Creep in single crystals

However, it is not yet homogeneous throughout the entire structure (observe Fig. 6.10a and b) until reaching 9 % strain. At 9 % strain, this ceU structure is observed throughout the entire sample (Fig. 6.10c and d). 

Considering Eq. (6.4), the experimental value of the stress exponent, n, obtained for the Al204Mg single crystal is 3.9 with an activation energy of Q — 5.3 eV. The creep range of this single crystal is 0.65 T -0.71 T and it follows a dislocation mechanism of the creep law. More specifically, the values of 

The stress exponent of Eq. (6.4a) may be obtained by the known stress-jump method, giving for n  

The creep of materials can also occur solely by diffusion, i.e., without the motion of dislocations. Consider a crystal under the action of a combination of tensile and compressive stresses, as shown in Fig. 7.4. The action of these stresses will be to respectively increase and decrease the equilibrium number of vacancies in the vicinity of the boundaries. (The boundaries are acting as sources or sinks for the vacancies.) Thus, if the temperature is high enough to allow significant vacancy diffusion, vacancies will move from boundaries under tension to those under compression. There will, of course, be a counter flow of atoms. As shown in Fig. 7.4, this mass flow gives rise to a permanent strain in the crystal. For lattice diffusion, this mechanism is known as Nabarro-Herring creep. The analysis showed that the creep rate e is given by 

---

Having described creep in single-crystal MgO, now it is appropriate to illustrate the behavior of its polycrystaUine form as well. But first, note that the experimental results (see Fig. 6.30) resemble the shape found in Fig. 6.1b. In addition to the transient curve seen in the Fig. 6.30, this experimental transient curve is joined with the steady-state creep curve. The solid line represents the fitting of both these parts of the creep along the experimental points. The transient creep, k, is expressed by an exponential decay relation, given as ... 

D. W. Lee, I.S. Haggerty. Plasticity and Creep in Single Crystals of Zirconium Carbide. Journal of the American Ceramic Society 1969 52 641. 

DW Lee, JS Haggerty. Plasticity and creep in single crystals of zirconium carbide. J Am Ceram Soc 52 641, 1969. 

The previous sections have been mostly concerned with the dislocations and microstructures observed in single crystals deformed to various strains under known experimental conditions. In some minerals, notably quartz and olivine, the macroscopic deformational behavior, as revealed by the creep and stress-strain curves, can be understood in terms of the micro-structural evolution during deformation and, furthermore, certain quantifiable characteristics of the microstructure correlate with the imposed... 

At r <, creep in single and polycrystals of the studied f.c.c. and h.c.p. metals appears to be due to quantum fluctuation surmounting of crystal lattice barriers by dislocations by way of zero oscillations of dislocation segments. 

Figure 6.62 shows the polycrystalline data plotted as a log (s. Td exp Q/kT) versus log a to normalize the temperature and grain-size dependence. Q is taken as 5.7 eV. Data from other experiments are included in the plot, which shows that the creep resistance of single crystals is better than that of ZrOa polycrystals having a similar composition under all stresses <100 MPa. From the best fit of the data, a slope of n = 4.1 may be obtained. In single crystals, p = 0. At lower temperatures, n rises to 7.5 and Q is 7.5 eV which is greater than its value at high... 

The properties of a single crystal creeping wave probe The a single crystal creeping wave probe is suitable for testing various artificial defects such as surface cracks, FBH, columned hole and SDH etc. and its distance amplitude cruve is shown in Fig.6... 

While for many years, metal single crystals were used only as tools for fundamental research, at the beginning of the 1970s single-crystal gas-turbine blades began to be made in the hope of improving creep performance, and today all such blades are routinely manufactured in this form (Duhl 1989). 

On top of this alloy development, turbine blades for the past two decades have been routinely made from single crystals of predetermined orientation the absence of grain boundaries greatly enhances creep resistance. Metallic monocrystals have come a long way since the early research-centred uses described in Section 4.2.1. 

Kohlstedt D. L. and Goetze C. (1974). Low-stress high-temperature creep in olivine single crystals. J. Geophys. Res., 79 2045-2051. 

---