BCC metals , Ta, alpha Fe, Nb, and its alloys have good workability, but show cleavage features even with 25-35% elongation. Why ? what are the possible reasons ?
One of the possible reasons is the test method. Recent studies have shown that one-dimensional uniform strain is limited to studying materials to 25-35% deformation rates. This is due to the induced single-axis centering deformation. It does not allow for a free reversal of all crystals and an even degree of deformation. Barriers have been created that cause meat disruption and early destruction. Under real operating conditions, materials are loaded with more complicated schemes.
The four-dimensional tensile test circuits are used to plot the degree of deformation. This scheme makes it possible to define deflection rates of up to 80%.
You can look at my thesis and our surveys:
https://www.researchgate.net/publication/275336186_Thesis_SHEET_MATERIALS_TESTING_USING_BIAXIAL_HYDRAULIC_BULGE_TEST
https://www.researchgate.net/publication/236201153_Research_highligths_of_sheet_metal_testing_by_hydraulic_bulging
https://www.researchgate.net/publication/249922089_About_the_sheet_metal_testing_by_hydraulic_bulging
Research Тhesis: SHEET MATERIALS TESTING USING BIAXIAL HYDRAULIC BULGE TEST
Article Research highligths of sheet metal testing by hydraulic bulging
Article About the sheet metal testing by hydraulic bulging
Cleavage fracture can be observed on the fracture surface by the use of the Scanning Electron Microscopy only locally. When the elongation is at about 25 or 28 % the majority of the observed area is characterized by the formation of dimples due to microvoid coalescence that may surround the cleavage fracture facets which are obtained in regions that have possibly triggered the fracture.
Not always fracture surface is characterized by ductile dimples after elongation up to 25%. We studied high chromium martensitic steel at temperature of 77 K. The steel exhibited elongation of 22% but fracture surface was characterized by cleavages. Probably such behavior is attributed with high ability of high chromium martensitic steels to strain hardening. Initially flow stress is much lower than stress required for cleavage. Strain hardening occurs during plastic deformation and flow stress approaches stress required for cleavage. Plastic deformation occurs until work- hardening rate, dσ/dε is higher than flow stress. When work-hardening rate decreases to value of flow stress, Considère criterion is fulfilled, plastic deformation is localized in the neck and flow stress approaches stress required for cleavage.
In general, there is a significant difference in the variation of the yield strength with temperature in the bcc and fcc lattices. For bcc materials, there is a strong increase in yield strength as the temperature is decreased. The stress required for cleavage thus can be less than the yield stress at a sufficiently low temperature, and brittle cracking becomes the dominant mechanism below the so-called ductile-brittle transition temperature. In contrast, the yield strength of fcc metals does not show as much variation versus temperature, and fracture mechanisms remain ductile even at cryogenic temperatures.
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