Indentation creep of the wrought AZ31 magnesium alloy

Creep behavior of a wrought Mg–3Al–1Zn (AZ31) alloy was investigated by long-term Vickers indentation testing under constant loads of 5 and 10 N and at temperatures in the range 423–523 K. Based on the steady-state power-law creep relationship, the stress exponents were determined. The creep behavior can be divided into two stress regimes with different stress exponents and activation energy values. The low-stress regime activation energy of 96.2 kJ mol⁻¹, which can be interpreted as that for the activation energy for Al diffusion in Mg, and stress exponents of about 3.0–3.4 suggest that the operative creep mechanism is dislocation viscous glide governed by the diffusion of aluminum atoms in magnesium. This behavior is in contrast to the high-stress regime, in which the average values of n = 6 and Q = 132.4 kJ mol⁻¹ imply that dislocation climb-controlled creep is the dominant deformation mechanism. Stress exponents and activation energies obtained by different analysis methods of the indentation tests are in good agreement with each other and with those of the conventional tensile creep tests on AZ31 magnesium alloy reported in the literature. The localized indentation creep tests are, thus, considered capable of acquiring reliable information on the creep behavior of wrought magnesium alloys.     

Room temperature nanoindentation creep of nanoscale Ag/Fe multilayers

This paper deals with room temperature indentation creep behavior of nanoscale Ag/Fe multilayers. The constant-load nanoindentation test results reveal that all the multilayers exhibit steady-state creep after transient creep occurring at first 150 s and decreasing periodicity leads to a decrease in the stress exponent and an increase in creep rate. The dependence of the stress exponent and creep rate on the periodicity indicates that the creep process is dominated by dislocation glide-climb mechanism and the increasing fraction of grain boundaries and interfaces provide effective diffusion paths for the creep climb that determines the whole creep rate.     

Loading rate sensitivity of nanoindentation creep in polycrystalline Ni films

Nanoindentation creep tests in Ni thin films with 3,000 nm thickness were performed with different loading times (5, 10, 20, 30, and 50 s) under the holding load 5,000 μN and holding time 30 s to investigate the dependence of the indentation creep behavior on the loading rate. The results show that significant indentation loading rate sensitivity on stress exponent and hardness was found, which shows that the stress exponent increases with indentation loading rate. In contrast, the elastic modulus decreases slightly (more or less 1%) due to a longer loading time. Based on the experimental results, we infer that the creep phenomena observed were probably induced by plasticity.     

Investigation of high-temperature plastic deformation using instrumented microindentation tests. Part IThe deformation of three aluminum alloys at 473 K to 833 K

Constant-load indentation tests were performed on wrought-2024, P/M-2024, and wrought-1100 aluminum alloys to assess the capability of the microindentation testing technique for measuring the high-temperature deformation rate controlling parameters of these alloys. The three alloys all display threshold indentation stress σ_th below which the indentation strain rate ε_ind approaches zero. The nominal inter-obstacle spacing, ℓ*, calculated from σ_th, increases with temperature in a way that is consistent with the known temperature dependence of the inter-particle spacing and dislocation cell size. The measured activation energy ΔG_o of ɛ_ind increases with temperature but remains within the range that is typical of deformation that occurs by dislocation glide limited by weak particles or dislocation/dislocation interactions. The three alloys tested show different trends of ΔG_o versus ℓ* and the trends are consistent with the known temperature dependence of the obstacles to dislocation glide. This study demonstrates that high-temperature indentation tests are sufficiently precise to detect changes in the operative deformation parameters between different alloys of the same general composition. This lays the groundwork for the use of this technique as a general tool for studying the local high-temperature deformation of a wide range of metal-based systems.     

Some features of indentation creep of LiF single crystals

The dependence of the size of the indentation and dislocation rosettes on loading time was investigated on the (001) plane of LiF single crystals. The measurements were performed in temperature range from room temperature to 170° C. The indentation time was varied from 0.2 to 10³ sec. It was revealed that the change of the indentation size during creep was more significant than the change in dislocation ensemble tracks in the field of the concentrated load. It was shown that the dependence of the length of the dislocation rosette edge arms on loading time, when plotting in double logarithmic scale, was linear. This fact allowed the determination of parameterm, characterizing the dependence of the dislocation velocity on stress, using creep experiments. The values ofm proved to be in good agreement with the results obtained by different methods.     

The high-temperature creep behaviour of an Al-1 wt% Cu solid-solution alloy

The creep behaviour of an Al-1 wt% Cu solid-solution alloy is investigated at a temperature of 813 K under stress range of 0.5–5 MPa. The creep characteristics of the alloy including the stress dependence of the steady-state creep rate (n=4.4), the shape of creep curve (normal primary stage), the transient creep after stress increase, and the value of the true activation energy for creep, suggest that some form of dislocation climb is the rate-controlling process at higher stresses above 1 MPa. However, at low stresses (< 1 MPa), the creep curves show no distinguished steady state, and the stress dependence of the minimum creep rate is as high as ～ 8. The creep behaviour of the alloy is discussed based on recent theories available for describing creep in solid-solution alloys.     

Creep and creep fracture of polycrystalline copper

Normal creep curves are recorded over extended stress ranges at 686–823 K for fine-grain copper. Analyses of the curve shape variations, together with the results of stress change experiments, do not support the view that a transition from dislocation to diffusional creep mechanisms occurs with decreasing stress. Instead, the observed behaviour patterns suggest that dislocation processes are dominant at all stress levels. However, strain accumulation within the grains becomes progressively less important as deformation is increasingly confined to the grain boundary zones when the stress is reduced below the yield stress at the creep temperature. New approaches are then introduced for rationalization of creep rate and creep life measurements, which account for the data trends taken as evidence for major mechanism changes when the creep properties are described using power law relationships.     

Compressive creep behavior of as-cast and aging-treated Mg–5 wt% Sn alloys

The microstructure and compressive creep behaviors of as-cast and aging-treated Mg–5 wt% Sn alloys are investigated in this paper. The compressive creep resistance of aging-treated Mg–5 wt% Sn alloy is much better than that of as-cast alloy at the applied stresses from 25 MPa to 35 MPa and the temperatures from 423 K to 473 K, which is mainly due to the dispersive distribution of Mg₂Sn phase in the aging-treated Mg–5 wt% Sn alloy. The calculated average values of stress exponent n and activation energy Q_c suggest that dislocation cross slip and dislocation climb happen respectively in as-cast and aging-treated Mg–5 wt% Sn alloys during creep.     

Creep and creep recovery of cast aluminum alloys

Constant load uniaxial creep tests were performed on four aluminum alloys (designated M4032-2, 332, 332RR, and 333) at stresses of 31.5 MPa, 56.5 MPa, and 73 MPa and temperatures of 220°C and 260°C. Of the four materials, M4032-2 had the greatest resistance to creep, while 332RR alloy had the least. In addition to creep, the creep recovery phase was observed as well. It was found that, even for short loading periods, much of the time-dependent strain was not recoverable for all of the materials studied. Hardening was observed to occur in each of the alloys, resulting in a reduced creep rate on subsequent loadings. A constitutive equation for creep and recovery incorporating both stress and temperature dependence was developed for each of the alloys tested based on a viscous-viscoelastic model.     

Steady-state creep behaviour of Ti3Al-base intermetallics

The steady-state creep behaviour of Ti₃Al and Ti₃Al+10 wt% Nb was studied in the temperature range 550 to 825° C and in the stress range 69 to 312 MN m^–2. The temperature and stress dependences of the steady-state creep rates were determined for both intermetallics, and the activation energy and stress-exponent were measured. At temperatures above 700° C, the stress dependence of the steady-state creep rate indicated two distinct creep regimes: at stresses above 138 MN m^–2, the creep was controlled most probably by dislocation climb; at stresses below 138 MN m^–2, a transition regime with a lower stress-exponent value was obtained.     

Low-temperature deformation behaviour of polycrystalline copper

Low-temperature plastic flow in copper was investigated by studying its tensile and creep deformation characteristics. The dependence of the flow stress on temperature and strain rate was used to evaluate the thermal activation energy while the activation area was derived from the change-in-stress creep experiments. A value of 0.6 eV was obtained for the total obstacle energy both in electrolytic and commerical copper. The activation areas in copper of three selected purities fell in the range 1200 to 100 b². A forest intersection mechanism seems to control the temperature dependent part of the flow stress. The increase in the athermal component of the flow stress with impurity content in copper is attributed to a change in the dislocation density. The investigation also revealed that thermal activation of some attractive junctions also takes place during low-temperature creep. The model of attractive junction formation on a stress decrement during creep, yields a value of 45±10 ergs cm⁻² for the stacking fault energy in copper.     

Effect of loading rate on the creep behaviour of epoxy resin insulators by nanoindentation

In the present work, the effect of loading rate on indentation creep was studied. Indentation creep tests were conducted on epoxy resin to provide creep deformation under constant load, contact creep compliance and cut-off time using a Berkovich indenter. Several loading rates, ranging from 0.25 to 6 mN/s, were used to perform the tests. The results showed that there is a strong loading rate dependence on creep response of the epoxy resin under indentation. Contact creep compliance and cut-off time decreased with increasing loading rate. In contrast, an increase in reduced modulus, hardness, displacement variation and contact creep compliance variation during the holding time was noticed. The loading rate sensitivity on creep response under indentation can be attributed to viscoelastic response prior to holding segment and strain rate effect on yield stress of the epoxy resin. This study provided an insight to understand the loading rate dependence on creep behaviour of epoxy resin under indentation.     

Effects of Mn addition on the microstructure and indentation creep behavior of the hot dip Zn coating

The Zn coatings with different Mn additions were prepared by hot dip process, and the effects of the Mn addition on the microstructure and indentation creep behavior of the coatings were investigated through scanning electron microscope and constant-load holding indentation technique at the room temperature. Some twins can be observed in the microstructure of Zn coating, which may account for the formation of the large thermal misfit stress between the zinc coating and the steel substrate. The amount of twin microstructure in the Zn coating decreases with the Mn addition. It is also found that Mn addition could induce MnZn₁₃ phases to precipitate along the grain boundary and significantly refine the grains of Zn coatings. The steady-state stress of the Zn coating could be improved by Mn addition. The creep stress exponent values are in the range of 14–46 and increases with Mn addition. The creep process of the Zn coating is dominated by dislocation climb and twin formation.     

Investigation of the compressive creep behavior of AZ91D magnesium alloy

The creep resistance of AZ91D alloy has been studied in uniaxial compression tests at temperature ranges from 275 °C to 325 °C. The initial microstructure of the alloy consists of α phase and β phase precipitated in the grain boundary. The minimum creep rate dependence on applied stress and the temperature is also analyzed in detail. We find that the stress exponent n is close to the theoretical values (3 or 5) and the activation energy Q for creep varies from 121 kJ/mol to 171 kJ/mol. Creep could be controlled by high-temperature climb and cross-slip of dislocation at different temperatures.     

Creep and fracture behavior of as-cast Mg–11Y–5Gd–2Zn–0.5Zr (wt%)

The tensile-creep and creep–fracture behavior of as-cast Mg–11Y–5Gd–2Zn–0.5Zr (wt%) (WGZ1152) was investigated at temperatures between 523 and 598 K (0.58–0.66T _m) and stresses between 30 and 140 MPa. The creep stress exponent was close to five, suggesting that dislocation creep was the dominant creep mechanism. The activation energy for creep (233 ± 18 kJ/mol) was higher than that for self-diffusion in magnesium, and was believed to be associated with cross-slip, which was the dominant thermally-aided creep mechanism. This was consistent with the surface observations, which suggested non-basal slip and cross-slip were active at 573 K. The minimum creep rate and fracture time values fit the original and modified Monkman–Grant models. In situ creep experiments highlighted the intergranular cracking evolution. The creep properties and behavior were compared with those for other high-temperature creep-resistant Mg alloys such as WE54-T6 and HZ32-T5.     

Effect of initial temper on the creep behavior of a Mg–Gd–Nd–Zr alloy

The effect of initial temper on the tensile creep behavior of a cast Mg–Gd–Nd–Zr alloy has been investigated. Specimens in unaged, underaged and peak-aged conditions exhibit a sigmoidal creep stage between the primary and steady-state creep stage, while the overaged specimens have no such creep stage. Transmission electron microscope observations revealed that sigmoidal creep stage was induced by the dynamic precipitation in the microstructure, and the rapid formation of β₁-phase and β-phase plates takes responsibility for the softening of material in this stage. Comparative evaluation of creep properties of the specimens showed that alloy in overaged condition had creep resistance superior to those in other conditions. Stress and temperature dependence of the steady-state creep rate were studied over a temperature range of 250–300 °C and stress range of 50–100 MPa, and a dislocation creep mechanism was proposed for the alloy.     

The influence of a threshold stress for grain boundary sliding on constitutive response of polycrystalline Al during high temperature deformation

A continuum polycrystal plasticity model was used to estimate the influence of a threshold stress for grain boundary sliding on the relationship between macroscopic flow stress and strain rate for the aluminum alloy AA5083 when subjected to plane strain uniaxial tension at 450 °C. Under these conditions, AA5083 deforms by dislocation glide at strain rates exceeding 0.001 s⁻¹, and by grain boundary sliding at lower strain rates. The stress–strain rate response can be approximated by , where A and n depend on grain size and strain rate. We find that a threshold stress less or equal to 4 MPa has only a small influence on flow stress and stress exponent n in the dislocation creep regime (a threshold stress of 2 MPa increases n from 4.2 to 4.5), but substantially increases both flow stress and stress exponent in the grain boundary sliding regime (a threshold stress of 2 MPa increases n from 1.5 to 2.7). In addition, when the threshold stress is included, our model predicts stress versus strain rate behavior that is in good agreement with experimental measurements reported by Kulas et al. [M.A. Kulas, W.P. Green, E.M. Taleff, P.E. Krajewski, T.R. McNelley, Metall. Mater. Trans. A 36 (2005) 1249].     

The enhancement of creep in Al–22?wt% Ag alloy by cyclic stressing

The plastic deformation behavior of Al–22 wt% Ag alloy during phase transformation was investigated by studying the creep behavior under cyclic stress reduction of low frequencies. The cyclic creep curves obtained describe clearly the cyclic stress acceleration behavior. Increasing frequency of cyclic stress reduction enhanced the creep deformation depending upon the combination of the experimental variables as testing temperature, aging temperature and static creep rate. The irregularity in the creep parameters, n, β and ε_st with increasing aging temperatures, has been explained on the basis of structure transformations occurring in Al–Ag system and their mode of interaction with mobile dislocations.     

Effect of heat treatment on the micro-indentation behavior of powder metallurgy nickel based superalloy FGH96

The influences of solution treatment temperature on microstructure and micromechanics of P/M nickel-base superalloy FGH96 were investigated by micro-indentation methods. The alloy was heat-treated at the temperatures of 1050 °C, 1150 °C, 1220 °C and 1310 °C, respectively. The micro-indentation tests were conducted in the indenter load range from 500 mN to 4500 mN and the loading rate range from 5.19 mN/s to 103.71 mN/s at room temperature by using a sharp Berkovich indenter. The influence of solution treatment temperature on microstructure was analyzed based on microstructural observations using both optical and scanning electron microscope. The micro-hardness, Young’s modulus and yield stress were obtained by means of Oliver–Pharr method and reverse analysis algorithms, respectively. The results show that both of micro-hardness and Young’s modulus are significantly affected by indentation depth and solution treatment temperature. Based on microstructure analysis, these effects were attributed to the changes of precipitate properties, e.g., size, distribution and morphology, and the relationship between microstructure and micromechanics was established. Then, the deformation mechanism was explained on the basis of dislocation–dislocation and dislocation–precipitate interactions. In this paper, the descending Young’s modulus was related to localized stress concentration and microcrack propagation. The results reveal that the damage variable is high for P/M nickel-base superalloy FGH96 after high temperature solution treatments.     

Effects of cyclic stress on the creep behaviour and dislocation microstructure of pure copper in the temperature range 0.4 to 0.5T m

The creep deformation behaviour of polycrystalline pure copper under static and cyclic stress was studied in the temperature range 0.4 to 0.5T _m. Both cyclic creep acceleration and retardation occurred depending on the condition of peak stress and temperature combination. The comparison of dislocation microstructures, developed during steady state static and cyclic creep deformation, has also been performed to determine the effect of cyclic stress on the dislocation microstructure and evidence for the enhanced recovery of the cell wall under cyclic stress was found. These effects of cyclic stress on the creep rate and dislocation microstructure were interpreted on the basis of diffusion-controlled recovery creep theory and the cyclic creep acceleration mechanism is suggested as the enhanced recovery of the cell wall with the help of athermally generated excess vacancies.