High Fraction Of High-angle Grain Boundaries Research Articles

Microstructure and corrosion behavior of a hot-rolled pure Mg after annealing at various temperatures

The microstructure and corrosion behavior of a hot-rolled pure Mg in the rolled state and after annealing at various temperatures was studied in this paper. It was found that the hot-rolled pure Mg after annealing at 250 °C, 350 °C, 400 °C, 500 °C and 550 °C showed homogeneous microstructures and the presence of a partially recrystallised tissue in hot rolled pure Mg. Corrosion of annealed Mg mainly occurred at grain boundaries, which stemmed from the relatively higher energy and chemical activation on the grain boundaries. Initial corrosion sites were provided by different proportions of High angle grain boundaries (HAGBs). The corrosion rate tends to increase and then decrease with increasing grain size. The Mg annealed at 450 °C showed a non-uniformed microstructure, with a lower fraction of the recrystallization grains and higher fraction of HAGBs, leaving to the severe localized corrosion and the highest corrosion rates. Corrosion rate measurement indicated that there existed a curvilinear relationship between the corrosion rate and grain size.

Innovation in thermal cycling aging compared to isothermal aging for precipitation hardening stainless steel

The cyclic thermal process can assist and accelerate the kinetics of phase transformation. Conventional UNS S17400 grade stainless is characterized by a martensitic microstructure. After solution treatment, the steel was aged by thermal cycling between 600 °C and 25 °C and quenched in water in each cycle, completing under the self-designed system. The nano precipitates of very fine copper particles and larger NbC particles were found by using transmission electron microscopy (TEM). The fraction and quantity of high angle grain boundaries (HAGBs) after 36 cycles were the highest among the three numbers of thermal cycles. The peak hardness also occurred after 36 cycles and was attributed to the finest grains, high fraction of HAGBs, and the largest local microstrain. The microtwins and the reverted γ were formed by the thermal cycling process. The estimated fraction value of reverted γ was very low, below 0.1, with a calculated precipitation rate about 12.6 s−1 at t0.5.

Microstructural characterization and mechanical properties of additively manufactured 17–4PH stainless steel

The aim of the present study is to carry out a comprehensive microstructural characterization on the Selective Laser Melting (SLM) printed 17–4PH stainless steel and to compare the results with commercial 17–4PH wrought alloy (in H1150 condition). It was found out that the middle region of as-printed SLM had less microporosities as compared to bottom and top regions which was attributed to the lower cooling rate in the middle-area and greater extent of fusion during manufacturing. The as-printed samples mainly consist of ά-martensite and retained austenite (γ). The electron back scatter diffraction (EBSD) results showed that the middle region had less retained austenite as compared to top and bottom ones. The fraction of high angle grain boundaries (HAGB) and twin boundaries (TB) in martensite in wrought alloy was found to be greater than the as-built samples. In contrast, a high fraction of HAGBs and low fraction of TBs were observed in austenite in as-printed samples. The SEM-TKD revealed that the fraction of phase boundaries close to Kurdjumov–Sachs (KS) orientation relationship (OR), i.e., x < 10°, was 93.3% and 91.73% for the wrought alloy and SLM printed, respectively. The average Vickers hardness of as-printed sample was ≈ 25% less than the commercial wrought alloy (in H1150 condition) which can be attributed to more retained austenite, larger grain size, and more microporosities in the matrix. Monotonic uniaxial tensile test and in-situ tensile deformation with the aid of EBSD were used to examine the mechanical strength of wrought and SLM samples and microstructural evolution during plastic deformation. The ultimate tensile strength (UTS) of the as-printed sample (≈1118 ± 4 MPa) was quite comparable to commercial alloy (1017 ± 2 MPa) but the uniform elongation was considerably higher in as-printed sample (21.8% ± 0.3 in as-printed versus 9.00% ± 0.56 in wrought alloy). EBSD analysis during the in-situ tensile test showed transformation induced by plasticity (TRIP effect) of γ → ά. The area fraction of retained austenite in SLMed sample decreased from 26.2% in stage I (zero strain) to 0.49% in stage IV (failure). Lath and acicular shape retained austenite transformed faster to martensite than the blocky shape austenite.

Microstructure and mechanical properties of friction-stir-welded high-Mg-alloyed Al–Mg alloy plates at different rotating rates

Hot-rolled high-Mg-alloyed Al–Mg alloy (Al–9.2Mg–0.8Mn–0.2Zr–0.15Ti, labeled as 5A12) plates were successfully friction stir welded at rotating rates ranging from 750 to 1500 r·min−1 at a constant welding speed of 50 mm·min−1. The joints were characterized by optical microscopy (OM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), electron-dispersive spectroscopy (EDS), and tensile testing. All the joints are volume defect-free and exhibit fine, equiaxed dynamic recrystallization (DRX) grains with high-angle grain boundaries (HAGBs) fractions of 88.6%–93.3% in the nugget zones (NZs). The DRX grain size and the second-phase particle size in the NZs have a parabolic relation with the rotating rate. Furthermore, among the joints tested, the joint prepared at 1000 r·min−1, which has the highest ultimate tensile strength ((478 ± 3) MPa) and the largest elongation to rupture (22.5% ± 1.4%)—approximately 87.5% and 145.2% those of the base metal, respectively, exhibits the smallest grain size of 2.93 μm, as well as the smallest particle size in the NZs. These excellent mechanical properties can be ascribed to the combined effects of the fine DRX grains with high fraction of HAGBs and the fine second-phase particles with a uniform distribution.

Optimization of Homogenization Treatment Parameters and Microstructural Evolution of Large Size DC AA2014 Aluminum Alloy

The microstructural evolution of the as-cast large size AA2014 aluminum alloy ingot prepared by direct chill (DC) casting during homogenization was investigated by OM, SEM, EDS, DSC, EBSD, TEM and tensile tests. The results showed that serious dendrite segregation and numerous coarse nonequilibrium eutectics existed in the as-cast alloy were dramatically eliminated, while a small amount of Fe-rich phase was still present along the grain boundaries when the alloys were subjected to the optimized two-step homogenization. Compared with the nonhomogenized alloys, homogenization treatment can effectively reduce grain and second phase particles size, and increase the fraction of precipitates and high angle grain boundaries (HAGBs) during the hot deformation and T6 treatment. Besides, recrystallized grains with random orientation are formed due to dynamic recrystallization (DRX) for the alloys with hot deformation and T6 treatment. Additionally, the strength and elongation of the alloys were simultaneously improved with the application of optimum two-step homogenization because of the more homogeneous structures, finer grain and particles size, higher fraction of HAGBs and fine and uniformly distributed precipitates. The suitable homogenization schedule of the AA2014 aluminum alloy is 490°C × 10 h + 530°C × 8 h.

Effect of Dynamic Recrystallization on the Transformed Ferrite Microstructures in HSLA Steel

The flow stress behavior of high-strength low-alloy (HSLA) steel at different true strains was studied using a hot compression test. The effect of dynamic recrystallization (DRX) on the transformed ferrite microstructures was investigated with electron backscatter diffraction (EBSD). The EBSD analysis indicated that the fraction of high-angle grain boundaries (HAGBs) and DRX increased with increasing true strain. The low-angle grain boundaries (LAGBs) were gradually transformed into HAGBs with increasing DRX degree. When the true strain was increased to 0.916, the fraction of HAGBs increased to 85% and the fraction of DRX increased to 80.3%. The relatively high fraction of HAGBs was related to the complete DRX. The dislocations and substructures in the tested steel at different true strains were characterized by transmission electron microscopy (TEM). TEM observation shows that the nucleation of the dynamically recrystallized grains occurred by the bulging of the original grain boundaries. The DRX nucleation mechanism of the HSLA steel is the strain-induced grain boundary migration mechanism.

Dynamic recrystallization behaviors of high Mg alloyed Al-Mg alloy during high strain rate rolling deformation

The effects of the rolling parameters (rolling temperature, strain rate and thickness reduction) on the dynamic recrystallization (DRX) behaviors of the direct-chill cast high Mg alloyed Al-Mg alloy (Al-9.2Mg-0.8Mn-0.2Zr-0.15Ti) are carefully investigated. The as-rolled microstructures are characterized by optical microscope (OM), transmission electron microscopy (TEM), electron backscattered diffraction (EBSD) and X-ray diffraction (XRD). Strain rate plays a crucial role in enhancing DRX and the alloy experienced high strain rate rolling (HSRR) at 400 °C with a strain rate of 10 s−1 is featured with a fine-grained microstructure, with the DRX volume fraction of 95.4%, the average grain size of 2.63 μm and the fraction of high-angle grain boundaries (HAGBs) up to 91.9%. The relatively high fraction of HAGBs is associated with the full DRX related to the continuous DRX. The discontinuous DRX and the continuous DRX occur simultaneously in the high Mg alloyed Al-Mg alloy during HSRR and the strain-induced deformation bands (DBs) plays an important role in enhancing the DRX process.

The contribution of grain boundary sliding in tensile deformation of an ultrafine-grained aluminum alloy having high strength and high ductility

An as-cast Al–7 % Si alloy was processed by high-pressure torsion (HPT) for up to 10 turns at temperatures of 298 or 445 K. The HPT-processed samples had ultrafine-grained structures and they were tested in tension at room temperature at various strain rates in the range from 1.0 × 10−4 to 1.0 × 10−2 s−1. The contributions of grain boundary sliding (GBS) to the total strain were measured directly using atomic force microscopy. Samples simultaneously showing both high strength and high ductility contained the highest fractions of high-angle grain boundaries (HAGB) and exhibited the highest contributions from GBS, whereas samples showing high strength but low ductility gave negligible values for the sliding contributions. It is concluded that high strength and high ductility require both an ultrafine grain size and a high fraction of HAGB.

Correlation of microstructures and low temperature toughness in low carbon Mn–Mo–Nb pipeline steel

By adjusting thermomechanical controlled processing parameters, different microstructures were obtained in a low carbon Mn–Mo–Nb pipeline steel. The microstructural characteristic and its effect on low temperature toughness were investigated. The results show that under higher reduction in austenite non-recrystallisation region and faster cooling rate during accelerated cooling, the microstructure is dominated by acicular ferrite (AF) accompanied by a small amount of fine martensite/austenite (M/A) islands. In contrast, lower reduction and slower cooling rate lead to a predominantly quasi-polygonal ferrite microstructure with coarse M/A islands. The fine effective grain size (EGS) and the high fraction of high angle grain boundaries (HAGBs) make the cleavage crack propagation direction deflect frequently. The coarse M/A islands can lead to cleavage microcracks at the M–A/ferrite matrix interfaces. Compared with the microstructure mainly consisting of quasi-polygonal ferrite, the microstructure dominated by AF exhibits excellent low temperature toughness because of fine EGS, high fraction of HAGBs and fine M/A islands.

Effect of microstructure on low-temperature toughness of a low carbon Nb–V–Ti microalloyed pipeline steel

In order to further understand the relation between microstructure and low-temperature toughness of high strength pipeline steel, two kinds of microstructures with different features in a low carbon Nb–V–Ti microalloyed pipeline steel were obtained by adjusting thermo-mechanical controlled processing parameters. The microstructural characteristics, including detailed structure of microstructural constituents, as well as their effects on low-temperature toughness were investigated. The results show that under higher reduction in austenite non-recrystallization region and faster cooling rate during accelerated cooling, the microstructure is dominated by acicular ferrite (AF) accompanied by a small amount of fine martensite/austenite (M/A) islands. In contrast, lower reduction and slower cooling rate lead to a predominantly quasi-polygonal ferrite (QF) microstructure with coarse M/A islands. Like AF, QF contains high density tangled dislocations, well developing a lot of dislocation walls and cellular substructures. Compared with QF, AF appears to have finer effective grain size (EGS). The fine EGS and high fraction of high angle grain boundaries (HAGBs) make the cleavage crack propagation direction deflect frequently. The coarse M/A islands can lead to cleavage microcracks at the M-A/ferrite matrix interfaces. Acicular ferrite dominated microstructure exhibits excellent low-temperature toughness because of fine EGS, high fraction of HAGBs and fine M/A islands.

Grain-boundary structure of oxygen-free high-conductivity (OFHC) copper subjected to severe plastic deformation and annealing

The influence of grain-boundary structure on grain growth in copper subjected to severe plastic deformation has been studied using orientation imaging microscopy. The investigation was carried out on oxygen-free high-conductivity (OFHC) copper which was wire drawn to a true strain of about 4 and processed by equal-channel angular extrusion (ECAE) to 4 and 8 passes via “route Bc” (where the billet is rotated by 90° in the same direction between consecutive passes). The grain-boundary character distribution (GBCD) of the as-drawn wire was similar to that of ECAE-processed specimens, and both materials possessed a higher fraction of high-angle grain boundaries (HAGBs) than special coincidence-site lattice (CSL) boundaries. While the high fraction of HAGBs was retained in the annealed wires, they were transformed to CSL boundaries in the annealed ECAE-processed materials. In spite of an initially smaller grain size, when annealed at 750°C for 1h, the grain size of the 4-pass ECAE-processed material was larger than that of the wire drawn to a similar strain. This difference was attributed to a high density of high-mobility 35–50° 〈001〉 boundaries in the 4-pass ECAE materials. On the other hand, the presence of 50–60° 〈111〉 pinning boundaries in the annealed 8-pass material accounted for the smaller grain size after recrystallization.

Effect of Al 3Sc precipitate on the microstructural evolution during accumulative roll bonding in Al–0.2 wt.% Sc alloy

The cold-rolled Al–0.2 wt.% Sc alloy sheets were conducted the following two preliminary heat treatments: (1) as solution treatment at 630 °C for 1 h, and (2) as artificial aging at 300 °C for 5 h. The accumulative roll bonding (ARB) was followed up to eight cycles at ambient temperature. The stored energy release caused by annihilation of dislocation and recrystallization in aged alloy was higher than that in solution treated one in the 2-cycle sample, and this aspect was reversed with increasing cycles. The fraction of high-angle grain boundaries (HAGBs) in both alloys was increased gradually with increasing equivalent strain. However, the alloy of aged and ARBed had relatively higher fraction of HAGBs than the alloy of solution treated and ARBed at the same strain level. The former formed lamellar structure faster than the latter. This variation can attribute to the fact that the nanosized and semi-coherent Al 3Sc precipitates hinder the migration of boundaries and dislocations. However, those precipitates cannot generate the sufficient pinning effect because the driving pressure for recrystallization surpasses the Zener pinning pressure.