Papers by Keyword: High Temperature Creep

Abstract: Aiming at the evaluation of creep damage at elevated temperature of ferromagnetic materials in engineering application, the hysteresis loop measurement technology was adopted to study the creep damage behaviour of 1Cr5Mo steel. The creep testing at 600°C/90MPa of 1Cr5Mo steel were carried out to prepare specimens with various degrees of creep damage. The variation of magnetic parameter including coercivity (H_C) and remanence (Br) with creep damage was analyzed. The microstructure evolution of 1Cr5Mo steel with different degrees of creep damage was observed by optical microscope. The results show that the coercivity and remanence show certain regularity with creep damage at elevated temperature. Moreover, the regularity is repeatable which indicates that the measurement technology based on magnetic parameters can be used for the evaluation of high temperature creep damage.

Abstract: A recently developed 3D discrete dislocation dynamics (DDD) model is employed to study kinetics of dislocation ensembles subjected to high temperature creep in microstructures of metal matrix composites. We particularly focus on a migration of low angle tilt boundaries in a field of rigid impenetrable particles. This type of dislocation boundaries represents a typical microstructural feature mediating plastic deformation during the high temperature loadings. The article compares results of numerical studies that considered distinct dislocation-particle in-teractions in order to describe the response of dislocation structure to the applied stress. The resultssuggest that, regardless the details related to the dislocation-particle interactions, a critical applied stress always exists, below which the boundary migration process ceases [1,2]. The existence of crit-ical threshold is confirmed by creep tests of ODS materials. This critical threshold, contrary to theclassical Orowan stress, is proportional to the dislocation density. The displacements of individual dislocation segments on the micro-scale level reflect the changes in the dislocation-particle interactions quite sensitively. Atthemacro-scale level, the overall strain rate, which averages out velocities of all the individual dislocation segments, is also significantly influenced by the changes in dislocation-particle interaction

Abstract: The A508-III steel is widely used to manufacture the lower heads of commercial reactor pressure vessels (RPV). In severe accident, the reactor core in the RPV begins to melt and meanwhile the technology of in-vessel retention (IVR) exerts its role. In this case the inner surface of RPV will expose to temperatures over a phase transition temperature. However, the significant nonlinear feature of creep curve of A508-III steel suffered heterogeneous damage was not studied. In this work, the creep tests were performed for the steel at the phase transition temperature of 800°C. The microstructural evolution at different creep stages was characterized by scanning electron microscopy and transmission electron microscopy. The results show that, at the second creep stage, more coarsening second phase particles occur in the steel. With the creep processing, the grain size and diameter of second phase particles increase. At the tertiary creep stage, the grain size increases significantly, and the second phase particles coarsen during the process of atom migration. In addition, Micro-cracks and voids also come into being in the situation and they can become larger by combing each other during the creep process. At this stage, the growth of cavities and second phase particles coarsening become the main mechanism of creep damage. The trend of microstructural evolution is consistent with the creep constitutive equation obtained for the A508-III steel at the phase transition temperature of 800°C. The results obtained provide indispensable foundation to establish the relationship between the macroscopic creep and microscopic damage.

Abstract: Fe-Ni-Cr or known as Incoloy 800H and Haynes HR120 is a solid solution strengthened iron-nickel based superalloy which is extensively used in high temperature and corrosive environment. The effect of grain size in creep strength and creep rate comes through the grain boundary sliding and grain boundaries as barrier mechanism. This paper describes the effect of microstructural variation of Fe-Ni-Cr on the high temperature creep properties. The materials were heat treated at temperature 1050°C and 1200°C followed by water quenching process. The grain size of the samples of Incoloy 800H is 95.47μm for as-received, 122.81μm for solution treated at 1050°C and 380.95μm for solution treated at 1200°C. And the grain size of the samples of Haynes HR120 is 53.45μm for as-received, 61.50μm for solution treated at 1050°C and 158.27μm for solution treated at 1200°C. The creep damage investigation was carried out in the three different grain sizes of Fe-Ni-Cr superalloy at 900°C with stress at 100MPa. Rectangular section forms of specimens are used in the research. In all the tests conducted, the creep curves show primary, secondary and tertiary stages. The creep fracture surface were characterised by using scanning electron microscope. It was found that larger grain size results in lower creep rate for alloy Haynes HR120 but inverse result showed on alloy Incoloy 800H.

Abstract: Three kinds of combination schemes were designed about SMA-13 asphalt mixture. They were basalt aggregate SMA, limestone aggregate SMA, basalt and limestone aggregate SMA. Through the study of low temperature and high temperature test of three kinds of combination, the low temperature performance of basalt and limestone aggregate SMA was better than that of the basalt aggregate SMA; and the high temperature performance was in between, thus providing important experimental data for the limestones replacing basalt in SMA pavement.

Abstract: This article reviews the synthesis of Si-B-C-N ceramics, and the characteristics and principle of fabricating Si-B-C-N ceramics by using different precursors. So far, there only have two reports on preparation of Si-B-C-N ceramics, which are precursor pyrolysis and mechanical alloying. Therefore, this paper mainly discusses precursor pyrolysis and mechanical alloying. In addition, this paper proposed some prospects on the development and applications of Si-B-C-N ceramics in material science.

Abstract: GH3230 is a new type of superalloy,it is the first study about it. This paper studied the high temperature mechanical properties of the GH3230 base metal and argon-arc welding joints by high temperature tensile test,high temperature creep test and SEM analyses. The results showed thatthe ultimate tensile strength of GH3230 argon-arc welding joints is about 85.6% of that of base metal under the same conditions. In the high temperature creep test, GH3230 argon-arc welding joints had a longer creep life than that of base metal. Both GH3230 base metal and argon-arc welding joints showed a ductile fracture. High temperature tensile fracture is cavities fracture and high temperature creep fracture is intergranular fracture. The deformation of GH3230 argon-arc welding joints is much more uniformited and smaller than that of base material, and GH3230 argon-arc welding joints has less cracks.

Abstract: TiC/Ni composite material was synthesized by melt in-situ reaction from mixed Ti, C, Mo and Ni powders. Microstructure and phase composition of the composite were characterized by SEM and XRD. Creep behavior of the composite had been investigated under constant tensile stress, and the creep damage of the creep specimen was characterized by elastic modulus. The results show that the composite consists of nearly spherical TiC particle and Ni, the addition of Molybdenum has effect on refining the size of TiC particle; the mechanism of creep fracture is inter-granular fracture; relative variable curve of modulus resemble to creep curve, the method of elastic modulus is well to characterize the creep damage.

Abstract: Compressive creep behavior of hot-rolled (40%) Mg-Y and Mg-Y-Zn alloys are investigated at 480 ~ 650 K. Creep strength is substantially improved by the simultaneous addition of yttrium and zinc. The minimum creep rate of Mg-0.9mol%Y-0.04mol%Zn (WZ301) alloy decreases to 1/10 lower than that of Mg-1.1mol%Y (W4) alloy at 650 K. Activation energy for creep in W4 and WZ301 alloys are more than 200 kJ/mol at the temperature range of 480 ~ 550 K. These values are higher than the activation energy for self-diffusion coefficient in magnesium (135 kJ/mol). Many stacking faults (planar defects, PDs) are only observed on the basal planes of the matrix in Mg-Y-Zn ternary alloys. Stacking fault energy is considered to decrease by the multiple-addition of yttrium and zinc. The size and density of these planar defects depend on solute content, aging condition. TEM observation has been revealed that the decreasing of the stacking fault energy affects the distribution of dislocations during creep. Many a-dislocations on basal planes are extended significantly. Dislocation motion is restricted significantly by both of these two types of stacking faults (planar type and extended dislocations).

Abstract: High temperature oxidation / creep deformation behavior of a diffusion barrier coated Hastelloy-X alloy, with large grain size ~500μm, was investigated at 970°C in air with external tensile stress of 22.5, 27.5, 32, and 40MPa. The diffusion barrier coating formed on Hastelloy-X consisted of a duplex structure with an inner diffusion barrier layer of Re-Cr-Ni alloy, and an outer oxidation resistant layer of β-NiAl. Un coated bare Hastelloy-X alloy with same grain size was also examined under the same conditions for comparison. The composition of the as-coated diffusion barrier coating was (15~21)Ni, (33~37)Cr, (30~33)Re, (11~15)Mo, and (9~14)Fe. This composition corresponds to σ-phase in the Ni-Cr-Re ternary system, which is known as a topologically close packed, TCP phase. The composition of this diffusion barrier layer did not change during the experiment. The oxide scales formed after creep testing on the coated and un-coated alloy surfaces were needle-like θ-Al2O3, and Cr2O3 with small amount of FeCr2O4, respectively. Grain boundary oxidation was also found in the subsurface region of the un-coated alloy. The Al2O3 scale exhibited severe spallation, and many cracks were formed perpendicular to the stress direction. However, no spallation or cracks were observed in the Cr2O3. The creep rupture times for the diffusion barrier coated alloy were about 1.5 times longer than those for bare alloy at all creep stress conditions. The fracture surface after rupture indicates that fracture occurred along alloy grain boundaries in both the coated and un-coated alloy substrate. Many cavities and cracks were observed within the diffusion barrier coated alloy substrate. These cavities and cracks tended to propagate from the substrate toward the diffusion barrier layer, and then stopped at the Re-Cr-Ni / β-NiAl interface. Cracks formed in the un-coated alloy initiated at the tip of grain boundary oxides, and propagated into alloy substrate. However no major cavities were observed inside the alloy substrate. The stress index, n, for both specimens was about 6, and this indicates that the deformation mechanism of both samples was dislocation creep. These results suggest that the Re-Cr-Ni diffusion barrier layer acts as a barrier against the movement of dislocations at the interface with the alloy surface.