Creep deformation microstructure

More, K.L., Koester, D.A. and Davis, R.F., (1991), Microstructural characterization of a creep-deformed SiC whisker-reinforced Si3N4 Ultramicroscopy, 31, 263-278. 

In this section, the basic dislocation processes involved in the progressive deformation of a crystalline solid are discussed briefly to provide background for the detailed discussion of the deformation microstructures observed by TEM in specific minerals to follow. Particular attention is given to relating the nucleation, glide, climb, multiplication, and interaction of dislocations to the various stages of the creep and stress-strain curves. More discussion can be found in the texts referred to in Section 9.1. 

Equation 6.14 provides a formal connection between creep crack growth and the kinetics of creep deformation in that the steady-state crack growth rates can be predicted from the data on uniaxial creep deformation. Such a comparison was made by Yin et al. [3] and is reconstructed here to correct for the previously described discrepancies in the location of the crack-tip coordinates (from dr/2 to dr) with respect to the microstructural features, and in the fracture and crack growth models. Steady-state creep deformation and crack growth rate data on an AlSl 4340 steel (tempered at 477 K), obtained by Landes and Wei [2] at 297, 353, and 413 K, were used. (AU of these temperatures were below the homologous temperature of about 450 K.) The sensitivity of the model to ys, N, and cr is assessed. 

K. Kogure, G. Sines and J. G. Lavin, Microstructure and texture of pitch-based carbon fibers after creep deformation. Carbon, 32[8], 1469-1484 (1994). 

Fig. 6.32 Microstructure of MgO before creep deformation (50 % nitric acid etch X250) [53]. With kind permission of John Wiley and Sons...

The objective of the research is to use ar aiytical and high resolution electron microscopy to characterize the microstructure of a SIC whisker reinforced SI3N4 ceramic composite before and after creep deformation. This work represents a collaboration with North Carolina State University and GTE Laboratories. 

K.L. More, D.A, Koester, and R.F. Davis, "Microstructural Characterization of a Creep-Deformed SIC Whisker-Reinforced SI3N4 Composite," presented at the Frontiers of Electron Microscopy in Materials Science Meeting, May 20-24,1990, Oak Brook. Illinois. To be published in LUframlcroscopy. 

During the progression of creep deformation, the density of defects increases further. However, at some point, defects will begin to be annihilated at sinks, or be of such a density that they inhibit the motion of one another in the microstructure. By either course, the strain rate will then decrease with time, resulting in the traditional configuration of primary creep (curves C and D in Fig. 18 or as illustrated in Fig. 17). 

It was observed that the kinetic parameters (the sinh term exponent and apparent activation energy) were similar between Eq 16 and like parameters for the 96.5Sn-3.5Ag solder as shown in Eq 13. In fact, a direct comparison of creep rates between the higher-order Pb-firee solders and 96.5Sn-3.5Ag solders at similar stresses and temperature indicated that the former were only slightly more creep resistant than the binary alloy. Quantitatively, that difference stemmed from the relatively small variations in a and the A constant. Mechanistically, it indicates that the AgjSn and Cu Snj phases did not have a significant role in the creep deformation, either explicitly or even indirectly, such as by altering the Sn-rich matrix microstructure. 

The relatively high sinh term exponent value in Eq 16, 5.0 0.8, indicates that the Sn-Ag-Cu(-Sb) solders behaved more like a simple metal than an alloy. Like the bulk tensile creep data of the Sn-Ag alloy, the AggSn and Cu Snj phases did not have a significant role in the creep deformation. Rather, the Pb-free solders exhibited the creep behavior of a simple metal because the microstructure of the Sn-rich matrix was largely responsible for time-dependent deformation. Lastly, the apparent activation energy of 59 8 kJ/mol for creep in the Sn-Ag-Cu(-Sb) alloys was similar to that of the binary Sn-Ag solder, indicating that a fast-diffusion mechanism was likely responsible for creep. 

Kerr and Chawla observed the microstructure of 96.5Sn-3.5Ag solder after creep testing using TEM techniques (Ref 71). Dislocations were observed to extend between AgjSn particles within the ternary eutectic regions. In the same study, the fracture surfaces of creep rupture samples (in shear) indicated that grain boundary sliding did not contribute significantly to creep deformation. 

The creep behavior of Pb-Sn solder joints show similar trends as are observed in the bulk material [77,78]. A study examined the effect of cooling rate on the subsequent creep deformation of 40Pb 60Sn solder [79]. A faster cooling rate resulted in a finer starting microstructure. The resulting impact on creep behavior was most significant under low stress (low strain rates) where the steady-state strain rate increased with the finer microstructure. 

For second-phase sintered ceramics, these phases control the plasticity and they are responsible for the asymmetric behaviour when deformed in tension or compression, because there is a crucial difference in the microstructure evolution associated with tension and compression creep. There are few explanations for this asymmetry. 

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