Different Effects of Varying Cryogenic Temperatures on Different Properties of a Biocompatible Mg-10Se Alloy

Abstract

Mg has high potential as a base metal for biocompatible metallic implants due to its light weight, biocompatibility, and mechanical properties that are similar to bone. In the present study, Mg-10Se was synthesized via a powder metallurgy method followed by cryogenic treatment (CT). It was found that cryogenic exposure to −20 °C (RF20) resulted in the best combination of damping properties (38.5% and 12.1% gains in attenuation coefficient and damping capacity, respectively) and compressive yield strength (16.7%), while liquid nitrogen (LN) treatment (−196 °C) resulted in the best ultimate compressive strength (10% increase to 260 MPa), energy absorbed during compressive testing (17.5% increase to 40 MJ/m³), and optimal corrosion rate (reduction of 59.7% to 0.273 mm/year). This study clearly highlights the role and importance of not just compositional control in improving properties but that of cryogenic treatment temperature to selectively enhance the individual properties of metallic materials to best meet end application requirements.

1. Introduction

Modern medicine increasingly relies on advanced technologies to restore, maintain, or improve human health. Among these technologies, biomedical devices—such as dental implants, joint prostheses, and cardiovascular stents—play a crucial role in treating injuries and chronic conditions [1]. Over the past few years, significant progress has been made in designing materials that can mimic or support biological structures, enabling innovations in both recovery and replacement therapies. Desirable traits of such materials include good physical and mechanical properties, alongside critical characteristics such as biocompatibility, biofunctionality, corrosion resistance, osteoconductivity, a low friction coefficient, and minimal wear rates [2]. Among these, metals stand out due to their relatively superior mechanical properties, making them ideal for surgical implants and load-bearing clinical applications [3]. Alloys and metal-based composites are widely used in medical settings because they can be tailored to meet specific requirements.

Table 1. Properties of various biocompatible metals and human cortical bone.

Material	Density (g/cm3)	Elastic Modulus (GPa)	Reference
Mg	1.74	40–45	[4]
Zn	7.14	108	[5,6]
Ti	4.5	80–125	[7,8]
Human Cortical Bone	1.8	10–27	[9]

summarizes the properties of base metals suitable for use in biomedical contexts and that of human cortical bone.

Magnesium (Mg), a widely available element, has garnered significant attention for its potential in biomedical applications due to its unique combination of biocompatibility, biodegradability, and mechanical properties. Mg-based alloys show potential as biomaterials suitable for implants, offering ideal biomechanical properties with elasticity similar to human femoral cortical bone [10]. They promote bone growth through osteoconduction and are gradually replaced by the body’s own tissue after bioabsorption, leaving no residual metals in the body [11]. The apparent lighter weight of these implants (owing to the lower density of Mg) is also an advantage [12].

Furthermore, Mg degrades and dissolves naturally under in vivo conditions [13,14], which raises the potential of such implants dissolving within 12 months and being fully replaced by endogenous tissue closely resembling bone within three years as approximate timeframes. Further benefits can be engineered, such as attraction of osteoclasts and osteoblasts due to its osteoconductive properties, facilitating the bone remodeling process and the formation of osteoid (non-mineralized bone matrix) [15].

As an essential element for the human body, Mg is widely involved in biological metabolism. Under physiological conditions, Mg²⁺ ions are absorbed and excreted by the body, avoiding long-term complications caused by permanent implants. In addition, Mg²⁺ can promote osteoblast activity, stimulate bone formation and accelerate healing while reducing the risk of postoperative osteoporosis. In cardiovascular applications, Mg-based vascular stents can not only provide initial mechanical support, but their degradation products can also regulate heart rhythm, improve blood flow, inhibit platelet activation, lower blood pressure, and promote muscle relaxation, reducing the risk of postoperative restenosis and thrombosis [13,16].

At present, excessive degradation is a challenge associated with Mg-based materials. To meet degradation rate as well as mechanical (load-bearing) requirements, alloying with biocompatible elements is a possible approach [16,17]. To this end, this work seeks to develop a Mg alloy matrix incorporating selenium (Se). Selenium is known for its excellent antibacterial properties and its ability to promote cell proliferation, thereby enhancing tissue integration [18]. It also improves bone metabolism by participating in redox processes. Given the osteoinductive properties of metal ions, researchers have explored the effects of coating or implanting selenium and selenium nanoparticles in bone structures and tumors [19]. Additionally, it has been integrated into orthopedic medical devices as an effective therapeutic strategy to promote bone reconstruction [20].

Aside from compositional control, further processing can also be employed to enhance properties; cryogenic treatment (CT), a thermal process whereby the materials are subject to subzero temperatures (typically ranging from −80 °C to −196 °C) is one such option, with a slow cooling rate selected to minimize thermal stress and ensure uniform temperature distribution [4]. After a set duration, the material is gradually warmed to ambient temperature [5]. The volume contraction and reduction in lattice constants caused by CT promote the precipitation of secondary phases. Energy is stored in the matrix in the form of residual stress, which helps generate significant compressive stress and stores deformation energy, resulting in more twinning and changes in crystal orientation compared to untreated samples, as well as higher hardness and strength [6]. Due to the improved corrosion resistance conferred by cryogenic treatment, it is currently used in medical applications such as scissors, needles, and other medical instruments [5].

The results of a literature search indicate that no studies have been conducted so far to evaluate the effects of different cryogenic temperatures on the microstructure, physical, thermal, mechanical and electrochemical properties of ecofriendly and biocompatible alloys such as those based on Mg-Se systems. This work was accordingly undertaken to bridge this knowledge lapse, aiming at providing valuable insights in the context of different cryogenic temperatures (−20, −80 and −196 °C).

2. Materials and Methods

2.1. Synthesis

Raw materials constituting Mg powder (60 to 300 μm, >98.5% purity, Merck Group, Darmstadt, Germany) and Se powder (74 μm, 99.999% purity, Alfa Aesar GmbH and Co. KG, Haverhill, MA, USA) were synthesized using the powder metallurgy (PM) method, beginning with mixing in a sealed container using a roller mixer (Maxstech Pte Ltd., Singapore) at a speed of 200 rpm for 1 h. The resulting powder mixture was then compacted in a hydraulic press at 600 psi pressure for 2 min to form a green compact with a diameter of 35 mm. Colloidal graphite was sprayed onto the compacts as a protective measure against oxidation, and they were then subjected to hybrid microwave-assisted sintering with a target temperature of 200 °C using a Sharp R898C (S) microwave oven (900 W rated power, Sharp Corporation, Osaka, Japan).

The compact was then re-coated with colloidal graphite and homogenized at 400 °C for 60 min. Hot extrusion was performed immediately afterwards using a die with a diameter of 8 mm at 350 °C. Subsequently, the extruded rod was cut into samples for further characterization. Microwave sintering and hot extrusion were performed in an ambient air atmosphere.

A set of samples without further treatment (as-extruded, designated AE) were characterized. Additionally, several sets of samples were subjected to CT and exposed to −20 °C, −80 °C, and −196 °C (by immersion in liquid nitrogen, LN) for 24 h as outlined in

Table 2. Cryogenic treatment conditions.

Treatment and Condition	Material Designation Suffix
As-extruded	AE
Refrigerated at −20 °C for 24 h	RF20
Refrigerated at −80 °C for 24 h	RF80
Refrigerated in liquid nitrogen for 24 h	LN

. CT duration was set at 24 h as this was shown in past studies to be optimum for property enhancement [6,7,8]. Where applicable, characterization and property comparisons were performed on samples before and after CT.

2.2. Characterization

2.2.1. Physical Characterization

In this study, the experimental density of five representative samples for each material was measured using the Archimedes method with an AD-1653 density determination kit mounted on a GH-252 electronic balance (AND Co., Ltd., Tokyo, Japan).

Elemental analysis was performed on samples weighing approximately 2 mg; these were digested in a solution of HNO₃ and HCl (1:3 ratio) in a microwave at 240 °C for 15 min and then topped up to 10 mL with H₂O. The resulting solution was then analyzed using a using a Perkin Elmer Avio 500 Inductively Coupled Plasma–Optical Emission Spectrometer (ICP-OES, PerkinElmer, Inc., Waltham, MA, USA) to obtain the actual weight fractions of the individual elements in the materials.

Porosity of the materials was determined by calculating the pore area fraction, obtained by color segmentation analysis of SEM images using the ImageJ software (version 1.54m, National Institutes of Health, Bethesda, MD, USA).

2.2.2. Microstructure Characterization

Microstructure was investigated through X-ray diffraction (XRD), grain analysis, and SEM surface analysis. The longitudinal surface of the samples was exposed to Cu Kα X-rays, with a scan range of 2θ = 10° to 80° and a scanning speed of 2°/min. The experiment was performed using the Shimadzu XRD-6000 equipment (Shimadzu Corporation, Kyoto, Japan).

Microstructure analysis was done on polished cross-sectional sample surfaces. Representative images were then captured using a field emission scanning electron microscope (FESEM) with energy dispersive spectroscopy (EDS). A Hitachi S-4300 device (Hitachi, Ltd. in Tokyo, Japan) was utilized for this purpose. Secondary phase analysis was conducted on SEM images utilizing color segmentation techniques within the ImageJ software.

Grain analysis was conducted after the etching outlined in

Table 3. Surface etching parameters for samples under different CT.

Condition	Etchant	Etching Duration (s)
AE	5% citric acid in H2O	18
RF20	2% citric acid in H2O	55
RF80	2% citric acid in H2O	15
LN	2% citric acid in H2O	12

with a Leica DM2500 optical microscope (Leica Microsystems (SEA) Pte Ltd., Singapore) used for image acquisition, following which the MATLAB software (version R2013b) was utilized to characterize the grain diameter in accordance with ASTM E112-13 (2021) [21] on a minimum of 100 grains per material.

2.2.3. Thermal Characterization

Thermal properties of the materials were evaluated through thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and coefficient of thermal expansion (CTE) analysis.

Samples with approximate dimensions of 2 mm × 2 mm × 2 mm were analyzed over a temperature range of 30 to 1400 °C, with a ramp-up rate of 10 °C/min. The tests were conducted in a purified air environment at a flow rate of 50 mL/min using a Shimadzu DTG-60H thermogravimetric analyzer (Shimadzu Corporation, Kyoto, Japan) to characterize the ignition response.

Samples of similar dimensions to those detailed above were subjected to a temperature range of 30 to 600 °C, with a ramp-up rate of 5 °C/min. The experiments were carried out in an argon gas environment at a flow rate of 25 mL/min using a Shimadzu DSC-60 digital scanning calorimeter (Shimadzu Corporation, Kyoto, Japan) to characterize the thermal response.

CTE was characterized on samples with a height of approximately 5 mm; they were tested over a temperature range of 50 to 400 °C, with a ramp-up rate of 5 °C/min. The tests were performed in an argon gas environment at a flow rate of 100 mL/min using a TMA PT1000 Thermomechanical Analyzer (Linseis Messgeraete GmbH, Selb, Germany).

2.2.4. Mechanical Characterization

Impulse excitation (resonance testing) was performed on 50 mm length cylindrical samples using the Resonance Frequency Damping Analyzer (RFDA) software (version 8.1.2, IMCE, Genk, Belgium) in accordance with standard ASTM E1876-15 [9]. A minimum of 5 recorded vibration response curves per material were used to calculate the materials’ damping behavior, response, and elastic modulus. Elastic modulus was characterized by using the following formulae [22]:

E = 1.6067 × (L³/D⁴) × m × f_f² × T′

where E is Young’s modulus, L is the sample length, D is the sample diameter, m is the sample mass, and f_f is the fundamental flexural frequency measured. T′ is the correction factor to account for the finite diameter of the rod and Poisson’s ratio (µ), represented by the following:

$$\begin{gathered} \text{T′ = 1 + 4.939 (1 + 0.0752µ + 0.8109}{\mathsf{\mu }}^2\text{) (D/}{\mathrm{L}}^2\text{) − 0.4883} {\text{(D/L)}}^4 \\ - {\displaystyle \frac{4.691\left(1+0.2023\mathsf{\mu }+2.173{\mathsf{\mu }}^2\right) (\mathrm{D}/\mathrm{L}{)}^4}{1+4.754\left(1+0.1408\mathsf{\mu }+1.536{\mathsf{\mu }}^2\right) (\mathrm{D}/\mathrm{L}{)}^2}} \end{gathered}$$

Microhardness was characterized by indenting polished sample surfaces in accordance with the ASTM E384 standard [23] with a load force of 245.2 mN, a dwell time of 15 s, and a total of 15 measurements per material. A Shimadzu HMV 2 automatic microhardness tester (Shimadzu Corporation, Kyoto, Japan) was utilized for this.

Quasi-static compression was done on flat and parallel samples with a length-to-diameter ratio (L/D) of 1, tested at a strain rate of 0.0083%/s, following the ASTM E9-09 standard [24]. A minimum of three representative samples were compressed using an MTS E-44 hydraulic tester (MTS Systems, Eden Prairie, MN, USA).

2.2.5. Electrochemical Characterization

To evaluate corrosion resistance, disks with a thickness of 1.5–2 mm were subjected to immersion tests in Phosphate-Buffered Saline (PBS) (Thermo Fisher Scientific Inc., Waltham, MA, USA) at 37 °C to simulate human body conditions. Weight loss data were recorded at 24 h intervals until sample disintegration or 28 days, whichever was earlier. Prior to this, corrosion products were removed using a solution containing 20 g of CrO₃ and 1.9 g of AgNO₃ dissolved in 100 mL of deionized (DI) water. Corrosion rate was calculated using the following equation [25]:

Corrosion rate (mm/year) = 87.6 × W/(D × A × T)

where W is the weight loss in mg, D is the experimental density in g/cm³, A is the sample (disk) surface area in cm², and T is the immersion duration in hours.

3. Results and Discussion

3.1. Density

The actual composition of the materials after elemental analysis deviates from nominal values (

Table 4. Actual elemental compositions of Mg-10Se alloy post-synthesis.

wt.% Mg	wt.% Se	Total %
89.93	10.07	100

). The initially added Se content was 15 wt.%. Significant Se loss likely occurred during processing (exposure to higher temperatures during the DMD and/or homogenization and subsequent extrusion process), an observation similar to what was exhibited by Se-containing Mg materials processed with the same method [26,27]. This underscores the importance of judicious compositional control accounting for such losses. Hereafter, the material will be referred to as Mg-10Se.

Table 5. Density of experimental Mg-10Se in the work.

Condition	Theoretical Density (g/cm3)	Experimental Density (g/cm3)		Porosity (%)
		Pre-Treated	Post-Treated
AE	1.849	1.887 ± 0.006		1.84
RF20		1.894 ± 0.004	1.901 ± 0.003	2.23
RF80		1.885 ± 0.008	1.879 ± 0.008	1.52
LN		1.873 ± 0.008	1.876 ± 0.008	1.22

outlines the density and porosity results; theoretical density was calculated using the rule of mixtures accounting for the actual elemental composition presented in Table 4. The experimental density was consistently higher than the theoretical density, attributed to the formation of secondary phases as the rule of mixtures assumes the absence of ordered phases [28]. Porosity was calculated by estimating the pore area fraction of the materials based on SEM images and is independent of density measurements.

From the density results, RF20 and LN treatment led to densification, which primarily resulted from significant porosity reduction due to pore collapse, internal compressive stresses, and void elimination with exposure to cryogenic temperatures, coupled with increasing dislocation density, which possessed potential to sink into existing pores [5,8,29]. However, from independent porosity analysis on selected areas, RF20 resulted in higher measured porosity, but there is a clear trend of a reduction in porosity with decreasing cryogenic temperature exposure, which aligns with the increase in magnitude of pore collapse and compressive stresses.

3.2. X-Ray Diffraction

Figure 1 displays the X-ray diffraction (XRD) pattern of the Mg-10Se samples. The peaks of the Mg-10Se material were compared against JCPDS cards found in the Powder Diffraction File (PDF-5+, 2024) [30], with the following card numbers: 00-004-0770 (Mg), 00-006-0362 (Se), 00-004-0829 (MgO), and 01-079-5183 (MgSe).

The results indicate the formation of MgSe during the synthesis process, as supported by Mg- and Se-containing regions within the microstructure [31]. Mg-10Se exhibits weaker prismatic and pyramidal textures, with the maximum peak intensity appearing at 34°, indicating a strong basal texture perpendicular to the extrusion direction (

Table 6. Crystallographic peak data of Mg-10Se.

Condition	Crystal Plane	Relative Intensity (I/Imax)		Intensity
		Pre-Treatment	Post-Treatment	Pre-Treatment	Post-Treatment
RF20	10-10 Prismatic	0.15328	0.19272	124	381
	0002 Basal	1	1	809	1977
	10-11 Pyramidal	0.46972	0.58068	380	1148
RF80	10-10 Prismatic	0.16604	0.14667	351	324
	0002 Basal	1	1	2114	2209
	10-11 Pyramidal	0.56433	0.50973	1193	1126
LN	10-10 Prismatic	0.17202	0.15754	359	371
	0002 Basal	1	1	2087	2355
	10-11 Pyramidal	0.3771	0.42081	787	991

). The XRD data demonstrates a notable increase in diffraction intensity after CT for specific crystallographic planes, particularly in basal textures. A further observation was the prominent presence of elemental Se peaks; thus, Mg-10Se could be classified as an alloy–composite (where elemental Se acts as micron-sized reinforcements as previously indicated).

3.3. Microstructure

Figure 2 shows the micrographs of Mg-10Se after different treatment conditions, along with selected regions in the microstructure.

Table 7. Elemental composition of selected spectrum locations.

Condition	Spectrum No.	Detected Element (wt.%)
		Mg	Se
AE	1	99.2	0.8
	2	50.6	49.4
RF20	1	98.3	1.7
	2	71.9	28.1
RF80	1	99.3	0.7
	2	58.1	41.9
LN	1	100	0
	2	34.6	65.4

outlines the EDS results for the selected spectra locations. The regions containing Se are primarily confined to areas with bright particles or near pores (dark/black areas), while the matrix regions in the background (in grey) are predominantly composed of Mg with little Se content. These results can be explained due to the low solubility of Se in Mg [32]. The regions with a high Se content can also be attributed to the formation of MgSe, supported by the XRD results in Section 3.1 and DSC results in Section 3.4.

Secondary phase refinement was also observed after the RF80 and LN treatment (

Table 8. Secondary phase data of Mg-10Se.

Condition	Area Fraction (%)	Average Diameter (µm)	Aspect Ratio
AE	7.3	3.7 ± 2.3	2.1 ± 0.8
RF20	8.2	4.2 ± 2.6 (↑13.5%)	2.0 ± 0.8
RF80	8.5	2.8 ± 2.2 (↓24.3%)	1.9 ± 0.7
LN	3.3	2.9 ± 1.8 (↓21.6%)	2.3 ± 1.2

, up to 24% refinement). After the RF20 and RF80 treatments, the area fraction of the secondary phase (ratio of bright region area against entire image) increased to 8.2% and 8.5%, respectively, suggesting that CT at these temperatures encouraged the precipitation of secondary phases [8]. Though changes in secondary phase size (from Feret’s diameter, by characterizing each particle) were observed, these remained well inside standard deviation overlap and are thus not significant; the morphology of such secondary phases was also unaffected, as shown by the nearly unchanged aspect ratio (ratio of length to width).

3.4. Grain Morphology

Figure 3 shows the grain morphology and

Table 9. Grain data of Mg-10Se.

Material	Condition	Grain Diameter (µm)
Pure Mg [35]	AE	34 ± 2
Mg-10Se	AE	20.2 ± 7.3
	RF20	18.8 ± 5.8 (↓6.9%)
	RF80	19.4 ± 6.3 (↓4.0%)
	LN	21.6 ± 7.9 (↑6.5%)

outlines the grain sizes of the Mg-10Se, which remained unchanged (considering the standard deviation) when exposed to different cryogenic temperatures. Compared to pure Mg, which was synthesized under the same conditions barring the sintering temperature (640 °C), the grain diameter of the Mg-10Se conditions was refined by more than 40%, attributed to Particle-Stabilized Nucleation (PSN) imparted by the addition of Se (which acts as micron-scale reinforcements) [26,33,34].

3.5. Thermal Response

Figure 4 and Figure 5 show the ignition and thermal responses results of Mg-10Se. Between 650 and 700 °C, all samples exhibited a sudden temperature spike, the point just before which was designated as the ignition temperature. Mg-10Se exhibited a decrease in ignition temperature (resistance) after CT. Notably, under LN conditions, their ignition temperatures dropped to 651 °C, a reduction of 7.3%

CT induces internal stresses and promotes the precipitation of secondary phases, making the oxide layer more prone to cracking and thereby reducing its protective ability. As a result, oxidation occurs more readily [36]. Additionally, the Pilling–Bedworth Ratio (PBR) of MgO is 0.81, indicating that the oxide film formed at high temperatures is inherently fragile and struggles to act as an effective barrier [36].

DSC analysis (Figure 5) detected a small positive (exothermic) peak near 550 °C, which is the attributed to the formation of magnesium selenide (MgSe) from Mg and SeO₂ [31,37]. This is supported by the SEM and EDS analysis as well as MgSe peaks detected by XRD (Figure 1 and Figure 2).

3.6. Coefficient of Thermal Expansion

Table 10. CTE of Mg-10Se.

Condition	CTE (×10−6/K)
AE	21.8
RF20	23.8 (↑9.0%)
RF80	21.2 (↓3.0%)
LN	25.1 (↑13.0%)

summarizes the CTE data. The results demonstrate that the addition of Se to Mg improves the dimensional stability (owing to lower CTE), being lower than that of pure Mg (26 × 10⁻⁶/K) [38]. This is noteworthy considering that Se has a relatively high CTE of 37 × 10⁻⁶/K [39], suggesting that the formation of MgSe controls the overall thermal expansion of the alloy [26]. The progressive decrease in cryogenic temperature did not reveal any trend, as the RF20 and LN samples showed higher CTE while the RF80 samples showed lower CTE when compared to the AE samples.

3.7. Damping Response

Table 11. Damping results of Mg materials studied in this work.

Material	Condition	Attenuation Coefficient	Damping Capacity	E-Modulus (GPa)
Mg-10Se RF20	Pre-treated	46.6	0.001738	46.1 ± 0.3
	Post-treated	65.1 (↑38.5%)	0.001948 (↑12.1%)	40.9 ± 0.1 (↓11.2%)
Mg-10Se RF80	Pre-treated	83.0	0.002439	43.7 ± 0.1
	Post-treated	53.2 (↓35.9%)	0.001852 (↓24.1%)	44.6 ± 0.1 (↑2.0%)
Mg-10Se LN	Pre-treated	72.5	0.001902	47.8 ± 0.1
	Post-treated	57.9 (↓20.1%)	0.001640 (↓13.8%)	45.5 ± 0.1 (↓4.9%)

summarizes the damping behavior of the Mg-10Se. The attenuation coefficient was obtained from analyzing the vibration response curves of each material and then applying an exponential best-fit curve with the form A = Ke^(−bt), where A is the amplitude, K is a fitting constant, e is Euler’s constant, t is the time elapsed (in seconds), and b is the attenuation coefficient.

RF80 and LN CT demonstrated reduced attenuation coefficients, indicating a lower capacity to dissipate vibrational energy. This can be attributed to CT densifying the microstructure, reducing porosity and defects, thus limiting pathways for energy dissipation [40]. However, this does not apply for RF80, and furthermore, the same trend is not replicated for RF20 Mg-10Se (which exhibited an improved attenuation coefficient and damping capacity optimally by 38.5% and 12.1% respectively), suggesting that there are other mechanisms at play which influence the damping performance.

Young’s modulus of the Mg-10 Se samples remained significantly higher than that of human cortical bone (E = 17 GPa) [41] in all cases. In load-bearing implants, a mismatch in stiffness may lead to stress shielding, where the implant carries most of the mechanical load, potentially causing surrounding bone resorption. However, in certain orthopedic or trauma fixation scenarios like temporary bone plates or screws, stiffer material may be beneficial to provide sufficient mechanical support during recovery [42]. The variation in elastic modulus with the decrease in cryogenic temperature did not show any trend; RF 20 and LN exposures reduced it, while RF 80 exposure marginally increased it.

3.8. Hardness

Hardness remained within the standard deviation overlap despite reductions after CT treatment (

Table 12. Hardness of Mg-10Se studied in this work.

Condition	Average Hardness (Hv)
AE	72 ± 5
RF20	70 ± 6 (↓1.7%)
RF80	68 ± 8 (↓5.5%)
LN	62 ± 7 (↓13.3%)

). This shows that CT may lead to a slight softening effect, previously reported in Al alloys where residual stress relief influenced hardness without significant structural changes [43].

3.9. Compressive Response

Compared with pure Mg (

Table 13. Compressive response of Mg-10Se.

Material	Condition	Mean 0.2% Compressive Yield Strength (MPa)	Mean Ultimate Compressive Strength (MPa)	Mean Fracture Strain (%)	Mean Energy Absorbed (MJ/m3)
Pure Mg [34]	AE	72 ± 5	174 ± 7	16 ± 2	23 ± 2
Mg-10Se	AE	85 ± 3	234 ± 3	23 ± 1	33 ± 0
	RF20	102 ± 2 (↑16.7%)	229 ± 2 (↓2.1%)	21 ± 0 (↓8.7%)	30 ± 0 (↓9.1%)
	RF80	101 ± 13 (↑15.8%)	244 ± 1 (↑4.1%)	23 ± 1	36 ± 0 (↑8.3%)
	LN	90 ± 5 (↑5.6%)	260 ± 6 (↑10%)	23 ± 1	40 ± 2 (↑17.5%)

), clear gains in compressive properties were exhibited for Mg-10Se [34]. This improvement is mainly attributed to grain refinement, which leads to the Hall–Petch effect [44].

CT brought upon further improvements in compression response; RF20 resulted in the best compressive yield strength (CYS, 16.7% increase) while LN optimized the ultimate compressive strength (UCS) as well as energy absorption (10% and 17.5% increase respectively). These improvements were attributed to several mechanisms, namely compressive stresses induced by CT, which strain lattices (resulting in a higher stress threshold to initiate dislocation motion) [45], reduce porosity (Table 5), and increase dislocation density [5,8,46]. Overall, LN stands out as the optimal CT for compressive response considering the work of fracture, which takes into account all the strength and ductility values in the context of biomedical implant use cases.

The macroscopic photograph of the AE Mg-10Se sample post-fracture (Figure 6) shows a single diagonal fracture with a fracture angle of approximately 45° to the compression axis. Figure 7 outlines the microscale features found on the fracture surfaces. Some cracks and transverse shear bands were exhibited.

3.10. Corrosion Response

As seen in

Table 14. Corrosion rates of Mg-10Se.

Condition	Overall Corrosion Rate (mm/yr)
AE	0.677
RF20	0.978 (↑30.8%)
RF80	1.440 (↑35.2%)
LN	0.273 (↓59.7%)

, Mg-10Se suffered a significant increase in corrosion rate under the RF20 and RF80 conditions (by 30.8% and 35.2%, respectively), which could be due to the rise in residual stress or lattice defects. In contrast, under the LN treatment, the corrosion rate dropped sharply (by 59.7%), likely because of a more stabilized microstructure or the formation of a denser protective surface layer [47], which would require future work to fully ascertain via surface analysis studies. This indicates that Mg-10Se demonstrates excellent corrosion resistance after LN treatment but suffers after CT at less extreme temperatures, highlighting a potential compromise in properties with different processing parameters. Further work is required in this area. It is also noteworthy that the corrosion rate attained after the LN treatment (0.273 mm/year) is near the optimal range for that of bioresorbable orthopedic implants (0.1–0.22 mm/year) targeting total dissolution within 1–2 years after implantation [48,49,50].

4. Conclusions

Based on the characterization of Mg-10Se in this work, the following conclusions can be drawn:

• Cryogenic treatment is effective in the densification of materials produced via powder metallurgy, as seen with Mg-10Se. Se loss is apparent due to its low melting temperature (221 °C), necessitating pre-emptive compositional control to achieve intended material formulations.
• Damping performance was best enhanced with the RF20 treatment, with 38.5% and 12.1% gains in the attenuation coefficient and damping capacity, respectively, while also lowering Young’s modulus, which reduces stress shielding.
• The RF20 treatment resulted in optimal CYS (16.7% increase), while the LN treatment resulted in the highest increase in UCS and energy absorbed (by 10% and 17.5% respectively). Overall, the LN treatment stands out as the optimal CT for the overall combination of compressive properties in the context of best work of fracture value.
• The LN treatment was shown to be optimal for corrosion resistance, leading to a reduction in corrosion rate by 59.7%.

CT was found to be effective in inducing property changes, which enhances the suitability of Mg materials for biomedical contexts. This is a significant finding in context, as the existing literature mostly focuses on steels and medical instruments rather than biomedical implants.

Given the potential for different CT temperatures to selectively alter different properties, there is scope for future work on optimizing cryogenic temperature exposure to enhance or tailor properties of Mg materials beyond compositional control, catering for unique demands of specific use cases. Furthermore, it is evident that CT results in differing effects on different Mg-based materials, necessitating further work on different classes of Mg materials to provide a strong basis or reference for industry applications, especially on otherwise new and novel compositions.