Low Stress Creep Research Articles

Osmotic Pressure, Creaming, and Rheology of Emulsions Containing Nonionic Polysaccharide

We report measurements of the osmotic pressure, rheology, and creaming of concentrated oil-in-water emulsions (30 wt% mineral oil, 1 wt% surfactant, pH 7, ionic strength 0.05 M) as a function of the concentration of added nonionic polysaccharide dextran (5 × 105 Da). Experiments were carried out at 25°C on emulsions prepared with two different low-molecular-weight surfactants—nonionic Tween 20 (polyoxyethylenesorbitan monolaurate) and anionic SDS (sodium dodecyl sulfate). For both types of emulsifier, the addition of dextran was found to lead to a reduction in osmotic pressure, an increase in the limiting zero-shear-rate viscosity, and an enhancement in the rate of serum separation. These observations are consistent with the theory of depletion flocculation of the droplets by the nonadsorbing polymer. The effects of the surfactant type on emulsion properties are most evident at low or high dextran contents. The results are compared with those reported earlier for the same emulsions containing the anionic polysaccharide rhamsan. The combined data indicate a strong but complex relationship between creaming stability and low-stress creep rheological behavior. Substantial differences in properties between emulsions made with nonionic and charged surfactants indicate the importance of surfactant-biopolymer interactions in controlling the flocculation of emulsion droplets.

In defense of diffusional creep

Over the last decade, Ruano and coworkers have presented a series of papers in which they attempted to negate the evidence for diffusional creep as a deformation mechanism. In the present paper it is argued that, since diffusional creep theory is based on sound physical principles, it is more fruitful to investigate areas in which the theory needs modification to account for experimental departures from it, rather than to reject it entirely. Some of these modifications are discussed. Specific replies to several of the criticisms are given, involving some analysis and some additional metallographic evidence. Results on the influence of stress on thermal grooving during low stress creep are presented as unique evidence of a diffusional mechanism of deformation.

Evidence for Nabarro-Herring creep in metals: fiction or reality?

It is shown that all creep data in metals ascribed to the Nabarro-Herring (N-H) diffusional creep mechanism cannot in fact be definitively described by the N-H creep theory. Rather, the creep mechanism is associated with either Harper-Dorn (H-D) dislocation creep or grain boundary sliding (GBS). Specific responses are presented to work by Fiala and Langdon (F-L) who have presented opposing views to the above conclusions. Creep data for copper used by F-L to support N-H creep are shown to be incompatible with the theory, since the experimental creep rates are about three orders of magnitude higher than predicted. The copper creep data, however, can be predicted by phenomenological equations for GBS and H-D creep. It is concluded that as long as rigorous and physically sound theories for creep controlled by GBS and H-D creep are not forthcoming, researchers will continue to use, erroneously in the authors' view, the N-H diffusional creep mechanism to explain low stress creep behavior of polycrustalline solids.

Low-stress creep behaviour of superplastic Zn-22% Al alloy

Low-stress creep behaviour of microduplex Zn-22% Al alloy was studied using spring specimen geometry. The average phase size in the specimens investigated was 0.87, 1.48 and 1.98 μm. Experiments were conducted in the temperature range 393–473 K at stresses below about 1.0 MN m−2. The present study has established that the stress exponent of the creep rate is unity and, therefore, a viscous creep process dominates the flow in Region I superplasticity. The activation energy corresponds to that for boundary diffusion. However, the phase-size exponent was found to be −2 instead of −3, as predicted by the Coble creep theory. Further, the measured creep rates are three to four orders of magnitude slower than those predicted by the Coble theory. Transmission electron microscopy revealed precipitation, along α/α grain interfaces, whose inhibiting action on plastic flow should at least be partly responsible for the lower values of measured creep rates. There also exist two other interfaces, namely α/β and β/β, whose comprehensive role in diffusion creep is not yet fully understood. Therefore, it seems illogical to describe the creep behaviour of Zn-22% Al by the classical Coble theory, originally developed for single-phase polycrystals.

Low stress creep behaviour of zircaloy-2 vis-a-vis zirconium

Zircaloy-2 is a popular zirconium base nuclear fuel cladding material having tin as a major alloying additions along with Fe, Ni and Cr, each around 0.1 wt %. Low stress creep behavior of a-zirconium and zircaloy-2 has been studied employing helical spring specimens. Detailed experimental results are reported and analyzed elsewhere. Here we present data on a-Zr and zircaloy-2 obtained under identical test conditions of grain size and test temperature with the aim of bringing out the effect of alloying additions to zirconium through the results obtained on the low stress creep behavior of zircaloy-2. The high purity zirconium contained, by ppm, 110 Sn, 350 Fe, 37 Ni, 150 Cr and 880 O while zircaloy-2 contained, by wt %, 1.3 Sn, 0.16 Fe, 0.06 Cr, 0.04 Ni and 0.11 O. Helical spring specimens of wire radius 500{mu}m and spring radius 0.012m were employed in both the cases. Spring specimens were annealed under vacuum (Zr at 923K/1.8ks and zircaloy-2 at 1113K/2.7ks) while on mandrel to develop a matching grain size in both the materials. The mean linear intercept grain size was found to be 16.8{mu}m in alpha zirconium and 16.6{mu}m in zircaloy-2. Close coiled helices were creep tested at 873K using more » the set-up and procedure described elsewhere. The maximum applied stress was 5.3MN/m{sup 2} in the case of zirconium while the same in the case of zircaloy-2 was 3.9MN/m{sup 2}. Deflection versus time curves for tests conducted at 873K for both the materials are shown in this paper. « less

On the low stress creep in Cu-14Al alloy and α-zirconium at intermediate temperatures

Abstract It is shown that during creep in a Cu-14Al solid solution alloy at low stresses and intermediate temperatures, two flow processes act in parallel: (i) stress-directed diffusional transport of matter via the grain boundaries and (ii) grain boundary sliding accommodated by slip in thin zonesadjoining the sliding boundaries. However, creep in α-zirconium at low stresses and intermediate temperatures results from stress-directed diffusional transport of matter via the grain boundaries up to an intercept grain size of 120 μm, while for intercept grain sizes above this value, Harper-Dorn creep operates. An interpretation of the results of the investigations of creep in α-zirconium by Bernstein, Fiala and Cadek in terms of grain boundary sliding accommodated by slip as proposed by Ruano, Wadsworth and Sherby is shown to be incorrect.

Effects of grain-boundary configuration on the high-temperature creep strength of cobalt-base L-605 alloys

The effects of grain-boundary configuration on the high-temperature creep strength are investigated using commercial cobalt-base L-605 alloys with low carbon content in the temperature range 816 to 1038° C (1500 to 1900° F). Serrated grain boundaries are formed principally by the precipitation of tungsten-rich b c c phase (the same as α2 phase found in Ni-20Cr-20W alloys) on grain boundaries by a relatively simple heat treatment in these alloys. The creep rupture properties are improved by strengthening of grain boundaries by the precipitation of tungsten-rich bcc (α2) phase. The specimens with serrated grain boundaries have longer rupture lives and higher ductility than those with normal straight grain boundaries under low stress and high-temperature creep conditions, while the rupture lives and the creep ductility of both specimens are almost the same under high stresses below 927° C. The matrix of the alloys is strengthened by the precipitation of carbides at temperatures below 927° C and by the precipitation of tungsten-rich α2 phase at 1038° C during creep. It is found that there is an orientation relationship between tungsten-rich a2 phase particles andβ-Co matrix, such that (0 1 1)α2 ¶ (1 1 1)β-Co and [1\(\overline 1 \) 1]α2 ¶ [1\(\overline 1 \) 0]β-Co. The fracture surface of specimens with serrated grain boundaries is a ductile grain-boundary fracture surface, while typical grain-boundary facets prevailed in specimens with straight grain boundaries.

An analysis of low stress creep data for coarse-grained pure lead

An analysis has been carried out of low stress creep data for pure lead at 298 to 323 K and grain sizes from 200 to 500 μm, as compiled by Gifkins and Snowden. The results suggest pipe-diffusion-controlled Harper-Dorn creep to be the dominant creep mechanism for the data considered. This finding is consistent with the theoretical predictions.

Creep of zircaloy-2 at low stresses

Low stress creep behaviour of Zircaloy-2 of grain size 8, 12 and 17 μm has been investigated in the temperature range 783 to 993 K at stresses below 10 MN/m 2. Grain boundary diffusion controlled Coble creep governs flow in the material with 8 and 12 μm grain size. However, a switch to lattice diffusion controlled Nabarro-Herring creep occurs when Zircaloy-2 is treated to develop the coarser 17 μm grain size. It is suggested that an explanation for this change in creep behaviour is contained in the transmission electron microscopy observations with regard to grain boundary precipitation. In Zircaloy of 8 and 12 μm grain size, Zr 2 (Ni,Fe) type precipitates occur at grain boundaries, while grain boundaries appear to be the preferred sites for the more refractory Zr(Cr 2Fe) 2 type precipitates in the sample heat-treated to produce the 17 μm grain size.

Low stress creep of α-Zr at intermediate temperatures

Evidence is presented that the low stress creep behavior of polycrystalline α-Zr at intermediate temperatures is not controlled by diffusional flow. Rather, it is shown that the material deforms like typical fine-grained superplastic materials in which the rate-controlling process is grain boundary sliding accommodated by slip.

Analysis of Concurrent Grain Growth During Creep of Polycrystalline Alumina

During low-stress creep experiments in a fine-grained alumina, the strain rates decreased continuously with time due to the occurrence of concurrent grain growth. The grain-growth kinetics were independent of the applied stress and depended only on the time of exposure to the elevated test temperature. An analysis of the effect of concurrent grain growth was consistent with experimental observations.

Investigation of Solder Fatigue Acceleration Factors

Solder fatigue was investigated experimentally for effects of the strain loading waveform in isothermal mechanical fatigue, and frequency and strain range in true thermal cycling. Rapid strain loading followed by a long hold period at constant strain was found to be very damaging compared to constant rate cycling at the same peak amplitude and period. Thermal cycle fatigue data showed a stronger dependence on cycle period and weaker dependence on strain range than reported results in isothermal mechanical fatigue. Implications of the experimental findings are discussed with regard to solder joint fatigue and accelerated life testing of leaded chip carriers. Isothermal mechanical fatigue testing was carried out at room temperature with a custom torsional apparatus having an optical servo-loop for strain control. Mean joint compliance change per cycle was used as a fatigue metric. Experiments were carried out on 5/95 and 63/37 Sn/Pb solder joints. Mean stress in the solder joints was measured at 23°C and found to relax significantly after several minutes. Thermal cycle experiments were carried out with an air impingement apparatus capable of cycling the test samples between 30110°C in periods ranging from 2 to 120 min. Ceramic chip carriers were soldered to aluminum, zinc, and stainless steel substrates to permit fatigue life data to be taken from 0.3-14 percent strain range. A conventional crack criterion was used as the fatigue metric in the thermal cycle experiments. A simple finite element model of a 68 post molded leaded chip carrier (PLCC) was analyzed to examine expected stress loading of the solder joints under typical operating conditions. Transient thermal stress loading and low stress creep of the solder joint are discussed in light of this model and known creep properties of solder. It is shown that during typical operating cycles, the total creep strain can account for the majority of plastic or nonrecoverable strain.

On the applicability of diffusional flow to the creep of polycrystalline materials at low stresses and intermediate temperatures

It is shown that no evidence exists for diffusional (Coble) creep in single-phase metal-base polycrystals at intermediate temperatures and low stresses. At stresses above about 10 −5E (where E is Young's modulus), grain boundary sliding is more facile than diffusional flow and, at stresses below 10 −5E, plastic flow mechanisms not associated with diffusional flow dominate deformation. The recent statements of Sritharan and Jones regarding the inapplicability of a phenomenological relation based on grain boundary sliding in describing low stress creep are shown to be incorrect. Specifically, their arguments are invalid regarding (1) the representation of creep data as dotϵ ∫ σ 2 (dotϵ is the creep rate and σ is the stress), (2) the use of an “empirical” equation for describing the grain boundary diffusivity and (3) the position of the boundary between diffusional creep and grain boundary sliding in a deformation mechanism map. Phenomenological creep relations for describing grain boundary sliding and diffusion-controlled dislocation creep correctly predict the influence of grain size on the range of stress and strain rate where dotϵ ∫ σ 2.

48 Hour Multiaxial Creep and Creep Recovery of 2618 Aluminum Alloy at 200°C

Data are reported from 48 hour constant multiaxial stress creep followed by 48 hour creep recovery with the magnitudes of the effective stress ranging from 34.5 MPa (5.00 ksi) to 175.5 MPa (25.46 ksi). They differed from a previous data set in the much longer constant-stress durations and the inclusion of data from low stress creep, compression creep, and short term aging tests. Data were represented by a viscous-viscoelastic model in which the time-dependent strain was resolved into recoverable and nonrecoverable components. Previous stress-strain relations for constant stress creep and recovery were modified to include the current experimental observations of the nonexistence of creep limits, negligible aging effects, and symmetry in tension and compression. The time dependence was represented by a power of time with different exponents for the recoverable and nonrecoverable components. A homogeneous function of maximum shear stress was developed to represent the full range of stress dependence of the nonrecoverable time-dependent components; the third-order multiple integral representation was used for the recoverable component.

Low stress creep of fine-grained materials at intermediate temperatures: Diffusional creep or grain boundary sliding?

The work of Burton, Crossland, Jones and others on the creep of polycrystalline metals, of grain size about 10 – 100 μm, at intermediate temperatures (about (0.4 – 0.6)T m ) and low stresses ((10 −5 - 10 −4)E), is reviewed. These investigators suggest that such materials will deform by diffusional (Coble) creep accompanied by a threshold stress below which creep will not occur. Jones' observations that the creep rates of such metals are generally faster than those predicted by Coble creep and that the observed threshold stresses are unpredictably high has led us to consider alternative mechanisms for plastic flow. Our analyses indicate that the creep of such materials follows a relation of the form ϵ ̇ ∝ D gb δ 2/ d 3 , with no threshold stres, in agreement with the relation developed for describing fine-grained two-phase superplastic alloys. These correlations suggest that grain boundary sliding accommodated by slip is the principal mechanism of plastic flow. It is shown that diffusional creep (Coble) and grain boundary sliding accommodated by diffusional creep (Ashby-Verrall) are mechanisms which are not likely to contribute to plastic flow at stresses above 10 −5E.

Effect of quenching on the low stress creep behaviour of alpha-titanium

Effect of quenching on the low stress creep behaviour of alpha-titanium

Fluage haute temperature du sesquioxyde d'yttrium: Y2O3

Abstract HIGH-TEXPERATURE CREEP OF YTTRIUM SESQUIOXIDE : Y2O3 High-temperature and low-stress creep behaviour of yttrium oxide, Y2O3 has been studied by means of compressive experiments, conducted in air, between 1550 and 1800°C in the stress range 20 to 140 MPa. The creep testing apparatus and its technical limits are described in detail. Experimental results are related to a phenomenological analysis in terms of a mechanical creep law and macroscopic and microscopic aspects of the dislocation substructure of the crept samples. As in most high-temperature creep investigations, a phenomenological analysis of the creep curves leads to failure in finding a well-defined creep model. However, detailed analysis of the aspect of the faces of the crept samples, together with electron microscopy investigations of the dislocation substructure, suggest the following high-temperature creep behaviour for Y2O3. (a) The first step of the creep, which exhibits a short transient, involved glide of dislocations whose Burgers vector is b=a/2<111>. The dislocations density increases and induces dislocation junctions having a Burgers vector b = a100 >. Such a behaviour gives rise to a three-dimensional network of dislocations inside the crystal. (b) The dislocation junctions, b = a100 >, dissociates in a sessile manner by climb and pins down the three-dimensional network. Further glide development of this network is therefore difficult. The climb deformation process becomes quite competitive with respect to glide. Then, there begins a steady-state creep corresponding to the equilibrium between a strain hardening due to the multiplication of dislocations probably from Bardeen-Herring sources, and a recovery due to annihilation of dislocation by climb. In such a model, the dislocations of Burgers vector b = a100 >, which may largely dissociate by pure climb, plays the major role. It is suggested that such a pure-climb dissociation, not only inhibits the glide process, but is also an important ‘driving force’ in favour of a pure-climb deformation mechanism.

Microstructural stability during creep of a niobium-stabilized austenitic steel

Creep tests combined with transmission electron microscopy have been carried out on a niobium stabilized austenitic stainless steel which has been subject to a multi-mechanical thermal treatment (MMTT). It is found that the substructure introduced by the MMTT creates two distinct creep regimes, separated by a narrow transition temperature or stress range. In the low stress or low temperature regime creep is characterised by a low stress exponent (about 5) and a low activation energy (~300kJ/mole). The diffusion-controlled motion of jogged screw dislocations is suggested to be the most likely rate-limiting process. In the high stress or high temperature regime creep is characterised by a high stress exponent (about 10) and a high activation energy (~ 600 kJ/mole). Subgrains stabilized by the NbC precipates are observed to elongate during creep and the rate-limiting process is the breakaway of subgrain boundaries from small particles.

Creep of Alpha-Titanium at low stresses

Low-stress creep behaviour of alpha-titanium has been investigated over the range of temperature from 823–1088 K (0.43–0.56 T m ), grain size from 34–443 μm and stresses upto 2.0MN/m 2. Resistance heated helical specimens were crept under their own weight and the deflections of individual coils were monitored in situ as a function of time by direct observation through a transparent vacuum chamber using a cathetometer. Analysis of the results has been made with the objective of establishing regimes of stress, temperature and grain size within which a specific creep mechanism controls deformation. Coble, Nabarro-Herring, Harper-Dorn and power-law creep mechanisms have been considered. It is shown that low temperatures and fine grain sizes are conducive to the dominance of Coble creep while Nabarro-Herring and Harper-Dorn creep processes respectively predominate in the intermediate temperature-intermediate grain size and intermediate temperature-coarse grain size regimes. In all these situations Newtonian viscous behaviour with negligible threshold stress has been observed. Analysis of the strain rate vs stress data above a transition stress, corresponding to the stress at which an upward deviation from linearity occurs, has established the operation of power-law creep. On the basis of the experimental results, Ashby- and Langdon-type creep mechanism maps have been constructed for alphatitanium.

Self-diffusion of yttrium in monocrystalline yttrium oxide: Y 2O 3

Yttrium self-diffusion in monocrystalline yttrium oxide (Y 2O 3) is studied by means of the classical radio tracer technique. The few reliable diffusion data obtained in the temperature range 1600–1700°C lead to the following diffusion coefficient D=3.5×10 −9 exp− 72 RT ( kcal/mole) m 2 sec −1 . Experimental errors on the above numerical values are large and give, for the preexponential and energy terms, respectively: 2.10 −7<D 0<3.10 −10 m 2 sec − 62<Q<82 kcal/mole . Nevertheless these results seem in good agreement with those deduced from high-temperature and low-stress creep experiments. The theoretical aspect of self-diffusion of yttrium in Y 2O 3 is studied in terms of point defects and lattice disorder due to the equilibrium between the oxide and its environment. This last part is confined to the restricted range of high oxygen partial pressure in which oxygen interstitials are supposed to be majority defects. Intrinsic and extrinsic diffusion behavior are both considered on the basis of a vacancy diffusion mechanism.