Volume 110, Issue 3

In the direct quenching and partitioning (DQ&P) process, tough ultra-high-strength steel is made by combining thermomechanical processing with quenching and partitioning to obtain martensite toughened by thin films of retained austenite. The hot rolling stage with deformation and recrystallization between the rolling passes affects the state of the austenite before quenching and partitioning. This paper describes the static recrystallization kinetics of two steels with compositions suited to DQ&P processing, viz. (in wt.%) 0.3C-1Si-2Mn-1Cr and 0.25C-1.5Si-3Mn. The stress relaxation technique on a Gleeble thermomechanical simulator provided recrystallization times over a wide range of temperature, strain, strain rate and initial grain size. The higher levels of Si and Mn made the recrystallization kinetics less sensitive to strain, strain rate and temperature. The equations derived to describe the recrystallization kinetics can be used in the design of the rough rolling part of thermomechanical processing.

Hot dip aluminizing of mild steel at different temperatures was conducted to reveal the influence of reaction temperature and time on interfacial intermetallic compounds (IMCs). Scanning electron microscopy, energy dispersive X-ray spectrometry and X-ray diffraction were employed to investigate the interfacial microstructures. The IMCs of the dipping interface consisted of a thick layer of η-Fe 2 Al 5 between 4.2–132.2 μm next to the steel and a thin layer of θ-Fe 4 Al 13 between 0–5.5 μm close to the aluminum. With increasing dipping temperature and time, the total thickness of IMCs (Fe 2 Al 5 plus Fe 4 Al 13 ) increased. Specifically, the growth of the Fe 2 Al 5 layer can be described by parabolic rate laws. An activation energy of 93 kJ mol −1 was obtained, combining both the results from the present work and previous studies in the temperature range of 675–900°C. The change in Fe 4 Al 13 thickness is not significant compared with the Fe 2 Al 5 . However, the decrease in IMC thickness of the Fe 4 Al 13 with dipping temperature was observed for the first time and had never been reported before. Moreover, it can be clearly observed that the thickness of the Fe 4 Al 13 decreased with dipping time based on the linear fitting results by excluding the result of the initial 1 s. A possible mechanism is that interfacial dynamics and thermodynamics work for the dissolution and decomposition of the Fe 4 Al 13 layer. Higher temperature accelerates the dissolution of the θ layer.

The interfacial microstructures of borides in Fe–Cr–B cast steels with different Cr contents after hot dipping in Al–Zn alloys were investigated. The results showed: the Cr contained in Fe–Cr–B cast steel had a key influence on the formation of periodic layered structures (PLSs) between Cr-rich borides, which were (Cr,Fe) 2 B, and Al–Zn melt. The contents of Zn in the Al–Zn melts also significantly influenced the formation of periodic layered structures, which depended on the complex relationship between the content of Cr contained in Fe–Cr–B cast steel and that of Zn in Al–Zn melt. Within a certain range, the higher the content of Cr is, the larger the maximum value of Zn in Al–Zn melt that could form PLS at the (Cr,Fe) 2 B/Al–Zn interface could be.

The hot deformation behaviour of an Ni-based austenitic stainless steel (Fe-31Ni-15Cr austenitic steel) was investigated by isothermal tensile testing at temperatures ranging from 900–1100°C and at strain rates of 0.01, 0.1, 1 and 10 s −1 . The effects of the deformation parameters were studied through analyses of true stress–strain curves. Processing maps were built based on the dynamic material model. The fracture morphology was observed. The stress of the alloy has a dependence on strain rate and deformation temperature. A modified Johnson–Cook constitutive model was built and examined. The entire tensile fracture surface presenting typical plastic fracture morphology is covered by dimples and voids. The processing maps reveal that instability mainly appears in Ni-based austenitic stainless steel at low deformation temperatures ranging from 900–940°C.

Cu 0.5 FeNiVAl x high entropy alloys have been designed and prepared to study the microstructure and mechanical properties in as-cast and heat treated states. The as-cast Cu 0.5 FeNiV alloy is mainly composed of fcc and σ phases. With the increase in Al, the phase composition gradually changes to the fcc + bcc structure. In the Cu 0.5 FeNiVAl x alloy system, the compressive strength (1 801 MPa) of Cu 0.5 FeNiVAl 0.5 alloy is the best, and the Cu 0.5 FeNiVAl 1.2 alloy has the highest hardness (694 HV). In order to improve its strength, the Cu 0.5 FeNiVAl 0.5 alloy was annealed at 500 °C, 750 °C and 1 000 °C for 10 h, respectively. The highest compressive strength (2 154 MPa, increased by 20 % compared to the as-cast state) is reached at 750 °C annealing temperature. Meanwhile, the microhardness is increased from 516 HV to 670 HV, an increase of 30 % relative to the as-cast one. Study of the heat treatment process on the mechanical properties of high-entropy alloys contributes to the transformation of these materials from the laboratory research stage to the practical production stage.

Mg-9 %Al magnesium matrix nanocomposites reinforced by 5 wt.% nanometre-sized SiC particles were synthesized via semi-solid powder hot pressing assisted by ultrasonication. The effect of the temperature field on the microstructure and tensile properties of the nanocomposites was systematically investigated. The distribution of the SiC nanoparticles, grain size, and morphology of the Mg 17 Al 12 phase were found to be greatly affected by the hot-pressing temperature, resulting in strength and ductility first increasing and then decreasing with increasing hot-pressing temperatures. As the hot-pressing temperature increased to 510 °C, the nanocomposites consisted of hard SiC nanoparticles and isolated soft phases, and the SiC nanoparticles bonded well with the matrix without interfacial activity and exhibited the most uniform distribution in the nanocomposite. Moreover, compared to the Mg-9 %Al alloy, the nanocomposites exhibited significantly improved strength and excellent ductility both at room temperature and elevated temperature. The enhanced mechanical properties were attributed to the Orowan strengthening mechanism, the obvious grain boundary strengthening, and the load transfer effect.

A mixture of Nb powder and boron salt (KBF 4 ) was added to aluminum alloy melt to prepare particle reinforced aluminum matrix composites at 850°C. Microstructure, fracture morphology, and the chemical composition of the composite were characterized using scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. The average size of NbB 2 in the studied composites is found to be about 0.3 μm under optimal process parameters. The size of primary silicon particles is refined from 60–100 μm to 30–50 μm. Tensile testing results reveal that the reinforced composite exhibits higher microhardness, tensile strength, yield strength and poorer ductility than those of the matrix alloy. The fracture mode is found to be brittle fracture, which is mainly dominated by silicon particle cracking and NbB 2 particle–matrix debonding.

The viscosity of Al 2 O 3 /SiO 2 melts is important because it affects secondary steelmaking. In this study, the effect of the Al 2 O 3 /SiO 2 ratio on the viscosity was investigated for a CaO–Al 2 O 3 –SiO 2 –CaF 2 –MgO slag system. Softening point determination, structural analysis, and thermodynamic analysis were carried to explain and confirm the viscosity results. The lowest viscosity of 19 mPa · s was observed for 14Al 2 O 3 -18SiO 2 . An increase in the Al 2 O 3 /SiO 2 ratio to 27Al 2 O 3 -5SiO 2 resulted in an increase in the viscosity to 83 mPa · s at 1 600 °C. In the case of 6Al 2 O 3 -26SiO 2 , the main structural unit was [SiO 4 ] 4 − (tetrahedral) stretching vibration bands, whereas for 14Al 2 O 3 -18SiO 2 , the dominant [SiO 4 ] 4 − (tetrahedral) stretching vibration bands decreased after Si–O–Al vibration bands became dominant. [AlO 4 ] 5– (octahedral) bands were observed with increasing Al 2 O 3 /SiO 2 ratio in the CaO–Al 2 O 3 –SiO 2 –CaF 2 –MgO slag system.

Protective coatings can be used to enhance the performance of interconnects in solid oxide fuel cells. In the present work, AISI 304 steel was coated with Ni–Fe 2 O 3 composite from a modified Watt's type electrolyte by electroplating. The composite coating's microstructure and composition were investigated by scanning electron microscopy with energy dispersive X-ray spectroscopy and X-ray diffraction. Isothermal oxidation for 200 h in air at 850 °C converted the Ni–Fe 2 O 3 composite coating to Fe 2 O 3 /NiFe 2 O 4 . The coating layer also decreased outward migration of chromium and the growth rate of a Cr 2 O 3 layer, compared to an uncoated specimen.

0.6Li[Li 1/3 Mn 2/3 ]O 2 · 0.4Li[Ni 1/3 Mn 1/3 Co (1/3- y ) Al y ]O 2 ( y = 0, 0.03, 0.08, 0.13) was prepared by a high-temperature solid-state method as cathode material for lithium-ion batteries. X-ray diffraction and scanning electron microscopy were used to assess the structure and morphology of the samples. Electrochemical performance testing, AC impedance testing, and cyclic voltammetry testing were performed to study various aspects of the cathode materials. The results showed that the addition of Al 3+ had little effect on the charge–discharge performance, but the cycling performance and stability of the material were significantly enhanced. When the doping fraction of Al 3+ was 0.08, the cathode material 0.6Li[Li 1/3 Mn 2/3 ]O 2 · 0.4Li[Ni 1/3 Mn 1/3 Co (19/75) Al 0.08 ]O 2 had good electrochemical performance. The first discharge specific capacity reached 161.1 mAh · g −1 in the charge and discharge test at 0.1 C rate. After 20 cycles, the discharge capacity was still 159.7 mAh · g −1 . The charge–discharge specific capacity had almost no attenuation.

Rutile TiO 2 nanowire arrays have been synthesized on a fluorine-doped tin oxide (FTO) coated glass substrate by a hydrothermal method. The effect of synthetic parameters on the morphology and initial growth mechanism have been investigated by X-ray diffraction, field emission scanning electron microscopy, high-resolution transmission electron microscopy and selected area electron diffraction. Titanium n-butoxide concentration in the precursor solutions and growth temperatures are important parameters for synthesizing TiO 2 nanowires. Although the FTO glass substrate showed the preferred orientation in (110), (101) and (200) faces of rutile SnO 2 , [001]-oriented TiO 2 nanowire arrays were grown. Lattice matching between FTO and TiO 2 is important for the initial nucleation and subsequent growth process. The anisotropic growth of TiO 2 nanowire arrays along the [001] direction can be understood from the viewpoints of surface energy and growth rate of rutile TiO 2 .

The cathodic cage plasma nitriding technique is used for thin film deposition. As such, the hollow cathode effect on cage holes is directly related to deposition efficiency. The objective of this work is to study the influence of the cathode length-to-diameter ratio in the deposition of fine copper films on samples placed internally and externally to the cathodic cage, in an argon atmosphere, for 3 h at 420 °C. Compositional, transmittance and morphological characterization of films show copper deposition in all treatments. However, it was observed that substrate temperatures during film deposition influence its morphology. As such, the formation of continuous film on internal samples is observed, whereas external samples show uniformally dispersed nanoparticles as well as the absence of dense film on substrates. As far as the length-to-diameter ratio is concerned, the 1.5 ratio presented the highest deposition efficiency.