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The influence of intercritical tempering temperature on impact toughness of quenched and tempered 0.05C-5.42Mn medium-Mn steel was studied and compared with traditional tempering. The experimental steel had high hardenability because of high Mn-content. Lath-like α'-martensite without retained austenite was obtained over a wide range of quenching rate of 0.5–30 °C/s, and the quenched steel showed high strength but low impact toughness. On intercritical tempering at 625 °C and 665 °C, the impact toughness was enhanced, as compared to traditional tempering at 570 °C. The reversed austenite enriched with Mn and C formed between the martensite laths was the underlying reason for the increased absorbed crack propagation energy, and the ductile-brittle transition temperature (DBTT) was reduced because of increased stability of reversed austenite. Compared to the steel tempered at 625 °C, the steel tempered at 665 °C contained more reversed austenite, but the reversed austenite was less stable because of reduced enrichment of Mn and C. The enrichment or depletion of Mn and C in austenite and martensite was thermodynamically studied by DICTRA. In striking contrast to the steels tempered between 625 and 665 °C, twinned martensite was formed in the steel tempered at high temperature of 700 °C, and the steel exhibited impact toughness lower than the quenched steel. [ABSTRACT FROM AUTHOR]

Fatigue specimens with four types of surface were assessed under three exposure conditions (no exposure, block exposure, individual exposure–oxidation at 700 °C for 10,000 h) to quantify the effects of surface roughness, stress concentration, oxidation and inner microstructural embrittlement on fatigue strength of a near lamellar γ-TiAl alloy Ti–44Al–4Nb–4Zr–0.2Si–1B. With the yield strength of σ 0.1 =621 MPa, S – N fatigue is found to be always conducted under a loading condition of σ max < σ 0.1 . Local plastic deformation is difficult to occur on the maximum-stressed surface. The surface quality with or without defects and residual stresses therefore becomes critical for fatigue performance. Introducing compressive-stressed layer by shot peening and removing tensile-stressed layer and defects by grinding-electropolishing can improve the fatigue strength significantly, and the latter is more capable than the former for the high strength alloy. It is found that the fatigue performances of all types of surface are deteriorated to some degree when subjected to block exposure, owing to exposure-induced embrittlement. On the other hand, exposure-induced fatigue strengthening occurs after individual exposure–oxidation. The relaxation of residual tensile stress and dissipation of bulk stress in warm-air environment are found to outweigh the negative effects of oxidation layer at surface and exposure-induced embrittlement inside specimen. [ABSTRACT FROM AUTHOR]

Abstract: In this study temperature and grain size dependent plastic instability and strain rate sensitivity (SRS) were investigated for coarse grained (CG) and ultrafine grained (UFG) austenitic Fe–14Cr–16Ni alloys. As tensile testing temperature increased from 20 to 200°C, UFG alloys exhibited much more prominent decrease of uniform strain compared with CG counterparts. Transmission electron microscopy (TEM) analysis of deformed UFG alloys revealed temperature dependent evolution of microstructures. Through grain refinement, the SRS of Fe–14Cr–16Ni alloy increased and activation volume decreased, which may be related to the interactions of dislocations with grain boundaries. The SRS of UFG alloys exhibited greater temperature dependence than CG alloys. [Copyright &y& Elsevier]

Abstract: Equal channel angular pressing was used to refine the microstructure of a Fe–14Cr–16Ni (wt.%) alloy. The as-processed alloy had predominantly equiaxed austenite fine grains with an average grain size of 0.4μm and a low fraction of deformation induced martensite. Ex situ isothermal annealing experiments showed the fine microstructure was thermally stable up to 673K, and abnormal grain growth occurred at ∼873K. In situ annealing studies in a transmission electron microscope revealed the coarsening of grains. Analysis of grain growth kinetics from 873 to 1073K yielded average activation energy of grain growth to be ∼207kJ/mol. The grain growth mechanisms and annealing induced evolution of mechanical properties were discussed. [Copyright &y& Elsevier]

Confocal and electron microscopic observations of willemite (α-Zn2SiO4) crystalline ware glaze, fired to 1270 °C and then aged at 1080 °C for 5 h, indicated the glaze decomposed as (Zn, Ti, Ca)-rich and (Si, Al, K, Na)-rich regions, from which willemite crystallites and amorphous droplets were formed, respectively. Under the constraint of the glaze thickness, the willemite crystallites further developed into plate-like spherulites, which consists of lath-like crystals growing at the order of [0 0 0 1]>〈1 0 &1macr; 0〉>〈2 &1macr; &1macr; 0〉 and viable for coalescence over {2 &1macr; &1macr; 0} habit plane. The impinged plate-like spherulites formed a flat interface at the early stage of crystallization, but irregular interface in late stage due to weaving, truncation and lateral coalescence of the crystals. The {2 &1macr; &1macr; 0}-specific coalescence can be rationalized by Brownian-type motion/rotation of the crystals until parallel epitaxy is reached. [Copyright &y& Elsevier]

The annealing induced grain growth (GG) and heterogeneous interface evolution of Cu/Ag multilayers with individual layer thickness ( h ) varying from 5 to 50 nm were investigated by transmission electron microscopy (TEM). The results demonstrate that the thermal stability of Cu/Ag multilayers exhibits strong length scale dependence. For samples with h <20 nm, the heterogeneous interfaces completely disappear when the annealing temperature exceeds 200 °C. However, the temperature for stable layered structure can reach 300 °C as the h ≥20 nm, w h ere the interfaces remain remarkably intact. The existence of a large number of grain boundaries (GBs) decrease the stability of multilayers, while more heterogeneous interfaces contribute to resisting atomic diffusion, inhibiting GG. The equilibrium is achieved by a competitive process between GBs diffusion and heterogeneous interfaces resistance. Moreover, the formation of annealing twins in multilayer also significantly improve the microstructural stability. [ABSTRACT FROM AUTHOR]

This work is concerned with NiCrAlY and NiCoCrAlY coatings deposited on the superalloy IC-6 (Ni3Al-base superalloy) by arc ion plating (AIP). The results indicated that the presence of Al and Mo in alloy IC-6 impeded Cr atoms moving from coatings to substrates during the deposition process. As a consequence, the distribution of Cr is well proportioned in both NiCrAlY and NiCoCrAlY coatings. Vacuum heat treatment drastically increased diffusivities and the coating became more uniform. The interdiffusion also led to the phase transformation in the coatings. Although γ′-Ni3Al and γ-Ni were still the major phases in the coating, their lattice constant a0 decreased. The results of isothermal oxidation showed that the oxidation behavior of coatings obeyed the parabolic rate law. The oxidation resistance was not influenced very much by the presence of Co in the NiCrAlY coating at 900 °C static atmosphere. NiCoCrAlY coating had poorer oxidation resistance than NiCrAlY coating when they were exposed at 1000 °C. [Copyright &y& Elsevier]

The mechanical properties from room temperature to cryogenic temperature were investigated for a novel high-Mn cryogenic steel with 0.5C, 25Mn and 4Cr (in wt. %) based on understanding the relationship between stacking fault energy (SFE), microstructural evolution and deformation mechanism. The superior cryogenic toughness around ∼201 J was achieved and no ductile-brittle transformation temperature was found due to the highly thermal stable austenite. Only ∼2.5% ε-martensite was detected in impact sample fractured at −196 °C and consequently the ductile-dimpled fracture is dominant from room temperature to cryogenic temperature. The yield strength of 350 MPa, tensile strength of 810 MPa at room temperature increased to 820 MPa and 1380 MPa at −196 °C. It is controlled by the dominant twin deformation hardening mechanism with the SFE of 24.1 mJ/m2 at room temperature and combination of twin deformation hardening mechanism and martensitic transformation hardening mechanism at −196 °C. Due to the lower SEF at cryogenic temperature, and encountered by strain and stress, martensite transformation should be enhanced. [ABSTRACT FROM AUTHOR]

Abstract The thermal stability of nanoscale Cu/Ru metallic multilayers with different modulation ratio (a fixed Ru layer thickness of 3 nm and varied Cu layer thickness (h) from 10 nm to 200 nm) was investigated by transmission electron microscope (TEM) analyses and nanoindentation tests. All of the Cu/Ru multilayers retained the layered structures up to 400 °C annealing. The Cu grain growth slowed with Cu layer thickness decreasing, illustrating an unusual size dependence, that is, "smaller is more stable." Especially the average grain size was maintained as small as 25 nm for the Cu/Ru multilayers with h = 10 nm and the hardness only decreased by 6.5% after 400 °C annealing, which exhibits an excellent stability. The enhanced thermal stability of Cu/Ru multilayers originates from the textured columnar grains and semicoherent heterogeneous interfaces, which have a low interface energy. The amount of textured columnar grains and semicoherent heterogeneous interfaces increase with the Cu layer thickness decreasing is responsible for the enhanced thermal stability of Cu/Ru multilayers. [ABSTRACT FROM AUTHOR]

Abstract: The NiCrAlYSi coating with a diffusion barrier (DB) and/or buffer layer (BL) on DSM11 substrates was deposited by continuous arc ion plating (AIP) method. The interfacial strength, oxidation resistance and diffusion-barrier effectiveness were investigated. The tensile adhesion test result showed that the interfacial adhesive strength was stronger than that in the coating system with a diffusion barrier deposited by the conventional two-step AIP. The pre-deposition of the buffer layer into the DB/DSM11 interface could improve the interfacial strength further. The oxidation test results indicated that the multilayer could provide more effective protection for the DSM11 substrate than the single coating system. During thermal exposure, continuous and adhered Al2O3 scales were presented to the coating surface and the interdiffusion of alloying elements between the overlayer and the substrate was suppressed markedly. After thermal exposure or thermal cycle, the diffusion barrier was stable and kept efficient. [Copyright &y& Elsevier]

Abstract: With the aid of three-point bending test the mechanical behavior of the NiCrAlY coating systems with or without barrier layer was analyzed. Together with the results from tensile adhesion test the damage mechanism of the investigated systems was discussed. It is found that the damage behavior was mainly dependent on the interfacial adhesion and the overlayer property. With a strong or weak interfacial adhesion and the brittle or ductile overlayer different damage mechanism was developed during test. The presence of residual stress and brittle precipitates or flaws at the coating/substrate interface influenced the interfacial adhesion property. The thermal exposure treatment improved the interfacial strength and the overlayer ductility. [Copyright &y& Elsevier]

Abstract: The microstructural evolution of the NiCrAlY/CrON system during vacuum heat treatment and thermal exposure was investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The elastic modulus and residual stress of the coating system were measured by nanoindentation and photo-stimulated luminescence spectrum (PSLS) methods, respectively. It was shown that the series reaction of β→γ′→γ took place in the NiCrAlY coating, which contributed to a decrease in the Young’s modulus. The replacement of Cr2O3 by Al2O3 during thermal exposure resulted in an increase of the Young’s modulus of the barrier layer. A change in the residual stress in the barrier layer was due to a combination of thermal and growth stresses. [Copyright &y& Elsevier]

Abstract: Tensile and fatigue tests of ultrathin Cu films were conducted using a micro-force testing system. Fatigue strength as a function of film thickness was measured under the constant total strain range control at a frequency of 10Hz. The experimental results exhibit that both yield strength and fatigue lifetime are dependent on film thickness. Fatigue damage behavior in the 100nm thick Cu films with nanometer-sized grains is different from that in the micrometer-thick Cu films with large grains observed before. A comparison of the present results with those reported in literatures is conducted. Possible fatigue strengthening mechanism in the ultrathin Cu films is discussed. [Copyright &y& Elsevier]

Abstract: Thermal shock cycling behaviors of D-gun sprayed TBCs with a hollow spherical ZrO2–8Y2O3 (HSP-YSZ) top coat and NiCrAlY bond coat on directionally solidified Ni-base superalloys DZ125 were investigated at high temperature (1100°C)↔room temperature (RT) repeatedly by water quenching. Scanning electron microscopy (SEM) was used to characterize the coating microstructure and failure morphology. The results showed that failure of the D-gun sprayed TBC starts with crack initiation along the splats boundary in the ceramic top coat and the non-alumina oxides. The cracks propagate and coalesce with the increasing thermal cycling. The extensive cracking of the rapidly formed non-alumina oxides, resulting from the depletion of aluminum in the bond coat, aids to delamination of the outer ceramic layer. The stress distributions in TGO layer at different thermal shock cycles was measured by luminescence spectroscopy to investigate the failure mechanism of TBC system. [Copyright &y& Elsevier]

Abstract: NiCoCrAlYSiB coatings were obtained using an arc ion plating technique on a directionally solidified Ni-base superalloy. Thermal shock resistance of the bare and coated specimens were investigated by cycling between high temperature (900, 1000 and 1050°C) and room temperature (forced water quenching). The experienced attack during thermal shock cycling was studied by gravimetry, XRD, SEM and EDS. The results showed that bare alloys and coated specimens experienced aggravating oxidation under thermal shock cycling conditions. The coated specimens exhibited excellent thermal shock resistance and provided enough protective effect on the superalloys under thermal shock cycling conditions. The accelerated oxidation mechanism and crack formation and propagation in the coatings were also investigated. The accelerated oxidation can be attributed to the accelerating oxide formation and breakdown due to compressive stresses accumulation in the oxides due to oxide growth at high temperature and thermal mismatch during cooling. The formation and propagation of cracks formed in coatings can be attributed to tensile stresses in oxides generated by irreversible cyclic creep accumulation in the metallic coatings and the “wedging” effect of oxide protruded into the coatings. [Copyright &y& Elsevier]

Abstract: NiCoCrAlYSiB coatings were obtained using an arc ion plating technique on a directionally solidified Ni-base superalloy. Thermal shock resistance of the bare and coated superalloys were investigated by cycling between high temperature (900, 1000 and 1050°C) and room temperature (forced water quenching). The microstructure evolution in coatings and the underneath substrates was studied. The results showed that phase transformation from γ′ to γ occurred at the surface of the coated and bare specimens due to the consumption of Al and Ti by oxidation. A directional coarsening of the γ′ precipitates (γ′ rafting) just beneath the γ′-impoverishing zone (or interdiffusion zone) is observed. This kind of rafting is believed to come into being from the compressive stresses by temperature gradients due to high cooling rate during cooldown. The relationship between microstructure evolution and cracks formation and propagation is also discussed. [Copyright &y& Elsevier]

The high-Mn austenitic steel is expected to improve mechanical properties by grain boundary adjustment. The role of grain boundary on cryogenic toughness of high-Mn austenitic steel subjected to various welding thermal cycle was investigated by the means of instrumented Charpy V-notch impact tests and electron backscatter diffraction (EBSD) analysis. The absorbed energy of samples at −196 °C decreased from 203 J to 163 J with the increase of peak temperature from 600 °C to 900 °C. When the peak temperature increased to 1000 °C, 1100 °C and 1300 °C, the absorbed energy of sample increased to 185 J, 204 J and 196 J, respectively. Phase transformation and abnormal grain growth were absent in the samples. The lowest cryogenic toughness in sample with the peak temperature of 900 °C was thus attributed to the highest proportion of the special grain boundary ∑3. Furthermore, during the cryogenic impact test, the crack formation energy was corresponded to the proportion change of the low angle boundary. While, the crack propagation energy was closely related to the proportion change of special grain boundary of ∑3 rather than the high fraction of ∑3 by welding thermal cycle. It is proposed that the increasing low angle boundary of 2–5° and the activation ∑3 special grain boundary are responsible for the superior cryogenic toughness for high-Mn austenitic steel. [ABSTRACT FROM AUTHOR]