Microstructure twinning

Examination of the microstructure of the cavitated surface will commonly disclose evidence of deformation such as twins (Neumann hands) in carbon steel and general cold working in other metals (Case History 12.6). Damage from cavitation can be differentiated from attack by a strong mineral acid, which can produce a similar surface appearance, by observing the highly specific areas of attack characteristic of cavitation. Acid attack is typically general in its extent (Case History 12.6). 

Microstructural examinations revealed deformation twins (Neumann bands) in metal grains at wasted surfaces. The surfaces in these areas have a jagged, undercut profile. 

Some of the above discussed precursor phenomena are also observed prior to diffusion driven phase transformations. A typical example are the conventional EM tweed images obtained in the tetragonal parent phase in high Tc superconductors and other ceramics. In a recent survey by Putnis St e of such observations it was concluded that in these cases the tweed contrast resulted from underlying microstructures fomied by symmetry changes driven by cation ordering. These symmetry changes yield a fine patchwork of twin related domains which coarsen when the transfomiation proceeds. However, in view of the diffusion driven character of the latter examples, these cases should be clearly separated from those in the field of the martensites. 

In most specimens, albite twinning predominates over pericline twinning. Generally, the albite twinning is on a very fine scale and occurs in domains separated by untwinned domains, arranged to form several types of overall microstructure (McLaren 1984). 

A somewhat more complex exsolution microstructure involving albite twin lamellae in a peristerite of bulk composition Ab9i.3An4.oOr4.T was studied by Gjonnes and Olsen (1974) using a TEM fitted with an energy dispersive x-ray spectrometer (Chapter 7). In this specimen, the twin... 

Calcite and dolomite form large parts of the sedimentary continental crust. Consequently, their mechanical properties have been studied in some detail, and Wenk et al. (1983) have reviewed the rheology and associated microstructural development. Deformation takes place by both twinning and dislocation glide. 

Microstructures in deformed dolomite. The deformation characteristics of dolomite are markedly different from those of calcite and have been studied in detail by Barber, Heard, and Wenk (1981). Not only are the twin laws different, but twinning in dolomite occurs only at temperatures above about 250°C. The lower dislocation densities observed in twinned dolomite and at twin intersections is perhaps due to the greater ease of stress relaxation at the higher temperatures required for twinning. 

The deformation microstructures of monoclinic pyroxenes are considered in Section 9.9.2. Optical microscope observations (Griggs, Turner, and Heard 1960 Raleigh 1965) indicate that the dominant slip system in monoclinic pyroxenes is also (100)[100]. However, van Duysen and Doukhan (1984) found by TEM that in naturally deformed a-spodumene the activated slip systems are (110) [001] and [lT0) <110>. In specimens of a-spodumene deformed by scratching, they also observed interesting microstructures of dislocations and faults that may be related to the twins observed in deformed diopside by Kirby and Christie (1977). 

McLaren, A. C., Pitkethly, D. R. (1982). The twinning microstructure and growth of amethyst quartz. Phys. Chem. Minerals, 8, 128-35. 

Coarser biogenic ZnS nanoparticles are commonly twinned on very fine scales. We suggest that these microstructures arise as the result of an aggregation-based crystal growth mechanism (see below), probably commencing with clusters analogous to those described by Luther et al. (1999). 

Minerals are often riddled with microstructures when observed under the electron microscope. Although the observation and classification of microstructures such as twin boundaries, anti-phase boundaries, exsolution lamellae etc., has been a longstanding activity of mineralogists and crystallographers it has only been very recently that we started to understand the enormous importance of microstructures for the physical and chemical behaviour of minerals. 

Figure 6. Sequences of snapshots of the simulated twin microstructure corresponding to different armealing times in alkah feldspars, t (indicated in Monte Carlo steps per ordering atom). Only A1 atoms distributed over T positions of a single crankshaft are shown. Different symbols (heavy and hght dots) are used to distingitish between T o and Tim sites. The first snapshot corresponds to the initial, totally disordered Al-Si distribution. The armeahng temperature is 0.427) ...

Snapshots of the twin microstructure in the simulated sample annealed below the transition are shown in Figure 6. Only A1 atoms belonging to a single crankshaft and located at T1 sites are shown in Figure 6, and different symbols are used to represent A1 atoms at Tlo and Tim sites. The A1 atoms at T2 positions and all Si and host atoms are not shown in order to clearly distinguish the two variants of the ordered phase. At early... 

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