Because of having better corrosion resistance properties than crystalline uranium alloys, U-based metallic glasses show strong potential of applications in nuclear fields. U-Co and U-Fe are U-based important base glass systems, from which almost all the reported multi-component U-based amorphous alloys derive. However, which system possessing higher glass forming ability is unclear yet. Therefore, the relationship between the glass formation and the solidification rate is studied on two glassy alloys U66.7Co33.3 and U69.2Fe30.8 in this work, which are the best glass former in the corresponding system. A series of amorphous samples were prepared by modifying the cooling rate of their melts, and then were measured by using XRD and calorimetric analysis technique. The results show that both alloys were able to nearly amorphize completely at higher cooling rate, and tended to segregate U6Mn-typed crystalline phase when the cooling rate declined to some extent. In contrast, the U-Fe alloy needs a much lower critical cooling rate to achieve fully amorphous structure, directly demonstrating that U-Fe system possesses stronger glass forming capacity than U-Co. The reason for this conclusion is that the former system is of both thermodynamic and kinetic advantages for glass formation. This result can be applied as the foundation to exploit superior novel multicomponent U-based amorphous alloys.
Mo in the form of solid solution atom or compound phase is distributed in CoCrFeMnNi high entropy alloy, which has the effect of solution strengthening or second phase strengthening. The method of annealing was used to heat treated (CoCrFeMnNi)97.02Mo2.98 high entropy alloy to investigate effects of σ phase on mechanical properties of (CoCrFeMnNi)97.02Mo2.98 high entropy alloy. SEM, EDS and XRD were used to analyze effects of annealing temperature on precipitation σ phase (CrMo phase) in (CoCrFeMnNi)97.02Mo2.98 high entropy alloy. The mechanical properties were tested by microhardness and tensile test, and the influencing mechanism of σ phase on the mechanical properties was investigated. The results show that with increase of the annealing temperature, the quantity of precipitation σ phase increases in (CoCrFeMnNi)97.02Mo2.98 high entropy alloy, and the σ phase is first precipitated at the grain boundary, and is after precipitated in intracrystalline. The morphologies of σ phase at the grain boundary are changed gradually from tiny strips of discontinuous distribution to thick strip of continuous distribution. With the annealing temperature increases further, the morphologies of σ phase are changed from strip of continuous distribution to granular of continuous distribution. The precipitation σ phases in (CoCrFeMnNi)97.02Mo2.98 high entropy alloy by annealing have the effect of second phase reinforcement, with the annealing temperature increase, the numbers of precipitation σ phase increase, and the hardness and strength both increase, which is obviously at temperature higher than 900 ℃. The σ phase precipitation in intracrystalline, and its refinement, can improve the strength and plasticity of (CoCrFeMnNi)97.02Mo2.98 high entropy alloy synchronously.
Amorphous steels exhibit ultra-high strength but room-temperature brittleness and strain-softening behavior as loading, which restricted the application of amorphous steels as high-performance structural material. Developing in situ crystals is an effective way to toughen the amorphous alloys. However, the crystals may sacrifice the corrosion resistance of amorphous steels. In this work, austenite and ferrite duel phases were introduced to the amorphous phase, via transformation induced plasticity (TRIP) of the austenite as loading, to enhance the ductility and improve the work-hardening behavior; and via the synergy of ferrite and amorphous phase to ensure the corrosion resistance. A novel amorphous steel Fe-15Mn-5Si-14Cr-0.2C was fabricated by magnetic suspension melting in a water-cooled copper crucible, and negative pressure suction casting into a copper mold. The microstructure and mechanical properties of the amorphous steel were characterized by XRD, EBSD and the electronic universal testing machine. The corrosion behavior in artificial seawater was studied on an electrochemical work station with a three-electrode system, and the corrosion morphology and corrosion products were characterized by SEM with EDS analysis. The results showed that the as-cast amorphous steel consisted of the amorphous matrix, CFe15.1 super-cooled austenite and Fe-Cr ferrite phases. From surface to inner, amorphous phases mainly exist in the margin, while crystalline phases are abundantly distributed in the center. The amorphous steel exhibited excellent comprehensive mechanical properties at room temperature, and its yield strength, fracture strength and plastic strain were up to 978 MPa, 2645 MPa and 35.8%, respectively. In artificial seawater, compared with 304 stainless steel, the amorphous steel showed high self-corrosion potential, low self-corrosion current density and high polarization resistance, large resistance arc radius, only one high frequency resistance arc and low corrosion kinetic rate. Moreover, the stable and dense passivation film was observed on the corrosion surface. Their excellent corrosion resistance and mechanical properties endow the amorphous steel with the potential to become a novel corrosion-resistant structural material for marine engineering.
In recent years, high-entropy alloys have triggered broad research interests due to their unique and intriguing mechanical properties. In general, the increase in strength is accompanied by the reduction in ductility. Therefore, strong and ductile metallic materials have always been pursued by metallurgist. Heterogeneous structure has been reported to be very useful for overcoming the strength-ductility trade-off in metallic materials. In this work, typical partially recrystallized structure has been obtained in CoCrFeNiMo0.2 high-entropy alloy by cryogenic rolling and annealing. The effect of partially recrystallized structure on the mechanical properties has been studied. After 35% cold rolling (RTR35%) and 35% cryogenic rolling (CTR35%) and annealed at 800 ℃ for 30 min, CoCrFeNiMo0.2 high-entropy alloys developed partially recrystallization microstructures featured by coarse deformed grains and fine recrystallized grains. The yield strength of the CTR35% sample is 539.3 MPa and its elongation is 46.8%, which is similar in strength but 30% higher in elongation when compared with the RTR35% sample. This can be understood from the fact that samples rolled at cryogenic temperature showed a higher volume fraction of fine recrystallized grains, resulting in better strain hardening capability.
Zr-based amorphous alloys are characterized by high glass forming ability, high thermal stability and excellent mechanical properties. The amorphous alloys in thermodynamic metastable state have the tendency to change to metastable state with lower energy or even crystal structure in equilibrium state under certain temperature or pressure conditions. At present, few researches have been conducted on the mechanical behavior of partially crystallized Zr-Cu-Ni-Al-Nb amorphous alloys, especially the fracture behavior under dynamic loading. In this work, as-cast Zr-Cu-Ni-Al-Nb amorphous alloy was annealed to accomplish different levels of nano-crystallization by controlling holding time. DSC, XRD, HRTEM, SEM, quasi-static and dynamic compression tests were utilized to research the effect of nano-crystallization on compressive strength and fracture mechanism of Zr-based amorphous alloy under different strain rates. The results indicated that the volume fraction and size of nanoscale crystalline phase inside Zr-based amorphous alloy increased with the increasing of annealing holding time. The compressive strength of annealed Zr-based amorphous alloy increased first and then decreased with the increase of holding time. The variation of strain rates also affected the compressive strength, which decreased when the strain rate increased from 1×10-3 s-1 to 1×103 s-1, and increased when the strain rate continually increased to 3×103 s-1. Different degrees of nano-crystallization had an impact on the fracture characteristics of Zr-based amorphous alloy. As the degree of crystallization increased, the fracture morphology of compression samples changed from vein-like patterns to quasi-cleavage features and then to river patterns.
Laser additive manufacturing technology is a feasible technology for the fabrication of bulk metallic glass with complex geometry. It has the characteristics of small molten pool and high cooling rate. However, crystallization often occurs in heat affected zone (HAZ). In this work, laser multiple remelting of Zr55Cu30Al10Ni5 metallic glass by pulsed laser was carried out and the morphological evolution of the HAZ crystalline phase in the multiple remelting process was studied. The results show that with the increase of the remelting times, the crystalline grains number and size are both improved. With the growth of the grains, the crystallization caused by the growth of the crystalline grains in the molten pool also becomes more and more remarkable. Both the size and number of the grains in the HAZ increase linearly with the increase of the remelting times. The nucleation rate and growth rate of different metallic glass plates are close, whereas the initial crystalline grains number and size are different, which are attributed to the different cooling process in the copper casting of the metallic glass plates.
As a new alloy design concept, the high-entropy alloy (HEA) and the formation of simple solid solution introduce excellent properties such as high hardness, high strength and corrosion resistance. Investigations have shown that the single solid solution CrCoFeNi alloy possesses good corrosion resistance. The addition of Mo is beneficial to the corrosion resistance of the HEAs for potential industrial applications in 3.5%NaCl (mass fraction) simulating seawater type environments. The major effect of Mo is to promote the pitting potential of the alloy and inhibit the dissolution of the passivation film by forming and retaining molybdenum oxyhydroxide or molybdates (MoO42-). Considering that the cost of pure Co is higher, Ni and Co elements have similar atomic size and valence electron concentration, and the corrosion resistance of pure Ni is higher than that of pure Co, Ni2CrFeMox HEA was designed by replacing Co element with Ni element in CoCrFeNiMox HEA. As the Mo content increases in the Ni2CrFeMox HEAs, the interdendrite is a Cr and Mo rich σ phase, and the dendrite is a Cr and Mo depleted fcc phase. The potential difference between interdendrites and dendrites leads to galvanic corrosion, which accelerates the localized corrosion of alloys. Here, a solution heat treatment process is selected to reduce the precipitation phase and improve the corrosion resistance of the alloy. The effects of Mo element and heat treatment on the corrosion resistance of Ni2CrFeMox HEA in 3.5%NaCl solution were tested. The results show that the corrosion resistance of as-cast Ni2CrFeMox HEA is obviously higher than that of 316L stainless steel. The Ni2CrFeMo0.2 alloy has the best corrosion resistance because of its minimum dimensional passive current density and corrosion current density. However, the addition of excessive Mo leads to the precipitation of σ phase and galvanic corrosion, which reduces the corrosion resistance of the alloy. After solution treatment, the uniformity of alloy structure and element distribution weakens galvanic corrosion, and the corrosion resistance is obviously improved.
Room-temperature brittleness and strain-softening during deformation of bulk metallic glasses, and limited processability of shape memory alloys have been stumbling blocks for their advanced functional structural applications. To solve the key scientific problems, a new shape memory bulk metallic glass based composite, through the approach using transformation-induced plasticity (TRIP) effect of shape memory alloys to enhance both ductility and work-hardening capability of metallic glasses, and superplasticity of bulk metallic glass in supercooled liquid region to realize near net forming, was developed in this work. And the Ti-Ni base bulk metallic glass composites (BMGCs) rods were prepared by the levitation suspend melting-water cooled Cu mold process. Microstructure, thermal behavior, mechanical properties and high temperature deformation behavior of the alloy were investigated. The results show that the as-cast alloy microstructure consists of amorphous matrix, undercooled austenite and thermally-induced martensite. Besides, the size of the crystal phase precipitated on the amorphous matrix increases from the surface to the inside. The alloy exhibits excellent comprehensive mechanical properties at room temperature. The yield strength, fracture strength and the plastic strain of alloy are up to 1286 MPa, 2256 MPa and 12.2%, respectively. Under compressive loading in the supercooled liquid region, the composite exhibits approximate Newtonian behavior at lower strain rate in higher deformation temperature, and the optimum deformation temperature is T>480 ℃ and the intersection part with supercooled liquid region (SLR). When the temperature is 560 ℃ and the strain rate is 5×10-4 s-1, the stress sensitivity index m and the energy dissipation rate ψ are 0.81 and 0.895, respectively. Furthermore, the volume of activation is quantified to characterize the rheological behavior.
A new alloy design concept, high-entropy alloys (HEAs), has attracted increasing attentions and becomes a new research highlight recently. Different from traditional alloy design strategy which usually blends with one or two elements as the principal constituent and other minor elements for the further optimization of properties, HEAs are multicomponent alloys containing several principle elements (usually ≥5) in equiatomic or near equiatomic ratio. Due to their unique atomic structure, HEAs possess a lot of distinguished properties. Since the discovery of HEAs, a variety of HEA systems have been developed and shown unique physical, chemical and thermodynamic properties, especially the promising mechanical properties such as high strength and hardness, abrasion resistance, corrosion resistance and softening resistance. Here in this short review manuscript, starting from the research challenges for understanding the deformation mechanism of HEAs, this work briefly summarized the mechanical properties and deformation behavior of HEAs, reviewed the proposed strengthening-toughening strategies and their corresponding deformation mechanism in HEAs. A brief perspective on the research directions of mechanical behavior of HEAs was also proposed.
Metallic glasses (MGs) have disordered microstructure and no defects like in crystalline materials and possess a suite of outstanding mechanical and functional properties, showing thus promising potential for wide applications. Due to the lack of long range structural order, it is fraught with difficulties to construct the structure-property relationship in amorphous materials. The study of relaxation dynamics provides a very important approach to understand MGs, and is vital to understand their stability and deformation behavior and remains a core issue in the field of condensed matter physics and materials science. In recent years, with the use of more advanced research methods and the deepening of research, it was found that there exists rich dynamics covered by the extremely wide time scale and the different length scales of glassy state. Different dynamic modes not only correlate with each other but also show distinction. This article reviews recent progress in the study of relaxation dynamics in MGs, and its role in understanding and modifying material properties and optimizing material preparation.
It is easy to corrode the steel structural materials. In view of this problem, the Al-Ni-Zr amorphous and nanocrystalline composite coating with high amorphous volume was prepared by high velocity arc spraying on the 45 steel. The microstructure, macroscopic corrosion performance and microzone corrosion performance of the composite coating was investigated. XRD, SEM with EDS and TEM were applied to confirm that the gray zone of the composite coating microstructure was the amorphous enrichment zone. It was found by the scanning Kelvin probe microscopy (SKPM) that the corrosion failure order of each phase of the composite coating was arranged in order of the aluminum rich phases, the oxidation phases and the amorphous phase. The microhardness of the composite coating was about 364 HV0.1 which was greater than that of 45 steel. The EIS fitting results showed that the charge transfer resistance of the composite coating is 2~4 times of the aluminum coating and 45 steel. It has two time constants in the spectrum. The corrosion failure behavior of the composite coating in the low frequency was controlled by the diffusion process, which was related to the accumulation and diffusion of the corrosion products. The potentiodynamic polarization curves fitting results indicated that the self-corrosion potential of the composite coating was higher than those of the aluminum coating and 45 steel. And the self-corrosion current density of the composite coating was about 1.08 μA/cm2, which was 7/100 and 1/3 of that of the aluminum coating and 45 steel, respectively. According to the corrosion morphology of the composite coating, there was no obvious pitting. A large number of NaCl crystals were attached to the surface of the aluminum rich phase region as the preferred corrosion zone. But the surface of the amorphous enrichment zone was smooth. At the same time, the corrosion pits, micro-cracks and pitting enrichment occurred on the surface of the composite coating, which was mainly related to the effects of Cl- erosion and swelling.
High entropy alloys (HEAs) origin from a new alloy design concept with multi-principal elements, which have attracted significant interests in the past decade. The high configurational entropy in HEAs results in simple solid solutions with fcc and bcc structures. Especially, the single solid solution CoCrFeNi alloy exhibits excellent properties in many aspects, such as mechanical properties, thermal stability, radiation resistance and corrosion resistance. The excellent corrosion resistance of CoCrFeNi alloy is ascribed to the single-phase structure and uniform element distribution coupled with much higher Cr content than stainless steel. The single-phase structure and uniform element distribution can prevent the occurrence of localized corrosion, and higher Cr content can protect the alloy surface better with the form of oxidation film. Moreover, the corrosion resistance of CoCrFeNi-based HEAs, such as CoCrFeNiAlx, CoCrFeNiCux, CoCrFeNiTix, have also been extensively investigated. In most CoCrFeNi-based HEAs, the elements of Co, Cr, Fe and Ni are with equal-atomic ratio. However, the equal-atomic ratio is not necessary to obtain satisfactory properties and to ensure the single fcc structure in Co-Cr-Fe-Ni system. Accordingly, it is essential to further consider the effect of alloying elements on the corrosion resistance in Co-Cr-Fe-Ni HEA. In this work, the effect of Co, Fe and Ni elements on the corrosion resistance of single fcc Co-Cr-Fe-Ni system with concentrated constitution but different atomic ratios in 3.5%NaCl solution are investigated by using LSCM and EIS. The potentiodynamic polarization results indicate that the increase of Fe and the decrease of Ni will decrease the passivation current density of the alloys when the Co and Cr contents are equal. With the increase of Co and the decrease of Ni, the alloys show smaller passivation current density and better corrosion resistance when the Fe and Cr contents are equal. With the decrease of Co and the increase of Fe and Ni, the alloys show higher corrosion potential and smaller corrosion tendency when the Cr content is constant. These results will be helpful for the design of corrosion resistant HEAs in NaCl aqueous solution.