Yield stress single crystal

Fig. 2. Yield stress as a function of temperature for NiAl, Ni Al, and several commercial superaUoys where <001> is the paraHel-to-tensile axis for single crystals and (° ) are data points for NiAl + (2). See text. To convert MPa to psi, multiply by 145.

At room temperature, NiAl deforms almost exclusively by (100) dislocations [4, 9, 10] and the availability of only 3 independent slip systems is thought to be responsible for the limited ductility of polycrystalline NiAl. Only when single crystals are compressed along the (100) direction ( hard orientation), secondary (111) dislocations can be activated [3, 5]. Their mobility appears to be limited by the screw orientation [5] and yield stresses as high as 2 GPa are reported below 50K [5]. However, (110) dislocations are responsible for the increased plasticity in hard oriented crystals above 600K [3, 7]. The competition between (111) and (110) dislocations as secondary slip systems therefore appears to be one of the key issues to explain the observed deformation behaviour of NiAl. 

Yield Stress The effect of hydrogen on the yield stress of iron and steels is unpredictable. For very pure iron single crystals and polycrystals the yield stress is frequently found to be decreased by hydrogen, but it may increase or stay the same, depending on the dislocation structure, crystal orientation and purity of the iron . Little information is available for steels. 

The minimum stress necessary to introduce yielding occurs when a single crystal is oriented such that (p = X = 45°, for which... 

PLASTIC DEFORMATION. When a metal or other solid is plastically deformed it suffers a permanent change of shape. The theory of plastic deformation in crystalline solids such as metals is complicated but well advanced. Metals are unique among solids in their ability to undergo severe plastic deformation. The observed yield stresses of single crystals are often 10 4 times smaller than the theoretical strengths of perfect crystals. The fact that actual metal crystals are so easily deformed has been attributed to the presence of lattice defects inside the crystals. The most important type of defect is the dislocation. See also Creep (Metals) Crystal and Hot Working. 

Fig. 4.4 Yield stress normalized by the elastic shear modulus plotted against a size scale parameter (volume per surface area) illustrating the six orders of magnitude of stress levels and ten orders of magnitude of size related to plastic behavior of single crystal metals [152]...

Figure 10.6. Application of a tensile force to a cylindrical single crystal causes a shear stress on some crystal planes. When the shear stress is equal to the critical-resolved shear stress (the yield stress), glide proceeds along the slip direction of the planes.

Single-crystal and poly crystalline transition metal carbides have been investigated with respect to creep, microhardness, plasticity, and shp systems. The fee carbides show slip upon mechanical load within the (111)plane in the 110 direction. The ductile-to-brittle transformation temperature of TiC is about 800 °C and is dependent on the grain size. The yield stress of TiC obeys a Hall Petch type relation, that is, the yield stress is inversely proportional to the square root of the grain size. TiC and ZrC show plastic deformation at surprisingly low temperatures around 1000 °C. 

Figure 4.11. (a) Schematic representation of a screw dislocation in a lamellar single crystal of PE. The chain direction is [001], (b) Dependence of yield stress cr on crystal thickness ic for crystals of branched PE ( ) and linear PE ( ). The continuous line is calculated from eq. (4.10). (After Young, 1988.)... 

Although, the powder method was developed as early as 1916 by Debye and Scherrer, for more than 50 years its use was almost exclusively limited to qualitative and semi-quantitative phase analysis and macroscopic stress measurements. The main reason for this can be found in what is known as the principal problem of powder diffraction accidental and systematic peak overlap caused by a projection of three-dimensional reciprocal space on to the one-dimensional 26 axis, leading to a strongly reduced information content compared to a single crystal data set. However, despite the loss of angular information, often sufficient information resides in the ID dataset to reconstruct the 3D structure. Indeed, quantitative analysis of the pattern using modern computers and software yields the wealth of additional information about the sample structure that is illustrated in Figure 1. Modern... 

The yield strength of these uniformly composite materials abruptly decreases at temperatures over 1123 K when the concentration of PSZ in weight is under 50%. The specific yield strength (yield stress/density) of composites with more than 75 % PSZ is nearly equal to that of the CMSX-2 single crystal which is recently used for the turbine blades of the aero jet engines in the temperature range from 1173 K to 1273K. 

As mentioned previously, the boundary condition in DD and FE are different. Periodic boundary condition is used in DD analysis to take into account the periodicity of single crystals whereas confined boundary condition is used in the FE analysis to achieve the uniaxial state of strain. In order for the boundary conditions in FE and DD to be consistent, periodic FE boundary condition is implemented as well. This implementation of periodic FE boundary condition yields a relaxed state of stress with low peak pressure when compared to the experiment as illustrated in Fig. 9(a). Furthermore, both shear and longitudinal waves are generated which is discordant with plane wave characteristics as shown in Fig 9(b). Fig 10 shows the deformed shape when confined and periodic boundary conditions are used. In the confined case there is no distortion in the RVE. However, for the periodic case, considerable... 

Fig. IX-36. The influence of adsorption-active medium on the mechanical properties of lead single crystal a - the increase in the deformation rate, e, at a constant load, P b - the lowering of the yield stress, P at constant deformation rate, e[3]...

It is noted that one of the possible slip systems in single crystals shows an anomalous temperature dependence for the yield stress (Minonishi, 1991 Umakoshi etal., 1993b), i.e. the yield stress increases with rising temperature until a maximum is reached. Such an anomalous temperature dependence is characteristic of various in-termetallics and has been analyzed in much detail for the well-known case of Nij Al, which is discussed in Sec. 4.1.2. The findings for NijAl, however, do not apply... 

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