Chapter 4 Damage and fracture of dual-phase steels: experimental investigation and
4.2 Influence of martensite volume fraction on damage and fracture
4.2.1 Effect of martensite volume fraction on damage accumulation
4.2.1.1 Comparison of damage accumulation evolutions
Figure 4.1a and b reveal the effect of Vm on damage accumulation, quantified with (a) the void density and (b) the area fraction of voids. Only the voids larger than 110nm2 are taken into account for these plots.
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From the evolution of void density with thickness strain (Figure 4.1a), QT-700 with 15%, 19%, 28% and 37% of martensite all exhibit a continuous damage nucleation process. Indeed, the number of voids increases with increasing macroscopic strain.
Two considerations emerge from the comparison shown in Figure 4.1a. Firstly, the damage nucleation strain is only slightly decreased with increasing Vm. Some defects are initially present due to processing cold-rolled martensite (see Appendix A), and there is no significant damage evolution with deformation before necking. The damage nucleation strain here is defined as the thickness strain at which the number of voids starts to increase from the initial value. According to this definition, the strain at which damage starts to nucleate in QT-700-19% is about 0.12, while that of QT-700-37% is about 0.10, which are close to each other. Secondly, the damage nucleation rate is significantly increased by increasing Vm. This can be shown by the comparison of the slope of the curves in Figure 4.1a. As a result, for the same thickness strain, the void density is increased with increasing martensite volume fraction.
The area fraction of voids shown in Figure 4.1b is a characterization of both void density and void size, which is a general representation of damage accumulation. The area fraction of voids in QT-700-19% only starts to increase after a thickness strain of 0.25, while in QT-700-37%, it starts at around 0.1. This difference is much larger than that of damage nucleation strain defined above. However, for the same thickness strain, the area fraction of voids is also increased with increasing Vm.
Chapter 4 Damage and fracture of DP steels
Figure 4.1. The evolution of void density (a) and area fraction of voids (b) with thickness strain. Notice that only voids larger than 0.11μm2 are taken into account.
The effect of Vm on damage accumulation in dual-phase steels can also be revealed by the evolution of the void spectrum with strain. QT-700-37% has a higher flow stress than QT-700-19% during necking (Figure 4.2a). Figure 4.2b and c show the evolution of void size distribution in terms of void density and area fraction of voids for QT-700-19% and QT-700-37%. For QT-700-19%, there is a large population of voids smaller than 1.23μm2, but about 80% of the void area is contributed to by the voids larger than 1.23μm2. For QT-700-37%, there is also a large population of voids
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smaller than 1.23μm2, while about 50% of the void area is contributed by these small voids. That is, small voids are contributing more and more to the damage accumulation when Vm is increased. The void densities with various critical void sizes are all increasing with strain, indicating an on-going void growth process participating to the accumulation of porosity.
0.1 0.2 0.3 0.4 0.5
Chapter 4 Damage and fracture of DP steels
Figure 4.2. Evolution of microhardness (a), void spectrum of QT-700-19% and QT-700-37% with thickness strain in terms of void density (b) and area fraction of
voids (c).
4.2.1.2 Martensite volume fraction and damage mechanisms
Damage mechanisms in QT-700-15%
Figure 4.3a-d are SEM micrographs showing the damage observations in QT-700-15%.
The microstructure is significantly elongated and the distribution of martensite is aligned as a result of the large local deformation (Figure 4.3a). Elongated voids are observed and the so-called necklace coalescence [99] occurs in QT-700-15% (Figure 4.3a).
Both martensite fracture (Figure 4.3b) and interface decohesion (Figure 4.3c) operate as damage nucleation mechanisms in QT-700-15%. Notice that several cracks can be observed in a single martensite island (Figure 4.3b). Interface decohesion tends to occur at the triple junction between martensite island and ferrite grain boundary, and grows along the grain boundary as a void (Figure 4.3c) or propagates as a crack (Figure 4.3d). According to the observation, most of the damage occurrences are nucleated by interface decohesion in QT-700-15%.
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a) b)
c) d)
Figure 4.3. SEM micrographs showing damage mechanisms in QT-700-15%.
Damage mechanisms in QT-700-19%
Figure 4.4a-d are SEM micrographs showing the damage characterization for QT-700-19% specimen. As Vm is increased, the distribution of martensite islands is becoming more banded (see chapter 3). Large voids are observed to locate inside martensite clusters or bands due to premature growth and coalescence while small voids are formed at the isolated martensite islands (Figure 4.4a).
Both martensite fracture (Figure 4.4b) and interface decohesion (Figure 4.4c and d) are observed as damage nucleation mechanisms for QT-700-19%. In Figure 4.4b, the void formed from martensite fracture has deviated significantly from a penny shape, indicating substantial growth has occurred. Several observations support the statement that interface decohesion is probably initiated at triple junction between ferrite grain boundary and martensite islands (Figure 4.4c and d).
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a) b)
c) d)
Figure 4.4. Damage mechanisms in QT-700-19%.
Damage mechanisms in QT-700-28%
Figure 4.5a-d are SEM micrographs showing the damage mechanisms in QT-700-28%.
The comparison with the observations in Figure 4.3 and Figure 4.4 should potentially reveal the effect of martensite volume fraction on damage behavior of dual-phase steels.
With this volume fraction of martensite, wide continuous martensite bands have formed and large voids are observed to locate inside the bands (Figure 4.5a). Cavities nucleate as penny-shape voids by martensite fracture (Figure 4.5b) and this local fracture seems to be initiated from the edge of the martensite phase (Figure 4.5c).
The dominating damage nucleation mechanism for QT-700-28% is martensite fracture. But interface decohesion is still observed around small martensite islands and, again, is related to triple junctions (Figure 4.5d).
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a) b)
c) d)
Figure 4.5. Damage mechanisms in QT-700-28%.
Damage mechanisms in QT-700-37%
Figure 4.6a-d are SEM micrographs showing the damage mechanisms in QT-700-37%, which are supposed to be characteristic of the damage behavior of dual-phase steels involving large volume fraction of martensite.
Similar to QT-700-28%, large voids are located inside the wide continuous martensite bands (Figure 4.6a), and the coalescence between two adjacent large voids through martensite fracture is observed (Figure 4.6b). Penny-shape void can be formed by martensite fracture (Figure 4.6c), possibly along the block boundary as shown in Figure 4.6d.
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a) b)
c) d)
Figure 4.6. Damage mechanisms of QT-700-37%.