Optical Microstructure Analysis
Microstructural analysis of Solid solution and artificially aged Al-Zn alloy subjected to ECAP processing by route A was studied systematically. Figure 2(b) revealed an elongated dislocation cell structure with a few platelet precipitates of 120 nm and various fine spherical precipitates, including several GP zones as well as ή and a few η phases, after four passes of ECAP.Precipitates induced by the solid solution treatment prior to ECAP can speed up grain refinement and increase dislocation density by pinning dislocations [15]. Figure 2 shows standard micrographs of the Al-Zn alloy after post-ECAP heat treatments.The microstructure sample has nearly equiaxed grains that developed from the elongated dislocation cell structure after ECAP processing for 4 passes, as shown in Fig. 2(d).Since the elongated boundaries are not well-defined after 18 hour of artificial aged at 550oC sample, it is clear that the evolution is incomplete. In addition, there are several dislocations and dislocation tangles seen in Fig. 2. (c). Figure 2(c) shows that the microstructure is well-defined, with relatively equiaxed grains with an average size of 170 nm grains after increasing the holding time at 190oC to 16 hr.
Energy-dispersive X-ray spectroscopy (EDS, EDX, EDXS or XEDS), sometimes called energy dispersive X-ray analysis (EDXA) or energy dispersive X-ray micro analysis (EDXMA), is an analytical technique used for the elemental analysis or chemical characterization of a sample. Its characterization capabilities are due in large part to the fundamental principle that each element has a unique atomic structure. The microstructure and EDS spectra of eutectic phases of Al-Zn alloy subjected to Solid Solution treatment are shown in Fig. 3. It can be observed thateutectic Al-Zn alloy were mainly composed of laminar structures and massive structures. The EDS analysis result shows that the stoichiometries of the lamellar eutectic phase are 81.13% Al, 5.67% Zn, 2.5% Mg, and 1.4% Cu (molar fraction), as shown in Fig. 3, which are similar to the stoichiometries of Mg(Zn,Cu,Al)2 phases. The morphology and size of the massive Al7Cu2Fe phases show no change during the solution treatment due to its high melting point. While the base alloy shows discontinuity in the arrangement of precipitates in the matrix and along the grain boundary, the alloy after solid solution treatment continuous presence of precipitates in the matrix and along the grain boundary [20].
Microhardness
Table.2 shows the micro hardness values of Al-Zn alloy evaluated up to four passes through route A technique with an artificial ageing duration ranging from 0 to 20 hours.It has been observed that the hardness increases significantly up to 14 hours of ageing. Enhancement of hardness is due to the precipitates fragmentation and a rise in dislocation density and grain refinement [16].The micro hardness has increased from 67 Hv to 162 Hv from 0 to 14 Hr of aging duration ECAP processed samples in route-A which is the maximum among all aging duration.
Tensile Properties
Tensile tests were carried out in compliance with ASTM E8 at room temperaturespecimen dimension is shown in Fig. 4. Tensile properties were evaluated 0 to 20 Hr duration of artificially aged samples up to four passes ECAP processed in route A are shown in Table 2.Tensile strength continues to increase up to 16 Hr of aged sample at four passes compared to all artificial aging duration samples. These results are in line with the trend observed in hardness (Table 2). Route 'A' 16 Hr artificial aged sample shows UTS of 517 MPa after four passes which is almost 1.7 times higher than the untreated aged ECAP sample. This is due the additional presence of intermediate semi-coherent precipitates and intermetallic compounds such as CuAl2 may cause heterogeneities in the plastic flow during stress application. Additional tension is required in this method to shift the dislocation, which eventually results in an increase in strength [17].There was no definite trend in the decreased % of elongation of heat treated and ECAP processed Al-Zn alloy, but it was found to decrease with increasing ageing time.
Table 2
Mechanical properties of Al-Zn alloy after ECAP.
Artificial aged Condition, Hr
|
Average Microharndess, HV
|
Ultimate Tensile Strength, MPa
|
Tensile Yield Strength, MPa
|
Elongation %
|
O
|
67
|
297
|
279
|
10.42
|
2
|
87
|
345
|
289
|
10.67
|
4
|
96
|
369
|
344
|
10.8
|
6
|
105
|
440
|
427
|
10.91
|
8
|
108
|
460
|
437
|
10.37
|
10
|
122
|
497
|
480
|
8.8
|
12
|
137
|
502
|
489
|
8.02
|
14
|
162
|
509
|
494
|
7.7
|
16
|
158
|
517
|
497
|
7.93
|
18
|
143
|
490
|
464
|
9.15
|
20
|
136
|
487
|
460
|
9.35
|
Fracture Toughness Test
To determine the influence of grain size on Al-Zn alloy, fracture toughness test were performed on ECAP-processed samples in accordance with ASTM-E399 standards by using Bend Specimen SE (B).The testing specimens were taken from the ECAP processed billets' and cuts from central region using a wire cutting machine to achieve a specific geometry of Bend Specimen SE (B) with the dimensions shown in Figure. 5.A sharp crack was created by using a very thin wire with a diameter of 0.25 mm.Despite the fact that fatigue cracking was not carried out according to the standard, this study is still relevant since a comparative and qualitative analysis of the fracture performance of samples with similar geometry, material, and loading conditions was carried out.Constant loading rate of 0.05 mm/s was used to conduct a test [18].
Following the measures outlined in the ASTM standard, the force-CMOD (crack mouth opening displacement) plot can be used to determine the specimen's maximum sustainable load Pmax and PQ is the load at every point in the test record. As predicted because of the reduced ductility after ECAP, a linear relationship between force and CMOD was obtained for the processed samples.The force-CMOD curve for the artificial aged material was linearized according to the code. For linearization, a line with a slope of 5% less than the mother curve's initial linear portion is chosen.Pmax denotes the intersection point of this line with the curve. Table 3 shows the results for Pmax, with low data scattering serving as a strong indicator from a statistical standpoint.
Table 3
Pmax values for different ageing conditions ECAP processed samples
Material Aged conditions, Hr
|
Pmax, kN
|
0
|
0.685
|
2
|
0.745
|
4
|
0.812
|
6
|
0.866
|
8
|
1.049
|
10
|
1.066
|
12
|
1.158
|
14
|
1.239
|
16
|
1,278
|
18
|
1.098
|
20
|
1.053
|
The fracture toughness of each sample, KIC, can be calculated using the following equations, according to the ASTM-E399 standard:
The maximum applied load, specimen thickness, specimen width, span length, and crack sizeare represented by Pmax, B, W, S, and a respectively.For all specimens, the crack grows straight in the direction of the crack front, indicating symmetry and a pure opening mode loading state. After the fracture test, the specimens that failed are shown in Fig. 5.
Equation 1 is used to assess the fracture toughness of each sample. The determined Pmax and fracture toughness are within the valid range as indicated by the standard testing norm, taking into account the material and geometry conditions.The fracture toughness values of ECAP processed Al-Zn alloy samples were shown in Figure. 6. The fracture toughness of 2 hr aged condition samples improves from 12.94 to 21.68 MPa√m, as shown in this figure. This is attributable is due to longer ageing hours of Al-Zn alloy ultra-fine grain refinement in ECAP processed samples. After four passes, the fracture toughness of 16 hr aged samples is 81 percent higher than that of 0 hr aged samples.As indicated in our tests and in the literature, after a considerable reduction in ductility of material after 10 hr of aged condition the ductility remains approximately constant for the higher duration aging samples of ECAP processed samples while strength increases considerably between 12 hr to 18 hr aged samples.
In contrast, because of the higher grain boundary volume fraction, crack tip blunting and crack arrest play a much greater role in UFG materials' fracture resistance, resulting in increased fracture toughness. Furthermore, certain processes, such as grain boundary accommodation, grain boundary triple junction activity, grain nucleation, and grain rotation are intensifies, become more active as grain size decreases.Fracture toughness should improve after grain refinement because these mechanisms regulate the rate of plastic energy release at the nanoscale. Experiments on metallic materials have shown that after grain refining, the hardening area, such as the plastic zone, is widened.
Fracture Morphology Study
Fracture analysis of ECAP processed Al-Zn alloy for different ageing conditions was studied by taking photographs of fractured samples and high magnification micrographs were taken using SEM and are presented in Fig. 7. Photographs of fractured ECAP processed Al-Zn alloy samples shown in Fig. 7 depicts that cracks formed are not straight as seen in case of cast alloys. The cracks follow tortuous paths in most of the samples especially the one aged for longer ageing time durations. The formation of tortuous paths in these samples explains higher the fraction energy absorption capacity of these alloys is very high. Fine dimples and limited ductilefracture features are observed in Fig. 7 (a–f), which corresponds to the UFG ECAP-processed samples. This implies that, after the process, the number of dimples increased and the dimple size are reduced due to the grain refinement and work hardening. The shallower dimples are present in comparison with very deep holes in the annealed samples due to decrease in the ductility, which is attributed to the deformation in the UFG by the dislocation movements.