Effect of martensite volume fraction on fracture resistance

4.2.2.1 Evolution of fracture strain with martensite volume fraction

Figure 4.7a shows the effect of Vm on the fracture strain. The fracture strain decreases monotonically with increasing Vm. The scatter in the values of fracture strain is much less than the measurement of martensite volume fraction when Vm is larger than 28%. Since the tensile strength is increased with increasing Vm, the classical trade-off between tensile strength and fracture strain appears (Figure 4.7b).

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0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.2

650 700 750 800 850 900 950 1000 1050 1100 0.2

Figure 4.7. Evolution of fracture strain with martensite volume fraction (a) and fracture strain—tensile strength relationship (b) in QT-700.

4.2.2.2 Effect of martensite volume fraction on fracture behavior

Within this range of martensite volume fraction (from 15% to 37%), the DP microstructures generally fail by ductile fracture (Figure 4.8-11). The mean distance between dimple centers in QT-700-15%, 19%, 28% and 37% is about 2.9±0.7μm,

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a) b) Figure 4.8. Fracture surface of QT-700-15%.

a) b) Figure 4.9. Fracture surface of QT-700-19%.

a) b)

c) d)

Figure 4.10. Fracture surface of QT-700-28%. (a) and (b) are micrographs showing the

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center of fracture surface, while (c) and (d) are showing the features at the edge.

a) b)

c) d)

Figure 4.11. Fracture surface of QT-700-37%. (a) and (b) are micrographs showing the ductile fracture zone; while (c) and (d) are showing the transition zone with both

dimples and cleavage facets.

4.2.3 Brief discussion

4.2.3.1 Martensite volume fraction and damage mechanism

As presented above, damage accumulation rate is enhanced with increasing amount of martensite (Figure 4.1). Both the increasing rates of the density and area fraction of voids are enhanced with increasing Vm, which agrees with the conclusions in [52, 59]. Furthermore, as Vm is increased, the population of voids of all sizes is increased accordingly (see the comparison between QT-700-19% and QT-700-37% in Figure 4.2). In another words, QT-700-37% has a larger number of both large and small voids.

Actually, the effect of Vm is multi-fold. Regarding the microstructure, as Vm is

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increased, the average size of martensite islands is increased (Figure 2.23), and the banded distribution of martensite phase becomes significant until wide continuous bands are formed (Figure 3.1). An increase of island size leads to a higher probability of containing micro-defects, which tends to promote martensite fracture [99]. The martensite bands are figured out to be detrimental to damage resistance of dual-phase steels because of premature local fracture at these sites [98, 108]. It is observed that large voids are formed at the martensite bands (Figure 4.4-6) and they contribute of a large part to damage accumulation. Therefore, with an increased Vm, the damage accumulation is accelerated in that more large voids are formed at the large martensite phase. And there is a transition of dominating damage mechanisms that is schematically shown in Figure 4.12a and b, resulting from the increased martensite size and enhanced martensite connectivity.

Another consequence of higher Vm is that the flow stress is also increased. As a result of the higher connectivity and the efficient load transfer, the martensite phase bears higher stress, which leads to the earlier plastic deformation of the martensite (see the results in chapter 3). It is reasonable to assume that the small voids are formed at small martensite islands, and the small martensite islands are the less favorable sites for damage nucleation. However, the increasing stress in martensite by increasing volume fraction can promote the failure at such small martensite islands.

This point can explain the results in Figure 4.2 that QT-700-37% has also higher density of small voids than QT-700-19%.

As a summary, increasing Vm can increase the void density by the mechanisms that more large voids are formed at the wide banded martensite phase and that failure at small martensite islands is also promoted.

4.2.3.2 Martensite volume fraction and fracture behavior

Cleavage in DP steels has been reported to correspond to the ferrite grains but not the martensite phase [63, 104, 105, 157, 158]. This is also observed in this work (see Figure 4.23). The brittle fracture of DP steels is attributed to the interconnected martensite [104, 105] and/or to a coarse microstructure [63, 157, 158]. Comparing with the isolated martensite islands, the interconnected martensite in DP steels constrains the plastic flow in the ferrite matrix by confining the slip system [104]

and/or by imposing a high triaxiality state of stress. Once the martensite breaks, the cleavage crack in ferrite grain can be triggered due to the very large local stress

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building up at the crack tip.

The scale of microstructure is also important to understand the fracture behavior, and a coarse DP microstructure favors cleavage. Firstly, the large ferrite grains can lower the fracture stress, corresponding to the onset of cleavage, by enhancing stress concentration through dislocations pile-up [99]. Secondly, the martensite phase is larger in a coarse microstructure, producing larger initial cracks in the brittle phase. A large initial crack results in high stress intensity over larger region in the ferrite adjacent to the crack tip [159], which also increases the tendency for cleavage.

The results in this section show that DP steels mainly fracture in a ductile manner but cleavage becomes operating with high Vm. This trend is shown schematically in Figure 4.12c. With the increase of Vm, the connectivity of martensite phase should be enhanced. Also, the size of the martensite phase is increased (Figure 2.23), and the continuous wide martensite bands are formed (Figure 3.1). As discussed above, for the DP microstructure with high martensite fraction, the microstructure features favors the occurrence of cleavage in ferrite, which rationalizes the trend in Figure 4.12c.

But these points are not sufficient to reveal the full physical conditions for brittle fracture observed in this study. With the highest Vm, dimples are covering the major part of the fracture surface of QT-700-37%. Additionally, cleavage is not mixing with dimples and uniformly distributing over the fracture surface, but is only located at the edge of the tensile specimen (Figure 4.10c and Figure 4.11c). These observations indicate that ductile fracture is the intrinsic or dominating mode of failure in uniaxial tensile test but the occurrence of brittle fracture is probably due to geometrical and dynamic effects. A large main crack can be formed during the failure of tensile specimen, as shown in Figure 4.13 [92]. And the consequential high stress intensity and stress triaxiality at the crack tip can result in unstable and fast crack propagation.

Additionally, smaller load-bearing area remains after the formation of large crack, but the displacement speed is constant during tensile test. This leads to a high strain rate at the crack tip. The increased strain rate can increase the flow stress of ferrite [160], and an increased flow stress of ferrite favors the occurrence of cleavage [99].

As a summary, from the results in this section, the brittle fracture of DP steels can be triggered by martensite connectivity, size of martensite phase and also the geometrical and dynamic effects of crack propagation. However, since cleavage approximately occurs at the same stage as void coalescence, the fracture strain is still

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determined by the onset of void coalescence as for usual ductile fracture.

a)

b)

c)

Figure 4.12. Schematic representations of the evolution of damage initiation mechanisms (a), proportion of damage events (b), and fracture mechanisms (c) with martensite volume fraction. An accosiated increase of martensite size with increasing

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Vm is also shown in (b).

Figure 4.13. Formation of large crack in the tensile specimen before final fracture, which is observed in-situ in the SEM [92].

4.3 Influence of martensite composition on damage and