Mechanical Properties of NiTiBased Foam with High

Authors: Ying Qiu Hao Yu Marcus L Young

Publish Date: 2015/11/09

Volume: 1, Issue: 4, Pages: 479-485

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Abstract

In order to better understand NiTibased shape memory alloy foams for implant applications Ni40Ti50Cu10 foams were heat treated and then deformed under incremental and cyclic compression loading After heat treatment the microstructure consists of a NiCuTi matrix with small NiCu4Ti3 precipitates and a large Ti2NiCu secondary phase The heattreated Ni40Ti50Cu10 foam exhibits a twostep transformation involving B19′ → B19 and B19 → B2 on heating and B2 → B19 and B19 → B19′ on cooling respectively One Ni40Ti50Cu10 foam was compression loaded for 10 cycles at each subsequent strain level ie 1 2 3 4 5 and 6  strain In each set of compressive stress–strain loops the maximum stress level decreases due to plastic damage accumulation and/or retention of transformed martensite Crosssectional images from microcomputed tomography were collected during compression loading which shows very uniform deformation without severe structural damage even up to 5  strain Localized deformation is visible at 6  strainNiTibased shape memory alloys SMAs are excellent candidates for implant applications due to their unique shape memory effect and pseudoelasticity good biocompatibility high corrosion resistance and relatively low stiffness as compared to other biomedical alloys 1 2 3 By introducing porosity into NiTibased SMAs it is possible to tailor the stiffness and microstructure to more closely match that of bone as well as improve tissue and bone ingrowth 4 5 The addition of Cu which substitutes for Ni in NiTi SMAs such as Ni40Ti50Cu10 improves the stability of the phase transformation temperatures responsible for the shape memory effect and pseudoelasticity up to 8  reversible strain and narrows the phase transformation hysteresis 6 7 8 9 10 11 12 Heat treatments can be used to enhance the mechanical properties increase the phase transformation temperatures and likely decrease Ni release and thus decrease toxic allergic and carcinogenic effects 13 by forming Ni4Ti3 precipitates which also improve the phase transformation recoveryBricknell et al 7 showed that the substitution of Cu for Ni in NiTi SMAs decreases the lattice distortion needed to form the martensite phase from the austenite phase while still maintaining the ordered CsCl type structure at high temperature Gil et al 14 15 showed that NiTiCu SMAs exhibit a much more narrow stress hysteresis based on transformation stresses as compared to binary NiTi SMAs Fatigue testing of NiTiCu SMA actuator springs were conducted by Grossmann et al 16 17 They reported that the functional fatigue resistance of NiTiCu SMAs is strongly improved by the addition of Cu Sehitoglu et al 18 19 and Biscarini et al 20 investigated the mechanical properties of NiTiCu single crystal SMAs under compression loading and reported that the slip resistance increases when Nirich precipitates are present To date the mechanical behavior of bulk NiTiCu SMA is well established 7 14 15 16 17 18 19 21 22 23 24 however few studies have focused on the effect of porosity on NiTiCu SMAs 25 26 27 28 Goryczka et al 26 27 examined powder processing methods for producing homogenous NiTiCu SMAs They found that porous NiTiCu SMAs show a similar martensite transformation behavior as compared to NiTiCu bulk materials Porous NiTiCu SMAs can be optimized for implant applications due to the tunable stiffness which can be tailored to match with the surrounding tissue or bone structure by controlling the porosity 9 Young et al 28 produced NiTiCu foams with 60  porosity by casting a bulk NiTiCu SMA into SrF2 salt preform and then dissolving away the SrF2 salt preform with an acid solution In this earlier study the method for producing castreplicated NiTiCu foams was presented as well as the microstructure and compression loading/unloading data for seven consecutive loops at body temperature 311 K for a ~60  porous NiTiCu foam which was homogenized at 1173 K for 5 h These foams exhibited 4  recoverable strain and relatively low stiffness under single loading and unloading conditions at body temperature 28 However the mechanical behavior of these NiTiCu foams under cyclic loading conditions and the microstructural evolution after cyclic loading and unloading at room temperature have not been investigated In the study presented here a 76  porous NiTiCu foam was produced from the same method as presented in Young et al 28 resulting in a martensitic SMA at room temperature This NiTiCu foam was additionally heat treated to obtain austenite at room temperature The mechanical response and microstructural evolution of this austenitic NiTiCu foam was further examined at room temperature with microcomputed tomographic µCT imaging during incremental and cyclic in situ compression loading and unloading The combination of mechanical loading and imaging allows for analysis of both the macroscopic and microscopic mechanical behavior of the NiTiCu foamNi40Ti50Cu10 foams referred to as NiTiCu foams throughout the paper were prepared by a salt replication casting process 28 29 The process involved the following steps 1 SrF2 powders with 180 µm to 355 µm size were packed into an alumina crucible and sintering at 1673 K for 10 h under a vacuum with 10−4 Pa pressure 2 a bulk NiTiCu SMA was then placed in an alumina crucible on top of an alumina spacer disc which was on top of the partially densified SrF2 preform 3 the crucible with the bulk NiTiCu SMA alumina spacer disc and the porous SrF2 preform was then heated under high vacuum to 1648 K After holding at this temperature for 1 h the chamber was then backfilled with Ar gas to a pressure of 1 atm which forced the fully melted NiTiCu SMA into the porous SrF2 preform 4 the resulting NiTiCu/SrF2 composite structure was then sectioned using an oilcooled diamond wire saw MTI Corporation STX202 into compression samples with approximate dimensions of 25 × 25 × 50 mm3 5 the compression samples were ultrasonicated in an aqueous solution with 20  nitric acid for 2 h in order to remove the space holder SrF2 preform and 6 the NiTiCu compression samples were then sealed in evacuated quartz capsules and homogenized at 1173 K for 5 h followed by water quenching and then heat treated at 673 K for 8 h to improve the mechanical behavior by forming Ni4Ti3like precipitatesA Skyscan1172 Xray µCT scanner was used to perform combined cyclic compression testing and 3D Xray tomography The engineering strain measurements were based on the crosshead displacement which was corrected and verified by testing a Teflon sample with similar dimensions as that of the NiTiCu foam compression sample 3D Xray tomographic images of the NiTiCu foam sample were collected before compression and at compression up to 1 2 3 4 5 and 6  strain where 10 loading/unloading cycles were performed at each strain level Xray µCT scanning was performed at a tube voltage of 100 kV and a tube current of 100 µA with an Al + Cu filter Xray radiographs were taken with an angular step of 04° and compiled to create a reconstructed 3D structure of the ascompressed NiTiCu foam sample About 860 radiographs with a pixel size of 495 µm were obtained for each tomographic image Image analysis softwares NRecon CTvox and Dataviewer were used to perform the 3D reconstruction and analysis

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