Level Of Cavitation Research Articles

Microstructural aspects of superplastic tensile deformation and cavitation failure in a fine-grained yttria stabilized tetragonal zirconia

A fine-grained yttria stabilized tetragonal zirconia exhibits an optimum superplastic elongation to failure of ∼ 700% at 1823 K and a strain rate of 8.3 × 10 −5s −1. A detailed microstructural investigation of the superplastically deformed specimens reveals the occurrence of extensive concurrent grain growth and internal cavitation. An expression is developed to characterize the extent of deformation enhanced concurrent grain growth, as influenced by experimental factors such as true strain, strain rate and temperature. The variation in the level of concurrent cavitation with strain rate conforms closely to the variation in elongation to failure with strain rate. It is demonstrated that the tendency towards cavity interlinkage in a direction perpendicular to the tensile axis is an important factor influencing the total elongation to failure obtained in superplastic materials.

Effects of dwell time on the creep damage generated in AISI type 347 weld and 1CrMoV steel during creep fatigue loading

Creep fatigue is an important failure mode in many applications. Recent studies have shown that the effect of dwell time on creep fatigue crack growth is strongly dependent upon the material type. AISI type 347 weld metal shows a strong dependence of crack growth rate on dwell period, but 1CrMoV rotor steels show no such trend. In an attempt to understand this phenomenon, the distribution of cavitational damage has been examined ahead of creep fatigue crack tips. The extent of cavitation was found to remain approximately constant in 1CrMoV steel irrespective of dwell times up to 200 h, whereas in 347 weld metal the level of cavitation was found to increase with increasing dwell period. These results have been used to explain the observed differences in the effect of dwell time on cyclic crack growth rates in these materials.

Role of Concurrent Cavitation in the Fracture of a Superplastic Zirconia—Alumina Composite

Superplastic zirconia‐alumina composites exhibit very large elongations to failure of ≳500% under optimum conditions. A quantitative microstructural study reveals that the composite is susceptible to considerable internal cavitation, with the level of cavitation increasing with increasing strain rate. The observed inverse relationship between the level of cavitation and the elongation to failure suggests strongly that the tensile ductility is limited by the evolution of cavitation damage during superplastic deformation.

The characteristics of cavitation during superplastic deformation of a warm-rolled AlLiCuMgZr alloy

The process of superplastic forming has become important in numerous commercial applications. However, there may be some deleterious defects on the properties of superplastically-formed components due to the presence of minor levels of cavitation introduced during the forming process. Thus, it is important to obtain information on nucleation, growth and interlinkage of cavities in a wide range of superplastic materials. Two different thermomechanical processing routes may be employed to develop fine grain sizes conducive to superlasticity in Al-Li alloys. In one of them, a fine grain size microstructure is obtained by static recrystallization prior to superplastic deformation, and in another, a fine grain size microstructure is obtained by deformation-induced recrystallization during the initial stage of superplastic deformation. The deformation-induced recrystallization of a cold (or warm)-rolled Al-Li alloy is strongly dependent on the strain rate microstructural change during the initial stage of superplastic deformation due to deformation-induced recrystallization. There have been some studies dealing with cavitation in superplastic Al-Li alloys. However, there is no research work reported on the effect of deformation-induced recrystallization on the characteristics of cavitation during superplastic deformation of a warm-rolled Al-Li alloy. This paper deals with the relationship between deformation-induced recrystallization and cavitation characteristics during superplastic deformationmore » of a warm-rolled Al-Li-Cu-Mg-Zr alloy. The results show that there are three kinds of cavities. The first includes the fine cavities formed at the beginning stage of deformation, the second the large cavities formed around the intermetallic particles and the third the grain boundary cavities. All three kinds of cavities are closely related to deformation-induced recrystallization.« less

An experimental investigation of the superplastic forming behavior of a commercial al-bronze

The superplastic behavior of thermomechanically processed complex commercial Al-bronze (Cu-10Al-4.5Fe-6Ni-2Mn) sheet material has been investigated under biaxial bulge-forming conditions using constant gas pressures at different temperatures. The results of bulge testing were systematically analyzed usingm values determined from uniaxial tensile tests on the same sheet. The variations of dome height and thickness were examined relative to forming time. The rate of change of dome height was observed to be comparable to the corresponding apex strain rate, and both dome height and m value were seen to influence the thickness gradient. It was also demonstrated that the geometry of a dome was not constant during bulge formation. It could be flatter than, equivalent to, or sharper than a spherical shape. The grain size was relatively small (2 to 3 μm@#@) and very stable, and this is likely to be responsible for the large superplastic strains and relatively low levels of cavitation observed for the alloy.

A Comparative Study of Superplastic Deformation and Cavitation Failure in a Yttria Stabilized Zirconia and a Zirconia Alumina Composite

ABSTRACTA fine grained yttria stabilized zirconia and a zirconia composite containing 20% alumina exhibit superplastic characteristics, with optimum elongations to failure of ≥500%. There is microstructural evidence for deformation enhanced grain growth and concurrent cavitation in both of these superplastic ceramics. Grain sizes after superplastic deformation may be several times larger than the initial grain size, and the level of cavitation can attain values as high as ≥30% under some experimental conditions. This report presents in a comparative format some recent experimental results on the microstructural aspects of superplastic deformation and concurrent cavitation in these superplastic ceramics.

Mechanical Properties and Cavitation during Superplastic Deformation of Al–Mg Alloys

The effect of temperature and strain rate on cavitation during tensile straining in Al–Mg alloys has been studied using precision density measurements, metallography and semi-empirical model of cavity growth. Cavitation occurs throughout the gauge length of the specimens, with most of the cavities being concentrated in regions near the fracture tip. The extent of cavitation has been found to increase with increasing temperature and decreasing strain rate. In fact, the cross-sectional area at fracture increased with increasing level of cavitation and its coalescence. However, cavitation is not found to be optimum around the superplastic temperature, i.e. 673 K and at an initial strain rate of ε=1.4×10−3 s−1. This is the combination of temperature and strain rate at which the alloys show a maximum elongation to failure and strain rate sensitivity index. Cavity growth at high temperatures may be controlled either by vacancy diffusion or the power law growth process. These two growth mechanisms are examined with reference to Al–Mg alloys. It is found that diffusion growth is favoured at low strains and there is a transition to the power law growth process at a critical cavity radius (rc). The value of (rc) increases with increasing temperature and decreasing strain rate.

Application of Signal Analysis to Cavitation

The diagnostic facilities of the cross power spectrum and the coherence function have been employed to enhance the identification of not only the inception of cavitation, but also its level. Two piezoelectric pressure transducers placed in the downstream chamber of a model spool valve undergoing various levels of cavitation allowed for the use of both functions—the phase angle of the complex cross spectrum and the dimensionless coherence function—to sense clearly the difference between noise levels associated with a noncavitating jet from those once cavitation inception is attained. The cavitation noise within the chamber exhibited quite a regular character in terms of the phase difference between instruments for limited cavitation. Varying cavitation levels clearly illustrated the effect of bubble size on the attendant frequency range for which there was an extremely high coherence or nearly perfect causality.

Effect of cavitation on post-deformation tensile properties of a superplastic copper-base alloy

The effects of cavitation, introduced during superplastic tensile flow, on post-deformation tensile properties have been studied for a microduplex Cu-Zn-Ni alloy. Increasing levels of cavitation of up to 4 percent by volume caused a progressive reduction in tensile strength, while ductility was decreased to a substantially greater extent. Increasing the volume fraction of cavities also changed the appearance of the fracture surface from cup and cone to macroscopically brittle. A post deformation annealing treatment reduced the level of cavitation and caused an increase in room temperature ductility, but the proportion of cavities removed during annealing became progressively less as cavitation was increased.

Large strain behaviour of a superplastic copper alloy—II. Cavitation and fracture

A quantitative study of cavitation damage and fracture of a superplastic copper alloy, Coronze 638, has been made. Cavities are found to nucleate at large particles present in the form of stringers. The size and shape of cavities, as well as the level of damage up to fracture are essentially independent of strain rate over regions I and II of the σ−ϵ curve, as are the true strain to fracture and the development of t instabilities. As the strain rate increases into region III, the level of damage to failure decreases, while the true failure strain increases and necks become sharper. Extensive cavity coalescence is observed up to strains of about 1.5, producing a number of large (> 100 μm) cavities which exhibit a high stability, and little tendency to coalescence. This allows the sample to sustain a very high level of cavitation without failure. The mechanism of cavity growth for small isolated cavities (< 10 μm) is thought to be diffusive growth constrained by matrix creep at low strain rates, with a transition to plasticity controlled growth at large strain rates. For larger cavities growth appears to be entirely creep controlled. Final fracture occurs by the material exhaustion in the ligaments between voids once the reduction in the cross section exceeds about 30%. No large instability either in flow or damage seems to be involved in this process.

Cavitation and cavity sintering during compressive deformation of a superplastic microduplex Cu–Zn–Ni alloy

A superplastic microduplex α–β nickel silver alloy has been deformed in tension and compression. Cavitation occurred during superplastic tensile flow, but not during homogeneous compressive deformation. In those regions of the compression specimens which were subjected to barrelling and transient tensile stress, however, a measurable level of cavitation was detected. Application of compressive stress caused cavities introduced during prior tensile deformation to sinter and the degree of sintering increased with increasing compressive strain, but was relatively independent of strain rate. The closure rate of cavities was found to be consistent with a viscous mechanism of sintering and was much greater than the closure rate predicted by stress-directed vacancy diffusion.

Cavitation and cavity growth during superplastic flow in microduplex Cu-Zn-Ni alloys

A study has been made of cavity growth during superplastic tensile deformation of two microduplex α/β nickel-silvers, one a Cu-Zn-Ni alloy and the other a Cu-Zn-Ni-Mn alloy. For cavities with radii of >0.5 /gmm, measured growth rates were found to be in good agreement with values calculated on the assumption that cavity growth was controlled by viscous flow of the matrix. For smaller cavity sizes a diffusional growth mechanism could predominate. Metallography revealed that cavity morphology changed with strain in a manner consistent with diffusion-controlled growth at small sizes, and matrix deformation controlled growth at intermediate and large cavity sizes. Density studies showed that the overall level of cavitation was independent of both strain rate and temperature, and was influenced only by strain.

Effect of grain size on cavitation in superplastic Zn-Al eutectoid

The effect of initial grain size on cavitation during superplastic deformation in two commercially available Zn-Al eutectoid alloys has been studied using metallography and precision density measurements. Cavitation was found to be minimal for initial grain sizes below about 5 μm. Superplastic deformation caused grain growth in both alloys under all testing conditions, and when the grain size exceeded about 8 μm a significant level of cavitation was produced. The grain size and extent of cavitation increased with increasing strain along the specimen gauge length, with cavities concentrated in regions adjacent to the fracture tip. Although never very large, the cross-sectional area at fracture increased with increasing levels of cavitation. It was concluded that cavitation in Zn-Al eutectoid results from incomplete accommodation of grain-boundary sliding when excessive grain growth leads to restricted grain-boundary diffusion and/or to restricted grain-boundary migration.

Cavitation during the superplastic deformation of an α/β brass

Cavitation during superplastic tensile flow has been studied in anα/β brass using metallography and precision density measurements. Cavities nucleated primarily at triple points and were sometimes associated with small second phase particles. The level of cavitation increased as strain, strain rate and grain size were increased and as the temperature was decreased. The influence of these variables can be interpreted in terms of their effects on cavity nucleation and/or cavity growth rates.

Cavitation during superplastic flow of ternary alloys based on microduplex Pb-Sn eutectic

The effect of phases having a range of hardnesses on the superplastic tensile behaviour of microduplex Pb-Sn eutectic has been studied. The presence of relatively hard particles induced cavitation at particle/matrix interfaces during deformation in an otherwise noncavitating system, and the growth and interlinkage of cavities led to brittle superplastic fractures. Density measurements showed that cavitation increased as the volume fraction, hardness and size of intermetallic particles was increased. Increasing strain rate and decreasing deformation temperature also led to an increased level of cavitation. Cavity nucleation was attributed to the limited ability of the relatively hard phases to contribute to the accommodation processes occurring during superplastic flow.

Predicting Cavitation in Sudden Enlargements

In recent years incipient, critical, and choking cavitation levels have been introduced as design criteria for orifices and valves in pipelines. Recent studies have evaluated a fourth level, incipient cavitation damage, for orifices that is applicable to the design of sudden-enlargement energy dissipators. Design data for these four levels are included in the paper. Since cavitation in prototype structures can differ from that predicted by model studies, research has also been directed toward evaluation of scale effects. The existence of scale effects due to pressure, size, and geometry for four levels of cavitation are examined herein and methods and equations for adjusting reference test data to structures of other sizes operating under different pressure conditions are presented. The location and distribution of cavitation pitting on the pipe wall downstream from the sudden enlargement are identified. The pitting is a function of expansion ratio and cavitation intensity.

Cavitation in alloy steels during superplastic deformation

A study of cavitation during superplastic tensile straining of two microduplex steels has been made using density measurements and quantitative optical metallography. The steels were of basically similar composition with the exception of a trace addition of boron made to one alloy. During deformation cavities formedα/γ boundaries and matrix-carbide interfaces; the growth and coalescence of these cavities led to failure. Density measurements showed that the extent of cavitation increased with increasing strain and decreasing strain-rate, but the level of cavitation was reduced by the presence of boron. A time dependence of overall void volume of 1.4 to 2.0 was observed. Quantitative metallographic studies of the nucleation and growth contributions to the overall rate of void formation showed that boron inhibited each of these processeS. However, both the nucleation rate and the magnitude of the time exponent of void volume increase suggested that a substantial number of voids grew from pre-existing nuclei which were probably present as non-coherent carbide-matrix interfaces.

Brittle Failure of Superductile Alpha-Beta Brass

The tensile behaviour of fine grained Cu-40%Zn has been studied between 400-700°C at strain rates between 3 x 10-2sec-1 and 3 x 10-5sec-1. A maximum ductility greater than 500% is observed at 600°C but failure occurs with negligible necking in a mechanistically brittle manner by intergranular cavitation. For commercial application further alloy development is needed to reduce the level of cavitation.

The kinetics of cavity growth in 20 Cr/25 Ni stainless steel

The development of cavitation during creep has been examined in an unstabilised 20 Cr/25 Ni austenitic stainless steel, using precise density measurements as an indicator of the level of cavitation. The change in density was proportional to the duration of creep to the power of 3.0 increasing to 4 to 5 just before fracture, and this time exponent was not affected by either grain size or irradiation. Metallographic examination showed that wedge-cracks were the predominant mode of cavitation and helium bubble growth in irradiated specimens increased the rate of crack propagation, thus reducing the ductility. A simple model for the growth of triple-point cracks is used to explain the experimentally observed changes in density.