Search

We present detailed investigations of structural and static/dynamic magnetic properties of hydrogenated hcp-Co8057Fe4Ir16 soft magnetic thin films. Two different kinds of defects, i.e., destructive and non-destructive, were demonstrated by controlling the negative bias voltage of the hydrogenation process. Our results show that the structure and magnetic properties of our sample can be tuned by the density of the induced defects. These results provide better understanding of the hydrogenation effect and thus can be used in the future for materials processing to meet the requirements of different devices. [ABSTRACT FROM AUTHOR]

In present work, a novel Co-free (CrFeNi 2) 100- x Y x (x = 0, 3, 6, 9) medium entropy alloys (MEAs) were successfully prepared by vacuum arc-melting method. The results revealed that the addition of rare-earth element Y had a great effect on the phase composition of CrFeNi 2 MEA, which was transformed from the single FCC phase structure to the duplex structure of the FCC and HCP (YNi 5 type) phases. Meanwhile, the phase evolution of the current alloy system was evaluated using the previous criteria (ΔH mix - δ , Ω - δ , Δ χ , VEC and Λ). In addition, the volume fraction of the HCP phase increased significantly with the increasing Y content, which caused the obvious grain refinement. On the other hand, the nanoindentation test results revealed that the nanohardness of the HCP phase (∼10.7 GPa) was more than three times of the FCC phase (∼3.4 GPa). The dislocation nucleation types of the FCC phase (3.68 GPa) and the HCP phase (6.46 GPa) were discussed by calculating their maximum initial plastic shear stress. With the increasing Y content, both the hardness and strength of the alloy were significantly improved, but the plasticity deteriorated. The second phase strengthening played a dominant role in the strength improvement of (CrFeNi 2) 100- x Y x MEAs, while both the solid solution strengthening and grain boundary strengthening contributed slightly to the strength increment. • The expensive Co elements were replaced with atoms by the lower-cost Ni element to reduce costs. • Phase precipitation behavior and mechanical properties of (CrFeNi 2) 100- x Y x (x = 0,3,6,9) MEAs were investigated for the first time. • The differences in mechanical properties and deformation behavior of FCC and HCP phases were explored. • The yield strength and hardness of (CrFeNi 2) 100- x Y x (x = 9) MEAs were increased to 1265 MPa and 485HV, respectively. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Ultrafine-grained Ti/AZ31 magnesium matrix composites were prepared by powder metallurgy. • AZ31 alloys distributed uniformly by submicron/nanometer sized titanium were obtained. • The strengthening mechanisms for Ti/AZ31 composites at room temperature were analyzed quantitatively. • Outstanding strength at elevated temperature of ultrafine-grained Ti/AZ31 composites was confirmed experimentally. Abstract The microstructure of ultrafine grain for magnesium alloys can result in drastic enhancement in their room temperature strength, but the issue of low strength at elevated temperature becomes more serious as well due to grain boundary slide. Here ultrafine-grained Ti/AZ31 magnesium matrix composites with high strength at both room and elevated temperature were prepared by vacuum hot pressing and subsequent hot extrusion. The microstructure of the composite samples before and after consolidation processing was characterized, and the mechanical properties of the as-consolidated bulk samples were measured at room and elevated temperatures. The results indicate that after extrusion ultrafine-grained magnesium alloys were obtained and Ti particulates with particulate size of ∼310 nm disperse in Mg matrix. The magnesium grain of AZ31-15at.%Ti grows from 66 nm to 800 nm. Meanwhile, the relative densities of Ti/AZ31 composites are higher than 99%. The yield strength (YS) of extruded AZ31-15at.%Ti composite at room temperature is 341 MPa, being 2.4 times higher than original AZ31 alloy. Theoretical estimation shows that remarkably enhanced room-temperature mechanical strength attributes to grain boundary strengthening with the contribution ratio of 74%. In addition, the peak stress of extruded AZ31-15at.%Ti composite at 573 K is 82 MPa and ultrafine Ti dispersions are responsible for the enhanced strength. [ABSTRACT FROM AUTHOR]

The nanocrystalline AZ31 magnesium (Mg) alloy with 18 wt% titanium (Ti) addition was isothermally annealed in the temperature range of 300–400 °C for different durations. A study on the thermal stability, including solid solubility evolution, phase transformation, grain growth kinetics and hardness evolution, was carried out. The microstructure evolution at elevated temperatures was investigated by the means of X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and high angle annular dark field (HAADF). Based on lattice parameters evolution and TEM observation, it was confirmed that the Ti element precipitated from Mg matrix as Ti phase. After the material being annealed at 400 °C for 300min, as to the original Ti particulates, there is no segregation and growth with the average particulate size being approximately 367 nm, meanwhile, magnesium matrix shows limited grain coarsening with the average grain size increasing from 95 nm to 210 nm. In addition, the grain growth kinetics could be well described by the kinetics equation, D 7 − D 0 7 = k t . The activation energy for grain growth was calculated to be 120 kJ/mol, which is about 28 kJ/mol greater than that of pure Mg. Simultaneously, the stability of mechanical property was investigated by microhardness evolution at different annealing temperatures. It was found that after this material being annealed at 400 °C for 300min, the microhardness remained 105HV, which is two times larger than that of the as-cast AZ31 Mg alloy. [ABSTRACT FROM AUTHOR]

In this work, vanadium particles (VP) were utilized as a novel reinforcement of AZ31 magnesium (Mg) alloy. The nanocrystalline (NC) AZ31–VP composites were prepared via mechanical milling (MM) and vacuum hot-press sintering. During the milling process, the presence of VP contributed to the cold welding and fracture mechanism, resulting in the acceleration of the milling process. Additionally, increasing the VP content accelerated the grain refinement of the matrix during the milling process. After milling for 90 h, the average grain size of AZ31-X wt % Vp (X = 5, 7.5, 10) was refined to only about 23 nm, 19 nm and 16 nm, respectively. In the meantime, VP was refined to sub-micron scale and distributed uniformly in the matrix, exhibiting excellent interfacial bonding with the matrix. After the sintering process, the average grain size of AZ31-X wt % VP (X = 5, 7.5, 10) composites still remained at the NC scale, which was mainly caused by the pinning effect of VP. Besides that, the porosity of the sintered composites was no more than 7.8%, indicating a good densification effect. As a result, there was little difference between the theoretical and real density. Compared to as-cast AZ31 Mg alloy, the microhardness of sintered AZ31-X wt % VP (X = 5, 7.5, 10) composites increased by 65%, 87% and 96%, respectively, owing to the strengthening mechanisms of grain refinement strengthening, Orowan strengthening and load-bearing effects. [ABSTRACT FROM AUTHOR]

The influence of electropulsing treatment (EPT) parameters on the static recrystallization (SRX) microstructure in a cold-deformed Ni-based superalloy was investigated. During EPT, both the volume fraction of SRX grains and the average grain size increased with the increasing EPT temperature, which was attributed to the thermal effect and athermal effect induced by EPT. The mobility of SRX grain boundaries was promoted at the higher temperature due to the thermal effect, while the nucleation rate would be increased by EPT through decreasing the thermodynamic barrier. The formation of parallel dislocations caused by electron wind force could also play an indirect role in promoting SRX process. Moreover, the volume fraction of SRX grains increased significantly with the extension of EPT time at 700 °C, while the EPT time had a trivial effect on the average grain size. In addition, the sufficient deformation was essential to the occurrence of SRX behavior during EPT, and the localized Joule heating effect could promote the SRX behavior in the samples with the larger strains. Besides that, the influence of twining and carbides on the SRX behaviors was also investigated. [ABSTRACT FROM AUTHOR]

In this work, nanocrystalline AZ31 magnesium alloy powders, reinforced by submicron TiB2 particles, were prepared via mechanical milling. It was found that TiB2 particles stimulated the fracture and welding of AZ31/TiB2 powders, leading to the acceleration of the milling process. Meanwhile, the TiB2 particles were refined to submicron-scale size during the milling process, and their distribution was uniform in the Mg matrix. In addition, the matrix was significantly refined during the milling process, which was also accelerated by the TiB2 particles. The formation of grain boundary segregation layers also led to the weakened TiB2 peaks in the XRD patterns during the mechanical milling. The grain sizes of AZ31–2.5 wt % TiB2, AZ31–5 wt % TiB2 and AZ31–10 wt % TiB2 powders were refined to 53 nm, 37 nm and 23 nm, respectively, after milling for 110 h. Under the combined effect of the nanocrystalline matrix and the well-dispersed submicron TiB2 particles, the AZ31/TiB2 composites exhibited excellent micro-hardness. For the AZ31–10 wt % TiB2 composite, the micro-hardness was enhanced to 153 HV0.5. [ABSTRACT FROM AUTHOR]

By introducing nanoscale disordered interfaces into the ordered superlattice grains can achieved a combination of ultra-high strength and excellent ductility in Ni 3 Al-type high-temperature alloys, but limited by the complex preparation process. In the present work, a Ni 3 (Al, Ti)-type multicomponent alloy of Ni 43.9 Co 22.4 Fe 8.8 Al 10.7 Ti 11.7 B 2.5 was successfully fabricated using laser metal deposition (LMD) technique. The microstructural evolution, tensile properties and strengthening mechanism were investigated. Microstructural and phase analyses of the sample reveale the presence of dendritic γ + γ′ phases, γ′ envelopes and interdendritic γ + γ′ phases in the alloy. The LMD prepared Ni 43.9 Co 22.4 Fe 8.8 Al 10.7 Ti 11.7 B 2.5 alloy exhibits superior mechanical property at both room temperature and elevated temperature. Compared to Ni 3 Al-type intermetallic alloys, the alloy exhibited excellent strength-ductility synergy at room temperature. In addition, the alloy's yield strength rises as temperature rises from 298 K to 973 K, reaching a maximum value of 1043 MPa at 973 K. The strength of the prepared Ni 43.9 Co 22.4 Fe 8.8 Al 10.7 Ti 11.7 B 2.5 alloy is mainly contributed by lattice friction strengthening, solid solution strengthening, precipitation strengthening and the inherent yield strength of γ′ envelopes with precipitation strengthening, and the inherent yield strength is the key factor. This work provides insight into the preparation of Ni 3 (Al, Ti)-type multicomponent alloy with superior mechanical property at both room temperature and elevated temperature by LMD. [ABSTRACT FROM AUTHOR]

The hot deformation behavior of a nickel-based superalloy was investigated by means of isothermal compression tests in the strain rate range of 0.001–10 s −1 at 1110 °C. Transmission electron microscope (TEM) and electron backscatter diffraction (EBSD) technique were used to study the effect of strain rate on the microstructure evolution of the alloy during hot deformation. The results revealed that the dynamic recrystallization (DRX) process was stimulated at high strain rates ( ε ̇ ⩾ 5 s - 1 ) due to the high dislocation density and adiabatic temperature rise. Meanwhile, high nucleation of DRX and low grain growth led to the fine DRX grains. In the strain rate rage of 0.001–1 s −1 , the volume fraction of DRX grains increased with the decreasing strain rate, and the grain growth gradually governed the DRX process. Moreover, the strain rate has an important effect on DDRX and CDRX during hot deformation. On the other hand, particular attention was also paid to the evolution of twin boundaries during hot deformation. It was found that there was a lower fraction of Σ 3 boundaries at the intermediate strain rate of 1 s −1 , while the fractions of Σ 3 boundaries were much higher at both the lower strain rates ( ε ̇ ⩽ 0.1 s - 1 ) and higher strain rates ( ε ̇ ⩾ 5 s - 1 ). [ABSTRACT FROM AUTHOR]

Nanocrystalline Ti/AZ61 magnesium matrix composite, prepared by mechanical milling, was annealed in the temperature range of 573–723 K for various durations. During annealing treatment, diffusion of Al and Ti elements, formation of precipitates and grain growth behavior were observed and the evolution of hardness was obtained. Based on microstructural evolution, the mechanism for the precipitating of Ti and Al from magnesium matrix was analyzed. And the corresponding schematic illustration of microstructural evolution model was proposed. By the observation from high-resolution transmission electron microscopy, it was found that dispersing nanometer sized particulates, precipitated from Ti and Al supersaturated Mg solid solution, pined up magnesium grain boundaries. During annealing treatment, the nano-scaled precipitates together with original submicron Ti particulates hindered the movement of grain boundaries. Thus, the thermal stability for magnesium grains was improved. After annealing treatment at 723 K for 600 min, this magnesium matrix composite with the average grain size of ∼93 nm was still nanocrystalline. In addition, influence of temperature and time on hardness was investigated. After annealing treatment at 723 K for 600 min, the hardness of annealed Ti/AZ61 composite is approximately 116 HV, presenting outstanding thermal stability. [ABSTRACT FROM AUTHOR]

Nanocrystalline AZ61 powders, dispersed with submicron/nanometer Ti particulates, were mechanically milled and a comparative study on thermal stability and mechanical properties was performed. After mechanical milling, Ti dispersions were cracked to submicron/nanometer particulates and Mg grains were refined to nanocrystalline. As illustrated by high-resolution transmission electron microscopy, coherent lattice and semi-coherent lattice interfaces between magnesium and titanium were achieved. Based on high angle annular dark field and corresponding energy dispersive spectrometer maps, Al dissolved into Ti particulates after mechanical milling. Moreover, the emergence of Ti 3 Al phase was verified by X-ray diffraction during mechanical milling and heat treatment. According to heat treatment results, this nanocrystalline magnesium composite exhibits a preeminent thermal stability among nano-structured magnesium alloys. After annealed at 673 K for 600 min, the composite still remained nanocrystalline with average grain size being 68.6 nm. Due to such microstructure, the composite revealed prominent mechanical property with the hardness being 146.6 HV. Simultaneously, the hardness of annealed composite remained high, with the value being 134.7 HV, which could ascribe to the excellent thermal stability. [ABSTRACT FROM AUTHOR]

The effects of selective laser melting (SLM) processing parameters on the microstructure and corrosion behavior of CX (Corrax) stainless steel were investigated via microstructure characterization, electrochemical tests and passive layer analysis. Different processing parameters (including the laser power of 150-170 W and scanning speed of 0.9–1.3 m·s−1) were used to manufacture the SLMed CX samples. The results revealed that the relative density of SLMed CX samples reached above 96%, which exhibited a higher densification effect. Besides that, the SLMed CX samples had the high-density of geometrically necessary dislocations (GNDs, ranging from 9.63 × 1014 m−2 to 9.86 × 1014 m−2), and the average grain size mainly distributed within 2 μm, in which the sample S1 (manufactured with P = 170 W, V = 0.9 m·s−1) exhibited the smallest average grain size of 1.1 μm. The proportion of Σ 3 boundaries varied slightly in samples, ranging from 4.13% to 4.87%, which was positively correlated with the content of recrystallization. In addition, the electrochemical test results indicated that the scanning speed had a greater effect on corrosion resistance of SLMed CX samples, comparing with the laser power. The sample S1 exhibited both the largest relative density of 99.13% and the best corrosion resistance, including the largest value of corrosion potential (E Corr , −259 ± 12mV SCE) and the lowest value of corrosion current density (I Corr , 0.484 ± 0.012 μA·cm−2). The corrosion resistance of SLMed CX sample was ascribed to the synergistic effects of its relative density, grain size, grain boundary character distribution and GNDs density, and these influence factors were also discussed in detail. • The CX stainless steel based SLM were prepared under different process parameters. • The average grain size of SLMed CX samples mainly distributed within 2 μm. • After SLM, the high-density GNDs ranging from 9.63 × 1014 to 9.86 × 1014 m−2 were formed. • The S1 prepared with P = 170 W, V = 0.9 m·s−1 exhibited the best corrosion resistance. • The effect of V on corrosion resistance was more notable than P. [ABSTRACT FROM AUTHOR]

• A series of novel Co-free CrFeNiSi x MEAs was prepared via vacuum arc-melting. • Si induced the formation of (Cr, Si)-rich σ phase in CrFeNiSi x MEAs. • The growth accident model was the main formation mechanism of Σ 3 boundaries. • The increasing σ phase resulted in the transformation of fracture mechanisms. • There is a good trade-off between strength and ductility in CrFeNiSi 0.15 alloy. In this paper, a series of novel Co-free CrFeNiSi x (x = 0.05, 0.15, 0.25) MEAs was prepared via vacuum arc-melting. The addition of Si induced the formation of (Cr, Si)-rich σ phase in CrFeNiSi x MEAs, which effectively hindered the migration of dislocations and grain boundaries, resulting in the grain refinement. Meanwhile, the existence of σ phase inhibited the formation of twinning, leading to the decreasing fractions of low- Σ CSL boundaries with the increasing Si addition. Compared with CrFeNi alloy, both the strength and microhardness of CrFeNiSi x MEAs were significantly improved, but the plasticity had deteriorated. Overall, there was a good trade-off between strength and ductility in CrFeNiSi 0.15 MEA, including the yield strength of 550 ± 5.5 MPa, ultimate tensile strength of 795 ± 7.1 MPa, and elongation of 20 ± 0.4%. In addition, the experimental results demonstrated that the grain boundary strengthening and precipitation strengthening played a dominant role for the strengthening of CrFeNiSi x MEAs. The decreased plasticity was mainly attributed to the formation of σ phase and the reduced twinning. Furthermore, the increasing σ phase also resulted in the transformation of fracture mechanisms from ductile fracture to the mixed fracture. Our work provides significant reference for the further development of high performance Co-free MEAs/HEAs. [ABSTRACT FROM AUTHOR]

[Display omitted] • Corrax stainless steel based SLM were prepared under different process parameters. • Many lath martensite (above 99%) and LAGBs (above 50%) were formed in SLMed samples. • The best process parameter was determined as 190 W-1.1 m·s−1. • The sample under the optimal parameter, exhibited superior mechanical properties. • Dislocation strengthening dominated the strengthening mechanism of the SLMed sample. In this paper, selective laser melting (SLM) was used to manufacture corrax stainless steel samples under different parameters. It was found that the SLMed samples were mainly composed by lots of fine martensite (including cellular structure, cellular dendritic grains and blocky grains), and trace austenite. During SLM forming process, a large number of low-angle grain boundaries (LAGBs) and high-density dislocations were formed in the matrix. Meanwhile, the samples showed weak texture and no obvious preference orientation. Moreover, the relative density of all SLMed samples reached above 90%, and the relative density was above 97% when the laser energy density was 54.13–78.19 J·mm−3. Under the optimal process parameters of P = 190 W, V = 1.1 m·s−1, the relative density of sample reached above 99.52 ± 0.09%, while the sample exhibited the best mechanical properties, including the highest microhardness (374.2 ± 6.5HV), yield strength (YS = 946 ± 7.3 MPa), ultimate tensile strength (UTS = 1084 ± 3 MPa) and elongation (EL = 17.64 ± 0.18%). Moreover, the strengthening mechanisms of SLMed samples mainly included grain boundary strengthening, dislocation strengthening and precipitation strengthening, while dislocation strengthening played a dominant role. Besides that, the fracture mechanisms of SLMed samples belonged to ductile fracture, except for the samples prepared with laser energy density below 54.13 J·mm−3. [ABSTRACT FROM AUTHOR]

Evolution of grain structure and grain boundary character distribution in a cold-deformed Ni-based superalloy during the electropulsing treatment (EPT) was investigated by EBSD technique. Compared with the conventional heat treatments (CHT), the nucleation and growth of SRX grains were significantly accelerated by EPT, which was mainly attributed to the high mobility of dislocations and solute atoms, and the efficient energy transfer under the effect of EPT. Especially, with the introduction of electric current, the activation energy of dislocation motion could be reduced by generating additional energy near the dislocation line, which made the motion of dislocation easy to start. The dislocations would also be easy to move in the form of slipping and climbing, under the effect of electron wind force. On the other hand, most of Σ 3 boundaries were newly formed through growth accidents in the form of coherent Σ 3 boundaries during EPT, and the mutual interactions between different Σ 3 n (n = 1, 2, 3) boundaries were rather limited. In addition, EPT could indirectly promote the formation of Σ 3 boundaries by accelerating the SRX process, but the formation of Σ 9 and Σ 27 boundaries was inhibited at the temperatures higher than 700 °C. Moreover, the connectivity of random boundaries in the cold-deformed alloy treated by EPT was also discussed. • Compared with CHT, the SRX behaviors in the cold-deformed Ni-based superalloy were significantly accelerated by EPT. • Most of Σ 3 boundaries were newly formed through growth accidents in the form of coherent Σ 3 boundaries during EPT. • EPT could indirectly promote the formation of Σ 3 boundaries by accelerating the SRX process. • Compared with CHT, the cold-deformed alloy treated by EPT exhibited the better connectivity of random boundaries. [ABSTRACT FROM AUTHOR]