*2.1.5 Rheocasting technique*

*Magnesium - The Wonder Element for Engineering/Biomedical Applications*

reactions between principal alloy and the additives [46–50]. The type and number of additives are chosen based on final formulation of the matrix and volume fraction of the reinforcement. Reaction temperature is a key parameter in process design of in-situ MMNCs to ensure the desired matrix and the reinforcement phase. An example of creating in-situ Mg-Zn/MgO composites includes the use of Mg and ZnO as starting materials and heating them to a predetermined temperature to

Mg + ZnO = > MgO + Zn (1)

 Mg + Zn = > Mg—Zn (2) Chelliah et al. [46] synthesized magnesium-polymeric derived ceramic (PDC) silicon carbonitride (SiCNO) nano composite by liquid pyrolysis using stir casting technique. The liquid poly (urea-methyl-vinyl) silazane (PUVMS) was used to formulate magnesium nano-composite. In this method, magnesium was melted in a steel crucible using a resistance furnace at a temperature of 700 °C and shielded with Ar-5%SF6 gas. The melt was stirred mechanically at 600 rpm to form vortex and the liquid PUVMS was injected to the melt. The melt was stirred for 15 min to ensure thorough pyrolysis. The liquid melt was bottom-poured into a rectangular metal mold preheated at 300 °C. Mg/nano- SiCNO composite was fabricated

The benefits of in-situ MMNCs include: (a) uniform distribution of the reinforcement, (b) elimination of particle wettability issue, and (c) clean and strong matrixparticle interface. The disadvantages of in-situ techniques, in general, are scalability and

ensure the feasibility of the following two reactions (Eqs. (1) and (2)):

exhibiting uniform distribution of the reinforcement (**Figure 4**).

the amount of reinforcements that can be created using the in-situ reactions.

**16**

**Figure 4.**

*Experimental setup of liquid pyrolysis stir casting [45].*

Rheocasting is a semi-solid casting method where the matrix is processed in liquidus-solidus (L-S) zone. In this so called semi-solid zone, the reinforcement particles are added, and the resultant slurry is thoroughly stirred to ensure uniform distribution of the reinforcement. Following stirring, the semi-sold composite melt is tapped into a permanent mold. Often cleaning and degassing of the slurry is carried out to avoid oxidation and formation of inclusions.

A MMNC of Mg (AZ91E) with Al2O3n (50 nm) was synthesized using a semisolid Rheocasting process [51]. The Mg ingots were placed in boron nitride coated mild steel crucible. The melt was formed in the metal crucible at 750 °C using electric resistant furnace. The slurry was degassed using argon to avoid oxidation. The reinforcement (Al2O3n) was then added to the slurry at semi-solid (L-S) temperature (~590 °C). The melt slurry was stirred using a mechanical stirrer. The MMNCs slurry was subsequently poured into a permanent mold for further characterization.

The benefits of this technique include: (a) spheroidal/equiaxed grains and no dendrites, (b) less shrinkage and porosity and (c) lower operating temperature.
