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During powder sintering, it is widely accepted that the three unfavorable features, i.e., closed-pore hollow powder, satellite powder, and low excess free energy, can affect the densification behavior; however, their specific influences have previously never been determined. In this work, we report the efficient elimination of the three unfavorable features for atomized spherical titanium powder via discharge plasma modification and further clarify their influences on the densification behavior during spark plasma sintering. Our results show that the activation energy of power-law creep for the modified powder is two times lower than that of the atomized counterpart.

Phase transformation is one of the essential topics in the studies on high entropy alloys (HEAs). However, characterization of the nucleation behavior in the phase transformation for HEAs is still challenging through experimental methods. In the present work, HfNbTaTiZr HEA was chosen as the representative material, and molecular dynamics/Monte Carlo (MD/MC) simulations were performed to investigate the nucleation behavior in temperature-induced BCC-to-HCP transformation for this HEA system. The results indicate that Nb–Ta, Ti–Zr, Hf–Zr and Hf–Ti atom pairs are preferred in the BCC solid solution of HfNbTaTiZr HEA and Hf–Ti–Zr-rich atom cluster with chemical short range order acts as the nucleation site for HCP phase. The nucleation process follows the non-classical two-step nucleation model: BCC-like structure with severe lattice distortion forms first and then HCP structure nucleates from the BCC-like structure. Moreover, at low temperature, the BCC-to-HCP nucleation hardly occurs, and the BCC solid solution is stabilized. The present work provides more atomic details of the nucleation behavior in temperature-induced BCC-to-HCP phase transformation for HEA, and can help in deep understanding of the phase stability for HEAs.

The emergence and development of high-performance materials have benefited from the revolution in modern manufacturing technology, in which additive manufacturing (AM) is the most representative over the last four decades [...]

Supercavitating vehicles, compared with other underwater vehicles, have amazing high speeds. However, the supercavitating vehicles have the unique tail-slap phenomenon in motion due to the lack of buoyancy and the existence of the planing force. In addition, there are large and unpredictable uncertainties in the hydrodynamics of the supercavitating vehicle because of the approximate understanding of this kind of mechanics. All these together impose big challenge to the controller design. In this paper, the longitudinal model of the supercavitating vehicles is adopted to carry out control investigation. A cascade control strategy is designed to coordinate depth control and avoid the planing force. In the outer-loop, the depth is controlled by the classical proportional-integral-derivative (PID) method; in the inner-loop, the control of longitudinal velocity and pitch rate use the linear active disturbance reject control (LADRC). The feasibility of the method is verified by mathematical simulation. The simulation results demonstrate the effectiveness and the superiority of the proposed strategy.

Chemical short-range order (CSRO) in high entropy alloys (HEAs) has attracted interests recently and is believed to be capable for tuning their mechanical properties. However, the characterization of CSRO in HEAs through experimental methods remains challenging. In this work, a modified embedded-atom method interatomic potential with good accuracy for studying CSRO in HfNbTaTiZr alloy system was developed. By employing the potential, molecular dynamic/Monte Carlo simulation was performed to investigate the CSRO in HfNbTaZr HEA. The results indicated that Hf-Zr and Nb-Ta atom pairs were preferred in the BCC solid solution of HfNbTaZr, and a new type of CSRO with topological B2 order was predicted, which can help to understand the mechanical properties of HfNbTaZr HEA. It was also found that forming of CSRO was an incubation process for the precipitation in HfNbTaZr, implying the significance of CSRO on the phase stability or precipitation behavior of HEAs. The findings in the present work can help in understanding CSRO and establishing its relationship with precipitates in HEAs, and more topics related to CSRO and phase stability in HfNbTaTiZr alloy system can be further investigated by atomistic simulation.

Liquid structure of Al-Si alloy is a long-standing issue. Using the newly-developed Al-Si potential, we studied the liquid structure of Al-10.3 wt% Si alloy by molecular dynamics simulation for the first time. We found that Al and Si atoms tend to be well mixed and the aggregation of Si-rich clusters is weak. The results exclude the possibility of regarding the aggregation of Si-rich clusters as the origin of the hysteresis in liquid Al-Si alloy. We didn't find obvious abrupt structure change in liquid Al-10.3 wt% Si near the melting point of Al.

An improved modified embedded-atom method (MEAM) potential for Si was optimized. The potential could quite well fit the potential energy curve and the elastic constants of diamond Si, the pair correlation function, the structure factor and the bond angle distribution function of liquid Si. The potential could also well fit the melting point and the solid-liquid phase transformation enthalpy. The potential could be used to study the solid-liquid transformation behaviors and the liquid structure of Si, and it could also be used to develop a MEAM potential for the binary Al-Si alloy.

Physical correlation between structural heterogeneity and plastic flow of metallic glasses (MGs) is crucial for the understanding of MGs' deformation mechanism. In this work, different degrees of structural heterogeneity are introduced into Zr-based MGs through different cooling rates casting. The effect of structural heterogeneity on serrated flow behavior was studied. The findings demonstrate that there exists a tendency that serration flow dynamics of the MGs transforms from a chaotic state to a self-organized critical state with increased inhomogeneity. The established correlation between structural heterogeneity and serrated flow behavior shows that the higher degree of structural heterogeneity facilitates a higher frequency of interaction and multiplication of shear bands by increasing nucleation sites, and then promotes serrated flow behavior to be more homogeneous in time and space, thereby induces transformation of dynamics behavior and improves the plasticity. The obtained results shed light on deformation mechanism of plastic flow and provide a new insight into the plasticity of MGs. (C) 2018 Elsevier B.V. All rights reserved.

With the addition of Al element, the microstructure of high entropy alloy AlxCoCrCuFeNi (x = 0.5, 1.5 and 3.0) changes from single FCC to FCC + BCC and then to single BCC phase. Hardness and elastic modulus of different phases in the HEAs are measured utilizing nanoindentation method. It is found that with the increase of the Al element, the hardness rises more significantly in the BCC phase than that in the FCC phase. The reason is attributed to that the Al element has a stronger solid-solution strengthening effect in the BCC phase, due to the larger elastic modulus mismatch. The precise characterization of local mechanical properties of different phases in the HEAs provides us opportunity to design alloys with desired performance.

The ambient-temperature brittleness of bulk metallic glasses (BMGs) is a long-standing problem in materials engineering. Herein, we report a route to overcome this Achilles’ heel of BMGs via high-pressure die casting (HPDC) at a high filling rate and pressure to create bi-continuous interpenetrating-phase composites. Our results show that large tensile plasticity of 5.1% can be achieved in the Zr-based BMG/stainless steel interpenetrating-phase composite prepared by HPDC, which is superior to most of as-reported ex-situ secondary phase reinforced BMG composites. The excellent mechanical properties of the BMG composite mainly originate from excellent metallurgical bonding between matrix and reinforcement as well as high efficiency suppression of shear band propagation by three-dimensional metal skeleton. Our findings provide a promising approach for developing cost-effective BMG composites suitable for industrial applications.

Borides have been widely used in cast TiAl alloy for grain refinement and a variety of stoichiometry and crystal structure of borides were reported. Here the effects of alloying elements Nb, Ta, and Mn on the structural stability of fine boride precipitates in TiAl alloys have been studied combining transmission electron microscopy (TEM) and first-principles calculations. The results show that most boride particles have the TiB stoichiometry. In the alloy containing Nb and Mn, all the TiB particles have the B27 structure and are highly enriched with Nb but depleted with Mn. In the alloy containing Nb and Ta, however, the intergrowth of B f and B27 structure has been observed, and the TiB particles are enriched with both Nb and Ta. First-principles calculations reveal different effects of Nb, Ta, and Mn on the structural stability of TiB polymorphs. Nb stabilizes B27 but destabilizes B f . Ta strongly stabilizes both B27 and B f structures. Mn strongly destabilizes both B27 and B f structures.

We report on the formulation of a factor, f , that when applied together with the activation energy for viscous flow ( Q ), can be used to provide important insight into the densification mechanism that are active during powder sintering. We ascertain the validity of this formulation by comparing the densification behaviour of atomized and milled powders for Ti-6Al-4V alloy and pure Ti during spark plasma sintering, and identifying the underlying mechanisms.

Two kinds of atom distribution in Mg-9wt.%Al melt, undissolved particles and atom aggregation, were investigated by synchrotron small-angle X-ray scattering (SAXS) and molecular dynamics simulation, respectively. The SAXS results indicated that no undissolved particles exist in Mg-9wt.%Al melt after short-time equilibration. The simulation results indicated that atom Mg and Al have a tendency to form bonds in Mg-9wt.%Al melt and the sizes of aggregated Al-centred clusters are within the characteristic scale of medium-range order; the degree of atom aggregation in Mg-9wt.%Al melt increases slightly with increasing temperature.

Large-sized industrial grade Zr55Cu30Ni5Al10 bulk metallic glass (BMG) was fabricated by a two-step arc-melting process and adding a trace of rare earth yttrium elements. A model to calculate critical cooling rate for forming BMG was established by considering the effect of pressure. Based on the model, the most important indicator, namely the critical size of forming BMG, for design and fabrication of BMG components by high pressure die casting (HPDC) is determined in consideration of different processing parameters, including the pressure and casting temperature. Theoretical calculation and numerical simulation indicated that increasing applied pressure during casting suppresses the nucleation of crystal nuclei but has little effect on the growth of nuclei at a deep supercooled temperature, thereby decreasing critical cooling rate and thus increase critical size. An increasing casting temperature would reduce critical size caused by more heat to dissipate. The critical size of the optimized Zr55Cu30Ni5Al10 BMG cast by using steel mold is determined to be about 4–7 mm under different HPDC parameters. Our study reveals that the forming ability of HPDC is large enough for fabricating low cost Zr-based BMGs with suitable size for applications.

In this paper, the output feedback path following problem for marine vessels is investigated with model uncertainties and external disturbance. A Serret-Frenet frame based path following kinematics is first introduced. For the output feedback control design, an iterative learning observer (ILO) is established to estimate the system states, and a novel sliding mode backstepping approach is presented, which makes the control gain parameters easy for tuning. The overall closed-loop system is uniformly ultimately bounded based on Lyapunov stability theory. Comparative simulation results validate the effectiveness and disturbance rejection performance of the proposed methodology.

For the electromagnetic transport (EMT) of liquid aluminum alloy during high pressure die casting (HPDC) by the plane induction electromagnetic pump (EMP), how to improve EMT efficiency and control EMT stationarity are key problems. Magnetic-flow coupling analysis was used to reveal effects of structural design and transport process parameters on EMT efficiency and stationarity. The output pump height of plane induction EMP was optimized by matching of iron core width W, coil width W′ and pump ditch width b, i.e., b values of bopt corresponding to 90% of the maximum output pump height, bopt/W = 1.27 and bopt/W′ = 1. Both EMT efficiency and stationarity are achieved under the optimum transport current 32 A. With the increase of the transport height from 350 mm to 500 mm, the EMT flow rate decreases from 4.28 kg/s to 3.59 kg/s, and the fillings of shot sleeve are always stationary. The transport tubes suffer a maximum positive pressure of 1.8 × 104 Pa and a minimum negative pressure of −1.42 × 104 Pa during EMT. For the liquid aluminum alloy soup occasions of 4.5 kg, 6.5 kg and 12.0 kg, the transport time could be shortened significantly from manipulator’s 16 s, 22 s and 38 s to EMT’s at most 2.195 s, 2.75 s and 4.28 s, respectively. The developed EMT process with plane induction EMP for HPDC is a process with low cost, high transport efficiency and stationarity.

Forming complex geometries using the casting process is a big challenge for bulk metallic glasses (BMGs), because of a lack of time of the window for shaping under the required high cooling rate. In this work, we open an approach named the &ldquo, entire process vacuum high pressure die casting&rdquo, (EPV-HPDC), which delivers the ability to fill die with molten metal in milliseconds, and create solidification under high pressure. Based on this process, various Zr-based BMGs were prepared by using industrial grade raw material. The results indicate that the EPV-HPDC process is feasible to produce a glassy structure for most Zr-based BMGs, with a size of 3 mm &times, 10 mm and with a high strength. In addition, it has been found that EPV-HPDC process allows complex industrial BMG parts, some of which are hard to be formed by any other metal processes, to be net shaped precisely. The BMG components prepared by the EVP-HPDC process possess the advantages of dimensional accuracy, efficiency, and cost compared with the ones formed by other methods. The EVP-HPDC process paves the way for the large-scale application of BMGs.

High-performance titanium alloys with an equiaxed composite microstructure were achieved by sintering and crystallizing amorphous powder. By introducing a second phase in a β-Ti matrix, series of optimized Ti–Nb–Fe–Co–Al and Ti–Nb–Cu–Ni–Al composites, which have a microstructure composed of ultrafine-grained and equiaxed CoTi2 or (Cu,Ni)Ti2 precipitated phases surrounded by a ductile β-Ti matrix, were fabricated by sintering and crystallizing mechanically alloyed amorphous powder. The as-fabricated composites exhibit ultra-high ultimate compressive strength of 2585 MPa and extremely large compressive plastic strain of around 40%, which are greater than the corresponding ones for most titanium alloys. In contrast, the alloy fabricated by sintering and crystallizing Ti–Zr–Cu–Ni–Al amorphous powder, which possesses significantly higher glass forming ability in comparison with the Ti–Nb–Fe–Co–Al and Ti–Nb–Cu–Ni–Al alloy systems, exhibits a complex microstructure with several intermetallic compounds and a typical brittle fracture feature. The deformation behavior and fracture mechanism indicate that the ultrahigh compressive strength and large plasticity of the as-fabricated equiaxed composites is induced by dislocations pinning effect of the CoTi2 or (Cu,Ni)Ti2 second phases and the interaction and multiplication of generated shear bands in the ductile β-Ti matrix, respectively. The results obtained provide basis guidelines for designing and fabricating titanium alloys with excellent mechanical properties by powder metallurgy.

Densification mechanism of Ti 40.6 Zr 9.4 Cu 37.5 Ni 9.4 Al 3.1 metallic glass (MG) powder with different heating rates during spark plasma sintering process was investigated using annealed crystalline counterpart powder as the object of reference. Results show that compared with single-stage densification process in the whole sintering process for the annealed crystalline counterpart powder, MG powder contains a two-stage densification process. Meanwhile, higher heating rate promotes obviously densification behavior for the both kinds of alloy powders. Further densification mechanism analysis based on Frenkel model indicates that the activation energy of the viscous flow for MG powder in the supercooled liquid region decreases with increasing heating rate, thus promoting densification behavior of the MG powder in the first stage. In contrast, for the annealed crystalline counterpart powder, the promoting densification behavior with increasing heating rate is attributed to the higher temperature gradient between the sample center and the edge. The variation in the viscosity of melt-solidified Ti 40.6 Zr 9.4 Cu 37.5 Ni 9.4 Al 3.1 bulk MG with temperature at different heating rates confirms the correctness of the obtained densification mechanism in our case. The results obtained can provide a new insight to understand the advantage of MG powder in fabrication of nano/ultrafine-grained titanium alloys with nearly full density.

Ultrafine grained Ti-based composites with equiaxed microstructure were fabricated by sintering and crystallizing from glassy powder precursors. The optimized composites exhibit ultimate strength of 2585 MPa and plasticity of exceeding 30%, occupying an unprecedented regime in the strength and ductility space of advanced titanium alloys. Dislocation pile-ups induced by precipitated phases and slip band propagation in the β-Ti matrix are the main reasons for the ultrahigh strength and plasticity respectively. The results provide an innovative route for fabricating titanium alloys with higher mechanical properties.

Nanocrystalline (NC) materials have fascinating physical and chemical properties, thereby they exhibit great prospects in academic and industrial fields. Highly efficient approaches for fabricating bulk NC materials have been pursued extensively over past decades. However, the instability of nanograin, which is sensitive to processing parameters (such as temperature and time), is always a challenging issue to be solved and remains to date. Herein, we report an ultrafast nanostructuring strategy, namely ultrasonic vibration consolidation (UVC). The strategy utilizes internal friction heat, generated from mutually rubbing between Ti-based metallic glass powders, to heat the glassy alloy rapidly through its supercooled liquid regime, and accelerated viscous flow bonds the powders together. Consequently, bulk NC-Ti alloy with grain size ranging from 10 to 70 nm and nearly full density is consolidated in 2 seconds. The novel consolidation approach proposed here offers a general and highly efficient pathway for manufacturing bulk nanomaterials.

Ultrafine-grained Ni50.2−xTi49.8Cux (x = 0, 2.5, 5, and 7.5) bulk shape memory alloys were fabricated by sintering of metallic glass (MG) powder and crystallization of amorphous phase. Non-isothermal crystallization kinetic analysis reveals that the crystallization mechanism of the synthesized x = 5 MG powder is typical interface-controlled two dimensional growth of nuclei followed by volume diffusion-controlled three dimensional growth of nuclei. In contrast, the crystallization mechanism of the synthesized x = 7.5 MG powder is typical volume diffusion-controlled three dimensional growth of nuclei in whole crystallization process. Correspondingly to different crystallization mechanisms, the two sintered and crystallized (SCed) bulk alloys have the same crystallized phases of bcc B2, fcc NiTi2 phases, and monoclinic B19′, but these phases display different morphologies and distributions. The SCed x = 5 bulk alloy has a microstructure of bcc B2 matrix surrounding fcc NiTi2 phase region, while the SCed x = 7.5 bulk alloy possesses discontinuous bcc B2 phase region. Consequently, the different crystallization mechanisms and microstructures causes extreme high yield strength and large plasticity for the SCed x = 5 bulk alloy and low strength and no plasticity for the SCed x = 7.5 bulk alloy. Especially, the yield strength of the SCed x = 5 bulk alloy is at least two times of that of the counterpart alloy prepared by melt solidification. The results provide a method fabricating high performance bulk alloys by tailoring crystallization mechanism using powder metallurgy.

Ti 64 Nb 12 Cu 11.2 Ni 9.6 Sn 3.2 bulk alloys were fabricated by consolidation of metallic glass (MG) powder synthesized by mechanical alloying and crystallization of amorphous phase. The Johnson–Mehl–Avrami–Kolmogorov method was employed to investigate non-isothermal crystallization kinetic of the synthesized MG powder. The average local Avrami exponent n determined from crystallization kinetic increases from 1.7 to 2.9 and then decreases to 1.8–2.0 when the crystallized volume fraction α is in the range of 0.1–0.3, and around 0.5 and 0.9, respectively. This indicates the corresponding crystallization mechanism is diffusion-controlled three-dimensional growth of nuclei with decreasing nucleation rate, volume diffusion-controlled three-dimension growth with increasing nucleation rate, and diffusion-controlled growth with decreasing nucleation rate, respectively. Meanwhile, the values of n imply that the nucleation rate increases with increasing heating rate. Microstructure analysis indicates that the fabricated bulk alloys mainly consist of three kinds of crystallized phases, β-Ti, (Cu, Ni)–Ti 2 , and Nb 3 Sn, but exhibit different microstructures and mechanical properties. The effect of the heating rate on the microstructure and mechanical properties can be explained based on different crystallization mechanisms. Especially, obvious plasticity is resulted from formation of ductile (Ti, Nb) 3 Sn from the Nb 3 Sn at relative high sintering temperature. The results obtained provide insight into fabrication of large-sized crystallized phase-containing bulk alloys with excellent mechanical property by powder metallurgy.

We report an equiaxed grained (EGed) structure in Ti 66 Nb 13 Cu 8 Ni 6.8 Al 6.2 bulk alloys with higher compressive mechanical properties, which was fabricated by the spark plasma sintering of milled alloy powders. Microstructural analysis indicates that all of the alloys consolidated from the alloy powders with different milling times consist of the same phases, β-Ti and (Cu, Ni)–Ti 2 , but exhibit different phase contents, morphologies, and distributions. The alloys consolidated from alloy powders that were milled for less than 30 h exhibit a typical bimodal microstructure with EGed (Cu, Ni)–Ti 2 regions surrounded by a Widmanstatten structure-like matrix that encompasses bcc β-Ti regions and the nanosized interleaving of plate-like fcc (Cu, Ni)–Ti 2 lamellae. In contrast, the alloys consolidated from alloy powders that were milled for more than 30 h have predominant EGed structures, i.e., EGed (Cu, Ni)–Ti 2 regions embedded in an EGed β-Ti matrix. With an increasing volume fraction of the EGed structure or a decreasing volume fraction of the Widmanstatten structure, the alloys exhibit an invariant compressive yield strength and increasing compressive fracture strength and strain. Fracture mechanism analysis based on TEM observations indicates that under compression stress, cracking occurs preferentially at the grain boundaries of soft EGed (Cu, Ni)–Ti 2 regions and at the interfaces between soft EGed (Cu, Ni)–Ti 2 and hard EGed β-Ti regions, while the penetration of the hard β-Ti regions due to cracking in the middle of the interleaving of the plate-like (Cu, Ni)–Ti 2 lamellae deteriorates the compression fracture strength and strain of the alloys with a Widmanstatten-structure matrix. The obtained results further confirm a promising method for fabricating fine EGed bulk titanium alloys with excellent mechanical properties by powder metallurgy.

Fe79.3Mo4.5P8.1C6.75B1.35 amorphous alloy composite powder from respective element powders of Fe, Mo, C, B, and Fe–P intermediate compound, was synthesized by mechanical alloying. Microstructure evolution analysis indicates that the synthesized amorphous alloy composite powder after a milling time of 70 h encompasses predominately amorphous matrix embedded by nanocrystalline α-Fe with a grain size of about 5.5 nm. However, unlike other Fe-based amorphous alloys, the synthesized amorphous alloy composite powder exhibits no obvious supercooled liquid region with only crystallization temperature. The corresponding crystallization onset temperature and exothermic enthalpy measured from DSC curves are about 762 K and 15.86 J/g, respectively. The results obtained provide good candidate materials for fabricating bulk metallic glass composites and related bulk nanocrystalline materials.

High-strength Ti66Nb13Cu8Ni6.8Al6.2 composites with intermetallic matrix reinforced by nanocrystalline WC particles were fabricated via spark plasma sintering and crystallization of amorphous phase. Microstructure analysis indicates that the bulk composites contain isolated nanocrystalline WC particles surrounded by crystallized intermetallic matrix, including ductile bcc β-Ti regions surrounded by a mixed-phase region of (Cu, Ni)-Ti2, TiC, and remaining amorphous phase. With increasing sintering temperature the crystallized intermetallic matrix transforms from nanocrystalline to an ultrafine-grained structure. The composite with ultrafine-grained intermetallic matrix reinforced by 4.5 vol.% WC exhibits high yield and fracture strengths of 2194 and 2277 MPa, respectively. The variation of fracture strength for the fabricated composites can be explained based on the volume fraction of the crystallized ductile β-Ti phase and the scale of the crystallized phase regions.

The multidimensional finned surface can greatly improve the capability of separation, extraction, absorption and heat transfer, so it is wildly applied to modern environmental protection, petrochemical industry, medical industry, and micro-electronics cooling, etc. By ploughing-extrusion process, this paper obtained a novel micro-groove array structure composed of micro-grooves, primary fins and compound fins, which have macro-structure, meta-structure, and micro-structure. The depth of micro groove is about 200 μm, and the height of the primary fin is about 80 μm. The height of the compound fins is larger than 100 μm, and the space of two compound fins is about 400 μm. In this process, the metal undergoes plastic deformation and turns into primary fins. Owing to the friction stress between the extrusion surfaces and the metal surpass the break limit of the metal, some regions of the primary fin undergo ductile fracture and turn into denticular compound fins. The whole process can be divided into four steps, i.e. splitting, extruding, frictional tearing and fin forming. The four stages are continuous and happen at the same time. The novel finned micro-groove array structure can be manufactured in a single procedure.

It is well known that semi-solid forming could only obtain coarse-grained microstructure in a few alloy systems with a low melting point, such as aluminum and magnesium alloys. This work presents that semi-solid forming could also produce novel bimodal microstructure composed of nanostructured matrix and micro-sized (CoFe)Ti2 twins in a titanium alloy, Ti62Nb12.2Fe13.6Co6.4Al5.8. The semi-solid sintering induced by eutectic transformation to form a bimodal microstructure in Ti62Nb12.2Fe13.6Co6.4Al5.8 alloy is a fundamentally different approach from other known methods. The fabricated alloy exhibits high yield strength of 1790 MPa and plastic strain of 15.5%. The novel idea provides a new insight into obtaining nano-grain or bimodal microstructure in alloy systems with high melting point by semi-solid forming and into fabricating high-performance metallic alloys in structural applications.