Effect of Extrusion Temperature and Isothermal Severe Plastic Deformation on Magnesium and its Alloys

Abstract

This study deals with the effect of extrusion temperature on ZX10, a high strength low alloy (HSLA) biodegradable material, from 180oC to 300oC at 40oC interval to study the impact on corrosion and mechanical properties in 3.5% NaCl saturated with Mg(OH)2. The effect of addition of neodymium (Nd) and calcium (Ca) in magnesium has also been investigated at fixed extrusion temperature of 300oC. All the results were then compared with commercially pure magnesium processed under similar conditions.

The microstructures, respective phases and corrosion morphologies were characterized by optical microscopy (OM), x-ray diffraction (XRD), scanning electron microscope (SEM) and electron backscattered diffraction (EBSD), respectively. The electrochemical corrosion behavior was investigated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) while immersion study was conducted using fishing line specimen arrangement to simultaneously measure corrosion rates from weight loss (Pw) and hydrogen evolution (PH) after 168 hours. The mechanical properties including tensile and compression properties were measured using a 2-ton Unitech R&B machine at fixed strain rate of 2 x 10-4.

As the extrusion temperature decreases, fine dynamic recrystallized (DRX) grains (~1.67μm) and elongated coarse un-dynamic recrystallized (unDRX) grains turned out to be more dominant. The binary neodymium alloy precipitates metastable Mg12Nd and thermodynamically stable Mg41Nd5 phases while ternary Mg-Nd-Ca alloys precipitated Mg2Ca and Mg41Nd5 phases. The Mg12Nd phase is known to act as weak cathode phase and is better for corrosion resistance.

Plastic deformation is a convenient method to change the microstructure and improve corrosion behavior of materials. Since magnesium and ZX10 extruded at 180oC has the best mechanical and corrosion properties in as-extruded condition, they were subjected to isothermal severe plastic deformation processes, such as screw rolling (SR) and multi-directional forging (MDF) at 300oC.

In case of ZX10, by observing screw rolled sample at 300oC, it is found that the corrosion rate was reduced appreciably with increase of yield strength. However, MDF increased the grain size along with increased porosity and Mg-Zn-Ca phase leading localized attack. When ternary Mg-Nd-Ca alloys were subjected to screw rolling, the corrosion resistance of EX33 was increased by impressively 78% at the expense of strength when compared with as-extruded condition. This elevated temperature decline in yield strength and increment of corrosion resistance could be associated with proportions of unDRX grains and reduction of second phases (galvanic cells).

Finally, the corrosion rates of studied materials were plotted on a corrosion model recently proposed in the literature. The calculated and predicted data values were found to be in good agreement for most materials.