SEM micrographs of as received aluminium alloys reveal the intermetallic particles of several shapes and sizes.
E-DAX and X-RD analysis have been used to confirm that, as-received 6082(T6) alloys revealed five types of intermetallic compounds located at grain boundaries. Intermetallic phases are identified as α-Al15 (FeMn)3Si, Al9Mn3Si, Mg2Si, Al6Mn, and AlFeSi. E-DAX analysis indicated that those phases are generally contained, apart from Al, Si, and Mn, significant amounts of Fe is also present.
SEM micrographs of as received aluminium alloys reveal the intermetallic particles of several shapes and sizes.
Typical intermetallic particles distribution in as received condition and with various deformation stages, are depicted in Fig. 3. It shows, distribution of all intermetallic particles is strongly heterogeneous.
Figure 3 shows the SEM micrographs of shear band during progressive deformation (a) as received (b) 0.15t (c) 0.30t (d) 0.45t and (e) completely sheared specimen. To analyze the size effect, equivalent diameter (Deq) of the particle was estimated. The Deq was defined as diameter of a circle, which has the surface area equal to the surface area of a given particle.
In Fig. 3(a), as-received sample shows larger intermetallic particles. With progressive deformation, size of these particles reduces and count will increase subsequently, as shown in Fig. 3(b,c,d). Increase in imposed strain, enhances the number of fractured particles. The particle size (Deq) gets reduced with an increase in its distribution density. As seen in Fig. 3 (c, d) for deformation at punch penetration 0.15t, particles were scattered randomly over the entire micrograph. However, the microstructure after 0.30t deformation shows that the particles in the shear band are aligned with the flow direction. This phenomenon is more pronounced as the deformation proceeds. Figure 3(e), shows completely sheared off specimen. Ductile fracture in this alloy is a result of void nucleation, growth and coalescence as shown in the Fig. 3.
As observed, the particle size (Deq) has reduced after 0.15t of deformation and subsequently voids get nucleated. It is related to the particle fracture and fragmentation by means of void nucleation. The result of particle fragmentation is the geometrical change in particle size (Deq) and their relevant aspect ratio (AR) and angle orientation (θ), as discussed below.
3.1 Nomenclatures of intermetallic particles
For simplicity in understanding of fracture evolution through intermetallic particles, they are classified into four classes, with respect to progressive deformation. Figure 4 (a) and 4 (b) shows the schematic and actual nomenclature of particles, respectively i.e.
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Unbroken Particles (UBP) [Never broken particles]
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Voids (V) [Broken & pulled-out particles leads to voids formation]
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Prior Unbroken Particles (PUP) [union of all the fragments and still combined with voids]
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Fracture Particle Fragments (FPF) [Fully broken particles]
These were obtained from SEM microscopy and were performed using image analysis software ImageJ. To quantify particle distribution, such as particle size (Deq), shape (aspect ratio) and angle orientation (θ) were considered.
Figure 5 shows, all sizes of unbroken particles are present in a measured area of the shear band at maximum deformation of 0.45t. All sizes (Deq) of particles are present with different count per unit area (/µm2). The highest fractions (count/unit area) of Unbroken Particles (UBP) are in the range of 1.5 to 2 µm.
3.2 Effect of equivalent diameter (Deq)
Figure 6 (a, b, c, and d) shows the distributions of Deq of Unbroken particles (UBP), Voids (V), Prior unbroken particle (PUP) and Fracture particle fragment (FPF) with respective to progressive (punch) deformation in the rolling-thickness view plane.
Figure 6 (a) shows the measurement of only unbroken particles (UBP) i.e., equivalent diameter (Deq) distribution. As shown in Fig. 6 (a) with an increase in the strain the count per unit area of UBP decreases and the peak shifts towards the smaller value of its Deq. This means particles are breaking continuously with increase in the imposed strain.
Figure 6 (b) Shows, the count of voids increases with an increase in a strain. Void Deq shifts to the right side of the curve i.e. towards larger Deq, with an increase in strain. There is no significant change for the count of voids for the deformation stages from 0.15t to 0.25t.
From Fig. 6 (a) it can be seen that, particles are breaking at the deformation stages of 0.15t to 0.25t, but void count does not change much for this deformation stages as seen in Fig. 6 (b), this means that fragmentation of unbroken particles (UBP) is taking place without significant void growth for this particular deformation stage. For other deformation stages, there is continuous decrease of unbroken particles (UBP) and increase of voids (V) takes place with increase of strain, as shown in Fig. 6 (a) and (b).
In Fig. 6 (c), shows the equivalent size of the prior unbroken particles (PUP) which is a union of all the fragments and voids of a prior particle (Fig. 4). Depending upon the prior particle size, a particle can break into several pieces (with voids in between), each piece separately called a fracture particle fragment (FPF), a measure shown in Fig. 6(d). Since the prior unbroken particles (PUP) definition contains both the fragmented particles and voids, it is a combined measure of fracture and void growth. Up to 50% of deformation these particles equivalent diameter (Deq) falls in the range of 2 to 4 µm. After 50% of deformation, Deq range has increased to 4 to 6 µm, there is an increase in equivalent diameter (Deq) with imposed strain.
From deformation of 0.15t to 0.25t, there is an increase in breaking of larger size particles to a smaller size (Fig. 6a); therefore, the count of fracture particle fragment (FPF) also increases (Fig. 6d) when going from deformation of 0.15t to 0.25t.
However, on further increase in strain, the broken particles further fragment to provide yet smaller fracture particle fragments (FPF) (Fig. 6d). Thus, there is a change in the fracture mechanism from 0.25t onwards, where multi-fragmentation and re-fracturing of previously broken particles are taking place. Qualitetive and few quantitative analysis of fracture particles has been reported in literature [30–31].
Based on the above quantitative data, the particles fracture and void formation mechanism can be summarised as follows:
a. Initial Phase (up to 0.15t): Initial breaking of particles without a significant increase in voids. In this phase only the large particles are broken.
b. Intermediate phase (0.15t to 0.25t): In this phase not only large, but also smaller particles start to fracture. This phase is also characterized with a significant formation of voids,
c. Final Phase (0.25t to 0.45t): Re-fracturing and multi-fragmentation of already broken particles with large void growth.
d. Fracture Phase (beyond 0.45t): In this stage the various isolated voids coalescences, forming a large crack leading to sample fracture.
3.3 Effect of aspect ratio (A.R)
Figure 7 shows the SEM micrograph for prior unbroken particle (PUP) with major length ‘a’ and minor lengths ‘b’ and angle orientation ‘θ’ with respect to the compression axis. The detailed quantitative measurements of the particles aspect ratio (AR) and angle orientation (θ) were carried out.
The aspect ratio of particle is defined as the ratio of minor length ‘b’ to major length ‘a’. The angle ‘θ’ represents the orientation of particle with respect to radial axis on the sheet plane.
From Fig. 8, it is clear that the probability of fracture of a particle increases with an increase in the range of aspect ratio from 0.1 to 0.7. An aspect ratio much smaller than unity indicates elongated particles, while an aspect ratio closer to unity denotes more circular particles.
Almost all the particles with an aspect ratio in the range of 0.1 to 0.7 will undergo fracture regardless of deformation conditions. This observation tells that larger and elongated particles are more prone to cracking and is consistent with previous observations [32].Therefore, elongated particles are more effective in terms of initiation of fracture, as compared to circular particles of the same equivalent size. Also, more circular particles required more deformation to break, while elongated particles can break down at a smaller deformation.
3.4 Effect of angle orientation (θ)
Figure 9 shows the variation of orientation of prior unbroken particles (PUP) with respect to the loading axis. The prior unbroken particles (PUP) orientation angle varies from 0 to 90°. From deformation of 0.15t to 0.35t, there is no significant change in count per unit area but as the deformation increases, from 0.35t and 0.45t, there is a sudden increase in count per unit area for prior unbroken particles (PUP) as shown in Fig. 9.
The probability of fracture is more for particles oriented with their major axis nearly perpendicular with the loading axis. This is because of the plastic flow due to compressive loading of the sample results in tensile loading of the particle [33]. It is easier to cleave the particles along the minor axis than along the major axis. As a result of this, the cracks in the particles are also oriented in a narrow band of angles.
During the initiation of formation of the shear band, particles are randomly oriented in all direction with the angle from 0 to 90° from the reference plane (as shown in Fig. 9). After more than 50%t deformations most of the prior unbroken particles (PUP) are oriented with maximum count ranging from 20 to 60°. It observed that prior unbroken particles (PUP), density decreases as orientation angle deviates from 20 to 60 degrees.
3.5 Evolution of prior unbroken particles (PUP)
Figure 10 (a) Shows that, the relation between all four stages of nomenclature i.e. unbroken particles (UBP), voids (V), prior unbroken particles (PUP) & fracture particle fragment (FPF). As the shear strain increases there is an increase in voids (V) and prior unbroken particles (PUP). It is clearly visible from the diagram that, after the shear strain of 2, there is drastic increase in fracture particle fragments (FPF), similarly from the same point there is a drastic decrease in unbroken particles (UBP), so it shows that UBP and FPF are functions of shear strain.
From Fig. 10 (a), it is seen that beyond 2.4 strains, there is a sudden increase in count of fracture particle fragment (FPF). This is strongly correlated to decrease in unbroken particles (UBP) count at the same strain level.
Figure 10 (b) shows that for particles which are having an equivalent diameter (Deq) less than 2µm; there is no drastic change in count per unit area with the increase in percentage deformation over the entire strain range (only a slight increase in the count can be seen after 50% of deformation). However, for larger particles (Deq >2 µm) there is a significant increase in count per unit area with increase in strain. Nonetheless, increase in count per unit area with increase in strain for the larger particles (Deq >2 µm) saturates at high strain. This indicates that the larger particles have a major contribution in the fracture in the shearing process throughout the deformation.
From Fig. 11 (a), it can be seen that the mean of unbroken particles (UBP) decreases marginally, whereas mean of voids increases gradually. Also, the mean of prior unbroken particles (PUP) decreases as shown, but the mean of Deq of fracture particle fragment (FPF) decreases more gradually, which means that the multiple fragmentations are occurring with an increase in imposed strain, and FPF are breaking down into the smaller particles as the deformation proceeds. Usually, larger particles nucleate voids at much lower strains than smaller particles, and void growth takes place more rapidly at larger particles as shown in Fig. 11 (a).
The mean size of fracture particle fragments (FPF) decreases with increase in the strain (Fig. 11a) this supports the hypothesis that larger particles are first to fracture and the extent of damage in the smaller particles size range gradually increases with strain.
The volume fraction of the fracture particle fragment (FPF) increases drastically with an increase in strain. This non-linear growth rate indicates a change of fracture mechanism at around 1.25 strains as seen in Fig. 11 (b).
3.6 Particle cracking
Figure 12 SEM micrograph shows the cracking of particles from an interrupted test, at the deformation of 0.45t. It demonstrates the void growth process in shear zone. Note that the macroscopic cracks have not yet reached to this region. Figure 12 (a, b) presents the enlargement of a particular group of voids. Figure 12 (c) depicted the process of void coalescence.
The nucleation of voids occurred by fragmentation and re-fragmentation of the particle-matrix interfaces. Usually, particles which have more equiaxed shape nucleate voids by interfacial de-cohesion while particles with more irregular shapes and large aspect ratios often break by internal fragmentation as shown in Fig. 12(d).
Table 2 Summary blanking shear fracture process
As discussed earlier, the fracture in blanking process can be summarised as shown in Table 5.1.
3.7 Crack initiation and propagation
Figure 13 (a) Shows the enlarge view of crack initiation and propagation in the shear band. Crack has initiated from the punch side, as in this case punch is sharper than the die. Cracks are found almost parallel to the loading axis.
3.8 Conclusions
A detailed microstructural analysis has been carried out to quantify the fracture evolution of intermetallic particles present in AA6082 (T6) alloys, as a function of strain. In asymmetric blanking (punch side is sharper than the die side) crack initiates in the shear band from the sharper (punch) side. Analysis of the experimental data leads to the following major conclusions on fracture of particles and crack formation.
1. Intermetallic particles trigger the fracture process in blanking. Larger, elongated and unfavorably oriented particles fracture first (Initial phase) and with progressive deformation (blanking), smaller and rounded particles start breaking.
2. There is a change in the fracture mechanism (Final phase) with higher deformation, where multi-fragmentation and re-fracturing of previously broken particles are taking place which enhances the fracture process.
3. The broken particles become the crack nucleus for matrix cracking and coalescences.
4. Shear fracture process can be summarized in four phases as, (a) Initial phase: shows the fracture of large, elongated and unfavorably oriented particles. (b) Intermediate phase: shows more particle fracture and void formation. (c) Final phase: represents multi-fragmentation (Re-fracturing) of previously broken particles with large void growth. And (d) Fracture phase: finally void coalescences and sample fracture takes place.