Liyuan Sheng

Microstructure and room temperature mechanical properties of NiAl-Cr(Mo)-(Hf, Dy) hypoeutectic alloy prepared by injection casting

The NiAl–Cr(Mo)–(Hf,Dy) hypoeutectic alloys were prepared by conventional casting and injection casting techniques respectively, and their microstructure and room temperature mechanical properties were investigated. The results reveal that with the addition of Hf and Dy, the Ni2AlHf Heusler phase and Ni5Dy phase form along the NiAl/Cr(Mo) phase boundaries in intercellular region. By the injection casting method, some Ni2AlHf Heusler phase and Ni5Dy phase transform into Hf and Dy solid solutions, respectively. Moreover, the microstructure of the alloy gets good optimization, which can be characterized by the fine interlamellar spacing, high proportion of eutectic cell area and homogeneously distributed fine Ni2AlHf, Ni5Dy, Hf solid solution and Dy solid solutions. Compared with conventional-cast alloy, the room temperature mechanical properties of injection-cast alloy are improved obviously.

Effect of extrusion process on microstructure and mechanical properties of Ni3Al-B-Cr alloy during self-propagation high-temperature synthesis

The well-densified Ni3Al-0.5B-5Cr alloy was fabricated by self-propagation high-temperature synthesis and extrusion technique. Microstructure examination shows that the synthesized alloy has fine microstructure and contains Ni3Al, Al2O3, Ni3B and Cr3Ni2 phases. Moreover, the self-propagation high-temperature synthesis and extrusion lead to great deformation and recrystallization in the alloy, which helps to refine the microstructure and weaken the misorientation. In addition, the subsequent extrusion procedure redistributes the Al2O3 particles and eliminates the γ-Ni phase. Compared with the alloy synthesized without extrusion, the Ni3Al-0.5B-5Cr alloy fabricated by self-propagation high-temperature synthesis and extrusion has better room temperature mechanical properties, which should be ascribed to the microstructure evolution.

Microstructure and elevated temperature tensile behaviour of directionally solidified nickel based superalloy

Abstract In this paper, a nickel based superalloy with good corrosion resistance is fabricated by directional solidification, and its microstructure and tensile properties were investigated. Microstructure observations reveal that besides the γ′ precipitates and γ matrix, some MC, Ni5Hf and M3B2 particles precipitate along the grain boundary. Tensile tests exhibit that the alloy has obvious anomalous yield and intermediate temperature brittleness (ITB) behaviour. The tensile properties depend on temperature significantly. Below 650°C, the yield strength decreases slightly but rises rapidly between 650 and 750°C, and after then, it decreases gradually with temperature increasing further. Examination by TEM exhibits that sharing of the γ′ by dislocation is almost the main deformation mechanism at low temperature, but the γ′ bypass dominates the deformation at high temperature and the transition temperature should be ∼800°C. In addition, the carbides and eutectic structure contribute partly to the ITB behaviours.

Nanocomposite films based on TEMPO-mediated oxidized bacterial cellulose and chitosan

Nanobacterial cellulose (BC) and chitosan (CH) have similar molecular structures. In the present work, nanocomposite films based on BC and CH were prepared by stepwise modification instead of by conventional physical blending. First, surface C6-carboxylated BC was prepared in a bromide-free system using 2,2,6,6-tetramethylpyperidine-1-oxyl (TEMPO) as a catalyst. The carboxylate groups of oxidised BC could couple to the amine groups of CH. The composite films were characterised by attenuated total reflectance Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and Carbon-13 solid nuclear magnetic resonance 13C NMR. The results showed that a cross-linking reaction occurred between TEMPO-mediated oxidised BC and CH. Even in the absence of cross-linkers, these two biopolymers could interact with each other because of their structural similarity. SEM images and tensile tests showed that the TEMPO-oxidized BC and CH composite film prepared at a 0.5:1 ratio was an exception. The mechanical properties of the composite films decreased with increasing CH content, passed through a minimum, and then increased. To explain this phenomenon, we propose that the hydrogen bonding in the original BC microstructure plays a decisive role in the modified nanocomposites. However, BC/CH composites with excellent properties could be synthesised at appropriate reactant ratios.

TEMPO-mediated oxidation of bacterial cellulose in a bromide-free system

A partially C6-carboxylated bacterial cellulose (BC) with a high carboxylate content was prepared in a bromide-free system by using 2,2,6,6-tetramethylpyperidine-1-oxyl (TEMPO) as a catalyst. ART-FTIR, X-ray diffraction, solid 13C-NMR, TEM analysis, and reaction kinetics measurements were performed to investigate the oxidation reaction of BC. Results show that C6 carboxylate was formed selectively on the microfiber surface without disrupting its highly ordered nanocrystalline structure. Given the extremely low accessibility of hydroxyl groups in d-anhydroglucopyranose units, the reaction can be described by second-order kinetics with very low reaction rate constants. pH exhibited a significant influence on the oxidation of BC and a higher activity at C6 was observed in a neutral medium.

Anomalous yield and intermediate temperature brittleness behaviors of directionally solidified nickel-based superalloy

Abstract A nickel-based superalloy with good corrosion resistance was fabricated by directional solidification, and its microstructure and tensile properties at elevated temperatures were investigated. Microstructure observations reveal that the γ′ precipitates are arrayed in the γ matrix regularly with some MC, Ni 5 Hf and M 3 B 2 particles distributed along the grain boundary. The tensile tests exhibit that the tensile properties depend on temperature significantly and demonstrate obvious anomalous yield and intermediate-temperature brittleness (ITB) behavior. Below 650 °C, the yield strength decreases slightly but the ultimate tensile strength almost has no change. When the temperature is between 650 °C and 750 °C, the yield and ultimate tensile strengths rise rapidly, and after then they both decrease gradually with temperature increasing further. The elongation has its minimum value at about 700 °C. The TEM examination exhibits that sharing of the γ′ by dislocation is almost the main deformation mechanism at low temperatures, but the γ′ by-pass dominates the deformation at high temperatures. The transition temperature from shearing to by-pass should be around 800 °C. The anomalous yield and intermediate-temperature brittleness behaviors should be attributed to the high content of γ′. In addition, the carbides and eutectic structure also contribute some to the ITB behaviors of the alloy.

Microstructure and compressive properties of NiAl-Cr(Mo)-Dy near eutectic alloy prepared by suction casting

Abstract The NiAl–Cr(Mo) eutectic alloy doped with Dy was prepared by suction casting technique and its microstructure and mechanical properties were investigated. It is found that with the addition of Dy, the Ni5Dy phase is formed along the NiAl/Cr(Mo) phase boundary in the intercellular region. By the suction casting method, the microstructure of the alloy get well optimisation which can be characterised by the fine interlamellar spacing, high proportion of eutectic cell area and fine homogeneous distributed Ni5Dy phase. The compression test results reveal that the room temperature and high temperature mechanical properties of the suction cast alloy improve significantly, compared with the conventionally cast alloy.

Effect of solution treatment on stress corrosion cracking behavior of an as-forged Mg-Zn-Y-Zr alloy

Effect of solid solution treatment (T4) on stress corrosion cracking (SCC) behavior of an as-forged Mg-6.7%Zn-1.3%Y-0.6%Zr (in wt.%) alloy has been investigated using slow strain rate tensile (SSRT) testing in 3.5 wt.% NaCl solution. The results demonstrated that the SCC susceptibility index (ISCC) of as-forged samples was 0.95 and its elongation-to-failure (εf) was only 1.1%. After T4 treatment, the SCC resistance was remarkably improved. The ISCC and εf values of T4 samples were 0.86 and 3.4%, respectively. Fractography and surface observation indicated that the stress corrosion cracking mode for as-forged samples was dominated by transgranular and partially intergranular morphology, whereas the cracking mode for T4 samples was transgranular. In both cases, the main cracking mechanism was associated with hydrogen embrittlement (HE). Through alleviating the corrosion attack of Mg matrix, the influence of HE on the SCC resistance of T4 samples can be greatly suppressed.

Microstructure and wear behaviour of ceramic particles strengthening NiAl based composite

Abstract In the present paper, the NiAl based composite strengthened by TiC and ZrO2 ceramic particles (NiAl–TiC–ZrO2 for short) was prepared by the hot press aiding exothermic synthesis (HPES) technique to get a material with good wear properties. OM, SEM and TEM were employed to investigate the microstructure of the NiAl–TiC–ZrO2 composite. The mechanical properties and wear behaviour were studied by compression and dry sliding wear tests. The results showed that the NiAl–TiC–ZrO2 composite possesses fine microstructure, but the TiC and ZrO2 particles tended to agglomerate along NiAl phase boundary. Further observations revealed that the ultrafine ZrO2 particles contained stacking faults and microtwins inside and thin amorphous layer was generated along some interfaces of NiAl and TiC phases. Compression tests exhibited that the addition of TiC and ZrO2 particles could increase the strength of the NiAl–TiC–ZrO2 composite significantly. Wear tests results demonstrated that the NiAl–TiC–ZrO2 composite had relative good wear resistance at high temperature. Especially at 973 and 1073 K, the composite exhibited self-lubrication behaviour, which might be attributed to the lubricant film formed on the worn surface. TEM observation on the lubricant film revealed that it was composed of amorphous matrix and nanoparticles. Moreover, TEM observation found the composite near the worn surface contained ultrafine microstructure. When the temperature increased or decreased, the lubricant film would be spalled partly, which led to the increasing of the friction coefficient and wear rate.

Influence of Tantalum Addition on Microstructure and Mechanical Properties of the Nial-Based Eutectic Alloy

NiAl-28Cr-6Mo or NiAl-Cr(Mo) eutectic alloys with different degree of tantalum (Ta) addition have been fabricated and investigated. The microstructural characterization shows that minor Ta addition results in the formation of Cr2Ta Laves phase with C14 crystal structure. Small Laves phases are prone to precipitation along Cr(Mo) phase boundary, while large/bulk ones are formed in the intercellular region. As compared with the NiAl-Cr(Mo) eutectic alloy, minor Ta addition can refine Cr(Mo) and NiAl eutectic lamella in the core of eutectic cell but coarsen the Cr(Mo) and NiAl phase in the fringe of eutectic cell. An increased Ta addition invokes coarsening of Cr(Mo) and NiAl phases in the intercellular region and enhances Laves phase precipitation in the intercellular region. When the Ta addition increases to 2.0 at.%, the modified alloy mainly comprises ultrafine eutectic cell core and coarse Cr(Mo) and NiAl phases, which are semi-separated by NiAl dendritic and bulk Laves phase. The results confirm that the appropriate Ta addition can significantly improve the high-temperature strength with a feeble effect on room-temperature ductility, which can be ascribed to the microstructure optimization induced by the Ta addition. However, further Ta addition strongly affects the cellular eutectic microstructure and is detrimental for the roomtemperature ductility. The appropriate Ta content in the NiAl-Cr(Mo) eutectic alloy should be about 1.0 at.%.