Influence of zirconium content on microstructure and creep properties of Mo–9Si–8B alloys

Mo–Si–B alloys with a molybdenum solid solution accompanied by two intermetallic phases and Mo₅SiB₂ are a prominent example for a potential new high temperature structural material. In this study the influence of 1, 2 and 4 at.% zirconium on microstructure and creep properties of Mo–9Si–8B (at.%) alloys produced by spark plasma sintering is investigated. Creep experiments have been carried out at temperatures of 1100 °C up to 1250 °C in vacuum. The samples exhibit sub-micron grain sizes as small as 450 nm due to the chosen production route. With addition of 1 at.% zirconium, formation of SiO₂ on the grain boundaries can be prevented, thereby enhancing grain boundary strength and creep properties significantly. Moreover ZrO₂ particles also enhance creep resistance of the molybdenum solid solution. Creep deformation is a combination of dislocation creep in the grains including dislocation-particle interaction and grain boundary sliding leading to intergranular fracture surfaces. It is promising to use grain size adjustments in order to balance the creep and oxidation resistance of the investigated material.     

Study on time-dependent behavior and stability assessment of deep-buried tunnels based on internal state variable theory

The creep model based on thermodynamics with internal state variables theory can simulate complex time-dependent deformation of rock mass, describe process of energy dissipation of material system, and can be used to evaluate the long-term stability of underground structures quantitatively. In this paper, the creep model proposed by author is improved further and recast to be central difference equation. The redevelopment interface of FLAC^3D is used to develop a new calculation program, which is based on thermodynamics for vsico-plasticity (PTV-P). Program validation has been conducted by comparing the results from FLAC^3D and Matlab software under uniaxial compression condition. Then the developed program has been applied to analyze the time-dependent behavior of deep-buried double tunnels. The integral values of energy dissipation rate and its time derivative in domain can be calculated and are used to evaluate the long-term stability of tunnels quantitatively, and the evaluation criterion is also proposed. Moreover, the contour map of energy dissipation rate is used to exhibits the local non-equilibrium region clearly.     

Effects of microstructure and C and Si additions on elevated temperature creep and fatigue of gamma TiAl alloys

The creep properties of K5 (Ti-46Al-3Nb-2Cr-0.2W) based alloys were analyzed in wrought processed microstructure forms. The brittle–ductile-transition-temperature (BDTT) depends distinctly on microstructure as well as strain rate, with the minimum value for each microstructure achieved at ∼10⁻⁶/s being about 680 °C and 780 °C, respectively. The greatest creep resistance is achieved in coarse-grained fully lamellar (FL) material and is related to the strong anisotropy of lath structure, large grain size and consequently high BDTT. Additional significant resistance improvement is realized with additions or increases of refractory elements (Nb or W) and decrease in Al content. The most remarkable improvements in primary as well as the minimum creep resistance are realized when small amounts of C or C + Si are added to generate incoherent (to gamma) carbide and silicide particles along γ/γ_T interfaces. The significance of primary creep is assessed for controlling subsequent creep behavior and discussed for its crucial role in satisfying the stringent design creep requirements for advanced rotational components. The accelerated or tertiary creep is used to explain the high temperature (870 °C) high cycle fatigue deformation that exhibits two-stage SN curves with the rapidly softening second stage.     

Simultaneous compensation of hysteresis and creep in a single piezoelectric actuator by open-loop control for quasi-static space active optics applications

Owing to their excellent properties piezoelectric actuators are studied as embedded elements for the quasi-statically active shape control of spatial optical mirrors. However, unwanted nonlinear effects in piezoelectric actuators, i.e., hysteresis and creep, severely limit their performance. This paper aims at developing a control methodology to compensate hysteresis and creep in a piezoelectric actuator simultaneously for quasi-static space active applications. In the methodology developed, hysteresis and creep behaviors are successively compensated by open-loop control. First, a derivative Preisach model is proposed to accurately portray the hysteresis while requiring relatively few measurements and describing the detachment between major and minor loops. The inverse derivative Preisach model is derived and inserted in open-loop to achieve hysteresis compensation. Then, the creep in the hysteresis compensated piezoelectric actuator is described by the use of a nonlinear viscoelastic model and a low pass filter is suggested to eliminate the effect of the inverse derivative Preisach model on the step reference input. To invert the creep model, the concept of “input relaxation” is implemented and an inverse multiplicative structure allows identifying the parameters of the inverse model while circumventing the difficulty of a mathematical computation. Finally, by cascading the low pass filter, the inverse model of creep and the inverse derivative Preisach model one after the other with the single piezoelectric actuator, the simultaneous compensation of hysteresis and creep is achieved. Experimental results show that in the case of step-like reference signals the hysteresis and the creep in a piezoelectric actuator can be significantly reduced at the same time. It implies that the developed methodology is effective and feasible in space active optics applications for which quasi-static distortions need to be compensated.     

Understanding low cycle fatigue and creep–fatigue interaction behavior of 316 L(N) stainless steel weld joint

The aim of this work is to study the time dependent effects on the low cycle fatigue (LCF) behavior of 316 L(N) stainless steel weld joint. Influence of strain rate, temperature, strain range, hold time and hold duration on fatigue life is evaluated. Occurrence of dynamic strain aging, creep damage, overall distribution of damage across the weld joint and the role of microstructure on the failure mode and failure location of the weld joint is discussed as a function of test parameters.     

A method to characterize asymmetrical three-stage creep of ordinary refractory ceramics and its application for numerical modelling

During operation, thermomechanical stresses occur in refractory linings. Under elevated stress and temperatures, these ceramics experience primary creep, which can further proceed to the secondary and tertiary creep stages. This necessitates a characterization of their three-stage creep behavior. Hence, two advanced uniaxial tensile and compressive creep testing devices are utilized. The Norton-Bailey creep equations and an inverse identification procedure are applied for the evaluation of the creep curves. To account for the full three-stage creep behavior in thermomechanical modelling activities, a creep-stage transition criterion is identified and subsequently implemented together with the Norton-Bailey creep-strain rate representations in a new developed creep model. The finite element simulation results from different creep testing procedures are in accordance with the corresponding experimental results of a magnesia-chromite refractory ceramic. The study also reveals the temperature-dependent asymmetrical creep behavior of the material in terms of the creep-strain rates and critical creep strains.     

Field trial of a reinforced landfill cover system: performance and failure

The geotechnical stability of an inclined multilayer capping depends on the shear strength available along the various interfaces. If the slope is very steep an additional reinforcing geosynthetic may be used to obtain a safer condition. Full-scale field trials can provide better resolution data on the reinforcement behaviour than conventional calculation methods based only on laboratory tests. The paper deals with a field trial carried out on multilayer capping, reinforced with a geogrid, in an Italian landfill. The geogrid behaviour was monitored for a month using displacement sensors and pressure cells located along the slope and in the anchor trench. Subsequently, the cover system was led to collapse by cutting the reinforcement and an analysis of the reinforcement behaviour and its relevance in the system stability were studied. This paper discusses in detail the setup of the field trial and the experimental data recorded during installation, monitoring, and failure phases of the system. The deformation behaviour of the geogrid during the entire test was recorded and analysed. The resulting data highlight the effects of the construction process on the geogrid behaviour including the contribution of geogrid creep characteristics until the failure.     

The effect of geotextile reinforcement and prefabricated vertical drains on the stability and settlement of embankments

The construction of four dikes on deep strata of very soft clay has required the application of several measures to improve the performance of the foundation, such as very wide berms, basal geotextile reinforcement and prefabricated vertical drains (PVDs). In order to control the rate of construction, the foundation and the dikes have been monitored with settlement plates, topographic stakes, inclinometers and piezometers. The use of back-analysis has allowed finding the adequate material model, the smearing of drains and the coefficient of secondary compression necessary to attain a good agreement between the measurements supplied by the instrumentation and the calculated values obtained with an elastic-viscoplastic (EVP) finite element (FE) program. Both the geotextile reinforcement and the PVDs produce an important increase in the safety factor (SF). The PVDs produce a significant acceleration in settlements, but the influence of the geotextile in the settlements is negligible. The combined use of the geosynthetic reinforcement and PVDs enhances embankment performance substantially more than the use of either method of soil improvement alone. The importance of flow in the results has been established.     

High-temperature strength and damage evolution in short fiber reinforced aluminum alloys studied by miniature creep testing and synchrotron microtomography

The creep behavior of a squeeze-cast, short fiber reinforced Al metal matrix composite (MMC), consisting of an Al-11 wt.% Zn-0.2 wt.% Mg alloy reinforced with 15 vol.% Al₂O₃ Saffil® short fibers is investigated using miniature creep specimens. The small dimensions of the miniature creep specimens permit them to be machined from regions of an MMC block with different microstructures, thus allowing the effect of grain size and fiber texture on creep to be investigated on a more local level than is possible using conventional specimen geometries. The miniature creep specimens are subjected to uniaxial tensile stresses ranging from 3 to 40 MPa at temperatures between 573 and 623 K. It is shown that tests performed using the miniature creep specimen geometry are in good agreement with results previously obtained with standard creep specimens. Through interrupted creep experiments, it is observed that the creep back flow that occurs after unloading increases with increasing accumulated plastic strain. In the as-cast MMC, synchrotron microtomography reveals a fine distribution of pores whose spatial density increases with the spatial density of the fibers. The presence of fractured fibers in the crept MMC is also revealed. Some of the regions between fractured fiber fragments appear to be filled with matrix material, while others are voided.     

Analysis of material degradation and life assessment of 25Cr–38Ni–Mo–Ti wrought alloy steel (HPM) for cracking tubes in an ethylene plant

Radiant tubes made of wrought 25Cr–38Ni–Mo–Ti alloy steel (HPM) have been in-service for 76,500 h as cracking tubes in an ethylene plant and they are expected to provide reliable service for 100,000 h (11.4 years) or more. During service, the tube inner surfaces were operated at temperature in the range of 820–835 °C within which thermal cracking process occurred. These aged tubes were assessed to ensure continued safe operation. The assessment of material degradation was carried out using optical microscopy, scanning electron microscopy (SEM) in combination with energy dispersive X-ray (EDX) analysis, X-ray powder diffraction (XRD) analysis, Vickers microhardness measurement and stress rupture test to obtain stress–Larson–Miller parameter (LMP) curves for remaining life prediction. Results showed that microstructural degradation was observed at the inner surface of the radiant tubes marked by the damage of protective oxide film containing Cr₂O₃, Fe₂O₃ and SiO₂. Once this film was removed, carburization occurred and free C atoms involved during cracking of ethylene easily penetrated along austenitic grain boundaries. In addition, carbon diffusion into the tube metal seemed to promote precipitation of Cr₂₃C₆ at grain boundaries and within the grains resulting in a sharp increase in hardness. The outer surface of the radiant tubes, on the other hand, was exposed to higher temperature, typically 1040–1100 °C during operation and creep damage seemed to be the main cause of material degradation. Based on stress rupture test, the remaining life of the radiant tubes is expected to be 21,107 h (2.4 years) consistent with the design life. In the present investigation, factors affecting creep are discussed.