Compressive Creep Deformation Research Articles

Identifying the activated slip modes and twin variants in compressive creep deformation of Mg-8.0Al-1.0Nd-1.0Gd

The compressive creep deformation mechanism of as-cast Mg-8.0Al-1.0Nd-1.0Gd alloy at 180 °C under constant stress of 60 MPa and 90 MPa was novelly analyzed through the distribution characteristics of in-grain misorientation axes (IGMA). Activated slip modes in the creep process was accurately identified and the structure of dislocations is revealed by the weak beam dark-field (WBDF) technology. The intensities of IGMA are concentrated on 011̅0 and 1̅21̅0 axis demonstrates that 0001112̅0(basal slip) and 112̅2112̅3̅ (pyramidal slip) dominate the deformation mechanism. Tensile twins 101̅21̅011 were extensively activated in compression-creep process and the 101̅21̅011 variants with a twinning trace angle close to 90 deg are 1̅012101̅1 and 101̅21̅011 with para-position. The creep mechanism of the alloy crept at 180°C/60 MPa is dominated by grain boundary sliding triggered by the precipitation of β-Mg17Al12, and pipe diffusion-controlled boundary sliding is the dominant creep mechanism of the alloy under 180 °C/90 MPa. In addition, the results reveal that tensile twins induced by dislocation slip contribute to coordinate the grain c-axis deformation, release the stress concentration caused by dislocation accumulation at the grain boundary.

Compressive creep behavior of selective electron beam melted high Nb containing TiAl alloy

In this study, the compressive creep behavior of Ti–45Al–8Nb alloy fabricated by selective electron beam melting technology at 800 °C and 850 °C was investigated. This alloy is mainly composed of equiaxed γ grains, and minor α2 and B2 grains. Creep results show that compressive creep behavior of this alloy is dependent on temperature, which is manifested in the significant increase of creep strain and creep rate with the increase of temperature. Through a multi-step creep test, the stress exponent n (2.65 at 800 °C and 3.65 at 850 °C) and activation volume V* (36.49 b3 at 800 °C and 53.51 b3 at 850 °C) were measured. Combined with kinetic analysis based on n and V* and TEM observation, compressive creep deformation of this alloy at 800 °C and 850 °C is dominated by dislocation motion, including dislocation slip and climb. Additionally, mechanical twinning also plays an important role in creep deformation. Driven by dislocation pile-ups or mechanical twins, discontinuous dynamic recrystallization (DDRX) occurs during creep. The degree of DDRX at 850 °C is greater. Sufficient DDRX greatly softens the alloy and accelerates creep rate, resulting in no steady-state creep stage at 850 °C.

Room-temperature compressive creep behavior of Ti-6Al-4V ELI alloys with basketweave microstructure under an alternating compressive stress

Titanium alloys have great potential as candidate materials for the deep-sea facilities. Long-term creep deformation behavior of the pressure-bearing structures made of titanium alloys under alternating compressive stress has a direct impact on the structural stability. The compressive creep evaluations of Ti-6Al-4V ELI alloys with basketweave microstructure under alternating stresses have been carried out by a uniaxial compressive creep tester. The compressive creep deformations have strong stress sensitivity due to the larger creep strain by the higher applied compressive stress. The dislocation densities in α and β phases increase with the increase of applied compressive stresses. The dislocation slip and dislocation multiplication occur in α phases accompanying with high dislocations density in β phases caused by compressive deformation. The large strain concentration occurs at the grain boundaries of α/β phase due to dislocation plug after creep, which causes the grain boundary pinning. The compressive creep deformation of Ti-6Al-4V ELI alloys under alternating stress is a decelerating creep process and controlled by <a> dislocation slip. The slight refinements of the grains and hardening have been confirmed after creeps. The synergistic effects of dislocation multiplication, grain boundary pinning and grain refinements lead to strengthening the Ti-6Al-4V ELI alloys and decelerating the compressive creeps.

Effect of Y2O3 dispersoids on microstructure and creep properties of Hastelloy X processed by laser powder-bed fusion

• The effect of Y 2 O 3 dispersoids on microstructure and creep properties of Hastelloy X processed by LPBF is investigated. • Grains are found aligned along build direction, with a strong <001>Ni texture along both build and laser scanning directions. • LPBF Hastelloy X exhibits anisotropic creep response, with elongated grains showing higher creep resistance when aligned along loading direction than when aligned perpendicular to loading direction. • Y 2 O 3 dispersoids improve alloy creep resistance at dislocation-creep regime. • Alloy grain structure remains resistant to recrystallization during high-temperature solutionizing and creeping. Laser powder bed fusion (LPBF) was used to consolidate powders of a Ni-Cr-Fe-Mo alloy (Hastelloy X) blended with 1 wt% Y 2 O 3 nanometric powders. The nearly-dense, crack-free specimens, with and without oxide dispersion strengthening (ODS), exhibit high-aspect ratio grains, aligned in the build direction and with a strong texture: 〈001〉 is aligned along both build direction and laser scanning direction. Creep tests performed at 950 °C, with compressive stresses aligned with the elongated grains (and the build direction), reveal two creep regimes: (i) at stresses below ∼50 MPa, diffusional creep dominates, and the ODS alloy is less creep resistant than the non-ODS alloy, consistent with a somewhat smaller grain size resulting from grain-boundary pinning by the oxide dispersoids; (ii) at stresses above ∼50 MPa, dislocation creep dominates and the ODS alloy is more creep resistant, as expected from oxide dispersoids impeding dislocation motion. When tested perpendicular to the build direction, both alloys have the same creep resistance, which is however much lower than when the stress is aligned with the build direction, reflecting a strong effect of texture and grain shape. After ∼10% compressive creep deformation, grain structure and texture remain largely unchanged as compared to pre-creep structure.

Compressive creep deformation of lithium foil at varied cell conditions

Lithium metal rechargeable batteries have great potential for higher energy density storage systems. Current configurations of all solid-state and anode-free cells depend on significant internal mechanical pressure (typically 0.5–1 MPa) to reduce interfacial resistance and suppress lithium dendrite formation. Mechanical stack pressure can increase over time and with use as cell components degrade. Lithium is soft at room temperature and may not be physically strong enough to support the required stack pressures in standard lithium-ion operating conditions. Compressive creep deformation of low aspect ratio lithium metal foils was measured in hermetically sealed, rigid but flexible cells at temperatures between 30–60°C and applied pressures between 0.6–3.6 MPa. Creep rates were typically on the order of a few μm h−1, depending on pressure, temperature, lithium thickness, and grain size. While difficult to notice during fast cycling these creep rates could have a pronounced effect during storage or practical use. Results suggest that pure lithium may not be strong enough for use in high energy density all solid-state devices, and that alternatives (e.g.lithium-rich alloys) should be considered.

Understanding raft formation and precipitate shearing during double minimum creep in a γ′-strengthened single crystalline Co-base superalloy

ABSTRACT The compressive creep deformation behavior of a Co-base single-crystalline superalloy is studied in the high temperature / low stress regime at 950°C/150 MPa. Emphasis is placed on the mechanisms causing the double minimum creep behavior consisting of a local and a global creep rate minimum. The local minimum occurs at ∼0.2% strain with dislocation accumulation at γ/γ′ interfaces after bowing out of dislocations in horizontal matrix channels. Plate-like rafting occurring normal to the applied stress axis in the early creep stages is regarded as the primary reason for the decreasing creep rate until the global minimum takes place at ∼0.8% strain. In the final creep stage following the global minimum the creep rate accelerates due to coarsening and extensive precipitate shearing. The shearing by partial dislocations leads to the formation of numerous stacking faults, nano/microtwins, and occasionally to the formation of small amounts of ordered D019-Co3W precipitates. The reasons for twinning and second phase formation as well as differences of the deformation mechanisms between Co- and Ni-base superalloys during double minimum creep are discussed in detail in this work.

Fatigue Crack Networks in the Die-Attach Joint of a Power Semiconductor Module During Power Cycling Testing and Effects of Test Parameters on the Joint Fatigue Life

The fracture mechanisms of a Sn-Ag-Cu lead-free solder joint and the effects of test parameters on power cycling life were investigated. In this study, three levels of average temperature (348, 373 and 398 K), two levels of each of current on-and-off time (2 s/10 s and 5 s/22 s), and three levels of ΔTj (75, 100 and 125 K) were used. Because the thermal expansion of the die-attach material was restrained by Si, equibiaxial tensile and compressive creep deformation occurred during power cycling. This deformation generated fatigue cracks in the central part of the die-attach joint, and the cracks connected to each other, resulting in a fatigue crack network. The fracture occupied 70% or more of the die-attach area and was the dominant factor affecting the joint’s fatigue life. As ΔTj increased, the strain energy density in the central part increased, resulting in a decrease in fatigue life. Although the strain energy density at an average temperature of 398 K decreased by about 10% compared with that at an average temperature of 348 K, the fatigue life decreased to about 40% when the average temperature increased. Continuous dynamic recrystallization occurred during power cycling, leading to fatigue failure in high-energy grain boundaries. Since continuous dynamic recrystallization tended to occur more readily at higher temperatures, the fatigue life decreased with increasing average temperature, even for the same junction temperature range ΔTj. Moreover, the area of equibiaxial tensile and compressive creep deformation in the die-attach material became larger with increasing current on-and-off times. As a result, the fracture area under long current on-and-off time conditions was increased by about 25% in comparison with that under short current on-and-off time conditions.

Compressive creep behavior of a γ-TiAl based Ti–45Al–8Nb–2Cr-0.2B alloy: The role of β(B2)-phase and concurrent phase transformations

The primary and steady-state compressive creep deformation behavior of an as-cast high Nb containing γ-TiA1 alloy having remnant β(B2)-phase has been studied over the temperature range of 750–850 °C at constant initial applied stress levels between 75-200 MPa. The creep curves derived over the entire creep testing conditions show a prominent primary creep regime which is attributed to the presence of dislocation debris of the coarse γ-grains at the colony boundaries. The stress and temperature dependence of the steady-state creep rate follows the Norton-Bailey power law and the corresponding average value of the stress exponent and apparent activation energy are estimated to be 3.6 and 375 kJ/mol, respectively. The average value of the stress exponent in conjunction with the dislocation substructure of the crept samples suggests that the kinetics of creep deformation within the studied experimental conditions is controlled by the non-conservative motion (climb) of dislocations. Further misorientation analyses of the phase-resolved EBSD maps imply that for most of the conditions creep strain is carried by the γ-TiAl phase. Consequently, contrary to the commonly believed idea, the β(B2)-phase does not appear to deteriorate the creep resistance of the present alloy below suitable combinations of temperature and stress. Beyond these conditions, the concurrent deformation of γ-TiAl and β(B2)-phases is found to enhance the steady-state creep rate. In addition, characteristics of the dynamic transformation of β(B2) phase into γ and α2 phases during creep and its effect on the steady-state creep rate are further considered. Finally, the application potential of the present alloy is compared through a composite plot of Larson-Miller parameter with several other potential alloys reported in the literature.

On the atomic solute diffusional mechanisms during compressive creep deformation of a Co-Al-W-Ta single crystal superalloy

We investigated the solute diffusional behavior active during compressive creep deformation at 150 MPa / 975 °C of a Co-Al-W-Ta single crystal superalloy in the [001] orientation. We report the formation of shear-bands that involves re-orientation of γ/γʹ rafts to {111} from {001} planes, referring to as γ/γ′ raft-rotation. In the shear-band regions, we observed abundant micro-twins, stacking faults (SFs), disordered zones within the γʹ termed as ‘γ pockets’ and also few geometrically-close-packed (GCP) phases. We used a correlative approach blending electron microscopy and atom probe tomography to characterize the structure and composition of these features. The SFs were identified as intrinsic and exhibit a W enrichment up to 14.5 at.% and an Al deficiency down to 5.1 at.%, with respect to the surrounding γʹ phase. The micro-twin boundaries show a solute enrichment similar to the SFs with a distinct W compositional profile gradients perpendicular from the boundaries into the twin interior, indicating solute diffusion within the micro-twins. The γ-pockets have a composition close to that of γ but richer in W/Ta. Based on these observations, we propose (i) a solute diffusion mechanism taking place during micro-twinning, (ii) a mechanism for the γ/γʹ raft-rotation process and evaluate their influence on the overall creep deformation of the present Co-based superalloy.

Experimental study of expanded cork agglomerate blocks – Compressive creep behavior and dynamic performance

The main goal of the present work is to experimentally evaluate the behavior of expanded cork agglomerate blocks subjected to static and dynamic loads. The study focuses on the compressive and creep effects and on the vibration isolation capability.First, compressive creep behavior and deformation recovery capability (after loading) tests were performed on blocks made of expanded cork agglomerate. The dynamic characterization then involved performing transmissibility tests for different static and dynamic loading conditions and evaluating the dynamic transfer stiffness and damping ratio of the blocks. The resulting transmissibility curves indicate how much vibration is transmitted through the isolator to its base at a given frequency. These tests were also performed under different conditions of strain to better understand the effect on the dynamic performance of the material, and to assess whether it might be a limiting factor.The results show that expanded cork agglomerate blocks, especially those with higher mass density, have physical and mechanical properties that provide this material with great potential for withstanding heavy static loads since small compressive creep effects and low permanent deformation was exhibited.The results also indicate that expanded cork agglomerate can be a good option for mitigating vibration. It was found that specimens with lower mass density and larger thickness performed better, with higher vibration isolation and a wider range of frequencies isolated. However, this behavior is limited by the influence of compressive stress on the blocks.

Gas permeability evolution of clayey rocks in process of compressive creep test

Low-permeability argillite is a potential material as the geological barrier in the context of radioactive waste disposal. This paper presents the influence of compressive creep deformation on gas permeability of the porous argillite by triaxial compressive creep test. The minerals and microscopic pore distribution of the argillite are analyzed from the SEM results. The triaxial compressive creep test is carried out to discover the evolutionary gas permeability of the material in the process of creep deformation and the corresponding evolution law of gas permeability is discussed closely related to both micro- and macro-characteristics of the material during creep deformation. The results suggest that gas permeability undergoes an evident decrease in initial deviatoric loading, tends to be constant in steady creep deformation, and demonstrates a slight increase in further creep when creep deformation creates cracks. The gas permeability does not exhibit a significant increase if material pores are not connected.

Bolt Load Loss Behavior of Magnesium Alloy AZ91D Bolted Joints Clamped with Aluminum Alloy A5056 Bolt

The bolt load loss behavior of AZ91D magnesium alloy bolted joints with a conventional SCM435 steel bolt and an A5056 aluminum bolt was investigated at elevated temperature. The A5056 bolt could reduce the bolt load loss compared to the SCM435 bolt due to smaller mismatch of thermal expansion between the bolt material and the plates. The mismatch of thermal expansion between bolt material and AZ91D plates was found to induce the compressive creep deformation in the AZ91D plates which performed as the main mechanism of bolt load loss. At higher tightening stress, the bolt load loss could be intensified by additional plastic deformation in bolt occurred during the test. Moreover, it is suggested that the plastic deformation could be reduced by decreasing the friction condition in the bolted joint.

Effect of stress and temperature on the micromechanics of creep in highly irradiated bone and dentin

Synchrotron X-ray diffraction is used to study in situ the evolution of phase strains during compressive creep deformation in bovine bone and dentin for a range of compressive stresses and irradiation rates, at ambient and body temperatures. In all cases, compressive strains in the collagen phase increase with increasing creep time (and concomitant irradiation), reflecting macroscopic deformation of the sample. By contrast, compressive elastic strains in the hydroxyapatite (HAP) phase, created upon initial application of compressive load on the sample, decrease with increasing time (and irradiation) for all conditions; this load shedding behavior is consistent with damage at the HAP–collagen interface due to the high irradiation doses (from ~100 to ~9,000kGy). Both the HAP and fibril strain rates increase with applied compressive stress, temperature and irradiation rate, which is indicative of greater collagen molecular sliding at the HAP–collagen interface and greater intermolecular sliding (i.e., plastic deformation) within the collagen network. The temperature sensitivity confirms that testing at body temperature, rather than ambient temperature, is necessary to assess the in vivo behavior of bone and teeth. The characteristic pattern of HAP strain evolution with time differs quantitatively between bone and dentin, and may reflect their different structural organization.

Effect of X-ray irradiation on the elastic strain evolution in the mineral phase of bovine bone under creep and load-free conditions

Both the load partitioning between hydroxyapatite (HAP) and collagen during compressive creep deformation of bone and the HAP residual strain in unloaded bone have been shown in previous synchrotron X-ray diffraction studies to be affected by the X-ray irradiation dose. Here, through detailed analysis of the X-ray diffraction patterns of bovine bone, the effect of X-ray dose on (i) the rate of HAP elastic strain accumulation/shedding under creep conditions and (ii) the HAP lattice spacing and average root mean square (RMS) strain under load-free conditions are examined. These strain measurements exhibit three stages in response to increasing X-ray dose. Up to ∼75kGy (stage I) no effect of dose is observed, indicating a threshold behavior. Between ∼75 and ∼300kGy (stage II) in unloaded bone the HAP d-spacing increases and the RMS strain decreases with dose, indicating strain relaxation of HAP. Furthermore, under constant compressive load creep conditions, the rate of compressive elastic strain accumulation in HAP decreases with increasing dose until, at ∼115kGy, it changes sign, indicating that the HAP phase is shedding load during creep deformation. These stage II behaviors are consistent with HAP–collagen interfacial damage, which allows the HAP elastic strain to relax within both the loaded and unloaded samples. Finally, for doses in excess of ∼300kGy (stage III, measured up to 7771kGy) the HAP lattice spacing and RMS strain for load-free samples and the rate of HAP elastic strain shedding for crept samples remain independent of dose, suggesting a saturation of damage and/or stiffening of the collagen matrix due to intermolecular cross-linking.

Ambient- and high-temperature mechanical properties of isochronally aged Al–0.06Sc, Al–0.06Zr and Al–0.06Sc–0.06Zr (at.%) alloys

Ambient- and high-temperature precipitation strengthening are investigated in Al–0.06Sc, Al–0.06Zr and Al–0.06Sc–0.06Zr (at.%) alloys. Following solidification, Sc is concentrated at the dendrite peripheries while Zr is segregated at the dendrite cores. During isochronal aging, precipitation of Al 3Sc (L1 2) commences between 250 and 300 °C for Al–0.06Sc, and reaches a 429 MPa peak microhardness at 325 °C. For Al–0.06Zr, precipitation of Al 3Zr (L1 2) first occurs between 400 and 425 °C and reaches a 295 MPa peak microhardness at 475 °C. A pronounced synergistic effect is observed when both Sc and Zr are present. Above 325 °C, Zr additions provide a secondary strength increase that is attributed to precipitation of Zr-enriched outer shells onto the Al 3Sc precipitates, leading to a peak microhardness of 618 MPa at 400 °C for Al–0.06Sc–0.06Zr. Upon compressive creep deformation at 300–400 °C, Al–0.06Sc–0.06Zr exhibits threshold stresses of 7–12 MPa; these values may be further improved by optimal heat-treatments.

Creep resistance of cast and aged Al–0.1Zr and Al–0.1Zr–0.1Ti (at.%) alloys at 300–400 °C

Cast and aged Al–0.1Zr and Al–0.1Zr–0.1Ti (at.%) alloys, upon compressive creep deformation at 300–400 °C, exhibit threshold stresses attributable to climb-controlled bypass of coherent Al3Zr and Al3(Zr1−xTix) precipitates. Al–0.1Zr–0.1Ti exhibits a smaller threshold stress than Al–0.1Zr, which is attributed principally to a reduced lattice parameter mismatch between the Al3(Zr1−xTix) precipitates and the matrix. The present alloys are less creep resistant than Al–Sc and Al–Sc–Zr/Ti alloys with similar precipitate radii and volume fractions.

Lattice distortion in γ′ precipitates of single crystal superalloy SC16 under creep deformation

Specimens of single crystal superalloy SC16 were creep deformed at 1223K along [001] up to ±0.5% creep strain using stresses of −150MPa and +150MPa, respectively. Line widths and peak positions of superlattice reflections were measured by means of X-ray diffraction parallel and perpendicular to the load axis in the temperature range between 293K and 1173K. The line widths were found to decrease with the increase of temperature for both directions on the two specimens after tensile and compressive creep deformation. After both kinds of creep deformation the crystal lattice showed tetragonal distortion which decreased with increasing temperature. The tetragonality after tensile creep deformation was larger than unity while it was smaller than unity after compressive creep deformation. The peak positions and widths restored after cooling back to room temperature. The experimental results can qualitatively be explained by the creation of dislocations during deformation and their anisotropic arrangement at the γ/γ′ interfaces.

Rapoport–Samoilenko test for cathode carbon materials—II. Swelling with external pressure and effect of creep

The creep strain of an anthracitic commercial cathode material used for aluminium production has been measured on solid cylinder samples. Experimental results have been obtained using a Rapoport–Samoilenko-type apparatus for virgin material at room and high temperatures as well as for electrolyzed material under loading. When an external pressure is applied to the material, the reduction of sodium expansion is explained by the effect of compressive creep deformation related to the sodium penetration into the binder phase of carbon. The creep strain is much larger when sodium is absorbed under pressure compared with the creep strain when an external pressure is applied to the material after binder phase sodium saturation. A constitutive model for creep deformation in the binder phase of cathode carbon materials which is able to reproduce the relationship between the creep strain, external pressure and time during the Rapoport–Samoilenko-type test has been developed. The model has been extended to a three-dimensional stress–strain state. The calculations demonstrate that creep has to be considered in order to obtain realistic stress–strain results.

Constituents and features of displacement vector angle of colluvial landslide

Based on an analysis of displacement-monitoring data of Xintan landslide, China, it was discovered that the whole stability of large-scale colluvial landslides can be more accurately described by taking the displacement vector angle into account, and the stability trend of a slope can be evaluated and predicted using the displacement vector angle. This paper also further indicates that the monitored deformation vector angle can be divided into sliding deformation vector angle, plastic-deformation vector angle, compressive deformation vector angle and creep deformation vector angle in terms of the displacement properties of the slope. Through the analysis it is found that the displacement vector angle, for which there is no substitute, is an important parameter serving as an explicit criterion for the stability of the slope, and hence its significance in the prediction of landslides. It is also shown that as far as the displacement vector angle is concerned, there are different constituents and features in the different deformation stage of colluvial landslide. On the basis of these and using the principles of statistics, a discussion on the stability of the colluvial slopes is carried out in terms of the monitoring data of F-series points on Xintan slope. The analysis results coincided with the destabilized time and the sliding laws, thus demonstrating that the parameter of the displacement vector angle has to a certain extent, very important significance in the forecasting of landslides.

Dynamic compressive creep of extruded ultra-high molecular weight polyethylene

To estimate the true wear rate of polyethylene acetabular cups used in total hip arthroplasty, the dynamic compressive creep deformation of ultra-high molecular weight polyethylene (UHMWPE) was quantified as a function of time, load amplitude, and radial location of the specimen in the extruded rod stock. These data were also compared with the creep behavior of polyethylene observed under static loading. Total creep strains under dynamic loading were only 64%, 70%, and 61% of the total creep strains under static loading at the same maximum pressures of 2 MPa, 4 MPa, and 8 MPa, respectively. Specimens cut from the periphery of the rod stock demonstrated more creep than those cut from the center when they were compressed in a direction parallel to the extrusion direction (vertical loading), whereas the opposite was observed when specimens were compressed in a direction perpendicular to the extrusion direction (transverse loading). These findings show that creep deformation of UHMWPE depends upon the orientation of the crystalline lamellae.