Phase transformation induced residual stresses

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. 

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. 

---