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Grain size has a significant impact on the superelasticity of alloys. Large-sized grains show superior superelastic properties because the grain boundaries are minimized and the grain constraints caused by triple junctions are reduced. Cyclic heat treatment (CHT) is commonly employed to generate subgrains, whose energy can be consumed to induce abnormal grain growth (AGG) and obtain large-sized grains. In this paper, the effects of adding Cu on the subgrain characteristics during AGG and microstructural evolution of FeMnAlNi-based superelastic alloys were systematically investigated. The addition of Cu reduced the temperature at which the γ phase precipitates and altered the morphology of the γ phase. After the dissolution of the refined γ phases, the average subgrain size became smaller and misorientation increased. These characteristic subgrain changes improved the driving force for AGG and accelerated the grain boundary migration rate. Due to the addition of Cu, the maximum grain size reached 28.2 mm. This study provides a new method for the preparation of FeMnAlNi-based superelastic alloys with large-sized grains. [ABSTRACT FROM AUTHOR]

HR3C steel is an austenitic high-temperature-resistant steel. Because of its good strength and high-temperature performance, it has been widely used in ultra-supercritical power plant boilers. With the increasingly frequent start-up and shutdown of thermal power units, leakages of HR3C steel pipes have occasionally occurred due to the embrittlement of HR3C pipe steel after a long service duration. In this study, the embrittlement mechanisms of HR3C pipe steel are investigated systematically. The mechanical properties of the pipe steel after running for 70,000 h in an ultra-supercritical unit were determined. As a comparison, the pipe steel supplied in the same batch was aged at 700 degrees Celsius for 500 h. The mechanical properties and the micro-precipitation of the aged counterparts were also determined for comparison. The results show that the embrittlement of HR3C pipe steel in service for 70,000 h is obvious. The average impact absorption is only 5.5 J, which is a decrease of 96.7%. It is found that embrittlement of HR3C steel also occurs after 500 h of aging at 700 °C, and the average value of impact absorption energy decreases by 70.4%. The comparison experiment between the in-service pipe steel and the aged pipe steel shows that in the rapid decline stage of the impact toughness of HR3C steel, the M23C6 carbide in the microstructure has a continuous chain distribution in the grain boundary. There were no other precipitated phases observed. The rapid precipitation and aggregation of M23C6 carbides leads to the initial embrittlement of HR3C steel at room temperature. The CRFe-type σ phase was found in the transmission electron microscope (TEM) image of the steel pipe after 70 thousand hours of use. The precipitation of the σ phase further induces the embrittlement of HR3C pipe steel after a long service duration. [ABSTRACT FROM AUTHOR]

Reducing the local stress in the vicinity of the grain boundaries is a favorable way to improve the super-elastic properties of super-elastic alloys. The crystal plasticity finite element method (CPFEM) was applied in this study to simulate the deformation behavior and local stress of a super-elastic Cu-8Al-11Mn (wt.%) alloy containing single grains with various orientations, columnar grains with different misorientation angles, and tri-crystals with distinct grain boundary morphologies. The results indicated that the stress distribution of single grains presented obvious orientation dependence during deformation. Uniformly distributed stress was observed in grains with orientations of 0° and 90° when more slip systems were activated during deformation. With the increase in the misorientation angles of columnar grains, the stresses in the vicinity of the grain boundaries increased, which was related to the difference in the shear stress of the slip systems in adjacent grains. When the difference in the shear stress of the slip systems in two adjacent grains was large, a local stress concentration formed in the vicinity of the grain boundary. Compared with the triple-junction grain boundaries, the local stresses of the straight and vertical grain boundaries were smaller, which was closely related to the number of activated slip systems on both sides of the grain boundary. The above results were obtained experimentally and could be used to design super-elastic alloys with high performance. [ABSTRACT FROM AUTHOR]

Directional recrystallization (DR) was utilized to obtain columnar grains in a superelastic CuAlMn alloy. The effects of hot-zone temperature and drawing velocity on the microstructural evolution of the alloy during DR, and the abnormal grain growth mechanism for the formation of columnar grains were systematically investigated by OM and EBSD. It was found that columnar grains could be obtained when the hot-zone temperature was above the α-phase solvus temperature of ∼730 °C. The optimum drawing velocities for obtaining columnar grains with the largest aspect ratio increased with increasing hot-zone temperatures, i.e. at 800 °C, 850 °C and 900 °C, the velocities were 2 μm/s, 5 μm/s and 15 μm/s, respectively. The apparent activation energy of the columnar grain growth was estimated to be ∼177 kJ/mol. The equiaxed grains that were produced by primary recrystallization all had a similar 111 1 1 ¯ 0 orientation. Thus, there was little driving force for grain growth. In contrast, grains that do not have this orientation are surrounded by high-angle grain boundaries, which have high mobility, and form columnar grains. • Columnar grains were successfully produced in a superelastic CuAlMn alloy by DR. • The apparent activation energy for producing columnar CuAlMn alloys were obtained. • The effect of DR process parameters on the maximum aspect ratio was derived. • The key factor for the formation of columnar grains is the LAGB pinning. [ABSTRACT FROM AUTHOR]

In this paper, the effects of Mo addition on the precipitation behaviors of L1 2 structured γ' (Ni 3 Al) phase and B2 structured β (NiAl) phases and the martensite transformation in Fe 43.5-x Ni 28 Co 17 Al 11.5 Mo x (x = 0/1/2/3 at.%) alloys were investigated. The results show that when Mo content reaches 2 at.%, Mo will be enriched at the grain boundary to form Laves phase of Fe 2 Mo. The grain boundary precipitation of Fe 2 Mo reduces the grain boundary energy, which inhibits the precipitation and coarsening behavior of the β phase. At the same time, the enrichment of high-sublimation enthalpy Mo element at the grain boundaries also improves the grain boundary cohesion, enhancing the alloy elongation. As β phase is suppressed by the addition of Mo, the content of γ' phase in the matrix increases. In addition, the size of γ' phase becomes smaller with the increase of Mo content, which increases its coherence degree with the parent phase. The Mo addition causes a significant decrease in the free energy and equilibrium temperature of alloy, which leads to a decrease in martensitic transformation start temperature with the increase of Mo content. The alloys with different Mo content have a low thermal hysteresis and exhibit a fully reversible thermoelastic martensitic transformation with temperature changes. • Mo reduces the grain boundary energy, inhibiting the formation of β phase. • Mo improves the grain boundary cohesion, enhancing the alloy elongation. • The addition of Mo promotes the coherent precipitation of γ' phase. • Mo reduces the equilibrium temperature and free energy of alloy, decreasing M s. [ABSTRACT FROM AUTHOR]

Compared with Ti–Ni-based and Cu–Al-based alloys, Fe–Ni–Co–Al-based superelastic alloys have received great attention in recent years because of their low cost and excellent performance. However, the precipitation of brittle precipitates on grain boundaries in Fe–Ni–Co–Al-based polycrystalline alloys significantly reduces the mechanical properties. In this paper, the effects of V on the formation of brittle precipitates on both grain boundaries and inside grains and mechanical properties in Fe–Ni–Co–Al–Nb alloy were investigated systematically. The results show that γ′(Ni 3 Al) precipitate with L1 2 structure precipitate out on grain boundaries and inside grains in the Fe–Ni–Co–Al–Nb alloy during aging without V addition. The γ′ precipitate with L1 2 structure on the grain boundaries grew to form a heavily-coarsened L1 2 precipitate (HCLP γ′ precipitate) due to the accumulation of Nb on the grain boundaries, resulting in a significant decrease in elongation. With the addition of V, intragranular Nb and V can combine together with γ′ to form Ni 3 (Al, Nb, V) (nano-sized γ′ precipitate) composite precipitates, which improves the strength. More important, since the diffusion coefficient of V was larger than that of Nb, V preferentially occupied the lattice positions on grain boundaries, repelling Nb from the grain boundaries to the matrix. Thus, the coarsening of HCLP γ′ precipitate on the grain boundaries was reduced, and the elongation of Fe–Ni–Co–Al–Nb–V alloy was significantly improved. [ABSTRACT FROM AUTHOR]

Large-size grain is one of the effective parameters to determine the superelasticity of Cu-based alloys. In this study, the effects of Ni addition on abnormal grain growth (AGG) and microstructure evolution of CuAlMnCoNi alloys are systematically investigated. The activation energy and driving force of grain boundary (GB) migration during AGG are evaluated. The results reveal that Ni significantly promotes AGG. Average grain size reaches 6.0 cm after three cyclic heat treatments (CHTs) in a CuAlMnCoNi alloy containing 2 at.% Ni. A maximum superelastic strain of 11.8 % can be obtained at 15 % applied strain. The addition of 2 at.% Ni increases the solvus temperature of the α-phase to about 818 °C and raises α-phase volume fraction at phase equilibrium. The precipitation of α-phase with high volume fraction causes high number density of dislocations. The misorientation angle of subgrain is increased by the addition of Ni, contributing to the AGG in CuAlMnCoNi alloys. The addition of Ni reduces the activation energy of GB migration and increases the GB migration driving force. [ABSTRACT FROM AUTHOR]

In this paper, the effects of annealing on grain-boundary character distribution and texture evolution were investigated in hot-rolled Fe-6.5 wt% Si steel. An annealing treatment performed at different temperatures affected the recrystallization behavior and changed the volume fraction of low-angle and coincidence site lattice (CSL) boundaries. High frequencies of coincidence boundaries Σ3, Σ9, Σ13, and Σ27 were observed in all annealed samples, while the annealing temperature can lead to a large difference in frequencies for Σ13. The effect of crystallographic texture on the directional dependence of the specific magnetic energy in the rolling plane of investigated sheets was calculated and discussed. These results demonstrate the possibility of optimizing grain-boundary character distribution (GBCD) and texture to improve the magnetic and mechanical properties of Fe-6.5 wt% Si steel by controlling annealing temperature. [ABSTRACT FROM AUTHOR]

Texture evolution is strongly dependent on the deformation mode during thermo-mechanical treatments. In this paper, we report the texture evolution in a duplex Mg–Li alloy. The results provide an evidence of deformation mode transition in the hexagonal-close-packed (hcp) α phase with various thickness reductions. The activation sequence of deformation modes is basal slip first, and then pyramidal slip during hot-rolling to a thickness reduction of 40%. The relative activity of slip decreases with further thickness reduction. After annealing, basal texture is strengthened and pyramidal component disappears due to static recrystallization and grain growth. The microstructure, specifically texture evolution in both hcp α and body-centered cubic (bcc) β phase and their effects on mechanical properties are quantitatively analyzed and assessed. [ABSTRACT FROM AUTHOR]

• The effect of Al/Ti ratio in a series of HEAs was investigated. • Layer-like L1 2 precipitates formed around the GBs, and spherical L1 2 precipitated in the grains. • The coarse L1 2 GB phases transform to Heusler phase due to the local supersaturation of Al and Ti. • The small amount of small size Heusler phases at the GBs have no negative effect on the plasticity. L1 2 -structured Ni 3 (Al, Ti) precipitates are promising strengthening particles in face-centered-cubic high entropy alloys (HEAs). However, if the Al/Ti ratio (R) is not carefully controlled, brittle intermetallic compounds can form, especially at the grain boundaries (GBs). In this study, a series of Ni 35 (CoCrFe) 55 Al x Ti 10−x HEAs with different R values, R = x/(10-x), were prepared, and the evolution of precipitates and their effects on the mechanical properties were investigated. The dispersed spherical precipitates within the grains act as an effective barrier to dislocation slip, producing high strength. However, increasing R (increasing Al) reduces the volume fraction of the precipitates within the grains, resulting in a decrease of yield strength. Both experimental measurements and first-principles calculations show that Al, Ti and Ni segregate to the GBs. This leads to the formation of L1 2 precipitates at the GB that, upon aging, coarsen to layer-like structures. When R ≥ 3 the local supersaturation of Al and Ti also leads to formation of a Heusler phase at the GBs, which reduces the ductility. However, when R > 3.5 the volume fraction of the Heusler phase decreases, and the plasticity increases again. These findings provide important guidance and inspire new strategies for the microstructural control in precipitate-strengthened HEAs. [ABSTRACT FROM AUTHOR]

The effects of deformation at 900 °C and 1050 °C along with various cooling rates on the microstructure and mechanical properties of a 460 MPa grade low carbon bainitic ferrite steel are investigated. Without prior high temperature deformation, the microstructures can be polygonal ferrite, pearlite, granular ferrite, acicular ferrite or bainite ferrite, depending on the cooling rate. High temperature deformation moves the transformation curves toward the left side in the continuous cooling transformation (CCT) diagram and promotes the formation of polygonal ferrite, granular ferrite, and acicular ferrite, which leads to a lower hardness. Electron backscattering diffraction (EBSD) results indicated that with a cooling rate of 40 °C/s, high proportion of low angle grain boundary (LAGB), large orientation gradient, and high dislocation density and residual stress can be obtained by deformation at high temperature before cooling. Low cooling rate reduces the proportion of LAGB, orientation gradient, dislocation density, and residual stress due to the release of residual stress and the consumption of dislocations. Deformation before cooling slightly lowers the transformation start temperature (Ar 3) while accelerating the transformation speed of the supercooled austenite. The yield and tensile strength increase with the increase in cooling rate. Furthermore, the impact energy value increases with the increase of cooling rate, and it slightly decreases when the microstructure fully changes to bainite ferrite owing to the significant increase in strength. Impact toughness is more sensitive to strength at low temperatures. [ABSTRACT FROM AUTHOR]