3.1. EBSD Analysis and Grain Boundary Strengthening
The microstructure characteristics of the two categories of Al-5Mg alloys after hot extrusion are shown in
Figure 1. The microstructure of as-extruded alloy 1, as shown in
Figure 1a, fully consists of fine, equiaxed grains with an average grain size of about 31.75 μm (
Figure 1e and
Table 2). By ranking the grain orientation spread (GOS) (
Figure 1c), recrystallized structure, substructure, and deformed structure were identified. There exist 74.3% recrystallized grains, indicating that nearly complete dynamic recrystallization occurred in alloy 1.
Figure 1b demonstrates the microstructure of the inverse pole figure (IPF) of as-extruded alloy 2, and there is a remarkably fine banding structure along the extrusion direction.
Figure 1d shows the banding structure as a combination of an initially deformed structure and a substructure, accounting for 33.9% and 44.3%, respectively.
The alloy 2 with a Sc content of 0.2% can form many Al
3Sc dispersoids to inhibit dynamic recrystallization, resulting in only 21.8% recrystallized structure in extruded alloy 2. This is consistent with previous findings that the addition of Sc has an anti-recrystallization effect in aluminum alloys [
19]. The microstructure of alloy 2 shown in
Figure 1b is much finer than that of alloy 1, even though the banding structure and the average grain size of alloy 2 are greatly decreased to about 5.31 μm.
Figure 2 shows the grain boundary misorientation (GBM) maps of as-extruded Al-5Mg alloys with different compositions. As shown in
Figure 2a,b, the green line (θ between 2° and 15°) represents low angle grain boundary (LAGB), and the blue line (θ ≥ 15°) represents high angle grain boundary (HAGB). The number fraction of LAGB in alloy 1 is extremely low at only 6.2%, while the number fraction of HAGB reaches 93.8%, as listed in
Table 3. The formation of HAGB is evidence of a dynamic recrystallization mechanism. Therefore, it can be inferred that recrystallization has maturely occurred during the extrusion of alloy 1.
Figure 2b reveals that the number fraction of LAGB in alloy 2 further increases by more than 7.5 times over alloy 1, and 47.2% grain boundaries are HAGB. By adding Sc to alloy 2, the average grain boundary angle is reduced to 21.70°, and the fraction of HAGB is reduced by 46.6%. It is suggested that the deformed structure and substructure with LAGB are retained in alloy 2 (
Figure 1d), which is beneficial to the improvement in tensile properties.
Previous research has shown that Al-Mg alloy is prone to dynamic recrystallization (DRX) during hot deformation, which is a kind of high-level stacking fault energy metal [
20]. In the early stage of hot extrusion, a large number of LAGBs (green lines in
Figure 2b) are formed with severe stress concentration, accompanied by the evolution of substructures (yellow grains shown in
Figure 1c,d). Then, the rate of partial dislocation formation increases on the LAGBs, resulting in an increase in the misorientation of the LAGB, which is converted into a HAGB (blue lines in
Figure 2b). Finally, dynamic recrystallization is completed, and new recrystallized grains are formed, red grains shown in
Figure 1c,d. There exist 74.3% recrystallized grains in alloy 1, indicating that maturely complete dynamic recrystallization occurred in alloy 1 (
Figure 1c). The addition of Sc in alloy 2 inhibits recovery and recrystallization during hot extrusion due to the pinning of grain boundaries by Al
3Sc particles, resulting in a banding structure composed of a deformed structure and a substructure (
Figure 1d) [
13].
In the above, alloy 2 containing Sc is partially recrystallized during hot extrusion, whose uneven fine grains have a great effect on the mechanical properties of as-extruded Al-Mg alloy. The high angle grain boundary strengthening (
) caused by the presence of HAGB in the alloy can be directly calculated by the Hall-Petch formula (Formula (1)) [
21]. While the low angle grain boundary strengthening (
) caused by the LAGB can be calculated by Formula (2).
and
are described by Formulas (1) and (2) as follows [
22]:
where
is the Hall-Petch coefficient.
,
,
, and
are HAGB fraction, LAGB fraction, average LAGB misorientation angle, and average grain size, respectively, and the values are shown in
Table 3.
and
are Taylor orientation factors,
G is the shear modulus, and
b is the Burgers vector whose values are shown in
Table 4. As can be seen in
Table 5, the sum of the yield strength increments due to grain boundary strengthening (
) in alloy 2 is 103.59 MPa, which is much higher than that of alloy 1.
is increased by a factor of 4.5, and
is increased by 73.4% compared to that of alloy 1. It is demonstrated that Sc has an excellent anti-recrystallization effect in Al-5Mg alloy during extrusion.
3.2. XRD Analysis and Dislocation Strengthening
Figure 3 displays the XRD patterns and full width at half maxima (FWHM) of as-extruded Al–5Mg alloys. As shown in
Figure 3c, a major increment in the intensity of the (111) fundamental peak compared to that of the other peaks is observed, which may indicate texturing of the microstructure along the (111) plane. FWHM of alloy 2 is wider than that of alloy 1 in
Figure 3b,d, and it shows that the lattice strain and dislocation density are increased in alloy 2.
The integral breadth analysis was used to calculate the coherent diffraction zone size and lattice strain from the XRD line broadening, leading to the following function [
26]:
where
is the measured FWHM,
is the peak maximum position,
is the wavelength of Cu-Kα ray,
is the size of XRD coherent diffraction zone, and
is the lattice strain.
Figure 4 shows the least-squares fit of the
against
for all measured peaks of as-extruded Al–5Mg alloys. The slope is
and the intercept is
. The size of the XRD coherent diffraction zone (
) and the root mean square lattice strain (
) were calculated, and the results are listed in
Table 6.
The relationship between dislocation density (
), coherent diffraction zone size (
), and square root of lattice strain (
) can be described by the following Equation [
27]:
where
is the Burger vector, and the value of
is 0.286 nm [
25]. The calculated dislocation density (
) values are listed in
Table 4.
The contribution of dislocation strength (
) to yield strength (
) can be calculated from Equation (3) [
28]:
where
and
are Taylor orientation factors,
G is the shear modulus, and
b is the Burgers vector. The values of these parameters are shown in
Table 4. The contribution of dislocation strengthening to yield strength in alloy 2 reaches 41.95 MPa, which is 15.09 MPa more than in alloy 1. It shows that the microalloying element Sc is beneficial in increasing the dislocation density of Al-5Mg alloy during extrusion deformation [
29]. At the same time, the Al
3Sc second phase particles can effectively pin the dislocations and further improve the strength of the Al-Mg alloy [
30].
3.3. Mechanical Property
Figure 5 depicts the room temperature tensile mechanical properties of as-extruded Al-5Mg alloys. It can be seen that the tensile strength (maximum resistance (R
m)) of alloy 2 is 380.7 MPa, which is 34.1% higher than that of alloy 1 without the Sc element. Meanwhile, the proof strength of non-proportional (proof resistance (R
p0.2)) increased by 93.0% over alloy 1. However, alloy 2 has a 46.2% reduction in elongation (A) compared to alloy 1.
Figure 6 illustrates the tensile fracture morphology of the extruded Al-5Mg alloys. The macroscopic morphologies of the tensile fracture of the two alloys show high ductility with typical cup-cone surface morphology in
Figure 6a,d, in which the shear lip region in the fracture of alloy 1 is wider than that of alloy 2, resulting in better fracture toughness of alloy 1 [
31]. It is found that the dimples of alloy 1 are deeper, and the dimple size is larger, as shown in
Figure 6b,c. While the dimples of alloy 2 are shallower, the dimple size is smaller, and the number of dimples is larger in
Figure 6e,f. It is inferred that alloy 2 has higher strength and lower elongation compared to alloy 1.
According to the microstructure characteristics of alloy 2 in
Figure 1b,d, it can be confirmed that the average grain size of alloy 2 is obviously refined, and a partially narrow banding structure is preserved, which leads to an increase in the hindering effect of grain boundaries on dislocation movement and further increases the strength [
32]. In addition, the existence of fine Al
3Sc particles in alloy 2 not only hinders dislocations from improving the strength but also causes stress concentration, resulting in a decrease in the elongation of alloy 2 [
33].