Habit plane

In the case of Cu—Zn and other systems that have relatively complex ordered martensite stmctures (9R or 18R), the reverse transformation is crystaHographicaHy restricted. This means that although there are many variants which can form on transformation from parent to martensite, only a single parent orientation is possible in the reverse, or shape-recovery, transformation. This process is illustrated in Figure 2a the shape recovery effect is shown in Figure 2b. A detailed discussion of the crystallography of martensitic transformations and lattice-invariant shear is available (3,4). When a martensite group is deformed to coalesce into a single orientation, the dominant mechanism is twinning, where each twin is actually an alternate variant of the martensite crystal. Thus, for the four variants that cluster about the (110) habit plane, each orientation is a twin of another, and by the degenerate variant-twin relationship, a group of martensite plates from a single parent crystal can, on deformation, coalesce to a single crystal (single variant) of martensite. The parent phase, usually ordered B2 or 1 0, transforms to one of the martensite crystal forms that exhibit shape memory. Examples are 2H, Cu—Al—Ni,  [c.463]

The orientational relationships between the martensite and austenite lattice which we observe are partially in accordance with experimental results In experiments a Nishiyama-Wasserman relationship is found for those systems which we have simulated. We think that the additional rotation of the (lll)f< c planes in the simulations is an effect of boundary conditions. Experimentally bcc and fee structure coexist and the plane of contact, the habit plane, is undistorted. In our simulations we have no coexistence of these structures. But the periodic boundary conditions play a similar role like the habit plane in the real crystals. Under these considerations the fact that we find the same invariant direction as it is observed experimentally shows, that our calculations simulate the same transition process as it takes place in experiments. The same is true for the inhomogeneous shear system which we see in our simulations.  [c.98]

One of the more recent definitions of a martensitic transformation is that of a shear-dominant, lattice-distortive, diffusionless transformation occurring by nucleation and growth . This means that the transformation starts at one or more nucleation points in the parent or austenite phase and the product or martensite structures, which often appear in typical morphological shapes such as plates or rods, will grow at the expense of the parent phase. At any given time during this transformation process, which can be athermal or isothermal, the austenite and martensite regions will be separated by a well defined interface or habit-plane. The actual change in atomic structure reflects a distortion of the basic lattice -opposite to shuffle transformations in which the contours of a given unit remain fixed - and is primarily driven by a shear or deviatoric component. The diffusionless character implies that the short as well as long range ordering remains unchanged. As the concept of a nucleation and growth mechanism is essential, martensitic transformations belong to the group of first order transformations. Its first order character, however, can be strong, moderate or weak.  [c.322]

For example, Scott et al. [191] take interest in the ripple phase in bilayers and have constructed a lattice model which assigns two integer degrees of freedom to each site ( lipid ) on the lattice one Ising spin cr = 1, which describes possible orientations of the head group, and one integer n e 0, 1, 2,..., which represents the displacement of the molecules perpendicular to the bilayer plane. Using a complicated Hamiltonian, borrowed from the chiral clock model, they indeed find a ripple phase with properties which are in decent agreement with experimental data. The interplay of head group orientation and ripple formation in hpid bilayers has also been studied by Schneider and Keller within a coarsegrained lipid model [192],  [c.663]

There are few engineering applications in which gears do not play an essential part. They can be used to reduce or increase speed, transmit power and change the direction or position of a rotating axis. There are several types of gears to suit these varying operational conditions, such as spur, helical, plain and spiral bevels, hypoid, worm and wheel (Figure 52.6).  [c.853]

A distinction exists in the habit and deformation characteristics of the 2H and 3R types and the 9R and 18R martensites. The former are internally twinned and deformation occurs by a detwinning of a variant plate. The latter are internally faulted and deformation proceeds by variant-to-variant coalescence, followed by group-to-group coalescence. Although these stmctural differences exist, the self-accommodating habit-plane grouping with respect to a (Oil) plane is common to all systems exhibiting the SME. The 9R martensite is derived from a B2 parent the 18R transforms from a DO supedattice. The difference in stacking of (110) planes in these two stmctures results from the requirement for an invariant plane strain that involves a restricted stacking of close-packed planes. In order to obtain the required plane strain condition, both the 9R and 18R contain stacking faults to provide the necessary accommodation. The atomic displacements required to yield these stmctures combine the processes of shuffling and shear. The 3R martensite twin plane is identical to the 9R and 18R fault plane. Eor the case of 2H martensites, no such twin-fault correspondence exists, and the twin is derived from a different parent (110) plane. A theory of thermoelastic martensite deformation and shape recovery has been developed (5).  [c.463]

Since the crystal shape, or habit, can be determined by kinetic and other nonequilibrium effects, an actud crystal may have faces that differ from those of the Wulff construction. For example, if a (100) plane is a stable or singular plane but by processing one produces a plane at a small angle to this, describable as an (xOO) plane, where x is a large number, the surface may decompose into a set of (100) steps and (010) risers [39].  [c.261]

Pullulan is generally produced in Hquid fermentations, and its accumulation causes a marked increase in the viscosity of the medium. However, as cultures age, the viscosity usually drops off. This lowering of viscosity results from enzymatic cleavage of the maltotetraosyl regions by an endogenous amylase-like enzyme (287,291). This phenomenon is a key factor in determining the physical properties of pullulan produced by fermentation. The fungus is capable of producing pullulan from a number of carbohydrate substrates (294,295), but the ones generally used are glucose, sucrose, or starch hydrolyzates. Biosynthesis is accompHshed via a Hpid-linked intermediate (296), with UDP—glucose serving as the initial glucosyl donor. Experimental evidence suggests that UDP—glucose is incorporated into lipid-linked panosyl or isopanosyl intermediates, which are then assembled into pullulan (297).  [c.300]

Simple uv—vis monochromators ate widely used with soHd-state imaging arrays (146,147), called optoelectronic imaging devices (OIDs) or optical multichannel analy2ets (OMAs), placed in the focal plane to record a spectmm neatly instantaneously. These ate the modem equivalent of the photographic plate, and have the advantage over an emulsion of rapid response and real-time results. An array detector consists of a set of photodiodes together with an integral electronic readout scheme. Arrays having time-gated windows as short as 5 ns ate useful for spectroscopy and kinetics of short-hved and unstable species, for which arrays have obvious advantages over mechanically scanned spectrophotometers, and ate often superior to Fourier-transform spectroscopy. Two-dimensional arrays designed for image recording can be used for time-resolved spectroscopy by rastering a temporally changing spectmm across the second dimension of the array.  [c.316]

Sensitivity on the part of the recipient is not considered, even though actual damage may result. Another legal quote is often used Ought this inconvenience to be considered, not merely according to elegant or dainty modes of habit or living, but according to plain and simple and sober notions amongst English people (Walter v. Selfe (1851)).  [c.759]

Specimens broken in dry ultra-pure argon at various high crosshead speeds have resulted in failures that were completely dimpled but such fractures became increasingly brittle as the crosshead speed was lowered, an effect seen previously and attributed to the presence of a small concentration of internal residual hydrogen. In aqueous solutions and under SSRHE conditions the resulting cleavage-like fractures were much more pronounced and indistinguishable from each other. The discontinuous transgranular fractures were accompanied by periodic acoustic emission signals Glancing angle electron diffraction revealed the presence of an fee hydride phase on the fracture surface. The propagation process was considered to consist of the repeated formation and fracture of the hydride phase. Cracking occurred on the 11017) plane, as had been previously reported by several workers but, in addition, two specimens with a texture which had this plane tending to lie parallel to the stress axis exhibited (100) fractures. Both these planes are hydride habit planes  [c.1268]

See pages that mention the term Habit plane : [c.462]    [c.256]    [c.758]    [c.792]   
Engineering materials Ч.2 (1999) -- [ c.83 ]