Solid-solution strengthening

Basically, solid solutions have a considerable impact on the physical properties of ceramics, influencing, among others, their magnetic, dielectric and optical 

The salient properties of this layered ternary ceramic are good damage tolerance, good machinabUity, low density and excellent thermal shock and oxidation resistance. Such unique properties make it possible to use Ti3AlC2 in structural components for high-temperature applications and as oxidation-resistant coatings. 

The addition of Re increases the flow stress and changes the high-temperature behavior from climb-controlled to viscous glide-controlled and the glide of the dislocations becomes restricted by the solute Re atoms. The dislocation substructures in Fig. 5.25b mostly have [100] Burgers vectors, but some V2[lll] dislocations are also observed. Those labeled Fig. 5.25 as a, b and d have [100] Burgers 

On average, the tests shown in Fig. 5.28 represent ten microhardness indentations per specimen. When cracks occasionally appear to emanate from the comers of the indentations (as also observed in SiC, seen in Fig. 5.29b, these values were not averaged in. The line in Fig. 5.28 is linear and, thus, microhardness values of the solid solutions may be represented as linear relations plotted against mol% chromia. The large scatter around the least-square line is assumed to be associated with the microindentation crossing many grains. Hot-pressed materials contain a 

Structure). In Fig. 14.9 on page 381 of Friedel s book, there is an illustration of a zigzagged dislocation line pinned at several points by impurities. A large enough applied stress is required to tear the dislocation away from its impurity. TEM observations of the dislocation substructure of a 9.4 mol% YSZ crystal, deformed to 14 % strain, is shown in Fig. 5.32. Many of these dislocations are curved and a few dislocation loops are also visible. The g-b = 0 conditions obtained at two 

Alloys are stronger than pure metals because impurity atoms that go into solid solution typically impose lattice strains on the surrounding host atoms. Lattice strain field 

FigMre 7.16 Variation with nickel content of (a) tensile strength, (b) yield strength, and (c) dnctility (%EL) for copper-nickel alloys, showing strengthening. 

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Strong materials either have a high intrinsic strength, /, (like diamond), or they rely on the superposition of. solid solution strengthening obstacles fo and work-hardening f i, (like high-tensile steels). But before we can use this information, one problem... 

As we learn from Sims s reviews, many other improvements have been made to superalloys and to their exploitation in recent decades. Solid-solution strengthening, grain-boundary strengthening with carbides and other precipitates, and especially the institution, some twenty years ago, of clean processing which allows the many unwanted impurities to be avoided (Benz 1999) have all improved the alloys to the point where (McLean 1996) the best superalloys now operate successfully at a Kelvin temperature which is as much as 85% of the melting temperature this shows that the prospect of significant further improvement is slight. 

The formation of solid solutions of metals is one way to change the properties (generally to increase strength) of the metals. Strengthening metals in this way is known as solid solution strengthening. The ability of two metals to form a solid solution can be predicted by a set of rules known as the Hume-Rothery rules, which can be stated as follows ... 

Hafnium is an effective solid solution strengthener at higher temperatures for other alloys such as nickel aluminides (39,40). 

Mickel-llase Superalloys. The nickel-basc superalloys are Ihe most complex in composition and microslrueuirex and. in must respects, the most successful high temperature alloys. The earliest superalloys were wrought, ie fabricated to final size by a mechanical working operation. Later alloys have incorporated higher aluminum plus tilanium contents, as well as molybdenum lor solid-solution strengthening (Nimonics 115 and 1201. 

Apait from y and solid-solution strengthening, many alloys benefit from the presence of carbides, enrhonitrides, ami borides. 

Protective scale formers Solid solution strengtheners Age hardening strengtheners Carbide strengtheners Improved scale adhesion (spallation resistance)... 

W Behaves similar to Mo, but less effective. Detrimental to thermal stability in high Ni-Cr-Mo alloys Provides solid solution strengthening... 

Other strengthening mechanisms include solid solution formation and strain hardening. Solid solution strengthening involves replacing a small number of atoms in the lattice with substitutional impurities of a slightly different size. This creates strain in the crystal. 

The three principle methods of strengthening materials are grain-size reduction, solid-solution strengthening, and plastic-deformation processes, like strain (work) hardening. 

In the case of solid-solution alloys, the size and valence of the solute atoms determine the degree of solid-solution strengthening. Differences in atomic size of about 5% between the solvent and the solute are necessary to produce a significant strain-hardening effect [6.1]. 

The hardness is a result of carbide precipitation and solid solution strengthening. The presence of carbides at the grain boundaries is important for the mechanical properties, because they retard grain boundary migration. 

Plastic Deformation and Fracture of Materials edited by H. Mughrabi. This book is Vol. 6 in the series Materials Science and Technology edited by R. W. Cahn, P. Haasen and E. J. Kramer, VCH Publishers, Inc., New York New York, 1993. Of especial interest concerning the present discussion see chap. 6 on the subject of solid solution strengthening by H. Neuhauser and C. Schwink and chap. 8 by B. Reppich on the subject of particle strengthening. 

Figure 8 shows the calculated stress distribution in NiCiluNi part. The peculiar form of the curve is due to the solid solution strengthening effect that occurs while changing from a soft, pure metal to an alloy. In the pure metal layers, the thermal stresses are relaxed by visco-plastic deformation. In the adjacent layers, the yield stresses of the alloys (for example Cu-20Ni and Ni-20Cu) are higher and thus are the residual stresses. 

The active strengthening mechanism at room temperature of the studied material was identified as solid solution strengthening, grain boundary strengthening, and Al3Zr precipitate strengthening. 

Taking into account that the volume content, size, distribution and properties of the reinforcing boride phase practically do not change with the alloying studied here, the strengthening of the ternary and quaternary eutectic alloys should be attributed practically in full to solid- solution strengthening of the titanium alloy matrix. 

Despite the fact that the reinforcing Ti5Si3 fibers are not continuously aligned within the a-titanium matrix the rule of mixture can be applied because the aspect ratio 11 /dF = 50 is fairly high and sufficient enough to transfer the load from the solid solution strengthened matrix to the fibers [25]. Assuming that the solid solution a-Ti(Si) matrix has a flow stress of about om = Em = 300 MPa at room temperature, the effective flow stress of the... 

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