*2.4.1 Microhardness test*

Microhardness test is also performed on as-received heat-treated and some of the deformed or ECAPed samples. The measurements were carried out at a load of 100gm and dwell of 13 s the microhardness was calculated using the expression [13]. Microhardness Model: MVH–S–AUTO from OMNI TECH, PUNE, INDIA.

#### *2.4.2 Tensile test*

The tensile test is used to evaluate the strength and ductility of as-received and equal channel angular extruded sample. Specimens were prepared according to the ASTM-E8 standard with 16 mm gauge length. The tensile properties of magnesium alloys were measured using UTM-Shimadzu AG-X plus™ equipped with 100 kN load cell and operated with a steady cross-head speed of 0.25 mm/min during all the tensile tests. Three samples were tested for each condition and uniaxial tensile testing was accomplished at room temperature and average reading was calculated and presented.

#### **2.5 Electrochemical corrosion test**

Corrosion study of AZ80/91 wrought Mg alloys was investigated using electrochemical corrosion analyzer, model: Gill AC-1684, supplied by Tech-science Pvt. Limited, Pune (India). The potentiodynamic polarization tests were conducted in 3.5 wt.% NaCl solution to estimate corrosion resistance or rate of corrosion of AZ80/91 wrought alloys. The auxiliary electrode (AE) was made of graphite (Gr) and the reference electrode (RE) was made of a saturated calomel electrode (SCE). 1cm2 area of the working electrode (AZ80/91 alloys) was exposed to the 3.5 wt.% NaCl solution. Before the electrochemical corrosion test, specimens were polished with 600, 800, 1000, 1200, 1500, 2000 grit emery papers and washed with ethanol. The specimens were kept in corrosion cell kit in NaCl solution for 20 min to stabilize the open circuit potential (OCP). Further, the AC impedance test of starting frequency 10 kHz and ending frequency 10 MHz with a scan speed of 5 mV/s and cyclic sweep experiments with −250 to +250 mV was carried on the electrochemical analyzer. Surface morphology of the corroded samples was examined by SEM. The corrosion product was removed using 200 g/L of chromic acid and 10 g/L of AgNO3 solutions. The corrosion rate of the alloy was calculated by using Eq. (1).

$$\text{CR} \left( \text{mm} / \text{y} \right) = 3.27X \, 10^{-3} \frac{i\_{corr} \, X \, A}{\rho} \tag{1}$$

where CR is the corrosion rate in miles per year, A is the molar mass (for magnesium 24.3 g/mol), *I*corr is the corrosion current density in μA/cm2 , n is the valance and ρ is the density (1.74 g/cm3 ).

## **3. Results and discussion**

#### **3.1 Effect of ECAP die channel angle on AZ80/91 magnesium alloy**

So far, many simulation studies have been executed to examine the impact of different die parameters on deformation homogeneity, strain rate, workflow etc. Although many researchers have been carried out on the efficiency of ECAP process routes and influences of various ECAP parameters on the strain behavior [14], there is limited work on a study of the effect of channel angle on grain size and other material properties through experimentally. In this chapter, the effect of ECAP channel angle on grain size, microhardness, tensile behavior and corrosion rate for different passes were analyzed using working temperatures of 598 K Furthermore, die A was used for examining above said material properties since this die gives the best results.

#### *3.1.1 Microstructure evolution of AZ80 Mg alloy*

The optical microstructures of as-received, homogenized at 673 K-24 h sample and those after ECAP processed specimens are shown in **Figures 6** and **7**. The microstructure of the as-received AZ80 Mg alloy presents the α-Mg and β-Mg17Al12 secondary phases along the grain boundaries indicated in **Figure 6(a)**. After homogenized at 673 K for 24 h secondary phases were partially dissolved along the grain boundaries as shown in **Figure 6(b)** this partial dissolution of secondary phases was achieved before ECAP and this sample is designated as 0P specimen. **Figure 7** presents the optical images of the ECAPed AZ80 Mg alloy processed

*Effect of ECAE Die Angle on Microstructure Mechanical Properties and Corrosion Behavior… DOI: http://dx.doi.org/10.5772/intechopen.94150*

**Figure 6.** *Optical images of (a) as-received (b) homogenized at 673 K-24 h.*

**Figure 7.** *Optical images for die A: (a) 2P (b) 4P and die B: (c) 2P and (d) 4P ECAP passes.*

through two ECAP die of 2 and 4 passes at 598 K processing temperature, in which the white and black contrast within the grains and along the grain boundaries represents α-Mg primary phase and β-Mg17Al12 secondary phases respectively. Also, the presence of α-Mg and β-Mg17Al12 phase in AZ80 alloys was confirmed through by the XRD analysis shown in **Figure 8**. The microstructure of the ECAPed Mg alloy showed significant grain refinement and bi-modal grains after ECAP of two passes for both die A and B, as shown in **Figure 7(a)** and **(c)**. These heterogeneous grains were typically obtained under the condition of lower deformation. When ECAP passes were gradually increased up to four passes bi-modal grain structure disappeared due to a large amount of induced plastic strain, as a result of the average grain size of ECAP-4P through die A was ~6.35 μm and the secondary phases are uniformly distributed throughout the material as shown in **Figure 7(b)**. Whereas ECAP-4P processed through die B exhibited slightly larger grains compared to

die A, the obtained grain size is of about ~9.77 μm. Hence, the effectiveness of grain refinement can be enhanced based on a channel angle, particularly, material processed through 90° channel angle exhibited better grain refinement.
