- There are many factors affecting cleavage fracture, such as temperature, loading speed, crystal structure, microstructure, size, shape, stress, stress state, sample shape, working environment, etc. of the second phase have important influence on the fracture.
The test temperature is low, the plastic deformation zone of the crack tip is small, the energy consumed when the crack propagates is small, the expansion resistance is small, and cleavage fracture is easily caused. Figure 4-34 is a schematic diagram showing the relationship between the test temperature and the cleavage stress and yield stress of a single crystal material. The material below Tc undergoes cleavage fracture before plastic deformation. On Tc, the stress first plastically deforms the crystal, and then cleaves the fracture.
Figure 4-34 Curve between test temperature and stress and strain, Tc is the critical temperatureIn addition to the influence of the test temperature, the cleavage fracture is also affected by the rate, and the high strain rate is favorable for the occurrence of cleavage fracture. The increase in loading speed releases the energy required to produce an unstable crepe, and the likelihood of cleavage fracture increases. For example, in high-speed impact and explosion conditions, some ductile materials often undergo cleavage and fracture.
The cleavage fracture usually occurs in a body-centered cubic, densely packed hexagonal lattice of metal or alloy, and the surface-centered metal and alloy generally do not undergo cleavage fracture. The coarser the grain, the more likely it is to cause cleavage fracture.The different cleavage fracture paths of the microstructure are different, and the cleavage fracture morphology is also different. The pearlite cleavage fracture river parallels the cementite sheet and expands along the interface of the iron body and the carbide. Rivers converge by continuously jumping from one interface to another. See Figure 4-35.
The ferrite fracture consists essentially of microscopic planes of {100} orientation, presenting typical river and tongue patterns. The upper bainite cleavage fracture is similar to pearlite, and the fracture facets pass through several upper bainite grains. The fracture path is mainly controlled by ferrite but is disturbed by carbide particles. The cracks in the lath martensite expand in approximately the same direction because the slab interface has a high dislocation density, which sometimes causes the cleavage plane to be microscopically uneven. The size of the facet on the fracture is the same as the size of the slat bundle.The effect of the shape and distribution of the second phase particles in the material on the cleavage fracture is that the coarse carbide particles promote cleavage fracture. Small and densely distributed particles, because they are difficult to form dislocations, do not become the point of initiation of cleavage fracture. Particle shape does not play a major role in cleavage fracture
Grain boundaries, subgrain boundaries, phase boundaries, twin boundaries and inclusions are prone to hinder the movement of dislocations, and the tensile stress caused by the plugging dislocations causes cracking.In addition, notches, cracks, shrinkage cavities, pores, machining marks and sudden changes in the cross section of the workpiece can promote the initiation of cleavage cracks.
Figure 4-35 Pearlite cleavage fracture × 10000
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