Interface Strengthening of α-Mg/C14–Mg2Ca Eutectic Alloy

This study investigates the effect of the α/C14 interface on the creep strength of α-Mg/C14–Mg2Ca eutectic alloy at 473 K under a stress of 40 MPa. The α/C14 interface is composed of terraces and steps, with terraces parallel to the (1101)α pyramidal plane of the α-Mg lamellae and to the (1120)C14 columnar plane of the C14–Mg2Ca lamellae. The creep curves of the alloy exhibit three stages: a normal transient creep stage, a minimum creep rate stage, and an accelerating stage. The minimum creep rate is proportional to the lamellar spacing, indicating that the α/C14 lamellar interface plays a creep-strengthening role. In the high-resolution transmission electron microscopy image captured of the specimen after the creep test, <a> dislocations can be mainly seen within the soft α-Mg lamellae, and they are randomly distributed at the α/C14 interface. In contrast, dislocations are rarely introduced in the hard C14–Mg2Ca lamellae. It is deduced that the α/C14 interface presents a barrier to dislocation gliding within the α-Mg lamellae and does not help rearrange the dislocations.

A Minimum Creep Rate for 2-1/4Cr-1Mo Steel Consistent With the ASME Section III, Division 5 Rules

This technical note describes a minimum creep rate model for 2-1/4Cr-1Mo steel that is consistent with the current creep strain equation embedded in the ASME Boiler & Pressure Vessel Code Section III, Division 5, Subsection HB, Subpart B isochronous stress–strain curves. Minimum creep rate models for all the Section III, Division 5 Class A materials are required for the development of improved high temperature design methods. Of all the Class A materials, only 2-1/4Cr-1Mo does not have a readily identifying minimum creep rate term in the current isochronous stress–strain curve model.

Effect of Low-Temperature Annealing on Creep Properties of AlSi10Mg Alloy Produced by Additive Manufacturing: Experiments and Modeling

The effects of postprocessing annealing at 225 °C for 2 h on the creep properties of AlSi10Mg alloy were investigated through constant load experiments carried out at 150 °C, 175 °C and 225 °C. In the range of the experimental conditions here considered, the annealing treatment resulted in an increase in minimum creep rate for a given stress. The reduction in creep strength was higher at the lowest temperature, while the effect progressively vanished as temperature increased and/or applied stress decreased. The minimum creep rate dependence on applied stress was modeled using a physically-based model which took into account the ripening of Si particles at high temperature and which had been previously applied to the as-deposited alloy. The model was successfully validated, since it gave an excellent description of the experimental data.

On the physical basis for the creep of ice: the high temperature regime

This work quantifies the increased temperature sensitivity of the constitutive behavior of ice with proximity to the melting point in terms of dislocation mechanics. An analysis of quasistatic and dynamic cyclic loading data for several ice types leads to the conclusion that high temperature (e.g. T ≥ −8°C) behavior is the result of a thermally induced increase in the number of mobile dislocations rather than an increase in the activation energy of dislocation glide or the introduction of a new deformation mechanism. The relationship between dislocation density and temperature is quantified and the model is shown to adequately represent the published minimum creep rate vs stress data for isotropic granular freshwater ice for −48 ≤ T ≤ −0.01°C.

The rupture life prediction in cold dwell fatigue of Ti-6Al-4V based on the creep deformation

Titanium a lloys have been found that the fatigue strength of Ti alloys decreases due to cold dwell fatigue (CDF) at room temperature. Ti and Ti alloys generate creep deformation at room temperature (T/Tm = 0.15). Thus, it is considered that creep affects the reduction in fatigue life in CDF tests. This research intends to clarify the effects of long time dwell under tensile stress and rupture life prediction from the view of creep deformation in CDF characteristics of Ti-6Al-4V. Rupture cycle decreased with increase of dwell time. Additionally, lower limit of rupture life ratio “NCDF/NLCF” was defined from rupture in creep test if it was assumed that creep test was extremely long time dwell CDF test. When strain change in whole dwell time was extracted in CDF tests, strain change was like creep curves and minimum creep rate changed depending on dwell time. Minimum creep rate was calculated by the formula based on experimental results, and then rupture time was calculated by Monkman-grant relationship. All of rupture cycle predictions were in factor of 2. Therefore, rupture cycle and time can be calculated if dwell time is known in CDF tests.

Comparative Study of Creep Behavior in 9Cr-1Mo Steel With Different Prediction Methods

Over the range of variables (temperature and stress) normally encountered in service applications, creep behaviors of 9Cr-1Mo steel were investigated with various models, such as Wilshire model, combination of exponential form and omega (CEO) model, and continuum damage mechanics (CDM) model. First, a series of short-term creep data was prepared to evaluate the material parameters occurring in these models. Then, creep curve, minimum creep rate, and long-term creep life of present 9Cr-1Mo steel were extrapolated with these estimated models. Based on the analysis of obtained results in detail, it suggested that both CDM model and CEO model can give reliable minimum creep rate predictions. However, the most reliable values of long-term creep life are obtained by the CDM model, followed by the Wilshire model and then the CEO model. In particular, the physically based CDM model can provide useful insights into the underlying creep mechanisms. Therefore, the CDM model has promising potential to study the long-term creep behaviors of 9Cr-1Mo steels.

Extrapolation of Imaginal Minimum Creep Rate in Compression by a Concept of SATO-Index

Creep characteristics of alloys and compounds have been evaluated mainly by the minimum creep rate or the steady-state creep rate, and by its stress and temperature dependences. In some cases, however, direct comparison of the minimum creep rate or the steady-state creep rate are not practically easy due to difficulties of experiment, i.e., a long duration of primary stage of creep deformation. The minimum creep rates are not always precise representative value, which is directly evaluated from experiments. It should be valuable, if one could estimate the minimum creep rate from creep curve in primary stage. I have proposed a method of quantitative evaluation of creep curve based on the evaluation of strain rate change and its strain dependence during creep [1-3]. The value that reflects a shape of creep curve is named “Strain Acceleration and Transition Objective-Index (SATO-Index)” [4]. SATO-Index and related differential equation show a strain dependence of strain rate and lead entre creep curve by numerical integration. This concept provides quantitative information of shape of each creep curve, and information of the entire creep curve. In this paper, examples of evaluation and extrapolation of creep rate from primary stage in compression are presented. It is concluded that the extrapolation with the concept of SATO-Index reasonably provides imaginal minimum creep rate. Usability of extrapolation of creep curve by the concept is presented.

Investigation on Creep Behavior of Grade 91 Heat-Resistant Steel at 923K

Creep deformation behavior of SA387Gr91Cl2 heat-resistant steel used for steam cooler has been investigated. Creep tests were carried out using flat creep specimens machined from the normalized and tempered plate at 973K with stresses of 100, 125 and 150MPa. The minimum creep rate and rupture time dependence on applied stress was analyzed. The analysis showed that the heat-resistant steel obey Monkman-Grant and modified Monkman-Grant relationships.

Time to Initiation of Tertiary Creep Property of Grade 91 Steels

Creep deformation property of Grade 91 steels was analyzed on more than 370 creep curves over a wide range of time to rupture from about 10 hours to beyond 100,000 hours, in order to evaluate time to 1% total strain, time to minimum creep rate and time to initiation of tertiary creep. Time to initiation of tertiary creep was assessed as a 0.2% offset with a slope of minimum creep rate. It is difficult to determine time to minimum creep rate precisely, which is a basis of 0.2% offset, however, it has been confirmed that time to initiation of tertiary creep is not sensitive to the time when the creep rate indicates minimum value. Life ratio of 1% total strain time against creep rupture time increases up to about 60% with increase of temperature and decrease of stress. Life ratio of time to initiation of tertiary creep also tends to increase with decrease in stress. However, change of it is in a range of 50 to 60% of creep rupture life over a wide range of creep rupture life from 10 hours to 100,000 hours, and it is not sensitive to creep test temperature. Over a range of temperatures from 500 to 600°C and up to about 200,000 hours, a temperature and time-dependent stress intensity limit, St is controlled by 67% of minimum stress to rupture. However, a difference between 67% of minimum stress to rupture and 80% of minimum stress to initiation of tertiary creep decreases with increases in temperature and time, and both values approach each other in the long-term beyond about 100,000 hours at 600°C. In the long-term beyond about 10,000 hours at 650°C, St is controlled by 80% of minimum stress to initiation of tertiary. The stable life fraction of time to initiation of tertiary creep establish a reliability of a temperature and time-dependent stress intensity limit value.