Atoms shearing

For Nitinol - at the transition Ms, atoms begins to shear uniformly throughout the crystal. As the temperature is lowered the atomic shear continues to increase. At temperature, Mf, the atoms shear to their maximum point and assume a new structure. Thus, between Ms and Mr temperature interval the crystal structure of Nitinol is undefined and belongs neither to austenite nor to martensite . Therefore, thermodynamically, it should be classified as the second-order transformation. This is illustrated in Fig. 3. Conventionally - above Ms temperature, the whole crystal assume a crystal structure identified as austenite . At Ms temperature, a new crystal structure of martensite begins to form through two-dimensional (planar) atomic shear. The two crystal structures of austenite and martensite therefore share an identical plane known as Invariant Plane. As the temperature is lowered, the two dimensional shear (or more correctly, shift ) continue to take place one plane at a time such that the Invariant Plane moves in the direction as to increase the volume of martensite at the expense of austenite . Ultimately, at Mt temperature the whole crystal becomes martensite . Since between Ms and Mf any given micro-volume of the crystal must belong to either the austenite or the martensite , the transformation is of the first-order thermodynamically. This case is pictorially illustrated in Fig. 4. 

Fig. 3. 2-dimensional pictorial illustration of atomic movements in Nitinol during the transition above Ms (O ), intermediate between Ms and Mf ( ) with arrow indicating direction of atomic shear, and below Mf(0 ) ... 

One of the well-known properties in the martensitic transition is that it undergoes macroscopic change in shape. This can be visualized by the illustration of Fig. 4. In Nitinol, no macroscopic shape change is observed. This can also be visualized by Fig. 3, in which the atomic shears take place in zigzag fashion. Since the atomic shears are all within interatomic distances macroscopically no shape change is observed. 

But, aside from these unique properties, Nitinol has a number of commonalities with other known martensitic transition systems (1) it is an athermal transformation, (2) it is diffusionless, (3) it involves displacive or shear-like movement of atoms, (4) the activation energy for the growth of martensite (continuous atomic shear in Nitinol) is effectively zero, i.e., the propagation rate of transformation (transition in Nitinol) is fast and independent of temperature. 

At Ms temperature TiNi initiates a uniform (inhomogeneous) distortion of its lattice — through a collective atomic shear movement. The lower the temperature, the greater the magnitude of shear movements. As a result, between Ms and Mr temperature the crystal structure is not definable. In sharp contrast, other known martensitic transformations initiate a nonuniform (heterogeneous) nucleation at Ms and thereafter the growth of martensite is achieved by shifting of a two dimensional plane known as invariant plane [28] at a time. Thus, between Ms and Mr temperature the crystal structure is that of austenite and/or martensite . 

These, therefore, are the reason I have avoided a full discussion of the mechanics of the Nitinol transition. For as long as the atomic shear is known to be continuous during the TTR, some one is bound to find a structure at a particular point within the TTR corresponding to his or her experimentation. In view of this dilemma, writing a review article on the subject of Nitinol transition based on one-sided view [66] should be avoided. By this I mean writing a review article on the mechanism of Nitinol transition must be written with a great care or should not be written at all. 

Most of the solid lubricants mentioned above owe their low-Mction characteristic primarily to a lamellar or layered crystal structure (see two of them in Figure 6.1 as typical examples). When present at a sliding contact interface, these solids shear easily along their atomic shear planes and thus provide low friction. Some of the solid lubricants do not have such layered crystal structures, but in applications, they too provide very low friction and wear. For example, certain soft metals (In, Pb, Ag, Sn, etc.), PTFE, a number of solid oxides and rare earth fluorides, diamond and diamondlike carbons, etc., can also provide fairly good lubrication despite the lack of a layered crystal structure like the ones shown in Figure 6.1 [1]. In fact, diamondlike carbon films are structurally amorphous but provide some of the lowest friction and wear coefficients among all other solid materials available today [8]. 

The heliarc butt-weld tensile data of Fig. 4 and Table III again show the typical increase in tensile strength between 70 and-320 F, followed by a decrease and wide scatter between -320 and -423 F. The scatter in tensile data at -423 F are typical of many notch-sensitive materials. The annealed material present in the welded joint has been shown to be even more prone to occur in the austenite-martensite reaction due to the smaller number of defects present in its structure (the defects presumably block atomic shear movements which result in the martensite transformation) [2]. 

Carpick R W, Agrait N, Ogletree D F and Salmeron M 1996 Measurement of interfacial shear (friction) with an ultrahigh vacuum atomic force microscope J. Vac. Sc/. Technol. B 14 1289... 

The shear viscosity is a tensor quantity, with components T] y, t],cz, T)yx> Vyz> Vzx> Vzy If property of the whole sample rather than of individual atoms and so cannot be calculat< with the same accuracy as the self-diffusion coefficient. For a homogeneous fluid the cor ponents of the shear viscosity should all be equal and so the statistical error can be reducf by averaging over the six components. An estimate of the precision of the calculation c then be determined by evaluating the standard deviation of these components from tl average. Unfortunately, Equation (7.89) cannot be directly used in periodic systems, evi if the positions have been unfolded, because the unfolded distance between two particl may not correspond to the distance of the minimum image that is used to calculate the fore For this reason alternative approaches are required. 

Atomization. A gas or Hquid may be dispersed into another Hquid by the action of shearing or turbulent impact forces that are present in the flow field. The steady-state drop si2e represents a balance between the fluid forces tending to dismpt the drop and the forces of interfacial tension tending to oppose distortion and breakup. When the flow field is laminar the abiHty to disperse is strongly affected by the ratio of viscosities of the two phases. Dispersion, in the sense of droplet formation, does not occur when the viscosity of the dispersed phase significantly exceeds that of the dispersing medium (13). 

The fundamental principle of Hquid disiategration Hes ia the balance between dismptive and cohesive forces. The common dismptive forces ia atomizer systems iaclude kinetic energy, turbulent fluctuation, pressure fluctuation, iaterface shearing, friction, and gravity. The cohesive forces within the Hquid are molecular bonding, viscosity, and surface tension. 

Liquid fuel is injected through a pressure-atomizing or an air-blast nozzle. This spray is sheared by air streams into laminae and droplets that vaporize and bum. Because the atomization process is so important for subsequent mixing and burning, fuel-injector design is as critical as fuel properties. Figure 5 is a schematic of the processes occurring in a typical combustor. 

Droplet size, particularly at high velocities, is controlled primarily by the relative velocity between liquid and air and in part by fuel viscosity and density (7). Surface tension has a minor effect. Minimum droplet size is achieved when the nozzle is designed to provide maximum physical contact between air and fuel. Hence primary air is introduced within the nozzle to provide both swid and shearing forces. Vaporization time is characteristically related to the square of droplet diameter and is inversely proportional to pressure drop across the atomizer (7). 

The dislocation cannot glide upwards by the shearing of atom planes - the atomic geometry is wrong - but the dislocation can move upwards if atoms at the bottom of the half-plane are able to diffuse away (Fig. 19.2). We have come across Fick s Law in which diffusion is driven by differences in concentration. A mechanical force can do exactly the same thing, and this is what leads to the diffusion of atoms away from the... 

Let US now look at how this contact geometry influences friction. If you attempt to slide one of the surfaces over the other, a shear stress fj/a appears at the asperities. The shear stress is greatest where the cross-sectional area of asperities is least, that is, at or very near the contact plane. Now, the intense plastic deformation in the regions of contact presses the asperity tips together so well that there is atom-to-atom contact across the junction. The junction, therefore, can withstand a shear stress as large as k approximately, where k is the shear-yield strength of the material (Chapter 11). 

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