Quasi-Static and Dynamic Compressive Behavior of Gum Metal: Experiment and Constitutive Model

ORIGINAL RESEARCH ARTICLE Quasi-Static and Dynamic Compressive Behavior of Gum Metal: Experiment and Constitutive Model KAROL MAREK GOLASIN´SKI, JACEK JANISZEWSKI, JUDYTA SIENKIEWICZ, TOMASZ PŁOCIN´SKI, MACIEJ ZUBKO, PAWEŁ S´WIEC, and ELZBIETA_ ALICJA PIECZYSKA The quasi-static and high strain rate compressive behavior of Gum Metal with composition Ti-36Nb-2Ta-3Zr-0.3O (wt pct) has been investigated using an electromechanical testing machine and a split Hopkinson pressure bar, respectively. The stress–strain curves obtained for Gum Metal tested under monotonic and dynamic loadings revealed a strain-softening effect which intensified with increasing strain rate. Moreover, the plastic flow stress was observed to increase for both static and dynamic loading conditions with increasing strain rate. The microstructural characterization of the tested Gum Metal specimens showed particular deformation mechanisms regulating the phenomena of strain hardening and strain softening, namely an adiabatic shear band formed at ~ 45 deg with respect to the loading direction as well as widely spaced deformation bands (kink bands). Dislocations within the channels intersecting with twins may cause strain hardening while recrystallized grains and kink bands with crystal rotation inside the grains may lead to strain softening. A constitutive description of the compressive behavior of Gum Metal was proposed using a modified Johnson–Cook model. Good agreement between the experimental and the numerical data obtained in the work was achieved. https://doi.org/10.1007/s11661-021-06409-z Ó The Author(s) 2021 I. INTRODUCTION Laboratories at the beginning of the twenty-first century.[1] The typical composition of Gum Metal is HIGH-STRENGTH construction materials, avail- Ti-36Nb-2Ta-3Zr-0.3O (in wt pct). The fabrication able on the market, usually do not possess good elastic process including cold working and the role of oxygen properties. In turn, elastic materials usually have low in the composition are critical for the unique mechanical strength parameters. However, at the turn of the performance of the alloy, i.e. low Young’s modulus, century, unique Ti-based alloys that combine high relatively large nonlinear recoverable deformation, good strength with excellent elasticity, named Gum Metals, ductility and high strength.[2,3] The mechanical proper- were designed. The name Gum Metal stands for a group ties combined with the nontoxic and nonallergic chem- of multifunctional b-Ti alloys, which were developed by ical composition of Gum Metal make it a promising the Toyota Central Research and Development candidate for orthopedic and dental implants.[4–6] Con- ventional Ti-based materials used in the medical indus- try, such as pure titanium or Ti-6Al-4V, are characterized by a relatively high Young’s modulus ´ _ KAROL MAREK GOLASINSKI and ELZBIETA ALICJA (> 100 GPa) which is over five times higher compared to PIECZYSKA are with the Institute of Fundamental Technological [4] Research, Polish Academy of Sciences, Pawin´skiego 5B, 02-106, that of a human bone. Moreover, aluminum is Warsaw, Poland. Contact-email: [email protected] JACEK considered a factor causing Alzheimer’s disease, and JANISZEWSKI and JUDYTA SIENKIEWICZ are with the the content of vanadium can be toxic.[7,8] Initially, the Faculty of Mechatronics, Armament and Aerospace, Military mechanism of the superelastic-like behavior of Gum University of Technology, Kaliskiego 2, 00-908 Warsaw, Poland. TOMASZ PŁOCIN´SKI is with the Faculty of Materials Science and Metal was not fully understood. The oxygen content in Engineering, Warsaw University of Technology, Wołoska 141, 02-507 the alloy after the process of cold working was said to Warsaw, Poland. MACIEJ ZUBKO is with the Institute of Materials hinder the martensitic transformation.[2] In,[9] Engineering, University of Silesia, 75 Pułku Piechoty 1A, 41-500, microstructures of oxygen-added cold worked b-Ti-Nb Chorzo´w, Poland and also with the Department of Physics, Faculty of Science, University of Hradec Kra´love´, Rokitanske´ho 62, 500 03, alloys were studied using in situ X-ray diffraction Hradec Kra´love´, Czech Republic. PAWEŁ S´WIEC is with the measurements and in situ TEM observations. It was Institute of Materials Engineering, University of Silesia. found that nanosized modulated domains (nan- Manuscript submitted October 12, 2020; accepted July 19, 2021 odomains) of the a¢¢ phase in the b phase act as local METALLURGICAL AND MATERIALS TRANSACTIONS A barriers to martensitic transformation, thereby sup- The mechanical behavior of Gum Metal at a high strain pressing long-range martensitic transformation in this rate is of special interest in view of its unconventional class of alloys. The nanodomain structure was also deformation mechanisms and the related outstanding studied in.[10,11] This unconventional deformation mech- characteristics, which make it suitable for implant anism was also investigated by analyzing the mean applications. So far, this issue has been seldom reported thermal response of Gum Metal under loading using in the literature. Both experimental and numerical infrared thermography.[12–14] In,[13] it was shown that a investigations are scarce. In,[30] the mechanical behavior drop in temperature during loading, related to the of the extruded Gum Metal and the alloy processed by thermoelastic effect, is also followed by a temperature equal channel angular pressing (ECAP) was investigated rise within the recoverable strain of the alloy. In other under compression at various strain rates. words, it was demonstrated that the large nonlinear However, the goal of this work is to examine the recoverable deformation of Gum Metal is a dissipative compressive behavior of Gum Metal under quasi-static process. This means that the underlying deformation and dynamic loadings including analysis of microstruc- mechanism is different from the classical elastic stretches tural changes and constitutive modeling. of atomic bonding. The findings were confirmed and broadened by a comprehensive investigation realized using infrared thermography in.[14] The peculiarities of the plastic behavior of Gum Metal were initially attributed to the hypothesis of disloca- II. EXPERIMENTAL METHOD tion-free plastic deformation,[1] since the ideal strength A Gum Metal rod of 700 mm length and 10 mm of Gum Metal calculated based on density functional theory in[15] was in the order of the alloy tensile strength diameter with nominal composition Ti-36Nb-2- [3] Ta-3Zr-0.3O (in wt pct) was obtained using a powder measured experimentally as shown in. Microstructure metallurgical method that consisted of cold isostatic characterization of Gum Metal found distinct surface pressing, sintering at 1300 °C for 16 hours in a vacuum steps called ‘‘giant faults,’’ plastic flow localization and À4 [16] of 10 Pa, hot forging and solution treatment at 900 C ‘‘nanodisturbances’’. Observations performed by ° for 30 minutes with final quenching in iced water. The high-resolution transmission electron microscopy bar was cold worked with a rotary swaging machine to revealed nanoprecipitates of the hexagonal x phase in Gum Metal.[17–20] The deformation mechanisms of Gum obtain a 90 pct reduction in area. Cylindrical samples of [21] 5 mm length and 5 mm diameter for compression Metal plasticity, including the activity of dislocations, loadings were made from the Gum Metal rod using dislocation channeling and stress-induced x–b transformation,[22] as well as the mechanism of twinning,[23] were electrical discharge machining. An MTS Criterion C45 electromechanical universal investigated. A more detailed study demonstrated that testing machine was used to measure the quasi-static the effective trapping of dislocations in Gum Metal is behavior under uniaxial compression at strain rates related to the average spacing of the nanometer-sized x À1 À1 [17] 0.001 s and 1 s . Grip platens made of tungsten phase (around 6 nm). carbide with polished contact surfaces were used. Understanding of the mechanical characteristics of Gum Metal subjected to loading under various condi- Additionally, a lubricant composed of mineral oil, lithium soap and molybdenum disulfide (MoS ) was tions is critical for its application in a variety of 2 applied between the contact surfaces of the specimen engineering solutions. In particular, a good material platens to mitigate the barreling effect during the plastic for implant applications must posses reliable characteristics under quasi-static and dynamic loadings. The deformation process. High strain rate compression tests were performed strain rate sensitivity of Gum Metal under quasi-static using the classical split Hopkinson pressure bar (SHPB) tension using digital image correlation was already [31,32] investigated in[24] and modeled in.[25] Anisotropy of technique. The basic parameters of the bar system [26] shown in Figure 1 are as follows: input and output bar Gum Metal was experimentally studied in. In general, 1200 mm long; striker bar 250 mm long; all bars 12 mm the mechanical behavior of many materials during dynamic loading differs significantly from that under in diameter. The bars were made of maraging steel (yield strength—2300 MPa; elastic wave speed—4960 msÀ1). quasi-static conditions. Recently, investigation of the The striker bar was also made of the maraging steel and mechanical performance of advanced materials, in was driven by a compressed air system with a barrel particular Ti-based alloys, at high strain rates has length and inner diameter of

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