Fractographic observations of cleavage initiation in the ductile-brittle transition region of a reactor-pressure-vessel
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Fractographic Observations of Cleavage Initiation in the Ductile-Brittle Transition Region of a Reactor-Pressure-Vessel Steel Fig. 7--Microstructure of 78.2 pct Zn-AI ice-water quenched and deformed 10 pct in tension at room temperature. TEM 200 kv.
absence of slip bands on the surface. The dislocation's density was observed to be the same before and after deformation (103 dislocations/cm2). The grains remain equiaxed and of the same size after deformation (Figure 7). It seems to be that deformation of these alloys at room temperature takes place by grain boundary sliding as they do at high temperature. Fracture at room temperature inhibits further deformation. One approach to explain the substantial difference between the yield stresses of the ice-water quenched material, when tested in compression or tension, is to consider that the shear stress for one grain to glide over another consists of two components: one is the shear stress (~'c) required to overcome the cohesion between grains, and the other is a frictional strength (/z) which is proportional to the normal stress (tT,,) on the shear plane. Thus, we may write 7 = %+p.o-n. The term /~r, is expected to be bigger in compression than in tension. When the material is tested in compression, there is a compressive normal stress (tr,) on the shear plane that increases the "packing" of the grains, increasing the friction. The bigger the friction, the more difficult it is for them to slide over one another. In tension this normal component of the stress is going to work in the reverse way, reducing the friction. The pearlite structure is not very sensitive to this "packing effect" because it is composed of "flakes" instead of grains, so its yield stress in compression is almost the same as in tension.
This work was partially supported by the Organization of the American States. We thank Dr. T.G. Langdon for his critical review of this paper. REFERENCES 1. J.W. Edington, K.N. Melton, and C.P. Cutler: Progr. Mater. Sci., 1976, vol. 21, pp. 61-170. 1934-- VOLUME 14A, SEPTEMBER 1983
A.R. ROSENFIELD, D.K. SHETTY, and A.J. SKIDMORE Unstable cleavage fracture of steels in the lower-shelf region is generally viewed as a process governed by a critical value (0~) for the maximum tensile stress (O-yy).Micromechanisms postulated for cleavage crack initiation and extension include cracking of grain-boundary carbides by slip or twinning and critical extension of these carbide cracks into the adjoining ferrite grain. ~It has generally been found that the critical tensile stress, 0-~, is independent of temperature and strain rate when carbide-crack nucleation is slip-induced. 2 These results and an additional postulate that O~y must exceed o~ over a "characteristic microstructural dimension" ahead of a precrack are the bases of the Ritchie, Knott, and Rice 3 model for temperature and strain-rate dependence of fracture toughness (Kit) in the lower-shelf region. The model has since been extended and applied to nuclear pressure vessel steels 4 as well as other steels. 5 In the duc
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