Metallurgical and Materials Transactions A最新文献

The evolution of the microstructure and mechanical properties of Al–5.76Zn–1.77Mg–0.33Fe−0.45Ni (wt pct) alloy bars produced by hot extrusion (HE) followed by radial shear rolling (RSR) and heat treatment (quenching + aging) has been studied. The results show that the suggested thermomechanical treatment (TMT) allows one to produce a specific type of microstructure. HE provides for defect-free deformation of the initial cast structure, while RSR leads to the formation of a fine microstructure due to unique temperature–strain conditions. During deformation, insoluble Al₉NiFe eutectic phase crystals are refined to a size of about 100 nm. These crystals are localized along the grain and subgrain boundaries and act as effective barriers to the migration of high-angle and low-angle grain boundaries. As a result, the formation of a deformed structure with an extensive network of low-angle grain boundaries (especially in the near-surface region where the equivalent strain is about 15) is observed in the central part and in the near-surface region of the obtained bars. The formation of a structure with an average characteristic subgrain size (~ 2 to 3 μm) comparable to that of grains produced by severe plastic deformation processes provides for a favorable combination of mechanical properties (UTS ~ 416 MPa, YS ~ 293 MPa, δ ~ 8.4 pct), which are comparable to the mechanical properties of a similar alloy after equal channel angular pressing.

The construction of the experimental liquidus surface projection and isothermal sections of the Co–V–Zr ternary system was based on the analyses of solidification microstructures and phase constituents. A new compound τ with Cr_0.5Fe_1.5Zr-type was identified, and the measured composition range of Zr in τ was ∼ 28.6 to 57.3 at. pct and 32.9 to 56.8 at. pct at 1100 °C and 1000 °C, respectively. The liquidus surface projection identified nine primary solidification areas, while two primary solidification areas were inferred from binary diagrams. Moreover, eight and ten three-phase equilibria were determined at 1100 °C and 1000 °C, respectively. The measured solubility of V in Co₂₃Zr₆, Co₂Zr and CoZr reached ~ 0.5, 7.6 and 4.1 at. pct at 1100 °C. Meanwhile, the solubility of V in Co₂₃Zr₆, Co₂Zr, CoZr and CoZr₂ was measured to be ~ 0.4, 7.6, 1.6 and 3.6 at. pct at 1000 °C. The solubility of Zr in Co₂V₃ was ~ 3.6 at. pct at 1100 °C, while the solubility of Zr in Co₃V, Co₂V₃ and CoV₃ was determined ~ 1.3, 3.0 and 4.5 at. pct at 1000 °C, respectively. The experimental data can help improve the thermodynamic parameters of the Co–V–Zr system and advance the development of databases for multi-component Co-based superalloys.

In this paper, the tensile properties, texture evolution, and deformation anisotropy of a cold-rolled Fe−0.3C–6.86Mn–3.5Al steel during superplastic deformation at 650 °C–750 °C and 0.25 × 10⁻³ s⁻¹− 4 × 10⁻³ s⁻¹ were studied. In particular, under 750 °C and 1 × 10⁻³ s⁻¹, the anisotropic microstructure evolution and texture characteristics were measured using EBSD. The results indicate that Medium Mn steel (MMS) sheets exhibit significant anisotropy under high tensile stress. The transverse direction (TD) specimens exhibit the highest peak strength of 145 MPa, and rolling direction (RD) specimens exhibit the highest elongation of 1295 pct. The plastic anisotropy r value of the RD sample varied more significantly than that of the TD sample. When the RD sample fractures, the aspect ratio reaches its minimum value (1.52), and ellipse fitting angle (θ) distributes from 0 to 90 degree and from 180 to 90 degree, which indicates that a large number of grains have undergone rotation.

The aim of this study is to evaluate the effect of titanium on the corrosion characteristics of bridge weathering steel plates in marine environments. The corrosion characteristics of steel containing different Ti additions were studied by simulating marine corrosion by cycles of the dry and wet environments. The addition of appropriate amounts of Ti can promote the production of γ-Fe₂O₃, which produces a protective rust layer. Steel containing 0.087 wt pct Ti, gave the best results. During long-term dry/wet cyclic corrosion experiments, the corrosion rate of the #0.087Ti steel first accelerated when a protective product layer has not completely covered the surface. The surface of the #0.087Ti steel was only fully covered after 144 hours of testing. With the further extension of periodic immersion testing the corrosion rate began to decrease gradually. After 576 hours of testing a stable protective product layer formed on the #0.087Ti steel, limiting further corrosion.

EH36 shipbuilding steels with varied Ti contents have been designed to investigate the roles of Ti upon inclusion evolution and microstructural characteristics. As the Ti content increases, the number density of Ti-containing inclusions appreciably multiplies, and corresponding area fraction of acicular ferrite boosts sharply from 5.0 to 29.9 pct. Additionally, through in-situ confocal scanning laser microscopy, it is clarified that acicular ferrite start temperature has been elevated, thanks to the population of Ti-containing inclusions.

Nickel-based superalloys with high Al + Ti content are considered non-weldable, and hot cracking is a major challenge in their thermal fabrication processes. In this study, the microstructure liquefaction characteristics and liquation cracking behavior of laser-remelted as-cast K447A superalloy have been investigated. In the heat-affected zone (HAZ), the special liquefaction phenomenon of the coarse script carbides consists of cracking, breakdown, and liquefaction successively. The whole liquefaction sequence observed in the as-cast K447A substrate encompasses: IMRs (Ni₇Hf₂-γ + γ–γ′ + M₅B₃-γ), primary γ′, MC-γ, matrix γ, and MC phases. Liquefaction of IMRs during the laser thermal cycle easily forms a continuous liquid film and evolves into liquation cracking. In the remelted zone (RZ), the fine MC particles and IMRs just form isolated liquefaction points. These isolated liquefaction points are interconnected due to micro-cracks generated by thermal stresses and solidification shrinkage stresses. Finally, crack-free remelted specimens have been obtained by process optimization and preheating the substrate. The tensile strength and elongation of the RZ are increased by 32 and 227 pct, which is beneficial for the RZ acting as a buffer layer to relax or inhibit the stress and cracks generated by subsequent additive repair of K447A.

Graphical Abstract

How the solid fraction f_S increases with decreasing temperature T during solidification, i.e., the f_S(T) of an alloy, can play a critical role in its susceptibility to cracking during solidification as demonstrated by various models of solidification cracking. In the present study the classic analytical modeling of microsegregation during rapid solidification was used to calculate f_S(T) using Al–Cu alloys as an example. The present study showed significant undercooling can occur during fast cooling and affect f_S(T) significantly. For the purpose of illustration, |dT/d(f_S)^1/2| near (f_S)^1/2 = 1 was used in the present study as a simple index for the susceptibility to solidification cracking. The f_S(T) calculated by any solidification model (e.g., the present analytical model or a phase-field model with undercooling, or the Scheil–Gulliver model without undercooling) can be used to calculate the curve of T vs (f_S)^1/2 and hence the index. The present study showed undercooling can increase the index and hence the cracking susceptibility significantly. It can also change the composition most susceptible to solidification cracking.

This work shows considerable advance in combining the exceptional properties of chemical vapor deposition (CVD) diamond with the toughness of steel, a subject that has been sought since the early 1990s. Combining both the previously developed techniques for vanadium carbide (VC) deposition, namely laser cladding vanadium carbide (LCVC) and thermo-reactive deposition (TRD) in a salt bath, made it possible to achieve the deposition of a CVD diamond film on a D6 tool steel, with a very low stress level of 1.8 ± 0.1 GPa. This was the lowest value for growth at 750 °C substrate temperature. The LCVC step was a fast processing to thicken the VC layer, while the short-term TRD (only 1 hour) closed the cracks left in the LCVC coating, relieved the residual stresses resulting from the rapid solidification after the laser incidence, and promoted a phase transformation from V₈C₇ to V₆C₅, a phase with lower thermal expansion coefficient. Hot Filament Chemical Vapor Deposition (HFCVD) was used to perform the CVD diamond deposition. The vanadium carbide layer has been an intermediate layer capable of acting as an excellent diffusion barrier and able to satisfactorily mitigate the thermal stress of the CVD diamond. The samples were characterized by Scanning Electron Microscopy with Field Emission Gun (FEG-SEM) equipped with Energy-Dispersive X-ray Spectroscopy (EDS), X-ray diffractometry, Rockwell A (588.6 N) indentation tester. Raman spectroscopy was used to further characterize HFCVD diamond, to compute the thermal compressive stress.

In recent years, high entropy alloys (HEAs), also known as multi-principal component alloys (MPCAs) have emerged as a new and exciting class of materials. This paper reports on the solid state reduction synthesis of a series of CoFeNiMn-based MPCA compositions, starting from a mixture of the corresponding oxides. One of the aims of the study was to test whether the degree of reduction of MnO, a highly stable oxide, could be enhanced by tailoring the alloy composition. Specifically, the influence of Ni content was studied because Ni exhibits a significant negative enthalpy of mixing with Mn. High purity precursor powders of Co(OH)₂, Fe₂O₃, MnO₂, and NiO were milled and mixed using standard ceramic processing methods. The nominal sample compositions (assuming complete oxide reduction) were (CoFeMn)_xNi_(1−x), for x = 0, 0.083, 0.166, and 0.25. The oxide samples were subjected to a series of isothermal reduction anneals in flowing 3 pct H₂–Ar at 1100 °C. The resulting microstructures were characterized using scanning electron microscopy (SEM), X-ray energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The composition of the resulting MPCAs was determined quantitatively using wavelength dispersive spectroscopy (WDS) in the electron microprobe. The study revealed that for each of the initial oxide compositions studied, it was possible to achieve an MPCA with ~ 25 at. pct Mn. These results were found to be consistent with the predictions of a thermodynamic model whereby a negative enthalpy of mixing (ΔH_mix), combined with a contribution from configurational entropy, can offset a positive free energy of reduction (ΔG_red). The incorporation of vibrational entropy into first principles calculations was found to have a significant effect on the predicted crystal structure of the MPCAs.

The laser powder bed fusion additive manufacturing (LPBF-AM) technique enables the production of near-net-shaped metal components, but the concentrated heat input employed during manufacturing leads to the development of significant internal residual stresses. These residual stresses may cause considerable issues such as distortion, crack initiation during fabrication, and premature failure during service. In this study, neutron diffraction experiments were performed on a nickel-based superalloy, Hastelloy-X, to evaluate the state of residual stresses below and near the surface of as-built specimens as a function of LPBF-AM core process parameters. In addition, a crystal plasticity method was used to determine the diffraction elastic constants to provide a better estimation of residual stresses for textured specimens. The results indicate large tensile residual stresses of about 660 MPa along the scanning direction, counterbalanced by compressive ones below the surface. It is shown that internal and surface residual stresses increase with the laser specific energy. The use of various diffraction peaks for determining residual stresses is discussed and it is shown that while trends do not change, the magnitudes of measured stresses vary.