Deformation at Low Temperatures

Low-temperature ductility is rarely observed in ceramics, which are inherently brittle, but some bulk ceramics show plasticity at ambient temperatures. One example of low-temperature plasticity in MgO is considered here. First, consider a single crystal, where i orientation-dependent properties are of interest. Orientation is one of the factors that influence mechanical properties. It was observed (by etch-pit technique) that the flow in MgO occurs on the 110 (110) slip system. However, it was also found [28] that the 110 (110) slip system contributes to deformation above 600 °C. Details on Plastic deformation in MgO single crystals were presented in Sect. 2.2, Figs. 2.33 and 2.38. Consequently, some information on deformation in polycrystalline ceramics may be of interest. 

In (a), the minimum critical resolved shear stress [henceforth CRSS] is 36 MPa. In (b), deformation occurs by a combination of delamination and kink-band formation in individual grains and also by shear-band formation. The multiple modes of deformation allow for plastic behavior in any arbitrary orientation of the compressive load. Notice that the 312- and 211-phases are layered hexagonal carbides and nitrides, having the general formula Mn+iAXn, (MAX), where n = 1 to 3, M is an early transition metal, A is an A-group element (mostly III A and IV A, or groups 13 and 14) and X is either carbon and/or nitrogen. 

Some aspects of the dislocation configurations in Ti3SiC2 polycrystalline ceramics are indicated in Fig. 4.7. 

The arrangement of these dislocations is parallel and they are positioned in different basal planes, one under another, so that the entire arrangement is normal to the basal planes and constitutes a low-angle boundary (a). Contrast analysis (b and c) reveals that the wall is composed of edge and mixed dislocations. For information on the various dislocation configurations observed during RT deformation, refer to the work of Barsoum et al. [14]. 

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One approach using deformation in tension is worthy of note. When the deformation at low temperatures is applied repeatedly the apparent modulus becomes lower until an equilibrium level is reached. Eagles and Fletcher20 described a dynamic low temperature test in which the test piece is continuously cycled in tension and the force monitored so that both initial and equilibrium moduli can be calculated. Furthermore, tests could be made at different applied strains. This method could undoubtedly provide more comprehensive precise data but, despite claims of better reproducibility, it was not adopted as a standard method, principally because it involved rather... 

It is noted, however, that both of the above CaF2 and Ti02 nanoceramics had some amount of porosity. This may account for an apparent soft behavior related to the superplastic deformation at low temperature, which does not yet reveal the plastic deformation characteristics in nanoceramics. Localized superplastic deformation under cyclic tensile fatigue tests was observed by Yan et al. on 3Y-TZP nanoceramics at room temperature [25], The micromechanism behind this phenomenon is argued to be essentially governed by grain-boundary diffusion. The contribution of dislocation slip might be in operation as a parallel mechanism to develop slip band-like microfeatures. 

It is well known that paramagnetic phase in austenitic stainless steels, such as an Fe-18Cr-8Ni, transforms into ferromagnetic a martensite phase by plastic deformation at low temperature [4,5]. Since the amount of the deformation-induced martensite increases at a larger... 

Figure 17.4b Veyssifere, P. and CBC. See also Veyssi re, P. and Carter, C.B. (1988) Dissociation of dislocations in MgAl204 spinel deformed at low temperatures, Phil. Mag. Lett. 57, 211. http //www.tandf.co.uk/journals Figure 17.10 Data from Kingery, W.D., Bowen, H.K., and Uhlmaim, D.R. (1976) Introduction to Ceramics, 2° Ed., Wiley, New York p. 740.

Stress-assisted transformation permits some p alloys to achieve a degree of low-temperatvire ductility otherwise unexpected in a bcc structure. For example, Ti 50Nb, whose deformation at low temperature is discussed in Sections 8.1.2 and 12.14.1, when tested to failure at 4.2 K, reveals the finely dimpled fracture surfaces characteristic of microscopic ductile fracture (Fig. 6.2). 

Do you intend to extend this program to include the added influence of mechanical stressing and deformation at low temperatures ... 

Thermal deformation of the material without load increases up to 700 C at which point begins its descending branch. Disabled deformations (with loading rates over 40%) indicate that the material is deformed at low temperatures (300 C) by over-charging and charging material itself disintegration of moisture loss when it is subjected to a heat source in the earliest stages. 

There is little information about dislocations in pyrrhotite, a mineral that can occur in several polymorphs, including monoclinic and hexagonal forms. Deformation experiments [103,391,412] at temperatures up to 500 °C to establish flow laws and to study textures have incidentally identified basal glide in hexagonal NC pyrrhotite. Fracturing contributes to deformation at low temperatures but decreases with temperature twinning occurs above 200 °C. 

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