High Stress Exponent Research Articles

Microstructure evolution by thermomechanical processing and high-temperature mechanical properties of HCP-high entropy alloys (HEAs)

The solid solution strengthening and deformation mechanisms of the hcp-high entropy alloys (HEAs) have not been elucidated yet because of the complexity introduced by the martensite structure, particularly in alloys undergoing phase transformation from bcc to hcp structures. This study attempted to fabricate an equiaxed hcp microstructure for TiZrHf, TiZrHfAl, and TiZrHfAlSc using the hot rolling process and subsequent heat treatment. The equiaxed hcp phase was successfully formed in TiZrHfAl, but the martensite structure remained in TiZrHf and TiZrHfAlSc. The 0.2 % proof stress at 400 and 600 °C increased with the increase of the average atomic size misfit δ and the mixing entropy ΔSmix. The low activation volume and high-stress exponent of TiZrHfAl with equiaxed hcp phase at room temperature suggest minor obstacles such as cluster or short-range order inhibit the movement of dislocation and leading to the significant solid-solution strengthening. However, the effect diminished at 600 °C.

The effect of constituent phases on microstructure and high temperature deformation mechanisms of IrRhRuWMo complex concentrated alloys

The alloys with different primary phases (BCC, BCC + HCP, HCP, FCC) were fabricated using the same alloying elements: Ir, Rh, Ru, W, and Mo. Comprehensive microstructure characterization and phase identification were performed to reveal all existing phases present within the selected alloys. In situ strain jump compression test was carried out to assess mechanical properties and explore underlying deformation mechanisms, both at room temperature (RT) and 1500 °C. While a noticeable discrepancy in 0.2 % proof stress was observed among the selected alloys at RT, the difference was significantly minimized at 1500 °C. Furthermore, while the dual-phase structure effectively increased the 0.2 % proof stress at RT, it had a detrimental effect at 1500 °C. All alloys obeyed the power-law relationship both at RT and 1500 °C, exhibiting relatively high stress exponents. Together with calculated activation volume, these findings suggested that different rate-limited deformation mechanisms were likely activated at RT and 1500 °C.

Role of threshold stress in creep of IN740H, a γ′-lean Ni-based superalloy

IN740H, a Ni-based superalloy comprising a low-volume fraction of γ′, is a candidate material to be used over long periods at high temperatures and stresses, as conditions prevalent in advanced ultra-supercritical (AUSC) power plants. In this study, IN740H has been tested at temperatures above 700 °C to gain mechanistic insights into its high-temperature creep behavior. The material showed classic signatures of the presence of threshold stress, marked by observation of a high apparent creep stress exponent, n, (e.g., 10 at 750 °C) and a high apparent activation energy for creep, Qc (e.g., ∼545 kJ/mol), along with a rapid increase in n at lower stresses. Accounting for the threshold stress led to a decrease in the values of n and Qc to 4 and 280 kJ/mol, respectively. Furthermore, the transmission electron microscopy and atomic scale compositional analysis reveal the pinning of dislocations at γ-γ′ interface and segregation of Co, Cr and Mo atoms in the regions of γ-γ′ interface rich in dislocations. The above combination of n and Qc and the observation of dislocation pinning at the γ-γ′ interface indicate the dislocation climb over γ′ as the dominant creep mechanism in this γ′-lean Ni-based superalloy, with the detachment of dislocations from the γ-γ′ interface, augmented by the segregation of Co, Cr and Mo, as the mechanism responsible for the realization of the threshold stress. This work, thus, provides a new impetus to research on the long-term structural integrity of γ′-lean Ni-based superalloys exposed to extreme service conditions.

Intermediate-temperature creep behaviors of an equiatomic VNbMoTaW refractory high-entropy alloy

As one of refractory high-entropy alloys (RHEAs), VNbMoTaW is considered to be a promising candidate for elevated-temperature application. However, its creep behavior is rarely reported. In the present work, the compressive creep behaviors of an equiatomic VNbMoTaW RHEA with a grain size of 138 ± 36 μm were well studied over an intermediate temperature range (973–1173 K) and under high applied stress (130–520 MPa). The stress exponent of the alloy is found to remain stable (∼1) at relatively low temperatures (973–1073 K), whereas a stress-dependent transition behavior occurs at high temperatures (1123–1173 K), i.e., the stress exponent of the alloy changes from ∼1 in the low stress region (130–390 MPa) to ∼ 4 in the high stress region (390–520 MPa). Meanwhile, the creep activation energy increases from 139 to 156 kJ mol−1 at low temperatures to 307–373 kJ mol−1 at high temperatures. The low stress exponent and low activation energy at low temperatures suggest that the creep is controlled by dislocation pipe diffusion. The low stress exponent and relatively high activation energy in the high-temperature low-stress region suggest that the creep is controlled by lattice diffusion. In the high-temperature high-stress region, the prevalent dislocations detected by the post-mortem microstructural observation, the high stress exponent, and high activation energy suggest that the creep deformation is controlled by a lattice diffusion mediated dislocation climb process. These findings provide a fundamental understanding of the creep behavior and deformation mechanism of VNbMoTaW, which can be applied to design advanced creep-resistant RHEAs.

Progression of creep deformation from grain boundaries to grain interior in Al-Cu-Mn-Zr alloys

Creep mechanisms are studied in θ′-Al2Cu-strengthened Al-Cu-Mn-Zr alloys at 300 and 350°C for (i) ACMZ, a base alloy without further alloying elements and (ii) RR350, a commercial alloy with additions of Ni and Co forming distinct grain-boundary precipitates. At high stresses, creep is dominated by dislocations bypassing θ′ precipitates within grains via the Orowan mechanism, as evidenced by (ⅰ) very high stress exponent (n∼20-25) and (ⅱ) α-Al and θ′ lattice strains (measured via in-situ neutron diffraction) evolving during creep in a manner consistent with load transfer from the plastically-deforming α-Al matrix to elastically-deforming θ′ precipitates. At intermediate stresses, both alloys exhibit a n∼3 regime, where α-Al and θ′ lattice strains scale near-linearly with applied stress while remaining largely unaffected by strain accumulation, indicating that Orowan looping or dislocation pile-up around θ′ is now inactive within the grains. Rather, dislocation motion occurs solely in θ′-precipitate-free zones (θ′-PFZ) where high dislocation densities are observed via TEM after creep deformation. Plastic flow at θ′-PFZ and/or localized pipe diffusion are expected to enable grain-boundary sliding (GBS), which is proposed as the rate-limiting mechanism in the n∼3 regime. Ni/Co-rich precipitates at RR350 grain-boundaries, with negligible θ′-PFZ around them, share load (as determined via neutron diffraction) with the α-Al matrix more effectively than θ-Al2Cu precipitates at ACMZ grain-boundaries, with wide surrounding θ′-PFZ. Thus, high creep resistance in the n∼3 GBS regime of RR350 is enabled by coarsening-resistant grain-boundary precipitates, forming without concomitant development of weak θ′-PFZ, which effectively share load with the grains.

Revealing the tensile creep behavior and mechanism of SiC particles reinforced AZ31 composite fabricated by liquid metallurgy

Constant engineering stress tensile creep behavior of AZ31 magnesium matrix composites with SiC particles in 20% volume fraction and a mean diameter of 8.6 μm, fabricated by liquid metallurgy, was performed at the temperature range of 473–623 K, and the stress level range of 7–70 MPa. The minimum creep rates of the as-cast composites reveal better creep resistance at high temperatures and low stresses compared with those of unreinforced AZ31 alloy. The high apparent stress exponents and apparent activation energies of the composites imply the presence of threshold stress in the creep behavior. The true stress exponent of the composite was calculated to be ∼5, indicating that the possible creep mechanism is the climb-controlled creep model controlled by lattice diffusion.

Creep properties and microstructure evolution at 260–300 °C of AlSi10Mg manufactured via laser powder-bed fusion

The aging behavior and creep resistance of eutectic AlSi10Mg (Al–10Si-0.6Mg, wt.%) manufactured by laser powder bed fusion (L-PBF) are studied at 260 and 300 °C. The Si phase, which forms a fine interconnected network of 100–200 nm filaments in the as-printed alloy, coarsens into blocky, 2–3 μm particles after 1 month exposure at 260 and 300 °C, with hardness decreasing following a power-law with exponent n = 0.06–0.08. AlSi10Mg with ∼1 μm Si particles (achieved by aging at 300 °C for 96 h) exhibits creep resistance at 260 and 300 °C, for test durations of a few days, comparable to those of (i) cast alloys: hypereutectic Al–Si alloys with coarse Si particles and eutectic Al–Ce, Al–Ce–Ni with much finer, and more coarsening-resistant eutectics phases (e.g., Al11Ce3, Al3Ni); and (ii) L-PBF Al–Mg–Zr alloys. At 260 and 300 °C, our L-PBF AlSi10Mg shows a power-law creep behavior with high apparent stress exponents (na = 10–13, higher than n = 4.0 for Al–Mg) for strain rates between ∼10−8 and ∼10−4 s−1. Also, a high apparent creep activation energy Qa = 256 kJ/mol is measured between 200 and 320 °C at 45 MPa, compared to Q = 142 kJ/mol for Al. These high apparent stress exponent and activation energy are consistent with power-law dislocation creep with a threshold stress, originating from load-transfer from the creeping Al(Mg) matrix to non-creeping Si particles.

Creep deformation and rupture behavior of 10Cr-3Co-2W heat-resistant steel weldments in ultra supercritical power units

Creep deformation and rupture behavior of a martensitic heat resistant steel (10Cr-3Co-2W) welded joints prepared by vacuum electron beam welding for supercritical power generation unit are investigated employing traditional uniaxial tensile creep, 898 K (625 ℃) to 948 K (675 ℃) and 105 MPa to 250 MPa, and room temperature nanoindentation on different welding micro-zones. The rupture location shifts from base metal (ductile rupture) for the shortest creep time of 74 h, to heat affected zone (brittle Type IV rupture) for the rest crept specimens. Accordingly, a softening phenomenon is observed in fine grain heat affected zone in crept specimens for longer creep times, where voids, precipitates and reduced dislocation density are found and further discussed. The extrapolating creep rupture time under various creep temperature and stress combinations and the 100,000 h creep rupture strength for different temperatures are predicted employing the Larson-Miller parameter method, which satisfies the industry service condition. Compared with tensile creep the nanoindentation creep yields faster creep rates and higher stress exponents; and the largest strain rate sensitivity value (m) in weld metal implies that weld metal has the best creep resistance while heat affected zone and base metal are more likely to appear creep rupture.

Solute-induced strengthening during creep of an aged-hardened Al-Mn-Zr alloy

We examine the precipitation and creep behavior of Al-0.5Mn-0.02Si (at.%) alloys, with and without the L12-forming elements Zr and Er (0.09 and 0.05 at.%, respectively), utilizing isochronal aging experiments as well as compressive and tensile creep tests performed between 275 and 400 °C. The Al-0.5Mn-0.09Zr-0.05Er-0.05Si alloy exhibits an unusually high creep resistance in the peak-aged state, which is significantly better than that observed generally in its Mn-free L12-strengthened counterparts; for example, the creep threshold stresses at 300 °C are 34-37 MPa, about three times higher than those in a Mn-free Al-0.11Zr-0.005Er-0.02Si alloy. Scanning transmission electron microscopy illustrates that nanoscale Al3(Zr,Er) L12-precipitates are formed in the dendritic cores and micron-sized Al(Mn,Fe)Si α-precipitates in the interdendritic channels. Moreover, the Al(f.c.c.)-matrix remains supersaturated with randomly distributed Mn solute atoms, as determined by atom-probe tomography and electrical conductivity measurements, for months at creep temperatures. Creep experiments on the Zr- and Er-free Al-0.5Mn-0.02Si solid-solution alloy reveal a small primary creep strain, a high apparent stress exponent, na ∼9-7, and a threshold-stress-type behavior. After ruling out other possible mechanisms, we provide evidence that the threshold stress in this precipitate-free alloy originates from dislocation/solute elastic interactions leading to a strong drag force exerted on edge dislocations, hindering their ability to climb. The relatively high creep resistance of Al-0.5Mn-0.09Zr-0.05Er-0.05Si is interpreted in terms of the synergy between this solute-induced threshold stress (SITS, from Mn in solid-solution) and the known precipitate-bypass threshold stress (from the L12-nanoprecipitates).

Modeling the Creep of Nickel

Abstract Many metals and alloys have a stress exponent for the creep rate that is considerably higher than the value of three that is typically predicted by creep recovery models. One example is pure Ni. Creep data from Norman and Duran that are analyzed in the paper give a stress exponent of about seven in the temperature range 0.3–0.55 of the melting point. It has recently been shown that the high creep exponent of Al and Cu in the power-law breakdown regime can be explained by the presence of strain-induced vacancies. By applying a creep recovery model that does not involve adjustable parameters, it is shown that strain-induced vacancies can also explain the high-stress exponent of pure nickel.

Compressive creep behavior of extruded Mg-4Sm-2Yb-0.6Zn-0.4Zr alloy

Abstract A high-strength Mg-4Sm-2Yb-0.6Zn-0.4Zr extruded alloy with bimodal microstructure was creep tested in compression at temperatures of 200 °C and 225 °C under applied stress in the range of 120–200 MPa. The creep curves of the alloy are dominated by a steady-state creep stage and a transient primary creep stage. An abnormal decrease of creep rate is observed in the later period of the steady-state creep stage, particularly the creep at high stress levels more significant, and the underlying reasons are proposed. Moreover, the alloy exhibits high stress exponents (n = 8.7 at 200 °C and n = 7.4 at 225 °C) and activation energies ranging from 225 kJ/mol to 310 kJ/mol. It is shown that the high stress exponents can be well rationalized by the commonly adopted threshold stress method in this work, which results in the modified stress exponents near 5, suggesting five power law, with some indicator of dislocation creep. Transmission electron microscopic observations revealed that cross-slip of dislocations is the dominant creep mechanism in recrystallized grains, while cross-slip of dislocations is operative in hot-worked grains. Besides, a fraction of γ phase in hot-worked regions was transformed to γ′ phase during creep while not in recrystallized regions, thus coexistence of γ and γ’ in hot-worked regions. Also, these γ-typed phases play an important role in creep of the studied alloy. Finally, the studied alloy owns better creep resistance than those of benchmark Mg alloys such as AE and AX series alloys.

High temperature creep behavior of an as-cast near-α Ti–6Al–4Sn–4Zr-0.8Mo–1Nb–1W-0.25Si alloy

The high temperature creep behavior of a near-α Ti–6Al–4Sn–4Zr-0.8Mo–1Nb–1W-0.25Si alloy prepared by induction skull melting has been studied under the conditions of 625 °C-675 °C and 150 MPa–250 MPa using uniaxial creep tests in this work. Results indicated that the incoherent secondary phase particle, silicide, mainly precipitated from the α/β lath interface, which has a positive effect on hindering the movement of dislocations and pinning α/β lamellae boundaries during creep. The stress exponent and apparent activation energy was 7.2 at 650 °C and 417.6 kJ/mol at 200 MPa, respectively. The high stress exponent and apparent activation energy indicated that the creep process was controlled by dislocation climb and slip impeded by silicides. The main failure mechanism of the as-cast alloy during creep was the initiation, growth, and coalescence of cracks at the grain boundary, which was proven from the intergranular fracture feature.

Creep deformation behavior of nano oxide dispersion strengthened Fe–18Cr ferritic steel

The objective of present work is to investigate the creep behavior of nano oxide dispersion strengthened Fe–18Cr ferritic steel (Fe–18Cr–3W–0.3Ti–0.35Y2O3) over a range of temperatures 923–1023 K and range of stresses 175–400 MPa. The analysis of creep data according to the power law equation suggests high apparent stress exponents (17–26) and high apparent activation energy (~690 kJ/mol). Microstructural analysis indicated that no change in the grain and nanoprecipitate size occurred in the sample crept at 973 K and 225 MPa after 2600 h. Transmission electron microscopy studies on crept samples revealed substantial dislocation activity in the grains and the interaction between dislocations and nanoprecipitates. Based on the analysis of results, dislocation detachment mechanism is identified as a creep mechanism in the present steel. Accordingly, the creep results are rationalized based on the dislocation detachment model and mechanical threshold stress model.

Creep analysis in thick composite cylinder considering large strain

Cylindrical vessels are widely used in petroleum or aerospace industry. In these applications, the cylinders have to work under harsh mechanical and thermal loadings where creep plays an important role. The present paper analyzes creeps behavior of an Al–SiCp composite cylinder under internal pressure. The strains are assumed to be large which necessitate the use of finite strain theory. Threshold creep law is used for analysis because the creep rate in aluminum-based composites using Norton’s law was not approved due to apparently high stress exponent and high apparent activation energy. Such analysis will help the designers in the prediction of correct creep rate and stresses in cases where large creep deformation of a cylinder is permissible. With the use of finite strain theory, it is found that design of the cylinder based on small strain theory leads to unsafe cylinder design.

Deformation of mafic schists from subducted oceanic crust at high pressure and temperature conditions

We conducted triaxial deformation experiments using a Griggs-type solid pressure-medium apparatus on two mafic schists that experienced different peak metamorphic conditions from the Sambagawa metamorphic belt; a greenschist and an epidote-amphibole schist. At a confining pressure of 1GPa, 400°C and strain rate of 10–5 1/s, differential stresses for all mafic schists were higher than 1GPa. The greenschist samples were weaker than epidote-amphibolite samples under all experimental conditions. Microstructures of recovered samples of greenschist indicate that deformation of mafic schist is accommodated by semi-brittle deformation. Samples of the epidote-amphibole schist deformed at 500°C show shear localization along a single fault oriented parallel to the foliation, while mechanical data shows no rapid stress drop. On the surface of the fault, we observe the formation of slickensides with little gouge. All sample types exhibit a high stress exponent (n > 15) and strain rate strengthening (i.e., positive a–b values); frictional behavior that inhibits earthquake nucleation. Differential stress increased with increasing confining pressure, while the friction coefficient decreased with increasing confining pressure and temperature. Greenschist and epidote-amphibolite facies metamorphism typically occurs at hot subduction zones such as Cascadia and southwest Japan. The lack of seismicity within the subducting oceanic crust at hot subduction zones could be explained by semi-brittle deformation, or frictional sliding with a positive (a – b) during deformation of these types of metamorphic rocks. Weak and aseismic fault zones in subducting slabs might promote slab-mantle decoupling at the corner of the mantle wedge, and detachment and underplating of oceanic crust from the subducting slab to forearc crust.

Stable second phase: The key to high-temperature creep performance of particle reinforced aluminum matrix composite

High-temperature creep behavior of 45vol%-SiCp/2024Al was studied at 250–350 °C under 40–140 MPa. True stress exponent and activation energy were measured 8 and 227.7 kJ/mol, respectively. The feature substructure of SiCp/2024Al was the second phase Al2Cu dispersed at the interface. The evolution of microstructure and the reason for its high creep activation energy were both analyzed and discussed. θ′ phase and S′ phase underwent different evolution processes during creep. θ′ phase gradually transformed to θ phase and remain stable from the steady-state creep until creep rupture. High content of the interface facilitated the dispersion of Al2Cu. The relationship between true stress exponent and interface content as well as the key to high true stress exponent of high volume fraction particle reinforced aluminum matrix composite were also discussed. This research provided new perspectives and guidance for designing and fabricating composites with superior high-temperature creep properties.

Cast near-eutectic Al-12.5 wt.% Ce alloy with high coarsening and creep resistance

This study investigates the creep behavior of a cast, coarse-grained Al-12.5 wt.% Ce (Al-2.7 at.% Ce) alloy, consisting of an eutectic microstructure (α-Al with ~11 vol.% submicron Al11Ce3 “Chinese script” platelets) with ~3 vol.% primary, micron-scale Al11Ce3 plates. Upon aging at 322 °C for 8 weeks or at 400 °C for 12 weeks, the microhardness of the alloy remains unchanged, demonstrating excellent coarsening resistance of the strengthening Al11Ce3 phase. In addition, no coarsening of Al11Ce3 is observed metallographically after 3 weeks under compressive loads of 13–70 MPa at 260–350 °C. When tested to failure under a constant tensile stress of 23 MPa at 300 °C, the alloy shows primary, secondary and tertiary creep regimes, and fails after 19 days at 17% tensile strain, demonstrating both high creep ductility and high creep resistance. Under compressive and tensile creep conditions, the alloy exhibits high apparent stress exponents (n = 9–11), which translate into threshold stresses for dislocation creep of 34, 22, and 14 MPa at 260, 300 and 350 °C, respectively. The creep resistance of Al-12.5 wt.% Ce is higher than that of Al-Sc-Zr-based alloys (with ~0.3 vol.% of coherent nanoprecipitates) and similar to cast, eutectic Al-6 wt.% Ni (with ~11 vol.% of incoherent Al3Ni micro-fibers). For as-cast grain sizes of 2–3 mm, Al-12.5 wt.% Ce exhibits a transition from dislocation creep to diffusional creep at strain rates of ~10−7 s−1, with a threshold stress of 19 MPa in compression at 260 °C and 5 MPa in tension at 300 °C.

Effect of Grain Orientation on Indentation Induced Creep in Pure Zinc

Abstract The study presents the anisotropy in the indentation creep response of zinc. Indentation creep tests were conducted on grains with three different orientations, namely, 〈0001〉, ⟨112¯0⟩, and ⟨101¯0⟩. Indentation creep along the 〈c〉 axis showed a pronounced creep exhibiting a 25% higher creep displacement as compared with indentation perpendicular to the 〈c〉 axis. However, the recovery observed was the highest for basal-oriented grains wherein a recovery of 50% to its indented depth was observed, whereas for ⟨112¯0⟩ the recovery observed was 30%. The stress exponent, n, was obtained by employing two different approaches for each of the orientations and a difference in the stress exponent value was also observed, highlighting the anisotropic creep response. Basal and non-basal oriented grains showed a lower (n ≅ 1.6) and higher stress exponent (n ≅ 5), respectively, highlighting the different operable creep mechanism. Surface topography using atomic force microscope (AFM) revealed twinning and sink-in for ⟨112¯0⟩ and ⟨101¯0⟩, whereas uniform pile up was observed for basal grain.

Microstructure evolution and creep behavior of high-pressure die-cast Mg–9Al–1Zn–1Sr alloy

The creep behavior of high-pressure die-cast Mg–9Al–1Zn–1Sr alloy was investigated in detail under temperatures ranging from 130 to 170 °C and stresses of 30–80 MPa. The studied alloy exhibited a high stress exponent of 8.4 and low activation energy of 102 kJ/mol. The obtained high stress exponent in studied alloy indicated the breakdown of the conventional power law. For rationalization, a threshold stress was introduced in the analysis, and then the high stress exponent was modified to be 5.2. Simultaneously, taking into account the significant influence of tested temperature on the minimum creep rate, a normalized stress exponent of 5 was achieved via the normalization method. Both methods suggested that the creep deformation of the studied alloy was mainly controlled by the dislocation climb. Moreover, transmission electron microscopy observations revealed that the origin of the threshold stress could be associated with the Orowan strengthening of γ-Mg17Al12 platelets that dynamically precipitated in Mg matrix during creep. Microstructure analysis results clarified that dislocation climb is the dominant creep mechanism of AZJ911 alloy, while the reticular eutectic Mg17Al12 phase disintegration, dynamical precipitation of γ-Mg17Al12, and twinning also influence the creep deformation of AJZ911 alloy.

An Experimental Investigation of the Relative Strength of the Silica Polymorphs Quartz, Coesite, and Stishovite

AbstractIn this study, quartz, coesite, and stishovite were deformed concurrently with an olivine reference sample at high pressure and 850 ± 50 °C. Olivine deformed with an effective stress exponent (n) of , which we interpret to indicate that the Peierls creep deformation mechanism was active in the olivine. Quartz and coesite had very similar strengths and deformed by a mechanism with n = and , respectively, which are consistent with previous measurements of power law creep in these phases. Stishovite deformed with n = and was stronger than both olivine and the other silica polymorphs. The high stress exponent of stishovite is greater than that typically observed for power law creep, indicating it is probably (but not certainly) deforming by Peierls creep. The rheology of SiO2 minerals appears therefore to be strongly affected by the change in silicon coordination and density from fourfold in quartz and coesite to sixfold in stishovite. If the effect of Si coordination can be generalized, the increase in Si coordination (and density) associated with bridgmanite formation may explain the tenfold to 100‐fold viscosity increase around 660 km depth in the Earth.