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INTRODUCTION: Developing biodegradable implants is of growing interest to help fight antimicrobial resistance. Magnesium alloys are candidates for orthopaedic applications, but there is a need for improving their corrosion resistance[1]. Alloying reduces corrosion rates, but further improvement could be achieved by microstructural tuning using additive manufacturing (AM). However, process parameter optimization is time and resource consuming, motivating the use of numerical modelling to accelerate alloy development. The aim of this study was to predict the microstructure-temperature relationship for a MgCaZn alloy using the CALPHAD method as input for a classical nucleation and growth theory (CNGT) simulation. To achieve this, raw powder feedstock was characterized, along with a minimal number of printed samples.

METHODS: The powder composition was identified as Mg63.2Zn32Ca4.8 by Inductively Coupled Plasma – Optical Emission Spectroscopy (ICP-OES) and compared to the results of ToF-ERDA (Time-of-Flight Elastic Recoil Detection Analysis). The composition of printed samples was also measured to estimate evaporation rates during printing.

X-ray diffraction (XRD) was used for phase identification. Differential Scanning Calorimetry (DSC) was used to characterize phase transitions and generate Time-Temperature-Transformation (TTT) and Continuous-Heating-Transformation (CHT) diagrams.

RESULTS: Three cold crystallizations were distinguished (see Figure 1) and analysed by XRD. Kissinger analysis[2] was done on four heating rates and the results are shown in Table 1 together with the transformation onset temperatures. Metastable phases were formed at the first (Tp1, phase Mg51Zn20) and third crystallisation (Tp3, IM4). Thermodynamic data and unit cell parameters from experimental results and literature[3] were used in a Python code to simulate CHT and TTT diagrams for comparison with the experimental results. The experimental data was used to validate the numerical model.

DISCUSSION & CONCLUSIONS: The overall transitions and phases agreed with previous literature, with only significant differences in the obtained energies of activation. The first simulated TTT diagram to our knowledge has been generated for this composition and phases. Synchrotron studies and more compositions should be tested for proper validation of the methodology.

REFERENCES:

[1] Radha, R., & Sreekanth, D. (2017), J. Magnes. Alloy, 5(3), 286–312.

[2] Kissinger, H. E. (1957) Anal. Chem., 29(11), 1702–1706.

[3] Mezbahul-Islam, M. et al. (2014) Calphad, 46, 134–147.

ACKNOWLEDGEMENTS: This work was supported by the Wallenberg Initiative Materials Science for Sustainability (WISE) funded by the Knut and Alice Wallenberg Foundation and the Swedish Research Council (grant numbers 2019-00191 and 2023-00155) supporting the Ion Technology Center.

Introduction

The use of biodegradable alloys in orthopaedic implants can reduce secondary surgical interventions, including those resulting from infections caused by bacteria attachment to inert implants. Mg alloys are promising materials, but a main challenge lies in improving their corrosion resistance to ensure their safe and effective use as large implants [1]. A widely explored system is the Mg-Zn-Ca ternary alloy. To achieve a further improvement in corrosion resistance of additively manufactured (AM) Mg-Zn-Ca alloys, a better understanding of the thermodynamic and kinetic phase transformations is needed. Hence, the aim of this study was to model the crystal nucleation and growth in Mg-Zn-Ca powders used for powder bed fusion with laser beam (PBF-LB), thereby providing critical insight towards designing more corrosion-resistant microstructures, while reducing the trial-and-error process that usually accompanies process parameter development.

Materials and Methods

Classical nucleation and growth theory (CNGT) was applied through an in-house Python code. CALculation of PHAse Diagrams-based (CALPHAD) thermodynamic simulations of the Mg-Zn-Ca system were used to predict steady state nucleation rates. X-ray diffraction (XRD) and Rietveld refinement was used to calculate cell parameters to use in the model. To validate our simulations of time-temperature-transformation (TTT) diagrams with experimental data, an extensive differential scanning calorimetry (DSC) study was realized, studying isothermal and variable heating rate thermal histories for different sized powders. The powder groups were three; denominated fine, medium and coarse, where the medium powder group had a normal distribution between 20 to 63 μm in diameter, which is typically used for printing. The fine and coarse contained particles out-of-range, namely those below 20 μm (fine) and those above 63 μm in size (coarse), typically considered waste.

Results

Constant heating rate DSC results revealed a difference in phase formation between powders with different size distributions, as shown in Fig. 1. Three common partial crystallizations are observed at around 150, 192 and 245°C with an additional peak at 290°C in the coarse powder, as well as an incongruent melting from 335°C. The additional peak in the coarse powder is also present in cast material in the literature [2]. The kinetics of these transformations were studied using the Kissinger method and were found to be in accordance with literature [3]. The experimental enthalpies of fusion were between 6.5 and 7 KJ/mol for all powders, which were substantially higher than expected from theoretical predictions. Results from the Rietveld refinement of the XRD patterns (Fig. 2) at different temperatures were comparable to previous studies on cast alloys [4]. Experimental points from isothermal DSC measurements overlapped with the simulated TTT diagram.

Conclusions

Experimental values from DSC and XRD allowed for the identification of the stable phases and their crystallization. As compared to cast material, there was one less transformation detected by DSC in fine and medium powders. The CNGT simulations gave good initial indicative results, but experimental validation was necessary for calibration of the model and to improve the accuracy of the predictions.

Aknowledgements

This work was partially supported by the Wallenberg Initiative Materials Science for Sustainability (WISE) funded by the Knut and Alice Wallenberg Foundation.

Magnesium (Mg) alloys are promising as resorbable implant materials, offering biocompatibility and gradual degradation in biological environments. To accelerate the development of these alloys for manufacturing processes such as Laser Beam – Powder Bed Fusion (LB-PBF) additive manufacturing, simulation plays a crucial role. However, the novel compositions of these alloys present challenges, as software such as ThermoCalc cannot calculate certain specific physical properties needed for the simulation of precipitation applying classical nucleation and growth theory (CNGT). The aim of this study is the development of an in-house Python code that allows the implementation of CNGT for a MgZnCa system, using TC-Python to access ThermoCalc’s databases.

The developed code currently centers on ternary systems but allows for easy modification to other configurations, such as binary systems. Two di5erent methods were used. The first method involves looping over a range of temperatures to obtain Gibbs energies and molar volumes. The data is stored in text files per temperature mapping the values in function of molar fractions. The second method extracts enthalpies and entropies of melting for each stable phase using a fixed point configuration to find the melting temperature of the system.

Key results include the determination of interfacial energies, nucleation rates, viscosities, and e5ective di5usion coe5icients. Some of these properties were calculated in function of the grain size, allowing for further calculation of growth parameters for a grain size distribution. These outputs formed the basis for constructing Time-Temperature-Transformation (TTT) diagrams with CNGT, although the rapid growth rates generated in the simulations showed that the precipitation kinetics need to be improved.

In conclusion, by integrating ThermoCalc’s database with an in-house Python code, limitations in calculating properties of MgZnCa were addressed. This approach enables precise and controlled modeling of interfacial energies and nucleation rates, supporting tools like TTT diagrams for optimizing the development of MgZnCa systems with LB-PBF. Further refinement of precipitation kinetics is needed to apply the results for real case processing.

This work was partially supported by the Wallenberg Initiative Materials Science for Sustainability (WISE) funded by the Knut and Alice Wallenberg Foundation.

Magnesium (Mg) alloys are gaining significant interest in biomedical applications, particularly as biodegradable implants. Mg alloys naturally dissolve in the body, eliminating the need for a second surgery to remove implants after healing [1]. Additionally, Mg is biocompatible and has mechanical properties close to that of human bone, which reduces stress shielding and promotes better bone regeneration. However, the main challenge lies in controlling the rapid corrosion rate of Mg alloys, which can lead to premature degradation. Their corrosion behaviour must be tailored to allow controlled degradation in sync with bone healing. Current research focuses on tuning of microstructure and alloying strategies, including eliminating the use of rare-earth elements (REEs) in the most recent studies[3], while maintaining the alloy’s biodegradability.

Additive manufacturing has potential for increased corrosion resistance through design of unique microstructures, while enabling additional flexibility in designing complex, patient-specific implants [2]. AM of Mg alloys is challenging due to the narrow processing window available to maintain the Mg between its melting and boiling temperatures, especially considering that oxides are also present in the initial powder. The aim of the present study is to accelerate the development and parameter optimization for the laser powder bed fusion (L-PBF) processing of a novel Mg-Ca-Zn-based alloy.

A finite element code implemented on Python was used to model the laser interaction with the material. The melt pool shapes were calibrated with experimental single tracks on printed samples. In addition, the application of the CALPHAD method and ThermoCalc databases were employed to predict the nucleation of stable phases during the solidification process with the classical nucleation and growth theory (CNGT).

Steady state nucleation of three main phases (Mg-hcp, Ca₂Mg₆Zn₃ and MgZn) was obtained using two different interfacial energies calculation methods. Local thermal history of single tracks calculated with the FE model were used to run the CNGT code. Growth parameters were also calculated with a first non-supersaturated approach.

This work represents the initial steps toward developing a comprehensive layer-by-layer simulation. Currently, the model remains semi-empirical, highlighting the need for further refinement. Future improvements should focus on leveraging experimental data for validation rather than calibration, ensuring a more robust and predictive simulation framework.

Magnesium is the lightest structural material and its alloys have attracted attention in research for aerospace and biomedical applications. As orthopedic implants one can benefit from their biodegradability, which could avoid secondary surgical interventions [1].

However, challenges still remain, e. g. in terms of their relatively low corrosion resistance and mechanical properties. Historically, this has been addressed by alloying with rare-earth elements (REEs). However, their production is not sustainable, mainly due to the environmental and health hazards generated in the extraction process. The end goal of this project is to decrease the corrosion rate of a REE-free composition based on Mg, Zn and Ca, by tunning its microstructure during additive manufacturing by powder bed fusion with laser beam (PBF-LB). To reduce the resource- heavy trial-and-error printing process, we aim to assist this development by numerical modelling of the composition of interest. A first step towards is the CALPHAD assessment of stable phases and the use of classical nucleation and growth theory (CNGT) to predict their precipitation in the undercooled liquid.

First system assessments using ThermoCalc TCMG7 database have showed HCP_A3, CA2MG6ZN3 and MGZN phases present at equilibrium. Here, a first approach without oxygen is undertaken due to the lack of information about magnesium oxides in TCMG7 and subsequent problems of databases compatibility. The composition shows a high glass-forming ability compared to various factors in literature [2], such as the deep eutectic point.

The melting temperature and enthalpy of melting were calculated and experimentally validated with DSC analysis. XRD results on PBF-LB samples showed peaks of crystalline Mg, but also some other intermetallic and oxides are in presence. WAXS was done to confirm the phases in presence in the final samples.

Due to limits in the ThermoCalc mobility databases and the results showing primarily HCP, only this phase and the liquid were used in the CNGT model using TC-Python. This code extracts Gibbs energy information from HCP and LIQUID phases (Figure 1) to calculate growth parameters at different temperatures for a future TTT diagram simulation. Driving forces are calculated and used to get the nucleation barrier of the precipitating phase.

Interfacial energies were obtained using enthalpies of melting to use the Turnbull expression [3] and were compared with calculations using extracted information from ThermoCalc (Becker’s energy approach). The viscosity of the liquid was simulated with ThermoCalc databases to obtain the high temperature viscosity constant and obtain growth parameters, but needs to be validated experimentally in future work.

This work was partially supported by the Wallenberg Initiative Materials Science for Sustainability (WISE) funded by the Knut and Alice Wallenberg Foundation, as well as by Sweden’s Innovation Agency through the AM4Life Competence Centre (grant number 2019-00029).

References

1. M. Mezbahul-Islam et al. “Critical assessment and thermodynamic modeling of Mg–Ca–Zn system supported by key experiments”. Calphad,46 (Sept. 2014), pp. 134–147.
2. Deok Kim, Byeong-Joo Lee, and Nack J. Kim. “Thermodynamic approach for predicting the glass forming ability of amorphous alloys”. Intermetallics 12.10–11 (Oct. 2004), pp. 1103–1107.
3. D. Turnbull, The Journal of Chemical Physics 1950, 18, 769.