majority of failure being intragranular. Crack propagation emanating from crack initiation sites 6
International Journal of Fatigue 128 (2019) 105177
D.I. Holsworth and D.L. DuQuesnay
macroporosity as detectable pores measuring over 20 μm [28]. It has also been documented that microporosity has little to no effect on fatigue properties of steels when compared to macroporosity [26]. Both Galfenol series possess large numbers of pores possessing Feret mean diameters well over 20 μm as well as surface area porosity of approximately 0.5%. Given the evidence provided in Figs. 12 and 13, it is therefore very likely that porosity is linked with the data scatter observed in Figs. 6 and 7. It is therefore suggested that a reduction in porosity would likely lead to increases in fatigue performance. Potential solutions. Hot iso-static pressing (HIP) is a well known mechanical process known to remove or reduce porosity in metals. Use of HIP on DS polycrystalline Galfenol might lead to significant reduction in porosity and therefore, enhancement in fatigue behaviour. Alternatively, bulk production of polycrystalline Galfenol by sequential hot rolling processes have also be shown to produce material with little porosity [9]. It is therefore suggested that fatigue behaviour of hot rolled and HIP DS Galfenol steels be conducted to further investigate the role of porosity in the observed fatigue results. It is suspected that fatigue life results for Galfenol specimens with little to no macroporosity would produce a more traditional stress-life curve than the results presented in Figs. 6 and 7, results with less scatter which align approximately with the right edge of the presented scatter bands. Another deduction from this work is that the range of magnetostrictive strains in the most promising of Galfenol steels is relatively limited with λs well below 400 ppm for the largest λ recorded on single crystal Galfenol [5]. Considering that polycrystalline Galfenol possesses a Young’s modulus ranging from 72 to 86 GPa [13], the stresses associated with fully saturated magnetostrictive strains would be small, well below 80 MPa. With careful design considerations, applications which could limit stress amplitudes to such levels could endure with 97.5% assurance at least 105, 107 and 108 cycles for C, F and M series Galfenol, respectively (based on two standard deviations removed on mean life calculations). Galfenol steels could still be durable enough for many engineering applications which require cyclical loading.
[2] Scott RS. Measurements on Galfenol material and transducers Ph.D. thesis The Pennsylvania State University; 2008. [3] Etrema inc. doi:24 Oct 17. https://web.archive.org/web/20160304003546/http:// www.etrema.com/terfenol-d/. [4] Atulasimha J. Characterization and modeling of the magnetomechanical behavior of iron-gallium alloys Ph.D. thesis University of Maryland; 2006. [5] Clark A, Restorff J, Wun-Fogle M, Lograsso T, Schlagel D. Magnetostrictive properties of body-centered cubic Fe-Ga and Fe-Ga-Al alloys. IEEE Trans Magn 2000;36(5):3238–40. [6] Atulasimha J, Flatau A, Summers E. Characterization and modeling of the magnetomechanical behavior of polycrystalline iron-gallium alloys. Smart Mater Struct 2007;16(4):1265. [7] Callister WD, Rethwisch DG. 8th ed. Materials science and engineering: an introduction vol. 7. New York: Wiley; 2007. [8] Boesenberg A, Restorff J, Wun-Fogle M, Sailsbury H, Summers E. Texture development in Galfenol wire. J Appl Phys 2013;113(17). https://doi.org/10.1063/1. 4794186. [9] Kellogg RA. Development and modeling of iron-gallium alloys Ph.D. thesis Iowa State University; 2003. [10] Glicksman ME. Principles of solidification: an introduction to modern casting and crystal growth concepts. Springer Science & Business Media; 2010. [11] Summers E, Lograsso T, Snodgrass J, Slaughter J. Magnetic and mechanical properties of polycrystalline Galfenol. Smart structures and materials. International Society for Optics and Photonics; 2004. p. 448–59. [12] Kellogg R, Russell A, Lograsso T, Flatau A, Clark A, Wun-Fogle M. Tensile properties of magnetostrictive iron–gallium alloys. Acta Mater 2004;52(17):5043–50. [13] Brooks M, Summers E, Lograsso T. Gallium content effects in low carbon steels. AIST Steel properties & applications conference proceedings and materials science & technology. ASM International; 2009. p. 681–92. [14] Restorff JB, Wun-Fogle M, Summers E, Kaufman R, Jones NJ. Development of a Galfenol actuator for operation under tension. Smart Mater Struct 2019;28(3):035013. [15] Ueno T, Yamada S. Performance of energy harvester using Fe-Ga alloy in free vibration. IEEE Trans Magn 2011;47(10):2407–9. [16] Berbyuk V. Vibration energy harvesting using Galfenol based transducer. In: Sodano Henry, editor. Active and passive smart structures and integrated systems 2013, Proc. of SPIE Vol. 8688; 2013. p. 86881. [17] Dezza FC, Cinquemani S, Mauri M, Maglio M, Resta F. A model of magnetostrictive actuators for active vibration control. 2011 IEEE international symposium on industrial electronics (ISIE). IEEE; 2011. p. 847–52. [18] Clark A, Restorff J, Wun-Fogle M, Hathaway K, Lograsso T, Huang M, et al. Magnetostriction of ternary Fe-Ga-x alloys (x = C, Y, Cr, Mn Co, Rh). J Appl Phys 2007;101(9):09C507. [19] Nolting A, Cheng L. Mechanical characterisation of Galfenol-based magnetostrictive alloys. In: Proceedings of the 12th international conference on fracture, Ottawa; 2009. p. 12–17. [20] Nolting A, Summers E. Tensile properties of binary and alloyed Galfenol. J Mater Sci 2015;50(15):5136–44. [21] Clark A, Wun-Fogle M, Restorff J, Lograsso T. Magnetostrictive properties of Galfenol alloys under compressive stress. Mater Trans 2002;43(5):881–6. [22] Swanson SR. Handbook of fatigue testing. Handbook of fatigue testing. American Society for Testing and Materials; 1974. [23] Designation A. E466-07: Standard practice for conducting force controlled constant amplitude axial fatigue tests of metallic materials; 2007. [24] Menezes PL, Kailas SV, et al. Studies on friction and transfer layer using inclined scratch. Tribology and interface engineering series, vol. 51. Elsevier; 2006. p. 262–79. [25] Fleck N, Smith R. Effect of density on tensile strength, fracture toughness, and fatigue crack propagation behaviour of sintered steel. Powder Metall 1981;3:121–5. [26] Chawla N, Deng X. Microstructure and mechanical behavior of porous sintered steels. Mater Sci Eng 2005;390:98–112. [27] DasGupta P, Queeney R. Fatigue crack growth rates in a porous metal. Int J Fatigue 1980;0142:113–7. [28] Sigl K, Hardin RH, Stephens I, Beckrmann C. Fatigue of 8630 cast steel in the presence of porosity. Int J Cast Met Res 2004;17(3):130–46. [29] Murakami Y. Metal fatigue: effects of small defects and nonmetallic inclusions. Elsevier; 2002. [30] Klesnil M, Holzmann M, Lukas P, Rys P. Some aspects of fatigue process in lowcarbon steel. J Iron Steel Inst 1965;203:47. [31] Designation A. E446: Nondestructive testing.
4. Conclusions DS Galfenol steels produced by ZMCG with gallium contents varying between 12.4 and 18.4 at.% and trace carbon contents below 0.15 at.% were tested to fatigue failure using fully-reversed loading cycles. The alloys tested were composed of Fe81.6Ga18.4 , Fe82.8Ga14.9Cr2.2 and Fe87.6 Ga12.4 , F, C and M series, respectively. The as-received material was laden with surface connected pores and further investigation of metallographic samples showed that the as-received samples also possessed significant internal macroporosity known to affect fatigue behaviour in metals. The fatigue tests produced scattered results which are likely linked with the observed porosity. It is expected that the same fatigue tests conducted on less porous material would produce less scattered results which would align closer to the longer lasting fatigue data obtained in this work. References [1] Joule JP. XVII. On the effects of magnetism upon the dimensions of iron and steel bars. London, Edinburgh, Dublin Philosoph Mag J Sci 1847;30(199):76–87.
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International Journal of Fatigue journal homepage: www.elsevier.com/locate/ijfatigue
Fatigue properties of Galfenol steels David I. Holsworth, David L. DuQuesnay
T
⁎
Royal Military College of Canada, Canada
A R T I C LE I N FO
A B S T R A C T
Keywords: Galfenol Gallium Fatigue Porosity Functional material
The fatigue behaviour of three Galfenol steels, with gallium contents of 12.4, 14.9 and 18.4 at.% was investigated. Specimens were machined from as-received 25.4 mm diameter Bridgman rods and were tested under constant amplitude R = −1 cyclical loading. Results were highly scattered with one standard deviation on life being of one order of magnitude. The as-received material was also very porous, with macropores as large as 500 μm. Fatigue results of each of the three alloys were approximately the same. It is suggested that the inherent porosity imparted on the as-extruded rods is the main source of the scatter.
1. Introduction 1.1. Background Magnetostriction. Galfenol is a relatively new functional Fe-based alloy best known for its unusual ability to be strained by application of applied magnetic fields. Such strains, most commonly referred to as magnetostrictive strains (λ ), are caused by the reorientation of magnetic domains within the Galfenol. Because domain reorientation may only occur up to 90 degrees, λ is limited. Such limits in strain are called saturated strain (λs ). Seeing as λs are small, on the order of 10−5 to 10−4, they are typically described in parts per million (ppm). Evolution of Magnetostrictive materials. First documentation of the magnetostrictive effect was reported by Joule in 1847 where it was determined that steel could be strained up to 30 ppm [1] by applied magnetic fields. First applications of magnetostrictive metals was seen only a century later, during WWII, when nickel-based alloys were used in newly invented sensing applications, sonar technology [2]. These Ni alloys possess λs nearly twice that of ordinary steel alloys. Since WWII, technological advances have led to the discovery of new materials with magnetostrictive potentials orders of magnitude larger than those observed in Ni-based alloys. Piezoceramics such as lead zirconate titanate (PZT) and Terfenol-D (Tb.27Dy.73Fe1.95)1 are examples of such advanced materials with λs as high as 2000 ppm [3]. . 1.2. Evolution of Galfenol Although PZTs and Terfenol-D possess λs suitable for the design of highly responsive sensing and actuating devices, these materials are
unfortunately inherently brittle and expensive to manufacture in the single crystal form. As such, Galfenol, a steel-like material discovered by research efforts at the U.S. Navy’s Naval Ordnance Laboratory (N.O.L.), has been found to be a potentially promising material for use in the design of instrumentation destined for use in explosive or rugged environments [4]. It was found by Clark et al. that single crystals of Galfenol, Fe-Ga alloys containing up to ∼20 at.% gallium content could produce magnetostrictive potentials as high as 400 ppm [5]. Given that this alloy also possesses mechanical properties and machineability comparable to that of steel, Galfenol has been found to be a good prospective replacement for more fragile magnetostrictive materials. Table 1 lists some magnetostrictive alloys along with approximate values of key mechanical properties. For Galfenol to be an economically viable replacement for more expensive and fragile alternatives, it needs to be manufactured in bulk, in the polycrystalline form. The major challenge with producing Galfenol in bulk remains the production of uniform grain orientation, a requirement for utilization of magnetostrictive properties. The three most used manufacturing methods for producing preferential grain orientation in bulk Galfenol are wire extrusions [8], sequential hot rolling processes [9] and directional solidification (DS) techniques such as Bridgman or Stockbarger extrusions [10]. The Zone Melt Crystal Growth (ZMCG) method developed by Summers et al. consists effectively of a modified Bridgman rod extrusion technique shown to produce λs as high as 220 ±25 ppm in Galfenol with Fe81.6 Ga18.4 [11]. These levels are over 75% of λs obtained in single crystals with the same composition. Given ZMCG has been shown to produce the largest λs in bulk Galfenol, it has been selected as the processing method for the samples obtained in this project.
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Corresponding author. E-mail address: [email protected] (D.L. DuQuesnay). 1 Subscript numbers indicate alloy compositions in atomic percent (at.%.). https://doi.org/10.1016/j.ijfatigue.2019.06.037 Received 3 May 2019; Received in revised form 3 June 2019; Accepted 25 June 2019 Available online 29 June 2019 0142-1123/ Crown Copyright © 2019 Published by Elsevier Ltd. All rights reserved.
International Journal of Fatigue 128 (2019) 105177
D.I. Holsworth and D.L. DuQuesnay
Table 1 Comparison of approximate material properties for Galfenol and other magnetostrictive materials [3,6,7]. Material Magnetostriction (ppm) Modulus of elasticity (GPa) Tensile strength (MPa) Ductile/Brittle
Terfenol-D
Galfenol
Ni alloys
Steel
2000 50 30 Brittle
400 200 500 Ductile
50 207 60–300 Ductile
30 200 400–1200 Ductile
1.3. Mechanical properties of Galfenol Complementary work on the mechanical behaviour of Galfenol was conducted by Kellogg [12] and Brooks et al. [13]. It was found that binary Galfenol with no carbon additions undergoes ductile-to-brittle (DTB) transition at gallium contents as low as 5 at.%. It was also found that increases in gallium content resulted in linear decreases in stiffness and tensile strength. Carbon additions to Galfenol steels was found to enhance intergranular cohesion hereby improving mechanical properties drastically. DTB was increased to 15.5–18.4 at.% and hardness was increased by 20%. With a composition of Fe81.6Ga18.4 , polycrystalline Galfenol steel produced by ZMCG was shown to possess tensile strengths of 370 MPa, a Young’s Modulus ranging from 72.4–86.3 GPa and ductility of 0.81–1.2% [13].
Fig. 1. Comparison of a as-received Galfenol sample and a completed fatigue specimen. Length of the cylindrical specimens was 53 mm and the specimen diameters were 4.5 and 8.7 mm, as depicted.
increase strength and ductility of binary Galfenol steel [20]. Clark et al. showed that λs was maximized in binary Galfenol with ∼19 at.% Ga [21] and it was also found that Fe100 − xGax for x ⩾ ∼ 20 proved too brittle for bulk production [9]. As such and because it was desired to broaden the spectrum of gallium contents tested, the third alloy tested possessed much lower gallium contents with a binary alloy composition of Fe87.4Ga12.4 . Specimens with this alloy composition were labelled as belonging to the M Series.
1.4. Motivations and objectives Galfenol steels have been considered for the design of more rugged and durable sensing and actuating devices because of their enhanced mechanical properties [14]. Given its strength and toughness, Galfenol has also been considered for design of new engineering applications, including energy harvesting and active vibration control [15–17]. Components in such applications experience cyclic loading. Therefore, the fatigue behaviour of Galfenol needs to be examined. As such, the two objectives of this work are to investigate the fatigue behaviour of three DS polycrystalline Galfenol steels and to determine the effect of gallium content on fatigue properties of the investigated alloys.
2.2. Specimen preparation and preliminary tests As-received rods were quartered along their length and each quartered segment was then further machined to produce a total of eight specimens per rod (see Fig. 1). Given the brittle nature of the as-received material, approximately 10% of the specimens fractured and were wasted during machining. Once machined, the gauge sections of the specimens were prepared in accordance with standards detailed in the Fatigue Handbook [22] and ASTM E466 [23]. Based on work published by Menezes et al., it is expected that the 400 grit emery paper used to polish the gauge sections produced an arithmetic mean roughness value (Ra ) of 0.3 μm [24]. Before fatigue testing, two hardness tests were conducted on each grip end of each specimen. Results of these tests are presented in Section 3.
2. Materials and methodology 2.1. Materials As-received Galfenol samples for this project were procured from ETREMA Inc. [3]. The DS cylindrical rods grown by ZMCG were 24 mm in diameter and 125 mm in length. Because carbon additions are known to increase strength and hardness of Galfenol [3] as well as enhance λs [18], all Galfenol samples for this project were Galfenol steels possessing 0.15 at.% carbon content. For the sake of brevity, carbon content will not be indicated when describing alloy contents in this paper. The first Galfenol steel alloy tested was a binary Galfenol steel possessing the same alloy composition as the polycrystalline Galfenol steel tested by Brooks et al. [13], Fe81.6Ga18.4 . Specimens machined from this alloy were labelled as belonging to the F series and data points belonging to this series are shown as red square boxes in all figures in this paper. As stated in Section 1.2, this alloy composition had been shown to possess the highest levels of λs for bulk polycrystalline Galfenol. The second alloy tested was Fe82.8Ga14.9Cr2.2 and specimens with this alloy composition are labelled as belonging to the C series. Data points from this series are presented as green circles in all figures. As with conventional stainless steel alloys, the chromium additions in Galfenol have been shown to enhance corrosion resistance, a highly desired characteristic for naval applications. Mechanical characterization of similar stainless Galfenol steels had previously been investigated by Nolting et al. [19]. In addition to enhancing corrosion resistance, it has also been shown that tertiary alloying elements such as Cr tend to
2.3. Testing Equipment. The equipment used in this project consisted of a servohydraulic loading frame and a computer to control and monitor load cycle amplitudes and cycle frequency. The electrohydraulic frame was equipped with a load cell and a linear variable displacement transducer (LVDT) used to monitor the displacement of the lower grip. A computer software was used to control the loading patterns which would produce the desired stress amplitudes (σa ), count the number of applied cycles as well as to monitor and minimize deviations and errors on the applied loads. Test details. Tests were conducted in laboratory air at an average temperature of 25 °C. Fatigue testing was performed in constant amplitude fully reversed load control, with applied stress derived by dividing the applied load by the initial gauge section area. Initial loading frequency was 10 Hz for the first few hundred cycles of loading and was then raised to 20 Hz for most specimens. Steady-state testing frequency was achieved at around 1000 cycles. The maximum testing frequency of 20 Hz was chosen so as to limit loading errors imparted on the specimens to ±3.5 MPa. Specimens tested at stresses above 400 MPa were tested at 10–15 Hz in order to also keep loading errors below ±3.5 MPa. 2
International Journal of Fatigue 128 (2019) 105177
D.I. Holsworth and D.L. DuQuesnay
Table 2 Average surface hardness of the tested Galfenol steels. Series
# specimens tested
Rockwell B hardness∗
56 19 60
99.7 ± 2.6 93.0 ± 3.6 90.2 ± 3.4
F C M ∗
± one standard deviation.
Fig. 2. Visible surface-connected porosity on a machined Galfenol rod.
Yield stress. Given that material costs were significant, effectively all machined specimens were used for fatigue testing. Although standard tensile strength testing practices could not be followed, the load–displacement path of the first cycle was captured for specimens tested near the yield stress (σy ). For such specimens, loading was done slowly for one fully-reversed cycle. After one full cycle, cyclical loading was started and controlled by software following standard fatigue testing practices. Because use of extensometers within the gauge section of the specimens was deemed to potentially cause surface damage, the stress–strain relationship in the first cycle could not be captured.
Fig. 4. Stress-displacement paths for specimens from each of the three alloys tested. Making the gross assumption that the majority of the specimen elongation occurs within the gauge length would produce an estimate that a displacement d = 0.1 mm equates to approximately 0.2% strain.
3. Results and discussions 3.1. Mechanical properties Hardness. The average of the measured hardness calculated for each alloy is tabulated in Table 2. Yield stress. Audible pings or metallic crunching sounds were heard for many of the tests, especially in the first few cycles for specimens loaded near the material’s estimated σy . These pinging sounds were also heard when gripping the specimens within the mechanical frame grips. Examples of stress-displacement paths for specimens from each alloy tested are depicted in Fig. 4. These results indicate that, as expected, σy increased with added Ga content. Since increases in gallium content have also been shown to increase embrittlement in Galfenol [13], σa for specimens from the more brittle F series were limited to the first audible signs of yielding, around 500 MPa. The C series was found to yield at 475 MPa ±20 and the softer M series, at 375 MPa ±20 MPa. These values are in line with what was presented earlier, in Section 1.3. A few F series specimens were tested in monotonic tension to failure with an axial extensometer mounted to measure strain. It is interesting to note that for F and M series, both binary Galfenol steels, material flow beyond the point of yielding was accompanied with sequential crunching or pinging sounds. Kellogg et al. observed similar sounds and discontinuous stress–strain behaviour [9]. It was suggested in this study that this behaviour was caused by deformation twinning. The pinging observed in this study coincides with the discontinuities in the stress-displacement paths captured in Fig. 4 and the stress–strain curve for an F series specimen shown in Fig. 5. This observation was not made for the C series Galfenol steel containing tertiary Cr additions. If this discontinuous behaviour during material flow is linked with propagation of damage at inter-grain boundaries, results would suggest that Cr additions enhance inter-grain cohesion. This reinforces recent observations from Nolting and Summers [20] which demonstrate that tertiary alloying additions such as Cr not only enhance strength and ductility of binary Galfenol steels but also help transition the predominant fracture mechanism from inter-granular to inra-granular fracture mechanisms. Further evaluation of the effects of Cr additions to inter-grain cohesion and fracture mechanisms was not pursued as it was not within the scope of this study.
2.4. Post-processing As can be seen in Fig. 2, some of as-received material possessed significant surface-connected porosity. Further observation of micrographic samples showed that porosity was also present within and throughout the as-received material. After fatigue failure, all fracture surfaces were examined and micrographed under magnification with an optical microscope. Fracture surfaces of specimens with abnormally long or short lives were further investigated with the aid of Scanning Electron Microscopy (SEM). Specimen fracture surfaces were prepared as depicted in Fig. 3b. for observation by SEM. Microstructure texture and porosity were further investigated on metallographic samples produced from sectioned halves of failed fatigue specimens, as depicted in Fig. 3a. Approximately 5% of the polished surface area of the metallogrpahic samples was captured by micrograph for analysis.
Fig. 3. Specimen sectioning for post-processing where the shadowed area indicates the examination surface. a) Preparation of grip for metallographic observation. The halved grips provided an observation surface area measuring approximately 8 mm × 20 mm, 160 mm2. b) Sectioning of fracture surfaces for investigation by microscope or SEM. Note that some of the long-lasting specimens which endured long fatigue lives, with Nf > 106 , were removed from testing before failure and entire gauge sections could be sectioned for observation. 3
International Journal of Fatigue 128 (2019) 105177
D.I. Holsworth and D.L. DuQuesnay
Fig. 5. Typical stress–strain path for F series specimens strained well beyond the point of yielding. The relative stress depicted is the ratio of applied stress over yield stress (σ / σy ).
Fig. 6. Stress-life (S-Nf ) data for all three as-received Galfenol steels with R = −1. Labelled solid lines and accompanying shadowed areas indicate mean S-N curves ± 2 standard deviations on log(Nf ). The error on σa is ± 3.5 MPa, smaller than the data point symbols. ∗ indicates the presence of two data points and arrows indicate run-on data points.
Fig. 7. Relative Stress-life (S-Nf ) data for all three as-received Galfenol steels with R = −1. labelled solid lines and accompanying shadowed areas indicate mean S-N curves ± 2 standard deviations on log(Nf ). The error on σa is ± 3.5 MPa, smaller than the data point symbols. ∗ indicates the presence of two data points and arrows indicate run-on data points.
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International Journal of Fatigue 128 (2019) 105177
D.I. Holsworth and D.L. DuQuesnay
Fig. 8. Mixed mode inter and intragranular fracture surfaces of F and C series Galfenol.
Fig. 9. Suspected sources of failure in short lived fatigue specimens.
Fig. 10. Micrograph of a particularly large surface connected pore. Fig. 11. Micrograph of a metallographic sample from specimen F46.
3.2. Fatigue results
Nf as the dependant variable. It was also assumed that the data was scattered in accordance with a log-normal Gaussian distribution. As can be seen in Fig. 6, there was significant scatter in the test results. The largest scatter on Nf was observed on the C series Galfenol with one standard deviation on life being approximately one order of magnitude (log (Mean) ±100.97 ). Given that the scatter bands for the M
Fig. 6 includes the fatigue test results obtained for fully reversed loading cycles (R = −1) on all three Galfenol steels. The test was conducted in load control and as such, even if there was up to 3.5 MPa error on the stress amplitude (σa ), the applied stress was effectively a controlled variable. The mean curve fits were therefore established with 5
International Journal of Fatigue 128 (2019) 105177
D.I. Holsworth and D.L. DuQuesnay
Fig. 12. Measured surface area % porosity of metallographic specimens from F and C series alloys. Hollow bars represent measurements made on one micrograph, 1.44 mm2 of the total sectioned surface area. The dark bars represent the average surface area porosity measured for each fatigue specimen, over the six micrographs (over 6.862 mm2 of surface area).
Fig. 13. Pore size distribution on F and C Series Galfenol over all of the micrographed surface area, 27.45 mm2.
were intergranular. This suggests that weak grain boundaries contributed to hastened crack propagation. For the F series Galfenol which is stronger and stiffer than the other tested alloys, fracture was predominantly intragranular. As can be seen in Fig. 8, crack propagation in the F series was limited to approximately 100 μm at the time of fracture while the more ductile C series specimens produced critical crack lengths closer to 500 μm. Fractographs were also useful in identifying potential crack initiation sites, as was the case for specimen C2 in Fig. 8a. In many instances, material artefacts such as macropores and grain boundary irregularities known to promote premature crack initiation were identified. This was especially true in the most short-lived specimens tested, as evidenced by the fractographs in Figs. 9 and 10. Other examples of surface-connected porosity were also observed on the surface of as-received samples (Fig. 2) and many of the micrographs obtained from metallographic samples, such as the micrograph in Fig. 10. Porosity. In an effort to further describe and quantify the degree of porosity in the as-received material, micrographs of polished metallographic samples were captured. A minimum of six micrographs each covering 1.14 mm2 were obtained per polished metallographic sample. A minimum of four fatigue specimens for the F and C series Galfenol were used to produce metallographic samples. Micrographs were then analyzed with the aid of image analysis software to quantify shape, size and number of pores. An example of how predominant the inherent porosity could be is depicted in Fig. 11. Evidence of porosity was captured in effectively all samples, regardless of the alloy. Fig. 12 summarizes the total percentage of surface area that was deemed to be porous in each micrograph and the pore size distribution by alloy is presented in Fig. 13. . ASTM standard E446 indicates that pore size description in cast steels is based on qualitative assessments [31]. Some further define
and F series fall within the larger C series scatter band, it is suggested that gallium content ranging from 12.4–18.4 at.% does not affect the fatigue behaviour of DS polycrystalline Galfenol steels. This being said, when analyzing the results as a function of relative stress amplitude (σa / σy ), the M series outperforms the higher Ga content Galfenol steels by nearly one order of magnitude on life as can be seen in Fig. 7. The scatter in these results was higher than what is typically expected for a steel-like alloy. Given that it has been well documented that porosity in steel is linked with reductions in mechanical properties [25] and fatigue behaviour [26–30] and that surface-connected pores were visible on parts of the as-received samples, it was believed that inherent porosity was the main contributor to the data scatter. 3.3. Post-processing of specimens Upon failure, specimen fracture surfaces were examined under microscope with up to x75 magnification. Specimens with especially long and short fatigue lives were further examined with SEM. Polished but unetched metallographic samples were also obtained from these same specimens so as to obtain information on material porosity and texture, as detailed in Section 2.4. Fracture surfaces. Fatigue striations and clamshell or thumbnail formations typically visible in failed fatigue specimens were only observed on the fracture surfaces of a few of the longer lasting specimens. One such rare example was captured on the fracture surface of specimen C2, relatively one of the longest lasting specimens from the C series, as depicted in Fig. 6 (σa = 350 MPa and Nf = 700 618 cycles). Thumbnail markings from the fatigue crack propagation can be seen at the bottom of the fractograph in Fig. 8a. Failure was very brittle in nature for the majority of the specimens. Although some specimens demonstrated intergranular failure, all specimens failed in mixed mode with the majority of failure being intragranular. Crack propagation emanating from crack initiation sites 6
International Journal of Fatigue 128 (2019) 105177
D.I. Holsworth and D.L. DuQuesnay
macroporosity as detectable pores measuring over 20 μm [28]. It has also been documented that microporosity has little to no effect on fatigue properties of steels when compared to macroporosity [26]. Both Galfenol series possess large numbers of pores possessing Feret mean diameters well over 20 μm as well as surface area porosity of approximately 0.5%. Given the evidence provided in Figs. 12 and 13, it is therefore very likely that porosity is linked with the data scatter observed in Figs. 6 and 7. It is therefore suggested that a reduction in porosity would likely lead to increases in fatigue performance. Potential solutions. Hot iso-static pressing (HIP) is a well known mechanical process known to remove or reduce porosity in metals. Use of HIP on DS polycrystalline Galfenol might lead to significant reduction in porosity and therefore, enhancement in fatigue behaviour. Alternatively, bulk production of polycrystalline Galfenol by sequential hot rolling processes have also be shown to produce material with little porosity [9]. It is therefore suggested that fatigue behaviour of hot rolled and HIP DS Galfenol steels be conducted to further investigate the role of porosity in the observed fatigue results. It is suspected that fatigue life results for Galfenol specimens with little to no macroporosity would produce a more traditional stress-life curve than the results presented in Figs. 6 and 7, results with less scatter which align approximately with the right edge of the presented scatter bands. Another deduction from this work is that the range of magnetostrictive strains in the most promising of Galfenol steels is relatively limited with λs well below 400 ppm for the largest λ recorded on single crystal Galfenol [5]. Considering that polycrystalline Galfenol possesses a Young’s modulus ranging from 72 to 86 GPa [13], the stresses associated with fully saturated magnetostrictive strains would be small, well below 80 MPa. With careful design considerations, applications which could limit stress amplitudes to such levels could endure with 97.5% assurance at least 105, 107 and 108 cycles for C, F and M series Galfenol, respectively (based on two standard deviations removed on mean life calculations). Galfenol steels could still be durable enough for many engineering applications which require cyclical loading.
[2] Scott RS. Measurements on Galfenol material and transducers Ph.D. thesis The Pennsylvania State University; 2008. [3] Etrema inc. doi:24 Oct 17. https://web.archive.org/web/20160304003546/http:// www.etrema.com/terfenol-d/. [4] Atulasimha J. Characterization and modeling of the magnetomechanical behavior of iron-gallium alloys Ph.D. thesis University of Maryland; 2006. [5] Clark A, Restorff J, Wun-Fogle M, Lograsso T, Schlagel D. Magnetostrictive properties of body-centered cubic Fe-Ga and Fe-Ga-Al alloys. IEEE Trans Magn 2000;36(5):3238–40. [6] Atulasimha J, Flatau A, Summers E. Characterization and modeling of the magnetomechanical behavior of polycrystalline iron-gallium alloys. Smart Mater Struct 2007;16(4):1265
4. Conclusions DS Galfenol steels produced by ZMCG with gallium contents varying between 12.4 and 18.4 at.% and trace carbon contents below 0.15 at.% were tested to fatigue failure using fully-reversed loading cycles. The alloys tested were composed of Fe81.6Ga18.4 , Fe82.8Ga14.9Cr2.2 and Fe87.6 Ga12.4 , F, C and M series, respectively. The as-received material was laden with surface connected pores and further investigation of metallographic samples showed that the as-received samples also possessed significant internal macroporosity known to affect fatigue behaviour in metals. The fatigue tests produced scattered results which are likely linked with the observed porosity. It is expected that the same fatigue tests conducted on less porous material would produce less scattered results which would align closer to the longer lasting fatigue data obtained in this work. References [1] Joule JP. XVII. On the effects of magnetism upon the dimensions of iron and steel bars. London, Edinburgh, Dublin Philosoph Mag J Sci 1847;30(199):76–87.
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