Compression Behavior of a Biocompatible We54 Alloy Reinforced by SiC

composite Abstract Magnesium based bio-composites and bio-alloys are used in biomedical applications such as bone fixation, cardiovascular stents, hip joints, screws/pins, and dental implants. Thence, the mechanical properties and corrosion behavior of magnesium-based bio-composites and alloys are of primary importance. In the last three decades, these properties were addressed to bust the development of modern Mg-based bio-composites for biomedical applications. Metallic and ceramic reinforcements such as Ti, Zn, TiO 2 , MgO, ZnO, ZrO 2 , TiB 2 , Al 2 O 3 , and SiC are known to be bioactive and bioinert. These, in turns, yield extra mechanical properties respect to the parent magnesium alloys with no reinforcements. Among the different technological and metallurgical processes of making such class of alloys, the most bio-compatible viable ones are powder metallurgy, melt deposition, and squeeze casting. In the present work, the microstructure and mechanical properties of WE54+15vol.%SiC under various compression temperature conditions were investigated by electron microscopy. Microstructure inspections revealed the formation of stable cuboid secondary phase particles, and lamellae and irregular-shaped intermetallic phases. A microstructure-based strengthening model was proposed and compared

bio-absorbable ceramics also have fast degradation rates compared to non-absorbable ceramics. Mg-based bio-alloys are considered to absorb within the human body at an appropriate resorption rate, but they also have a problem of a fast degradation rate during tissue remodelling, which limits their applications in clinical fields [13]. For this reason, Mg-based alloys are still considered to be a good choice in the fields of tissue engineering, orthopaedics and cardiovascular stents because of their suitable mechanical properties, reasonable biodegradation and lower toxicity [13].
Researchers have developed Mg-based bio micro-composites and bio nanocomposites to achieve the desired mechanical properties, optimized corrosion resistance, minimum cytotoxicity, and high biocompatibility [14][15][16][17][18][19][20][21]. Numerous opportunities and challenges still exist in developing Mg-based biodegradable composites and alloys for biomedical applications. In this sense, Mg-RE alloys (such as the WE series) provide better mechanical properties and corrosion resistance at both room and high temperatures [22]. Considerable improvement of the mechanical properties can also be achieved by reinforcement with ceramic particles or fibres. Metal matrix composites (MMCs) provide a substantial increase in strength and stiffness as well as creep resistance. The ductility of composites is significantly reduced as compared to unreinforced alloys. Bio-composites have been developed and used in tissue engineering, drug delivery, dentistry and bone implants because of their high performance. However, their low stiffness, poor mechanical properties, and inflammation issues during the implantation period have limited their use in the biomedical field [13]. The composite magnesium alloys usually contain at least two components, which are known as matrix and reinforcement. All the components of the composite must be biocompatible and nontoxic in any physiological environment.
The composite material allows the combination of matrix and reinforcement properties such as sufficiently high mechanical properties (tensile strength, elastic modulus, yield strength, compression strength), corrosion resistance and biocompatibility.
Thence, selection of matrix component and reinforcement component are very critical to obtain the desired properties [23,24]. In the present work a WE54 alloy added with 15%vol. SiC was tested by compression at high temperatures. Microstructure evolution was inspected by electron microscopy (TEM). The work presents a microstructure based strengthening model that was compared to the experimental compression tests carried out at temperatures ranging 50-to-300°C.

Experimental Procedure
The material used in this study was a WE54 magnesium matrix composite. Commercial WE54 (Mg-5wt.%Y-4wt.%RE, mainly Nd) alloy was reinforced by 15vol.% silicon carbide particles. Composite was prepared by squeeze casting technique. Compression tests were carried out at temperatures between room temperature and 300°C using an INSTRON testing machine. Cylindrical specimens of 8mm in diameter and 12mm long were deformed at an initial strain rate of 2.8⋅10 -4 s -1 . The microstructure was inspected by optical microscopy (OM) and by transmission electron microscopy (TEM).
Thin foils for TEM were mechanically thinned down to ~100 µm, punched, and then dimpled to a thickness of ~20 µm at the centre of the 3-mm disk. This was furthermore thinned to electron transparency using a precision ion polishing system (Gatan TM PIPS) with an initial tilt angle of 8°, followed by a running angle of 3°, at voltage of 4.5 V and cooled by liquid nitrogen. A Philips TM CM20 ® working at 200kV and equipped with a double tilt specimen holder was used. Detected secondary phase particles were identified by selected-area diffraction pattern (SAEDP).   It appeared that the microstructure remained stable up to 200°C, being quite like the one observed at room temperature (that is in the as-produced condition). Yet, the microstructure after the compression tests at 300°C clearly revealed a significant grain coarsening and secondary phase deterioration, that is coarsening of the reinforcing secondary phase particles. In addition, at 300°C compression test, most of the coarse secondary phase particles tend to align almost continuously at grain boundaries. This indeed correspond to a weakening morphology for the intergranular particles that can led to mechanical failure by grain decohesion.

Microstructure and Mechanical Properties
This microstructure degradation with compression temperature is ultimately believed to drive the corresponding mechanical failure of the present WE54-SiC alloy.
The microstructure inspections revealed the twin formation within the magnesium matrix. These were of nanometric scale and were found to be quite narrow and lying parallel to each other    That is, the precipitation hardening is an effective strengthening factor not only at room temperature but also at higher temperatures, MPa, after compression at 50, 150, and 300°C, respectively.

SiC Particle Composite Strength Contribution: According to
the shear-lag model proposed by Nardone and Prewo in [29], the composite particles do contribute to alloy reinforcement carrying a fraction of the load from the matrix. This alloy strengthening contribution strongly depends on the shape and morphology of the particles; it specifically depends on the particle aspect ratio [30].
Thus, the proposed relationship is (Eq. (4a)): where σ 0 is the unreinforced matrix yield stress, V V SiC the Sic particle volume fraction, L the particle size facing the load direction, t the mean particle thickness, A = L/t the particle aspect ratio. The Sic volume fraction, V V SiC , was determined using areal analysis (A A ) stereology method (ASTM EN-112). For equiaxed particles, or alternatively particles with 2D-shape close to circle, as in the present case, the Eq. (4a) reduces to Eq. (4b): A further strengthening mechanism acting in the composite WE54 alloy refers to the different thermal expansion coefficients (CTE) between the SiC particles and the magnesium matrix. This induces a dislocation density increment with the applied stress, yielding an additional strengthening contribution to the alloy.
The amount of the thermal stress induced by the presence of the reinforcement depends upon the particle volume fraction, morphology, and size, and on the effective temperature change.
Upon high-temperature compression, the relatively large thermal expansion coefficient between the matrix and the SiC particles creates a misfit strain at the SiC-Mg interface. Thermal stress can be partially released by the dislocation generation and accumulation in the surroundings of the reinforcement surfaces. Thus, according to [31][32][33] the induced extra dislocation density can be calculated as Eq. (5a): where C = 12 for equiaxed particles, ε = ∆α⋅∆T is the misfit strain, ∆T is the temperature variation, ∆α = 21⋅10 -6 K -1 is the difference between matrix and Sic thermal expansion, V V SiC the particle volume fraction, and t' the minimum size of the SiC particles. The thermally generated dislocation density yields a strengthening contribution of (Eq. (5b)): with α 1 = 0.35, and G = 17480 MPa is the shear modulus of Mg.
Since the average residual stress generated by the thermal expansion is of tension nature, it is a negative contribution to the strengthening to the magnesium composite alloy [34]. The different nature of the ceramic Sic particles respect to the metallic magnesium matrix induces geometrically necessary dislocations resulting in an additional strengthening contribution to the alloy. The resulting matrix-to-particle misfit depends on the reinforcement size and morphology [35]. The density of the geometrical necessary dislocations is given by Eq. (6a) [33,34]: where ε p = 0.28 is the plastic strain. The corresponding strengthening contribution is thus (Eq. (6b)): ( )  . This microstructure-based model was able to describe the microstructure factors contributing to the alloy strength at the different testing temperatures.