High-temperature Long-term Strength Research Articles

A first step towards computational design of W-containing self-healing ferritic creep resistant steels

ABSTRACT In this work, we combine a generic alloy-by-design model with a novel concept, the nucleation barrier for the formation of Laves phase to fill the creep cavities, in order to develop multi-component creep resistant steels with kinetically tuned self-healing behaviour. In the model the high-temperature long-term strength is estimated by integrating precipitation strengthening due to M23C6 carbides and solid solution strengthening, while the optimized compositional solutions are determined by employing the coupled thermodynamic and kinetic principles. W-containing Laves phase herein is selected as the self-healing agent to autonomously fill the grain boundary cavities, so as to prolong the creep lifetime. To achieve the effective healing reaction, the nucleation time for Laves precipitates are expected to coincide simultaneously with which creep cavities start to form or reach a healable size. Using experimental data from literature, an empirical relationship to estimate the incubation time for Laves phase formation has been constructed, from which the thermodynamic driving force for onset of precipitation as a function of temperature and intended precipitate nucleation time was derived. Three sample alloys have been selected among the desirable solutions, which are predicted to have the same strength but widely different Laves phase nucleation times. The calculations are also performed for different use temperatures to explore the compatibility between high temperature strength and timely cavity filling behaviour. In its current form the model is not expected to yield the truly optimal composition but to demonstrate how the kinetics of the healing reaction can affect the predicted optimal alloy compositions.

Improvement of mechanical properties of the Ti–45Al–5Nb–1Mo–0.2B (аt %) intermetallic alloy by means of microstructure controlling

The effect of heat and thermomechanical treatments conditions on the microstructure and main mechanical characteristics (obtained by tensile, high-temperature long-term strength, fracture toughness, and high-cycle fatigue tests) of the Ti–45Al–5Nb–1Mo–0.2B (аt %) alloy was studied. Before the treatments, the sequence of phase transformations in the alloy after its solidification was determined by testquenching method. The obtained data were used to develop conditions for the heat and thermomechanical treatments. It was found that a small but stable increase in the plasticity and strength of the cast alloy is observed after three-stage annealing at temperatures that correspond to the (α + γ)- and (α2 + β(В2) + γ)-phase region. The thermomechanical treatment at temperatures corresponding to the (α(α2) + β(В2) + γ)-phase region and subsequent two-stage annealing at temperatures that correspond to the (α + β(В2) + γ)- and (α2 + β(В2) + γ)-phase region lead to the formation of fine-grained duplex structure. This determined the substantial improvement of the low-temperature plasticity and strength (δ = 3.1% and σu = 860 MPa at 20°C, respectively) and retained high creep resistance to 700°C.

Prediction of High-Temperature Long-Term Strength of Materials

The authors present a method for predicting the long-term deformation of materials based on the base diagram concept. The long-term strength calculation of steels 18Cr–12Ni–Mo and 21Cr–32Ni–Ti–Al for different specified stress and temperature values showed the efficiency of its defining relations. The advantage this approach has over the most common parametric methods such as the Larson–Miller, Orr–Sherby–Dorn, Manson–Sakkop, and Trunin parameters is shown.

High-temperature long-term strength of shells in aggressive media with regard for the mass exchange

We study the long-term strength of long cylindrical shells placed in aggressive media under the conditions of complex stressed states. The influence of these states on the long-term strength of the shells is described with the help of equivalent scalar stresses. The time to fracture of the shell is found according to the Rabotnov kinetic theory by using two parameters, namely, the degree of damage to the material and the concentration of the medium in the material of the shell. As the boundary condition on the fracture front, we use the condition of mass exchange. We study the interaction between the diffusion and fracture fronts. The time to fracture of the shell is found by using the solution of a system of integrodifferential equations. Under the assumption of mean level of concentration of the medium, we deduce explicit relationships between the times to fracture of the shell in aggressive and neutral media.

Prediction of High-Temperature Long-Term Strength for Damaged Structures

A method is proposed for analytical estimation of the high-temperature long-term strength of structures with material discontinuities, which are stress raisers. We consider the processes involving the formation of diffusion pores. Diffusion processes in metals are strongly enhanced with increasing temperature and stresses.

Strength, Durability, and Safety of Engineering Systems

The evolution of approaches to the solution of machine dynamics and strength problems on the basis of determining static and dynamic nominal and local stresses of the operating load is discussed. It is shown how the material strength and plasticity characteristics, the cyclic strength characteristics in the high-cycle and low-cycle fatigue regions, the high-temperature long-term strength and creep characteristics, and the linear and nonlinear characteristics of fracture mechanics were sequentially used as the basic criterial parameters of the deformability and strength of structural materials. Particular emphasis was given to the results of studies on machine safety and catastrophe mechanics. Integrated approaches to the solution of strength and safety problems of potentially dangerous facilities (nuclear power stations, spacecraft complexes, aircraft, chemical plants, etc.) by analysis of all stages of their life cycle, including design, manufacture, testing, and operation, are discussed.

Comparative Creep Properties of Single-Phase Intermetallics of the Ti-Al System

Creep characteristics of polycrystalline single-phase intermetallics, α 2 -Ti 3 AI and γ-TiAl, at 1100 K have been summarized to evaluate their relative strength at elevated temperature at three strain-rate levels, 10 -1 10 −4 and 10 −8 s −1 , and are compared with those of α Ti-Al terminal solid solution alloys. The strength of these intermetallics is obviously higher than that of α, random solid solutions, especially at low strain-rates. However, the difference in high-temperature long-term strength among intermetallic alloys is not significant at any strain-rate level.

Method of analogy for estimating the high-temperature long-term strength properties of metal alloys

Method of analogy for estimating the high-temperature long-term strength properties of metal alloys

Use of the ?1 criterion in evaluating the long-term strength of structural steels in plane stress

1. Under plane stress conditions, an estimate of high-temperature long-term strength for structural steels obtained from the value of the maximum tensile stress will not be overstated only for the case of biaxial tension. 2. If a study of high-temperature long-term strength (creep strength) in biaxial tension is conducted on thin-walled tubular specimens and the results are compared with data for the simple axial tension of the same tubes, the presence of work hardening in the axial direction will lead to the appearance of a range of stress states in which estimates of long-term strength made according to the maximum tensile stress σ1 will be exaggerated. 3. The limits of the range of applicability of the criterion σ1 in biaxial tension are determined by Eqs. (15) and (16).