Phase transformation induced residual

Spontaneous Microcracking due to Phase-Transformation-Induced Residual Stresses... 

Numerous resistance measurements have been carried out under high-pressure shock compression [79D01]. Most of the work has been motivated by the desire to develop stress gauges to measure pressures in shock-compressed materials. Other measurements were undertaken to determine critical pressures to induce phase transformations. Although most of the work is not carried out in sufficient detail to relate resistance observations to defect characterizations, excess resistance at given shock pressures is observed in every case compared to comparably loaded static pressure observations. The presence of residual resistance for times after the loading is removed provides explicit evidence for irreversible changes in resistance due to defects. 

The effective mass of the electrons changes due to lattice strain, alloy additions, radiation damage, phase transformation, and phase content, directly relates to the ability to use electronic property measurements to assess microstructure phase stability. Electronic properties, such as thermoelectric power coefficients, resistivity and induced resistivity measurements, have a demonstrated correlation to solute and phase content, potential phase transformations, as well as residual strain. 

Doremus RH (1985) Rates of Phase Transformations. New York Academic Press Dreher KL, Jaskot RH, Lehmann JR, Richards JH, McGee JK, Ghio AJ, Costa DL (1997) Soluble transition metals mediate residual oil fly ash induced acute lung injury. J Toxicol Environ Health 50 285-305... 

There is some indication of a marginal increase in the weight fraction of tetragonal zirconia (Table 1) with depth. In the alumina-zirconia (90 10 by weight) control sample, the content of the t-phase is approximately 5 wt%. On the surface of the FGM sample, where AT is approximately 45 wt%, the weight fraction of t-phase is 1.0% and this value increases up to 2.4% at a depth of 1.2 mm. It is suggested that the presence of AT has induced tensile residual stresses which are responsible for enhancing the t->m phase transformation. 

Up to this point, it has been assumed that increasing zone size will be beneficial in terms of toughness. It must, however, be realized that there is a limiting zone size, above which the toughness must decrease. For example, if the critical transformation stress is reduced to a value such that the stress-induced phase transformation occurs throughout the specimen, the transformation zone will no longer be constrained. This implies that the zone will no longer be left in residual compression and the crack closure forces will vanish. The overall trend is shown schematically in Fig. 8.63. The stress required to induce the phase trans-... 

According to Fig. 1, residual stresses are induced mechanically, thermally, and by phase transformation (Brockhoff and Brinksmeier 1999). Each of these impact types can affect the degree of in-process distortion. The residual stress state of the finish parts results from the superposition of thermal and mechanical stresses. 

The wear rate of YPSZ on UHMWPE can be five times less than the wear rate of alumina on UHMWPE, depending on experimental conditions (Kumar et al., 1991 Davidson, 1993 Derbyshire et al., 1994). Wear resistance is a function of grain size, surface roughness, and residual compressive stresses induced by the phase transformation. The increased mechanical properties may allow for smaller-diameter femoral heads to be used in comparison with alumina. 

So-called stress-induced phase transformations can produce additional compressive residual stresses during crack propagation and thus increase the crack-growth resistance TTir. This is caused by particles in the matrix that can increase their volume by a phase transformation. Initially, the particles have to be in a metastable state which is thermodynamically unfavourable, but cannot transform to the thermodynamically stable phase because a nucleation barrier has to be overcome for this, similar to the process in precipitation hardening (see section 6.4.4). 

Transformation toughening increases the crack-growth resistance by producing compressive residual stresses in the material during crack propagation. These are caused by stress-induced phase transformations, described in section 7.2.4. To achieve this, particles are added to the matrix that perform a phase transformation that results in a larger volume of the particles when a sufficient tensile stress is applied. 

Support for the general mechanism outlined in Scheme 7-14 is provided by gas-phase Fourier-transform mass-spectrometric studies of the anionic reaction products of several substrates with Oa - (produced by electron impact with Oa HO can be produced by electron impact with HaO). 0,51 in these experiments neutral products are not detected. Both Oa - and HO react rapidly with 1,2-diphenylhydrazine in the gas phase (P = 10 7 Torr) to give the anion radical of azobenzene (PhN -NPh m/e = 182) and the anion from deprotonation (PhN NHPh m/e = 183), respectively. When Oa - is ejected from the experiment, the peak at m/e = 182 disappears. In contrast to the exponential decay that is observed for the HO peak with time, the ion current for Oa - decays to a steady-state concentration. Apparently, the PhN -NPh product reacts with residual Oa (which cannot be ejected from the FTMS cell) to give Oa - and azobenzene in a process that is analogous to the (Oa l-induced auto-oxidation in aprotic solvents (Scheme 7-14). 

By comparison, other methods for the preparation of alloys are less used in the laboratory, although in special cases the optimum methods may involve reduction (chemical or electrolytic) of metallic compounds. In addition, some intermetallic compounds are best obtained as residues remaining after the corresponding basic alloys are dissolved. However, a knowledge of the temperature-induced transformations of the phase diagram is of the greatest importance in all cases thus, the literature references below must be consulted, if at all possible. 

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