Effect of Effective Grain Size and Grain Boundary of Large Misorientation on Upper Shelf Energy in Pipeline Steels

X65, X70, and X80 belong to high grade pipeline steels. Toughness is one of the most important properties of pipeline steels when the pipeline transports the gas or oil, and the means to control toughness is very important for exploring even higher grade pipeline steels. We established the relationship between toughness and crystallographic parameters of high grade pipeline steels by studying the crystallographic parameters of X65, X70, and X80 using EBSD and analyzing Charpy CVN of X65, X70 and X80. The results show that the effective grain size, the frequency distribution of grain boundary misorientation and the ratio of high angle grain boundary to small angle grain boundary are important parameters. The finer the effective grain size, and the higher the frequency distribution of grain boundaries （〉 50~）, the more excellent toughness of high grade pipeline steels will be.

High-temperature fatigue strength of grain boundaries with different misorientations in nickel-based superalloy bicrystals

Nickel-based single-crystal superalloys are widely used in the manufacture of aeroengine turbine vanes for their excellent high-temperature performance. Low-angle grain boundaries (LAGBs) will be generated inevitably during their manufacture, which are often characterized by grain boundary misorientation (GBM) and will weaken the mechanical properties of superalloys. However, the relationship between GBM and the fatigue properties of superalloys at elevated temperatures has seldom been investigated due to the difficulty in the sample preparation and experiment process. Based on six kinds of bicrystals with different tilt LAGBs made by a second-generation single-crystal superalloy, the effects of misorientation on the grain boundary microstructure and fatigue properties (980 °C) of superalloys were studied systematically in this work. It is found that, with the increase of GBM, the GB precipitates combined with the cast micropores increase monotonically, accordingly both the fatigue life and fatigue strength decrease successively. Fatigue fracture observations show that the cracks of all the bicrystals initiated from the cast micropores at GBs, and then propagated along the GBs. Therefore, the coupling effect of cast micropores and GBM on the fatigue damage mechanisms of the bicrystals are evaluated according to their hindering degrees on the piled-up dislocations. Combining with a hysteresis energy model, a quantitative fatigue strength prediction model of superalloys is established and is well verified by abundant experimental data. This study could provide guidance for fatigue performance prediction and structural design of superalloys.

Static recovery of A5083 aluminum alloy after a small deformation through various measuring approaches

Static recovery was confirmed to be the dominant softening mechanism during annealing for the studied A5083 aluminum alloy.The kinetics of static recovery,described based on the activation parameters(activation energy and volume of static recovery)and the static softening fraction,were mainly studied through two thermomechanical tests,namely,double-pass compression and stress relaxation tests.A new approach was proposed to measure the static softening fraction in the stress relaxation test.In general,a higher temperature or strain rate accelerates static recovery,while interestingly,the effect of pre-strain on static recovery is opposite in cases with and without external stress,owing to the enhanced static recovery by external stress during annealing.In addition,microhardness tests and electron backscatter diffraction(EBSD)characterization were also conducted to verify the accuracy of the results.The grain average misorientation approach based on EBSD characterization was confirmed to be effective in distinguishing and quantifying static recovery.It is noteworthy that the special stress hardening phenomenon occurring at 400℃and 0.1/s is caused by strain aging,showing the complexity of the material behaviors after deformation of the studied aluminum alloy.

Characterizations of Dynamic Strain-induced Transformation in Low Carbon Steel

Dynamic strain-induced transformation of the low carbon steel Q（235） at 770℃ and 850℃ leads to fine ferrite grains. The microstructure characterization and mechanism of the fine ferrite grain were studied by scanning electron microscopy （SEM）, transmission electron microscopy （TEM） and electron backscattered diffraction （EBSD） technique. The results show that strain-induced microstructure is the mixed microstructure of ferrite and pearlite, with cementite randomly distributed on ferrite grain boundaries and the grains interiors. EBSD images of grain boundaries demonstrate that high angle grain boundaries （HAGBs） are dominant in both of the deformation induced microstructures occurring below and above A（e3） , with only a few low angle grain boundaries （LAGBs） existing in the grain interiors. It implies that the dynamic strain-induced transformation （DSIT） happens above and below A（e3） temperature and has the same phase transition mechanisms. The refinement of ferrite is the cooperative effect of DSIT and continuous dynamic recrystallization （CDRX） of ferrite. Besides, DSIT is deemed as an incomplete carbon diffusion phase transition through the analysis of microstructure and the previous simulated results. The strengths of the Q（235） steel with refined ferrite and pearlite structure get doubled than the initial state without treated by DSIT and the residual stress in the refined structure is partly responsible for the ductility loss.