Metallurgical and Materials Transactions A最新文献

The high temperature β/B2 phase field of Ti₃Al-Nb based alloys was rarely studied due to limitation in experimental technique, but different states of this phase field lead to difference in subsequent phase transformation path and resulting microstructure. By controlling the heating temperature and dwell time of α₂′ → β/B2 transformation and subsequent cooling rate, we achieved a new microstructure consisting of domains of the B2 phase with α₂′ martensite occupying the disordered BCC domain walls, effectively forming a pseudo sub-grain microstructure. It is shown that the domain walls evolved from the disordered β phase in the β/B2 phase field. Such pseudo sub-grain boundaries refine the prior β grains by subdividing them into smaller α₂ colonies that form during subsequent heat treatment in the α₂′ + β/B2 phase field upon reheating, and improve the mechanical properties of the alloy.

In this paper, a "multi-thin foil" structure + "two-stage" reaction strategy was employed to prepare B2-FeAl based MIL composite, and the effects of two different processes: conventional low-temperature hot-press sintering (CLT-HP) and fast high-temperature hot-press sintering (FHT-HP) on microstructure and mechanical properties were investigated. The results show that the MIL composites prepared by the two processes both exhibit multi-layer structure consisting of residual stainless steel layer, newly formed intermetallic layer, and intermediate transition layer. The metal layer is composed of both α-Fe and γ-Fe phases for CLT-HP, but only γ-Fe phase for FHT-HP counterpart. The intermetallic layer and transition layer mainly contain B2-FeAl phase for the two processes. Interestingly, multiple-sublayer structure was formed in the intermetallic layer and its thickness is obviously smaller and uniform for CLT-HP than FHT-HP. A kinetics calculation based on one-dimension diffusion equation was conducted to model the growth of B2-FeAl intermetallic layer, which exhibits a good consistency with the experimental results in terms of reaction rate and concentration profiles. CLT-HP MIL composites have better comprehensive mechanical properties than FHT-HP counterpart, especially for the compressive strength in the direction parallel to the layers, which is approximately 1 GPa higher for the former than the latter. This is mainly attributed to a good transitional role in properties through the transition layer and multiple-crack fracture mechanism in the intermetallic layer for CLT-HP MIL composite.

A Monte Carlo simulation method capable of replicating the kinetics of M₂₃C₆ precipitation in 347H stainless steels was developed for the purpose of producing synthetic microstructures that approximate its microstructural evolution under aging periods of up to 10,000 hours at temperatures between 600 °C and 750 °C. To accomplish this, experimental data from the literature was used to parameterize simulations and replicate the nucleation and growth kinetics of M₂₃C₆ particles within 347H and similar austenitic stainless steel alloys. These simulations were found to have considerable fidelity to previous efforts to study the precipitation of M₂₃C₆ in other 300 series stainless steel alloys. Synthetic 347H microstructures were then generated that accounted the effects of aging temperature, duration, dislocation density, and the presence of boron within the microstructure. These simulations predict several key trends, those being that (1) the size of M₂₃C₆ precipitates decreased with aging temperature and (2) the growth rate of M₂₃C₆ particles decreased with aging temperature. Further, while (3) the addition of dislocation density due to creep conditions resulted in increasing intragranular nucleation of M₂₃C₆ precipitates with increasing dislocation density and (4) B additions within the microstructure led to modest increases in precipitate size above 700 °C, which indicates that more complex physics are necessary to account for the presence of B.

The effects of an addition of 4 wt pct V on the mechanical properties and microstructure of a Ti–4Al–3Cu–2Fe (wt pct) alloy manufactured by extrusion of compacts of TiH₂/Al/Cu/Fe powder blend followed by vacuum annealing were investigated. It was found that the V addition changed the microstructure of the alloy from lamellar structure to basket-wave structure, increased the volume fraction of β phase from 47 to 53 pct, and reduced the average α lamella thickness significantly from 4.0 to 1.5 μm. Surprisingly, these compositional and microstructural changes cause only a small increase of the yield stress (from 1132 to 1151 MPa) and elongation to fracture (from 6.1 to 6.5 pct), but the strain hardening rate of the alloys are substantially enhanced over a narrow strain range of 0.9-1.7 pct, leading to a clear increase of the ultimate tensile strength from 1184 to 1252 MPa. The main mechanism for the microstructural changes caused by the V addition is the enhanced stabilization of β phase by V atoms and the growth restriction of α lamellae by V partitioning between α and β phases. The enhancement of strain hardening rate can be attributed to the enhance the number density of the α/β interfaces associated with the decrease of the α lamella thickness and which provides more effective barriers for the movement of dislocations.

The current research aims to develop a hybrid surface engineering process combining electrodeposition and ultrasonic shot peening (USP) to enhance surface-mechanical properties and coating strength. A thin nickel coating was deposited on the copper substrate through an electrodeposition process and the coating contained few microcracks and pits. The obtained coating was ultrasonically shot peened with different peening times and the number of peening shots (balls). The microstructural evolution, phase analysis, surface hardness, and scratch resistance of nickel coatings and peened coatings were characterized and discussed. The impact of peening based on duration and quantity of shots (ball), which induced grain refinement and compressive residual stress on the surface of the coating has been investigated. The multi-impact shots at a high velocity strike the coated surface and reduce the thickness of the electrodeposited nickel, creating better adhesion. The peening treatment resulted in enhancing the microhardness of the coated nickel from 123 Hv to 328 Hv. The tensile residual stress of coated nickel has been converted into compressive nature. The plastic deformation developed on the coated surface by USP and other factors lowered the coefficient of friction and enhanced the scratch resistance of the coating. Based on the result, it was established that USP has a broad and effective strengthening approach for the nickel coating deposited on soft substrate to increase its compactness and strength.

Graphical abstract

Essential understanding of the pearlite growth kinetics is of great significance to predict the lamellar spacing and the resultant mechanical properties of pearlitic steels. In this study, a series of eutectoid steels with Mn addition up to 2 mass pct were isothermally transformed at various temperatures from 873 K to 973 K to clarify the pearlite growth kinetics and the underlying thermodynamics at its growth front. The microscopic observation indicates the acceleration in pearlite growth rate and refinement in lamellar spacing by decreasing the transformation temperature or the amount of Mn addition. After analyzing the solute distribution at pearlite growth front via three-dimensional atom probe, no macroscopic Mn partitioning across pearlite/austenite interface is detected, whereas Mn segregation is only observed at ferrite/austenite interface. Furthermore, in-situ neutron diffraction measurements performed at elevated temperatures reveal that the magnitude of elastic strain generated during pearlite transformation is very small. Based on the thermodynamic model, these experimental results are used to estimate the contribution of various factors to the total energy dissipation. Compared with the Mn-free alloy, the retardation effect of Mn addition on pearlite growth kinetics, which is partly due to the reduced driving force for pearlite growth, can be well explained by further considering the solute drag effect of Mn.

A356 aluminum alloy has attracted enormous attention because of its excellent mechanical properties and good corrosion resistance. Refining the eutectic Si phase is required to improve the mechanical properties for practical applications. In this work, a new strategy is proposed by synergizing the permanent magnet stirring (PMS) with rare earth (RE) La and Ce elements simultaneously during the solidification of A356 aluminum alloy. Experimental studies reveal that the eutectic Si phase transforms from coarse needle-like and flake-like shapes to small granular morphology, and its average size reduces from 12.94 to 8.86 μm. It is noted that the additions of La and Ce can induce twins and stacking fault structures in the Si phase preventing its further growth. Meanwhile, the PMS can significantly refine the grain size of α-Al phase and generate more small size La–Ce precipitates at the front of Si phase. When PMS and rare earth are applied together, the electromagnetic force generated by PMS and the twins and stacking fault structures induced by rare earth elements La and Ce, collectively act on the A356 alloy to refine both the Si phase and α-Al phases. The refined alloy exhibits an outstanding combination of mechanical properties and wear resistance.

The recycling of scrap could cause continuous enrichment of Sn in steels. To reveal the influences of Sn on the mechanical properties and microstructure evolution of high-strength low-alloy (HSLA) steels, HSLA steels with varying Sn contents were designed and a series of continuous cooling tests were conducted. The results show that the effect of Sn on microhardness is mainly affected by the cooling rate. Under cooling rates lower than 10 °C/s, the addition of Sn increases the microhardness due to the solid solution strengthening effect of Sn. The ferrite grain size decreases with the increase of Sn content at a cooling rate of 0.1 °C/s because of the possible solute drag effect of Sn, while no refining effect was found for cooling rates between 0.5 °C/s and 10 °C/s. Under cooling rates higher than 10 °C/s, Sn reduces the ferrite and bainite transformation start temperatures and increases the bainite and martensite phase fractions. The higher hard phase fraction with increasing Sn content results in a significant increase in microhardness, and the contribution of solid solution strengthening plays a supplementary role under this condition. As one of the major residual elements in scrap, the strengthening and hardenability increasing effects of Sn should not be overlooked and may even be properly utilized in alloy design.

In this study, Cu–Zn alloys were deposited in citrate-based electrolytes on aluminum substrate by electrodeposition method. The effect of bath pH variation on the properties of the obtained Cu–Zn alloy coatings was investigated. The electrochemical behavior of the citrate-based baths and the crystalline structure, surface morphology and elemental content, electrical resistivity and thermal behavior of the alloy coatings were analyzed. According to the results of cyclic voltammetry (CV) analysis, increasing bath pH caused a negative shift in the cathodic deposition potential. In addition, the anodic dissolution peaks first shifted to the positive side with increasing pH and then shifted back to the negative direction. According to the results of XRD analysis, the phase structure of Cu–Zn alloys generally consists of α and β′ phases, but according to differential scanning calorimeter (DSC) analysis, it is possible that there is a γ phase in the structure in addition to these phases. In addition, pH increase (4.5 to 6.5) caused a relative increase in crystal grain size (~14 to ~ 25 nm). The Zn content of Cu–Zn coatings first increased (~pct 15 to ~ pct 55) with pH increase, then followed a horizontal trend (~pct 55 to ~ pct 59) with further pH increase and then exhibited a slight decreasing trend (~pct 59 to ~ pct 52). The pH increase significantly affected the surface morphology of the coatings and denser coatings were obtained with increasing pH. While the electrical resistivity of Cu–Zn coatings first increased (0.0408 to 0.0696 µΩcm for 297 K) with increasing pH, it tended to decrease (0.0696 to 0.0479 µΩcm for 297 K) again at higher pH values. In addition, the electrical resistivity of the coatings increased with increasing measurement temperature. According to DSC analysis of the coatings, endothermic peaks were obtained, possibly representing the transformation from γ to β′ phase.

Graphical Abstract

The magnetic behavior of two high-entropy alloys, CrMnFeCoNi and CrMnFeCoNi-CN, was investigated under varying degrees of deformation through uniaxial tensile tests. Microstructural, morphological, and crystalline structural analyses using XRD and SEM revealed a uniform and stable austenitic structure in all samples, with no presence of α’-martensite or ε-martensite phases. The main deformation mechanisms identified were twinning and slip dislocation for the CrMnFeCoNi-CN alloy, and slip dislocation for the CrMnFeCoNi alloy at room temperature. The alloys exhibited low magnetic moments attributed to magnetically frustrated configurations. At temperatures below 70 K, distinct magnetic states were observed ranging from paramagnetic to ferrimagnetic and spin-glass-like behavior. Antiferromagnetic interactions were confirmed by a negative paramagnetic Curie temperature for both alloys. The magnetization of the CrMnFeCoNi alloy increased with deformation, reflected in effective magnetic moments varying from 1.81 (0 pct) to 2.60 (20 pct) μB, while for the CrMnFeCoNi-CN alloy remained stable around 2.39 to 2.48 μB. The magnetization of the CrMnFeCoNi-CN alloy was found to be higher than that of the CrMnFeCoNi alloy, suggesting that the presence of C and N as alloying elements can enhance magnetization to some extent.