Metallurgical and Materials Transactions A最新文献

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.

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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.

The structural and superconducting properties of a new niobium- and titanium-rich high-entropy alloy (NbTi)(_{text {0.67}})(MoHfV)(_{text {0.33}}) were determined. The alloy was synthesized by arc melting and its physical properties were characterized by means of X-ray powder diffraction, energy dispersive X-ray spectroscopy, magnetization, electrical resistivity and specific heat measurements. Experimental data revealed that the (NbTi)(_{text {0.67}})(MoHfV)(_{text {0.33}}) crystalizes in body-centered cubic structure and exhibits type-II superconductivity below about 5 K. In addition, electronic structure calculations were performed using the Density Functional Theory (DFT) method. Their results suggest that in case of HEAs, the influence of the local atomic configuration on their electronic structure in the energy region close to E(_{mathrm {text {F}}}) is rather small or even negligible.

The effects of different annealing times on the microstructure, mechanical and damping properties of a rolled state ZK60 alloy were investigated. The results show that with increasing annealing time, the degree of static recrystallization gradually increases, and the grains grow gradually. The grain orientation changes from the (0001) orientation to the (01-10) and (− 12-10) orientations, and the basal texture intensity of the alloy decreases gradually. The tensile and yield strengths decrease slightly, and the elongation increases gradually. Because of the gradual reduction in dislocation entanglement, the room temperature damping properties improve with annealing time, and the damping value Q⁻¹ at ε_0.01 increases from 0.008 to 0.013. After annealing, the ZK60 alloy becomes a high-damping alloy (Q⁻¹ > 0.01). The recrystallization damping peak is more intense than the background damping peak, and recrystallization damping peak P₁ becomes increasingly obvious. The peak value of damping peak P₂ gradually decreases in the temperature range of 270 °C < T < 290 °C, and the peak width slowly increases, which is characterized by grain boundary-type relaxation. These results indicate that annealing effectively maintains the high strength of the ZK60 alloy while improving its damping properties.

The biocorrosion of a new surgical wire (0.6, 0.8, and 1.0 mm in diameter) obtained from a ZnMg0.004 alloy by hot and cold drawing was characterized using in vitro tests in a bovine animal serum solution and in vivo tests – through placement in the muscles of a rabbit and a in cat's broken paw. Corrosion gravimetric tests were carried out in a corrosive environment refreshed every 3 days and also one which remained unchanged throughout the test period. Electrochemical tests were performed in the same solution of serum as in gravimetric tests. Depending on the initial diameter, complete dissolution of the wire was observed after 36-78 days of immersion in periodically changed serum solution. None of the wires completely dissolved over 120 days in the unreplaced serum solution. The corrosion rate values determined in electrochemical and gravimetric tests were similar. After 90 days of in vivo tests, both in the body of the rabbit and the cat, the degradation of the wire was similar to in vitro tests in the unreplaced serum solution. No harmful effects were observed in the animals.

Aluminum matrix composite have shown the great prospects in improving service performance and life of high-end equipment under harsh conditions due to lightweight and high specific strength. However, the presence of coarse Mg₂Si phase with undesired morphology in Mg₂Si/Al composites limits their engineering application. This study proposes a strategy of TiB₂ ceramic particles regulating the morphology and size of Mg₂Si to strengthen the aluminum matrix composites by incorporating dual-scale hybrid reinforcements of Mg₂Si and TiB₂. The TiB₂ particles were introduced into the Al-15 wt pct Mg₂Si composite by a master alloy method, and the influence of TiB₂ particles on the morphology and size of Mg₂Si, and Al grains of composites were investigated. The results demonstrate that 3 wt pct TiB₂ particles can refine the primary Mg₂Si from 14.6 to 10.4 μm, resulting in a more regular morphology. Additionally, the eutectic Mg₂Si transforms from a long and slender Chinese character shape to a short rod shape, reducing the aspect ratio by 65.9 pct from 10.9 to 3.7. During the solidification process, TiB₂ particles promote the heterogeneous nucleation of primary Mg₂Si phases and impede the growth, leading to significant refinement. However, the improved morphology of eutectic Mg₂Si is mainly attributed to the inhibited preferential growth due to the formation of a nanoparticle layer, with no observed heterogeneous nucleation of TiB₂. Moreover, TiB₂ particles significantly refine the grain size of Al from 212.7 to 70.8 μm. The addition of 3 wt pct TiB₂ greatly improves the strength of the composites from 302 to 373 MPa, without losing ductility. After T6 heat treatment, Al-15 wt pct Mg₂Si composites exhibit simultaneous improvement in the strength and ductility. Compared with composites without TiB₂ particles (361 MPa and 32 pct), Al-15Mg₂Si-3TiB₂ composites have more superior strength–ductility combination of 434 MPa and 35 pct due to the significant modification of Mg₂Si phase, grain refinement, and Orowan strengthening.

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