A. KOCIJAN et al.: INFLUENCE OF DIFFERENT PRODUCTION PROCESSES ON THE BIODEGRADABILITY ... 805–811 INFLUENCE OF DIFFERENT PRODUCTION PROCESSES ON THE BIODEGRADABILITY OF AN FeMn17 ALLOY VPLIV RAZLI^NIH PROCESOV IZDELAVE NA BIORAZGRADLJIVOST ZLITINE FeMn17 Aleksandra Kocijan, Irena Paulin, ^rtomir Donik, Matej Ho~evar, Klemen Zeli~, Matja` Godec Institute of Metals and Technology, Lepi pot 11, 1000 Ljubljana, Slovenia aleksandra.kocijan@imt.si Prejem rokopisa – received: 2016-03-31; sprejem za objavo – accepted for publication: 2016-04-06 doi:10.17222/mit.2016.055 The purpose of this research was to evaluate the biodegradability of a cast FeMn17 alloy that was processed by hot rolling and annealing, which influenced both the mechanical and corrosion properties of the FeMn17 alloy. The corrosion behaviour and in-vitro biodegradability were investigated by light microscopy, scanning electron microscopy, X-ray diffraction and immersion tests in Hank’s solution. Compared to pure Fe, the cast FeMn17 alloy has a better biodegradability (higher corrosion rate). We showed that hot rolling additionally improves the biodegradability, while the annealing process lowers the biodegradability of the FeMn17 alloy. Keywords: biodegradability, FeMn alloy, corrosion, XRD, SEM Namen raziskave je bil oceniti biorazgradljivost zlitine FeMn17 s predelavo z vro~im valjanjem in `arjenjem. Vro~e valjanje in `arjenje vplivata na mehanske in korozijske lastnosti zlitine FeMn17. Korozijske lastnosti in biorazgradljivost smo preverjali s svetlobno mikroskopijo, vrsti~no elektronsko mikroskopijo, rentgensko difrakcijo in s testi potapljanja v Hankovo raztopino. V primerjavi s ~istim Fe, ima zlitina FeMn17 veliko bolj{o biorazgradljivost (ve~jo korozijsko hitrost). Pokazali smo, da vro~e valjanje dodatno izbolj{a biorazgradljivost, z `arjenjem pa se biorazgradljivost zlitine FeMn17 poslab{a. Klju~ne besede: biorazgradljivost, FeMn zlitina, korozija, XRD, SEM 1 INTRODUCTION Biodegradable metallic materials represent a novel class of bioactive biomaterials that can temporarily sup- port tissue healing and should progressively degrade completely without a negative effect on the healing pro- cess.1–3 Potential applications of these biomaterials are paediatric, orthopaedic (fixation screws and pins) and cardiovascular implants (coronary stents).1,4 Biodegrad- able polymers were first investigated as bioactive bio- materials; however, in recent years biodegradable metal- lic materials, especially Fe and Mg alloys, have received more attention due to their superior mechanical proper- ties and their cytocompatibility.1–3,5 Compared with Mg-based materials, Fe-based materials possess similar mechanical properties to stainless steel and are more at- tractive for applications that require high strength and ductility.1 Despite the immense potential of Fe and Mg alloys, experiments and clinical trials also exposed their weaknesses: too rapid degradation rates, poor mechani- cal properties and significant hydrogen evolution during the corrosion process of Mg-based alloys and a too slow degradation of Fe-based alloys.5–8 Fe-based alloys may also present problems with cer- tain imaging devices (magnetic resonance imaging, for example) due to the Fe’s ferromagnetic nature. However, alloying and heat treatment can modify the mechanical, corrosion, and ferromagnetic properties of pure Fe.1,2 The choice and the amount of alloying element is impor- tant with respect to the toxicity and degradation behav- iour of the Fe alloy.2 Mn represents a suitable alloying element based on microstructural, magnetic, corrosion, and toxicological considerations.2 Mn (austenite-forming element) transforms Fe into a nonmagnetic material, lowers the standard electrode potential of Fe and thus en- hances the degradation of the material, which represents an essential trace element necessary in many enzymatic reactions. Newly developed Fe-Mn-based alloys contain- ing up to 35 % of mass fractions of Mn have comparable mechanical properties to stainless steel, faster degrada- tion and improved MRI compatibility.1,2 Despite this, the low corrosion rate still represents the major problem fornewly developed Fe–Mn-based alloys. Non-conventional processing techniques such as powder metallurgy, electrodeposition and inkjet 3D- printing can achieve the faster degradation of Fe–Mn al- loys.1,2 However, these techniques are rather complex and expensive, therefore it is necessary to further investi- gate conventional methods, such as casting with addi- tional steel-processing techniques in order to find an eco- nomically favourable solution. Research has rarely been made on the influence of subsequent processing and heat treatment on the properties of any conventional, cast, Materiali in tehnologije / Materials and technology 50 (2016) 5, 805–811 805 UDK 620.1/.2:669.056:67.017 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 50(5)805(2016) biodegradable, metallic materials, including Fe–Mn- based alloys. The aim of the present study was to investi- gate the influence of three basic steel-processing meth- ods (casting, hot rolling and annealing) on the corrosion behaviour of the biodegradable Fe–Mn-based alloy due to the formation of less-corrosion-resistant deformational martensite.9 2 EXPERIMENTAL PART Material preparation – The investigated Fe-based al- loy with 17 % of mass fractions of Mn (FeMn17) was produced from relatively pure Fe and Mn. Both materials were melted in an induction furnace under air atmo- sphere at approximately 1700 °C and cast into two iron moulds. One mould was left to cool down in air atmosphere and the other one was after casting in mould, hot rolled at approximately 1000 °C for a 33 % reduc- tion. Parts of both samples were annealed at 1050 °C for 1 h and then furnace cooled. The materials were ana- lysed and the results were compared with pure Fe. The chemical compositions of the alloy and the pure Fe were determined using an X-ray fluorescence spectrometer XRF (Thermo Scientific Niton XL3t GOLDD+) and arepresented in Table 1. Metallographic investigation – Samples were cut with a water-cooled saw and cross-sections of the sam- ples were prepared by the standard metallographic tech- niques of grinding and polishing. The samples were etched with 3 % Nital, an ethanol solution of HCl(aq) and an ethanol solution of HNO3(aq). The characterization of the material was performed using a light microscope (LM, Microphot FXA Nikon with Olympus DP73) and a scanning electron microscope coupled with an en- ergy-dispersive spectrometer (SEM, JEOL JSM-6500F, EDS INCA ENERGY 400) for analyses of the inclusions and phases. X-ray Powder Diffraction – The samples were mea- sured using a Panalyitical XPERT Pro PW 3040/60 goniometer 2 between 15–90° with a step size of 0.002° and a scan step time of 100 s on each step. Cu with (K = 0.154 nm) anode was used with a current of 40 mA and a potential of 45 kV. Mechanical properties – The microhardness was measured by Vickers HV5 (5 kg load, 11s -Instron Tukon 2100B). Electrochemical measurements – Were performed on prepared specimens, ground with SiC emery paper down to 1200 grit. The experiments were carried out in a simu- lated physiological Hank’s solution, containing 8 g/L NaCl, 0.40 g/L KCl, 0.35 g/L NaHCO3, 0.25 g/L NaH2PO42H2O, 0.06 g/L Na2HPO42H2O, 0.19 g/L CaCl22H2O, 0.41 g/L MgCl26H2O, 0.06 g/L MgSO47H2O and 1 g/L glucose, at pH = 7.8. All chem- icals were from Merck, Darmstadt, Germany. The mea- surements were performed using a three-electrode, flat BioLogic corrosion cell (volume 0.25 L). The test speci- men was employed as the working electrode (WE). The reference electrode (RE) was a saturated calomel elec- trode (SCE, 0.242 V vs. SHE) and the counter electrode (CE) was a platinum net. Electrochemical measurements were recorded by using a BioLogic Modular Research Grade Potentiostat/Galvanostat/FRA Model SP-300 with an EC-Lab®software V10.44. The specimens were im- mersed in the solution 1 h prior to the measurement in order to stabilize the surface at the open-circuit potential (OCP). The potentiodynamic curves were recorded after 1 h of sample stabilisation at the open-circuit potential (OCP), starting the measurement at 250 mV vs. SCE more negative than the OCP. The potential was then in- creased, using a scan rate of 1 mV s–1, until the transpassive region was reached. The linear polarisation measurements were performed at ±25 mV according to the OCP, using a scan rate of 0.01 mV s–1. 3 RESULTS AND DISSCUSION 3.1 Microstructure characterization The microstructures of all four samples, i.e., cast, hot rolled and both samples after annealing at 1050 °C and furnace cooled down, are shown in Figure 1. There is no difference in the chemical composition, where as there are some differences in the microstructure. In the cast sample, the cast structure occurs upon cooling, though the cooling rate is too high for equilibrium phase trans- formation, as predicted from the Fe–Mn phase diagram. We observed some boundary segregations rich in Mn and inclusions of MnS and TiN, which were also confirmed by the EDS analyses. The observed average grain size for these samples was approximately 350 μm. There was also some smaller porosity present in the microstructure. The cast + annealed sample (Figure 1b) has a similar microstructure with the same precipitations and inclu- sions that are ubiquitous at grain boundary and in the grains. The difference is in the much larger grain sizes (average of approx. 1200 μm) and more segregation of Mn at the grain boundaries according to the prolongation of the time available for the cooling after annealing. In the hot-rolled sample was observed, beside austen- ite, also traces of strain-induced martensite and deforma- tion twins. Due partly to the recrystallization, the micro- structure consists of large and small grains. The A. KOCIJAN et al.: INFLUENCE OF DIFFERENT PRODUCTION PROCESSES ON THE BIODEGRADABILITY ... 806 Materiali in tehnologije / Materials and technology 50 (2016) 5, 805–811 Table 1: Chemical composition in mass fractions, w/% Tabela 1: Kemijska sestava v masnih dele`ih, w/% Material Mo Ni Mn Cu Ti Si C Fe Pure Fe