Creep Behavior Of Alloy Research Articles

Nanoindentation creep mechanism of gradient ultrafine lamellar γ+Ni5Zr eutectic using electromagnetic levitation coupled with fall casting

A quaternary Ni50Fe20Cr20Zr10 eutectic alloy was designed based on the Ni–Fe–Cr alloy, and the gradient ultrafine lamellar γ+Ni5Zr eutectic was achieved by means of electromagnetic levitation coupled with fall casting (EML-FC). The temperature field, the process of interface migration, microstructural characteristics and the nanoindentation creep property of the alloy were systematically analyzed. During the rapid solidification, since the conical surface served as the main heat dissipation channel, the solid-liquid interface moved from the conical surface towards the top of the cone axis. The gradual decrease in cooling rate from 105 to 103 K s−1 along this direction led to the formation of the gradient ultrafine lamellar γ + Ni5Zr eutectic with the size from 59 ± 12 to 140 ± 36 nm. The nanoindentation creep behavior and mechanism were analyzed by estimating the strain rate sensitivity and activation volume. As the interlamellar spacing of the eutectic decreased, the nanohardness and creep resistance of the alloy gradually increased owing to an increase in grain boundary density and the enhancement of coupling effect between dislocation and grain boundary. In addition, the creep behavior of alloy was sensitive to the loading rate. Increasing the loading rate led to an increase in strain rate sensitivity from 7.69 × 10−3 to 18.89 × 10−3 and a decrease in activation volume from 29 to 12 b3. This special eutectic structure and its improved nanohardness and creep resistance properties offered a potential opportunity to explore a new generation of alloys for the hot-end components of 972 K ultra-supercritical fossil-fired power plants.

Creep deformation and damage behavior of an alumina-forming austenitic alloy strengthened by intermetallic compounds

Based on Fe–30Ni–15Cr, a new alumina-forming austenitic (AFA) alloy, free of carbon, was designed for potential use in ultra-supercritical generator set. The creep behavior of alloy has been investigated by analyzing the creep constitutive equation, creep damage tolerance factor (λ), microstructure images, and oxide layer structure under different stresses at 750 °C. The creep microstructure and oxide morphology were obtained by electron microscopy. The minimum creep rate (ε˙m) is positively correlated with the stress, and when the stress increases from 120 MPa to 250 MPa, ε˙m increases from 6.26E-3 1/h to 6.34E-4 1/h, almost ten times of the latter. The creep constitutive equation at 750 °C was determined as: ε˙m=9.29×10−11σ3.3. B2–NiAl and L12-Ni3Al were precipitated during creep. The relationship between the misfit of nanoscale L12-Ni3Al phase and γ matrix are perfectly coherent. The L12-Ni3Al phase grows more slowly and remains at the nanoscale size after creep at 750 °C for 869h, which contribute to increase creep strength and reduce the creep rate of the material. However, the coarsened σ-FeCr phase reduced the creep productivity of the alloy. The microstructural degradation caused by precipitate coarsening was found to be responsible for creep failure, as indicated by the creep damage tolerance factor (λ) and creep fracture morphology. The elemental distribution of the oxide layer indicates the absence of a dense alumina protective scale at 750 °C. The extensive presence of oxidation cracks in the matrix suggests that the effect of oxidation cannot be ignored in the analysis of creep life. The fact that σ-FeCr is detrimental to creep damage indicates that the alloy composition needs further optimization.

Influence of punch diameter on impression creep behaviour of AZ91 magnesium alloy

The influence of punch diameter on impression creep behaviour of the AZ91 alloy in the cast and wrought conditions was studied. The stress exponents of 5.3–7.8 and 1.8–2.6 were found, respectively, for the cast and wrought alloy, and the activation energy is close to the lattice diffusion of magnesium for both conditions. The punch diameter has no influence on the deformation mechanisms and the obtained creep data introduce lattice diffusion-controlled dislocation climb and grain boundary sliding, which are the dominant deformation mechanisms during creep of the cast and wrought alloys, respectively. The punch diameter affected the impression creep behaviour of the alloys so that with an increase in the punch diameter, the steady-state impression rate in both deformation mechanisms was increased exponentially.

Creep of Cu-Bi Alloys with High Bi Content Near and Above Melting Temperature of Bi

Cu-Bi alloy with high Bi content can be used for thermal surge protection and energy storage. For these applications, creep at high temperatures, including temperatures above the melting temperature of Bi, Tm,Bi, becomes important. Accordingly, the creep behavior of Cu-Bi alloys, comprising 30 and 40 vol pct Bi, was studied under compression at temperatures above and below Tm,Bi. At 200 °C, which is below Tm,Bi, Cu-Bi showed a stress exponent of ~ 4 at high stresses and ~ 1 at low stresses. Finite element analysis revealed that the creep behavior of Cu-Bi at 200 °C was predominantly governed by Bi. On the other hand, at temperatures higher than Tm,Bi, Cu-Bi showed a short transient stage at high stresses, followed by sudden failure of the material. However, at low stresses, the sample first continued to expand and then started to accumulate compressive strain. A qualitative model based on interaction between liquid Bi and Cu is developed to explain the observed creep behavior at temperatures higher than Tm,Bi. The results obtained here shed light on the creep behavior of alloys with constituents having significantly different creep behavior and containing a non-reacting liquid phase.

Strain Rate during Creep in High-Pressure Die-Cast AZ91 Magnesium Alloys at Intermediate Temperatures.

During creep, magnesium alloys undergo microstructural changes due to temperature and stress. These alterations are associated with the evolution of the present phases at a microstructural level, creating different strain rates during primary and tertiary creep, and with the stability of the inter-metallic phase Mg17Al12 formed at these temperatures. In this paper, the results of creep testing of high-pressure die-cast AZ91 magnesium alloys are reported. During creep, continuous and discontinuous precipitates grow, which influences creep resistance. The creep mechanism that acts at these intermediate temperatures up to 150 °C is termed dislocation climbing. Finally, the influence of the type of precipitates on the creep behavior of alloys is determined by promoting the formation of continuous precipitates by a short heat treatment prior to creep testing.

Mechanical Behaviour of Materials during Creep with Changing Loads

A component in service experiences stress conditions that change continuously with time. Since service conditions are usually difficult and expensive to reproduce in laboratory, the creep behaviour of alloys in service has to be extrapolated from a limited number of creep tests at constant loads and temperatures. Empirical rules have been proposed to forecast the effects of variable load and temperature both on the time to rupture, as the life fraction rule (LFR), and on the accumulation of creep strain with time, as the strain hardening rule (SHR). Two directionally solidified (DS) nickel based superalloys have been investigated with creep tests at constant and variable loads and constant temperature. Nickel based superalloys, for the typical stresses experienced in service, are often characterised by a small negligible primary, a minimum of strain rate with no secondary state, and a dominant accelerating creep caused by dislocation multiplication. The damage mechanisms causing the final rupture appear only in the very last percentage of life. In the present work, simulation results are reported to show that the physical-sounded model used to describe the accelerating creep due to dislocation multiplication can be employed to better predict the times to rupture and the creep curves of the two DS nickel based super-alloys with step-like variable stress than the empirical LF and SH rules.

Investigation of Tensile and Compressive Creep Behaviour of AA2050-T34 during Creep Age Forming Process

The tensile and compressive creep behaviour of aluminium alloy 2050 with T34 initial temper (AA2050-T34) during creep-ageing process has been experimentally investigated and analysed in detail. Both tensile and compressive creep-ageing tests under various stress levels (ranging from 100 MPa to 187.5 MPa) have been carried out at a temperature of 155 °C for 18 hours. The results show that creep strains under tensile stresses are much larger than those under the same levels of compressive stresses and a new “double primary creep feature” with five-stage creep behaviour has been observed in the alloy during the creep-ageing tests. The conventional power-law creep equation was applied to analyse the new creep behaviour of the alloy at the steady-state creep stage. Furthermore, the power-law relationship between the applied stress and the corresponding creep strain rate was found to be effective in all creep-ageing stages of the alloy and was used for further analysis. These analyses indicate that the dislocation and diffusion mechanisms may both contribute to this new creep behaviour and may play different roles in different creep-ageing stages. Moreover, the evolution of the creep resistance or threshold creep stress of the alloy during the creep-ageing process was quantitatively investigated by the proposed relationship. These results help to not only understand the new creep behaviour of AA2050-T34 during the creep-ageing process, but also facilitate further constitutive modelling of this new creep behaviour for its creep age forming applications.

Effect of tin on the microstructure and impression creep behavior of MRI153 magnesium alloy

The effect of tin on the microstructure and creep behavior of as cast magnesium MRI153 alloy was investigated in the present study. Creep behavior of the alloys were studied using impression creep test under shear modulus normalized stress ranging from 0.025 to 0.04 at temperatures of 152–217°C. Results showed that adding tin to MRI153 alloy reduces the amount of Al2Ca via forming CaMgSn phase, whereas the continuity and value of Mg17Al12 phase are increased. Solubility of calcium in Mg17Al12 phase is also reduced in the presence of tin. It was shown that adding tin to MRI153 alloy decreases the alloy creep strength, which can be attributed to the reduced value of Al2Ca phase and formation of CaMgSn phase instead as well as reduced thermal stability of Mg17Al12 phase and its increased value and continuity. By calculating creep activation energy and stress exponent, it was shown that although climb-controlled dislocation creep is the dominant mechanism in the creep of these alloys, the creep activation energy is decreased with increasing the amount of tin.

Creep Model Based on Theta Projection and Omega Method for Predicting the Creep Behavior of Alloy

Creep testing of Alloy 718 has been carried out at various loads in the temperature range near 650°C in constant load control mode in order to understand how to predict the creep behavior including tertiary creep. The test results have been used for evaluating the existed models, such as Theta projection and Omega method that have been widely used for predicting long term creep strain and rupture time. After determining variables and material parameters of each method with the test results, estimated creep data from each model have been compared with the each measured creep data from the creep tests. The root cause of the discrepancy between estimated and measured data has been analyzed in order to improve the existed methods. The reliability of the improved model has been evaluated in relation to creep data.

A study of Carbon and Nitrogen Free New Alloys with Superior Creep Properties More over 700°C

In order to improve the creep strength of the heat resistant steels at elevated temperatures over 700°C, a new attempt has been demonstrated using carbon and nitrogen free Fe-Ni martensitic and austenitic alloys strengthened by Laves phase such as Fe2W and Fe2Mo. It is important that these alloys are independent of any carbides and any carbo-nitrides as strengthening factors. The high temperature creep tests over 700°C exceed 36,000 hours and the test is continued. Creep behavior of alloys is found to be completely different from that of the conventional high-Cr ferritic steels. The alloys exhibit gradual change in the creep rate with strain both in the transient and acceleration creep regions, and give a larger strain for the minimum creep rate. Effect of Cr on the Fe-12Ni-9Co-10W alloys on the creep properties more over 700°C was investigated. It became clear that the value for 100,000 hours was exceeded at 700°C and 100MPa calculated from the Larson-Miller parameter at C=20. And surface appearance of crept specimen was investigated in detail.

Oxidation assisted creep of alloy 600

The influence of oxidation on the creep behaviour of Alloy 600 at 550°C has been investigated by creep tests in air and in high vacuum (10 -2 Pa). Thin strips (200 μm) were chosen to emphasise the role of the surface reaction phenomena. An important effect of the environmental conditions was observed, the steady-state creep rate being 14 times higher in air than in vacuum. Careful examination of the specimens showed that this effect could be attributed to an enhancement of the recovery processes, which are the limiting phenomena in the dislocation creep mechanism evidenced for the samples tested. Therefore a detailed study of the oxide scale and of oxidation kinetics in air and in vacuum was carried out. Results indicated trial under vacuum a thin. protective Cr 2 O 3 layer grew on the surface of the samples, whereas the formation of a non-protective, nickel-rich oxide was evidenced in air. Thus. the oxidation mechanism was found to dramatically influence the creep properties. This effect was modelled in terms of an increase in the effective diffusion coefficient. and related to vacancy injection induced by the cationic, constrained growth of the non-protective oxide scale in air.

Modelling of the creep behaviour of a cobalt-based alloy by applying the kocks-mecking model

One possibility of evaluating the primary creep behaviour of the alloy CoCr22Ni22W14 is presented using the microstructural data obtained from TEM experiments. Due to the complex precipitation behaviour of the alloy at 1123 K it is necessary to investigate the evolution of the mean free path of the dislocations caused by forest dislocations and precipitations. Measured and modelled dislocation densities and the simulated creep curves are presented for creep under constant loading at 1123 K. The model used has no requirement for a weighting factor of strengthening caused by particles or dislocations and differs from Kocks and Mecking's approach using microstructural data from TEM.

The role of the alloy matrix in the creep behavior of particle-strengthened alloys

The more obvious strengthening microstructural features in heat-resistant super-alloys are oxide dispersoids and/or γ′ precipitates. An interesting problem is the role that the strength of the alloy matrix plays in the creep resistance of these alloys. We sought an answer to this problem through the evaluation of experimental data on a host of oxide-dispersion- and/or precipitation-strengthened nickel-based alloys with various levels of matrix solid solution strengthening and through a generalized expression for creep rates which separates the matrix contributions from the particle contributions to the resisting stress and creep strength of these alloys. It is concluded that the major role of the alloy matrix in the creep behavior of these alloys is in determining the apparent stress dependence of the creep rates. Specifically, the stronger the matrix through solid solution strengthening, the less applied stress sensitive is the creep rate of the alloy. The implications of these findings with respect to alloy design for better creep resistance in multiphase alloys are also discussed.