Papers by Keyword: Grain Boundary

Abstract: In Al_xCoCrFeNi high entropy alloys (x = 0.3–0.5), the NiAl phase with the B2 structure is precipitated rapidly along the fcc grain boundaries. During recrystallization after conventional cold rolling, the NiAl precipitates effectively suppress the grain growth, which results in the ultrafine-grained microstructure. It should be noted that no severe plastic deformation is necessary to obtain the microstructure. The volume fraction of the NiAl precipitates increases with increasing x. As a result, the average grain size of the fcc matrix (d_m) after the recrystallization decreases with increasing x, and therefore, a minimum d_m of 0.5 μm can be obtained at x = 0.5. The grain refinement by the NiAl precipitates is consistent with the Zener-Smith model. At x = 0.5, the alloy with d_m = 0.5 μm exhibits a yield stress of 1163 MPa and an elongation of 24% at room temperature.

Abstract: This paper deals with the analysis of microstructure and substructure of 9CrNB steel, after normalization at temperature of 1070 °C and tempering at 790 °C / 240 min. The tube was second time tempered at the following temperatures and holding times: 760 °C / 30 min (A1), 760 °C / 120 min (C1), 800 °C / 30 min (G1) and 800 °C / 120 min (I1). Microstructure after tempering consists of tempered martensite and bainite with lath morphology, while inhomogeneous redistribution of precipitates is visible. Substructure analysis of state A1 and I1 show, that a relatively large number of irregular, rod-shaped and oval carbide particles, often arranged in clusters, were precipitated at the primary original austenite grain boundaries. In case of state A1, the average size of these carbide particles is 300 nm and in case of state I1 the average size is 350 nm. A relatively large number of rod-shaped and oval shaped particles were found at the interface of the tempered martensite and bainite mainly in the form of clusters and also inside the tempered bainite with higher particle distribution. In the case of the state A1, they reached an average size of 150 nm. In some regions of substructure of the state I1, the fine carbide particles with an average size of 200 nm and coarse carbide particles with an average size of 400 nm were presented within the areas of tempered bainite. Particles were identified by EDX analysis and by selection electron diffraction. The mechanical properties after tempering were evaluated and compared with properties of P91 and P92 steel.

Abstract: The paper is devoted to some properties of grain boundaries: Segregation and concentration phase transitions – two important consequences of atomic interactions in grain boundaries. Except of a short description the Gibbs method of surface excesses and grain boundary segregation isotherms with the limited number of segregation sites in grain boundary, the paper concentrates on the effects of complexes formation, including thermodynamic and computer modeling, and concentration phase transition in the grain boundaries in systems with restricted solubility and intermediate compounds.

Abstract: Dislocation structures at crystalline scale play an important role in the scale effect of materials. The higher-order crystal plasticity, in which a dislocation information is introduced as the gradient of slip and affects the hardening behavior of slip, is a useful model to describe a scale dependency of metallic material. In this study, a large deformation finite element analysis of a bicrystalline micropillar is demonstrated to investigate the grain boundary effect on the dislocation motion. The effect of condition on the grain boundary is numerically discussed. It is suggested that the large angle grain boundary and the coherent twin boundary can be represented by boundary conditions of non-penetration and penetration of dislocation.

Abstract: The paper reports laboratory test findings on the impact of steel oxidation level on distribution features of non-metallic inclusions in low-alloyed structural steels. An analysis of the effect of various oxidation methods of steel on the distribution and formation of non-metallic inclusions is made. The results reveal a relation between the amount of sulphide and oxisulphide inclusions formed and steel oxidation level. The release of oxisulphide from the melt is accompanied with a decrease in the amount of both oxygen and sulphur. After oxygen content has achieved an equilibrium value, only “pure” sulphides are formed, which may deteriorate steel plastic properties. Thus, sulphides start precipitating only when oxygen content in the melt falls to a very low value. An increase in the amount of oxysulphides is accompanied with a decrease in sulphur concentration in the melt which reduces sulphide phase concentration at grain boundaries and stabilizes plastic properties. Thus the negative effect of sulphur can be reduced not only by decreasing its content in steel through expensive secondary steelmaking methods but also by controling the amount, shape and types of oxide, sulphide and oxisulphide inclusions in steel.

Abstract: The paper gives an assessment of the possibility of using local mineral raw materials to create foundry alloys that are operated under natural low temperatures. The feasibility of using iron ores of the Lena ore field (Yakutia, North-East of Russia) for the development of new cold resistant and high strength steels is analyzed. The chemical composition and microstructure of the melted alloy were studied. Also the phase composition of the alloy by X-ray diffraction method is analyzed. The alloy with respect to the chemical composition is naturally doped Mn, W, V, which are well preserved in the preparation of the iron semi-finished product by direct reduction technology. The obtained sample after processing has acquired a uniform structure, typical for tool carbon steel after forging. Elemental analysis data indicate a uniform distribution of iron, carbon, manganese, sulfur and phosphorus in the alloy.

Abstract: Behavior of hydrogen in tensile-deformed Al-9mass%Mg and Al-5.8mass%Zn-2.4mass%Mg alloys was investigated by means of hydrogen microprint technique, HMPT, a method to visualize the microscopic location of hydrogen evolution from specimen surface as silver particles. Both in the two alloys, surface relief was formed at most grain boundaries by the stretching, while hydrogen evolution was observed at some grain boundaries. The evolution of hydrogen was discussed with parameters such as the angle between grain boundary on the specimen surface and tensile direction, the angle between grain boundary on the surface and slip line inside the grain, the height of the surface relief, and maximum gradient of the surface relief. The results indicated that the shear deformation along grain boundary caused transportation of hydrogen atoms with gliding dislocations to the surface, breakage of surface oxide film. In the Al-Zn-Mg alloy, it was suggested that the preferential deformation in the precipitate free zone was attributed to hydrogen evolution.

Abstract: Ni-based single crystal superalloy turbine blades have excellent mechanical strength and resistance to corrosion and oxidation due to a uniformly distributed gamma prime phase in a gamma matrix. However, defect grains have been often found on the surface of turbine blades after manufacturing, which can be potential sites of crack initiation. In this study, several different types of surface defect grains formed in third generation Ni-based single crystal turbine blades, such as stray grains, freckle chain grains, equiax grains, and a new grain formed in surface scale, had been investigated. The grain boundary regions were observed by high resolution electron microscopy. Although the formation mechanism of each grain defect is different, secondary phases, such as rhenium-rich particles, have been always found in each grain boundary. In addition, depending on the existence of the secondary phases as well as the size of defect grains, different microstructures were observed even in the same defect grain boundary. Finally, the observed results suggest that if there is any boundary region in a turbine blade, secondary phases, such as Re-rich particles, can be found.

Abstract: Atomistic simulations of the structure, energy and relaxation under the action of high frequency cyclic straining are carried out for columnar nickel nanocrystals with [112] column axis, the grain boundaries (GBs) of which are in a nonequilibrium state caused by the presence of extrinsic grain boundary dislocations (EGBDs). A special method of introducing EGBDs is used to create initial structures with nonequilibrium GBs. Energy of GBs as a function of the degree of nonequilibrium is evaluated and qualitatively compared to the results of dislocation and disclination modeling. It is shown that under loading by symmetrically oscillating stresses the nonequilibrium GBs generate lattice dislocations, which travel across the grains and are absorbed by opposite GBs thus resulting in a relaxation of the structure, long-range stress fields and the energy of GBs.

Abstract: Since the mechanism of Sn-whisker growth is closely related with the behavior of deformation and recrystallization, understanding of the behavior is very important to establish the measure for mitigation of whisker growth. In this work, microstructural changes after heavy deformation by scratching were characterized by EBSD for a single crystal of β-Sn, and the following results were obtained. Three types of crystal grains appear immediately after the deformation; small grains in aggregation, large serrated grains and rim-grains. The small grains are considered to be formed by dynamic recrystallization. They continue to grow at a room temperature over a lengthy period of time. The large grain has a certain crystallographic relationship with the matrix where <100> axes of the large grain and the matrix are almost parallel to each other. The serrated boundaries of large grain are so stable that the large grain does not show a grain growth process. The stable boundary is considered to promote a continuous growth of whiskers.