Papers by Author: Guo Cai Chai

Abstract: Deformation twinning behaviors have been studied in high Ni alloys, Alloy 28 or Sanicro 28 and Alloy 625 at RT and at cryogenic temperature. The microstructures were evaluated using SEM-EBSD. Some constitutive approach has also been used in the discussion on the deformation twinning in high Ni-alloys. The results show that deformation twinning can occur in high Ni alloys depending on the chemical composition, strain range and stress conditions. TWIP can occur in the Ni based superalloy Alloy 625 at cryogenic temperature, which increases both strength and ductility. This is the first report of this phenomena for this alloy. Deformation twinning in high Ni alloys occurs heterogeneously in the material, depending on crystallographic parameters such as grain orientation and Schmid factor. Formation of deformation twins can lead to high texture in the material, which will contribute to the increase of strength. The mechanisms for the formation of deformation twins in high Ni alloys have been discussed.

Abstract: Fatigue damage in a metallic material during very high cycle fatigue can strongly be correlated to the microstructure. This paper provides a review and a discussion on the micro damage behaviors in a nickel base alloy and three steels during very high cycle fatigue using micro plasticity and material mechanics. The results show that cyclic plastic deformation in these materials can occur very locally even with an applied stress that is much lower than the yield strength. The fatigue damage occurs mainly at grain or twin boundaries due to local impingement and interaction of slip bands and these boundaries. The crystallographic properties, Schmid factors and orientations of grain and boundaries have very important roles to the fatigue damage. Subsurface fatigue crack initiation in the matrix is one of the mechanisms for very high cycle fatigue. In the fine granular area, high plastic strain localization and cyclic plastic deformation can lead to dislocation annihilation and consequently formation of vacancies, or eventually nanopores at the subcell boundary that leads to fatigue crack initiation and propagation.

Abstract: The influence of ageing at temperature 700°C for up to 20 000 hours on the deformation, damage and fracture behavior of Alloy 617 has been investigated by two toughness tests. Dense nano γ` phase and carbides are the main precipitates. However, the long-term aged material still shows high toughness. The mechanism has been studied using electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI). The results show that dense nano/micro-twins have been formed in the adjacent of the fracture front during the impact toughness or CTOD testing. Increase of ageing time reduces the number of nano/micro-twins, but they can be observed in all aged materials in spite of different strain rates. This indicates that besides dislocation slip, twinning is another deformation mechanism in long term aged material during the toughness tests. Formation of nano/micro-twins may be one of contributions to high toughness in the aged material, which is termed as twin induced toughening.

Abstract: Surface properties are essential for many engineering material ́s design issues, such as fatigue and corrosion performances. Austenitic stainless steels used in high-temperature applications, as for instance components in biomass-fired power plants, need sufficient corrosion resistance. At temperatures above 600 °C and in water vapor environment, Cr-vaporization will create Cr-depletion, causing a local change in chemical composition. This local change in chemical composition leads to phase transformation in some austenitic stainless steels. This paper reports the surface properties regarding the local phase transformation during thermal cycling in water vapor environment. Three commercial austenitic stainless steels are investigated, AISI 304, AISI 316L and Sandvik SanicroTM 28. The thermal cycling was performed up to 650 °C in a 15 mol.% water vapor environment. AISI 304 shows local surface phase transformation related to martensitic transformation due to locally changed chemical composition and increase in the martensitic transformation temperature (M_s). However, the other two austenitic stainless steels don’t show this martensitic transformation. The phase transformation and oxidation is discussed using microstructural investigations methods such as x-ray diffraction (XRD), electron backscatter diffraction (EBSD) and energy dispersive spectroscopy (EDS).

Abstract: Influences of dynamic strain ageing and long term ageing on deformation, damage and fracture behaviors of Alloy 617 material have been studied. Dynamic strain ageing can occur in this alloy at temperature from 400 to 700°C, which leads to a strain hardening and also an increase in fracture strain due to plastic deformation caused by twinning. Long term ageing at 700°C for up to 20 000 hours can cause different precipitation such as γ ́, M₆C (Mo-rich) and M₂₃C₆ (Cr-rich) carbides. These carbides are both inter-and intra-granular particles. The long term ageing reduces the fracture toughness of the material, but the alloy can still have rather high impact toughness and fracture toughness even with an ageing at 700°C for 20 000 hour. The mechanisms have been studied using electron backscatter detection and electron channeling contrast imaging. It shows that besides dislocation slip, twinning is another main deformation mechanism in these aged Alloy 617 materials. At the crack front, plenty of micro or nanotwins can be observed. The formation of these twins leads to a high ductility and toughness which is a new observation or a new concept for this type of material.

Abstract: Structural integrity is crucial for the safety of power plants with higher efficiency to meet the increasing global energy consumption. High-temperature environment will demand not only improved high-temperature corrosion resistance but also a maintained sufficient toughness. This study investigates how long term high-temperature environment influence the impact toughness of two austenitic stainless steels (AISI 304 and Sandvik SanicroTM 28) and one nickel-bas alloy (Alloy 617). Alloy 617 has shown increasing impact toughness with both increasing temperature and time, up to 700°C and 3 000 hours, while the two austenitic stainless steels have shown the opposite for the same conditions. At 10 000 hours the impact toughness of Alloy 617 has decreased but the alloy still possess great toughness. Both austenitic stainless steels show embrittlement due to brittle σ-phase and Alloy 617 seems to gain good impact toughness performance from small evenly distributed precipitates.

Abstract: The effect of strain path on work hardening and texture for a super austenitic stainless steel was investigated using both experiments and modeling. Compression deformation tests by stepwise changing loading direction in two and three dimensions were performed on cubic specimens at room temperature. The results were compared to uniaxial compression with equal accumulative strain, up to 20%, and uniaxial tension with equal final strain, up to 10% elongation of the longest side. The textures in all samples were analyzed using pole figures from EBSD analysis. Because of the high stacking fault energy of this super austenitic stainless steel, the texture was dominated by <110>-fiber texture in the compressive direction for the uniaxial compression, <111>- and <100>-fiber texture in the tensile direction for the uniaxial tensile test, and a combination of all these for the cube deformation. The density of the texture was much weaker for samples where the loading direction altered, if samples with equal accumulated strain were compared. The cube deformation was also modeled using a crystal plasticity model. The crystal plasticity model consists of a representative volume element (RVE) containing crystal grains with random orientations. The Taylor assumption was used for homogenization between the macro-and subscale. The material parameters in the crystal plasticity model were determined by calibration of its macroscopic response to experimental data. The simulated textures correspond rather well to the experimental results, but the work hardening should be completed to take into account kinematic hardening.

Abstract: In very high cycle fatigue, VHCF, regime, fatigue crack initiation can occur at subsurface defects such as inclusion or subsurface non-defect (matrix) origin. This paper provides a study on the fatigue crack initiation mechanisms at subsurface non-defect (matrix) origin in two metallic materials using electron backscatter diffraction and electron channeling contrast imaging. The results show that the strains in the material in the VHCF regime were highly localized, where the local maximum strain is greatly higher than the average strain value. This high strain localization can lead to the formation of fine grain zone and also fatigue damage or fatigue crack initiation at grain boundaries or twin boundaries by impingement cracking. High strain localization is caused by strain accumulation of each very small loading, and also increases the local hardness of the material. This may start quasi-cleavage crack origin, and consequently the formation of subsurface fatigue crack initiations. The results also show that fatigue damage and crack initiation mechanisms in the VHCF regime can be different in different metals due to the mechanisms for local plasticity exhaustion.

Abstract: Austenitic stainless steel grade UNS S31035 (Sandvik Sanicro® 25) has been developed for the next generation of 700°C A-USC power plant. This paper will mainly focus on the study of low cycle fatigue behavior and damage mechanisms of the material at room temperature, 600C to 700C by using electron back scatter diffraction and electron channeling contrast image techniques. At room temperature, the material shows a hardening and softening behavior as usual. At high temperature, however, it shows only a cyclic hardening behavior. Dynamic strain ageing can be one of the mechanisms. The damage and fatigue crack initiation mechanisms due to cyclic loading at different temperatures and loading conditions have been identified. The interactions between dislocations or slip bands with grain boundary or twin boundary are the main damage mechanism at low temperature or at high temperature with large strain amplitudes. Strain localization due to dislocation slipping is the main mechanism for the damage in grain.

Abstract: Fatigue crack propagation behaviors in a duplex stainless steel have been studied using an in-situ SEM/EBSD fatigue test and a conventional da/dN test. Crack propagation behaviors in grain, effect of Schmid factor, propagation cross the grain or phase boundaries have been discussed. Crack propagation occurs mainly in the grains with a high Schmid factor, but the crack can also propagate in the grains with very small Schmid factor. Crack deflection occurs mainly at the phase boundaries, but crack branching occurs mainly in the grains due to the dislocation slip. In-situ SEM/EBSD fatigue test confirms that crack propagation deflection can lead to a decrease in crack propagation rate. Formation of crack branches can significantly reduce the crack propagation rate, which can cause crack growth retardation in the main crack path in the worst case. The crack branches formed are usually not ideal. They can propagate almost transversely to the main crack direction with a mode II stress intensity factor, SIF, and a rate that is much higher than that of the main crack.