Tiebang Zhang

Microstructure and Tribological Properties of AlCoCrFeNiTi0.5 High-Entropy Alloy in Hydrogen Peroxide Solution

Microstructure and tribological properties of an AlCoCrFeNiTi0.5 high-entropy alloy in high-concentration hydrogen peroxide solution were investigated in this work. The results show that the sigma phase precipitates and the content of bcc2 decrease during the annealing process. Meanwhile, the complex construction of the interdendrite region changes into simple isolated-island shape, and much more spherical precipitates are formed. Those changes of microstructure during the annealing process lead to the increase of hardness of this alloy. In the testing conditions, the AlCoCrFeNiTi0.5 alloy shows smoother worn surfaces and steadier coefficient of friction curves than does the 1Cr18Ni9Ti stainless steel, and SiC ceramic preserves better wear resistance than ZrO2 ceramic. After annealing, the wear resistance of the AlCoCrFeNiTi0.5 alloy increases coupled with SiC counterface but decreases with ZrO2 counterface.

Solidification characteristics of high Nb-containing γ-TiAl-based alloys with different aluminum contents

The effect of Al content on the microstructure and solidification characteristics of Ti–Al–Nb–V–Cr alloys in as-cast and isothermally treated states was investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) equipped with energy dispersive spectroscope (EDS), and transmission electron microscopy (TEM). The typical solidification characteristics are due to the joint influence of both the crystal temperature range and the solidification path. The wide crystallization temperature range contributes to obtaining coarse dendrites in the as-cast Ti47Al7Nb2.5V1.0Cr (at%) alloy solidifying through the peritectic reaction. The β-solidifying Ti46Al7Nb2.5V1.0Cr (at%) alloy with the narrow crystallization temperature range is attributed to the formation of a homogeneous fine-grained microstructure. However, the crystallization temperature range of Ti48Al7Nb2.5V1.0Cr (at%) alloy is equivalent to that of Ti46Al7Nb2.5V1.0Cr alloy, but it is solidified by peritectic reaction, leading to the formation of finer dendrites.

Dehydrogenation behavior and microstructure evolution of hydrogenated magnesium–nickel–yttrium melt-spun ribbons

This paper reports porous Mg67Ni33−xYx (x = 0, 1, 3, 6) ribbons which are prepared by a melt spinning method. The dehydrogenation behaviors, including the thermal decomposition process, dehydrogenation capacities, percentage and kinetics, are investigated in a temperature range of 523–623 K. The microstructure evolution during the dehydrogenation process of metal melt-spun ribbons is discussed with respect to the microstructures of hydrogenated and dehydrogenated ribbons. It is considered that the porous structure in the as-prepared ribbons accelerates the diffusion of hydrogen gas and atoms, and increases dehydrogenation nucleation locations and decreases the diffusion distance. Additionally, the addition of yttrium and melt-spinning decreases the activation energy of dehydrogenation. A low activation energy and a refined microstructure play dominant roles in decreasing the dehydrogenation temperature and increasing the kinetics of Mg-based metal melt-spun ribbons.

Oxidation behavior of Zr-containing Ti2AlNb-based alloy at 800 °C

Abstract The oxidation behavior of Ti–22Al–(27–x)Nb–xZr (x=0, 1, 6) alloys at 800 °C for exposure time up to 100 h was examined. It is shown that oxidation rate of experimental alloys obeys the parabolic kinetics. Ti–22Al–26Nb–1Zr alloy demonstrates more excellent oxidation resistance than the other two alloys. The main oxidation products are TiO2, Al2O3 and AlNbO4 phases for all these alloys. For the Ti–22Al–26Nb–1Zr alloy, Zr addition can modify the growth mechanism of oxide scale, which can effectively hinder the diffusion of oxygen. Whereas, reaction of Zr with oxygen leads to the formation of ZrO2 precipitates for the Ti–22Al–21Nb–6Zr alloy, which promotes the oxygen ingress into the substrate. Meanwhile, oxidation affected zones, including internal-oxidation layer and oxygen-enriched zone, are present beneath the outmost oxide scale. The difference in these zones is derived from the phase constitution in the starting Ti–22Al–(27–x)Nb–xZr (x=0, 1, 6) alloys.

Microstructure and hydrogenation kinetics of Mg2Ni-based alloys with addition of Nd, Zn and Ti

Abstract In order to enhance the hydrogen absorption kinetics of the Mg 2 Ni-based alloys, metal elements (Nd, Zn, and Ti) were added during melting process, respectively. The Mg 2 Ni-based alloys were melted using an electric resistance furnace under the protection of the covering reagent to prevent the oxidation and the evaporation of magnesium. Phase compositions and microstructures of as-cast alloys were characterized by XRD and SEM equipped with EDS. Hydrogenation kinetics of experimental alloys were investigated by the constant volume method using a Sievert-type apparatus. The addition of Nd, Zn or Ti elements to Mg 2 Ni results in the formation of minor phases Mg 6 Ni and Ni 3 Ti. Nd and Zn are dissolved in α -Mg, Mg 2 Ni and MgNi 2 phases in Mg 2 Ni-based alloys. With the addition of Nd, the hydrogen content of the first absorption is 2.86% in mass fraction, which is higher than that of the Mg 2 Ni. Hydrogen absorption kinetics and activation properties of Mg 2 Ni-based alloys are improved evidently. During the initial three hydrogenation/dehydrogenation cycles, the hydrogen absorption capacity and kinetics properties have been improved for alloys with the addition of transition element Zn or Ti. The kinetics properties of the experimental alloys and absorbing reaction mechanism were also analyzed with the help of the Hirooka kinetics model.

Microstructural stability of long term aging treated Ti-22Al-26Nb-1Zr orthorhombic titanium aluminide

Abstract The microstructure development of lamellar structure of an orthorhombic Ti 2 AlNb-based Ti–22Al–26Nb–1Zr alloy, including B 2 decomposition and spheroidization of O phase, was investigated. The results show that the lamellar structure is fabricated by heating the samples in the single B 2 phase field and cooling slowly in the furnace. Aging treatments are conducted in the ( O + B 2) phases field by air cooling. After aging at 700 °C for a short time within 100 h, there is no significant change of microstructures, whereas the coarsening of lamellae is observed in the long-term aged microstructure. Ti–22Al–26Nb–1Zr alloy exhibits microstructural instability including the severe dissolution of B 2 lamella, discontinuous precipitation and spheroidization of O phase during the long term aging process at 700 °C up to 800 h. In addition, a pronounced formation of branch-shaped O phase lamella is observed for the alloy aged over 100 h.

Oxidation behavior of Hastelloy C-2000 superalloy at 800 °C and 1000 °C

Abstract The oxidation behavior of Hastelloy C-2000 alloy was investigated in air at 800 °C and 1000 °C for 100 h, respectively. Oxidation kinetics and oxide scales morphologies were examined by mass gain measurement, scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The oxidation behavior of the alloy approximately follows a parabolic rate law. Moreover, annealing twins defect structure in matrix deteriorates the oxidation resistance of alloy due to the improvement of diffusion rates for alloying elements and oxygen atoms. At 800 °C, the microstructure is primarily composed of NiO and Cr 1.3 Fe 0.7 O 6 and the initial annealing twins structure is visible and Mo-rich phases are emerged to approach boundary of oxide scales. At 1000 °C, however, the morphology microstructure of oxide scales consists of oxide particle with fine Cr oxides and large Ni oxides by inlaying each other, whilst Mo-rich phases hardly appear closing to the interface of oxide scales.

Hot corrosion characteristics of Ni–20Cr–18W superalloy in molten salt

Abstract The hot corrosion behavior of a Ni–20Cr–18W (mass fraction, %) superalloy in the mixture of 75%Na2SO4–25%NaCl melts at 700 and 800 °C was studied. The results demonstrate that the alloy suffers from serious hot corrosion attack in the mixture molten salt. Meanwhile, the degradation of the substrate accelerates with increasing the corrosion temperature. The corrosion layer has an obvious duplex microstructure, and the Cr-depletion zone is detected obviously nearby the inner corrosion layer. The main corrosion products at 700 and 800 °C are almost the same and mainly include NiO, Cr2O3 and Ni3S2, but a trace amount of NiCrO2 is detected at 800 °C for 20 h. The hot corrosion mechanism and formation mechanism of corrosion scales of the Ni–20Cr–18W superalloy in the molten salt are proposed.

Microstructure stability of Ti2AlN/Ti–48Al–2Cr–2Nb composite at 900 °C

Abstract Microstructure stability of in situ synthesized Ti 2 AlN/Ti–48Al–2Cr–2Nb composite during aging at 900 °C was investigated by XRD, OM and TEM, and the unreinforced Ti–48Al–2Cr–2Nb alloy was also examined for comparison. The result showed that in the TiAl alloy, α 2 lamellae thinned and were broken down, and became discontinuous with increasing aging time. The decomposition of α 2 lamella to γ which was characterized by parallel decomposition and breakdown of α 2 lamellae led to the degradation of the lamellar structure. While in the composite, lamellar structure remained relatively stable even after aging at 900 °C for 100 h. No breakdown of α 2 lamellae except parallel decomposition and precipitation of fine nitride particles was observed. The better microstructural stability of the composite was mainly attributed to the precipitation of Ti 2 AlN particles at the α 2 /γ interface which played an important role in retarding the coarsening of lamellar microstructure in the matrix of composite.

Microstructure evolution and nitrides precipitation in in-situ Ti2AlN/TiAl composites during isothermal aging at 900 °C

Abstract Microstructures of Ti 2 AlN/TiAl composites prepared by in-situ method were characterized in in-situ and aging treatment conditions and the nitride precipitation was investigated in Ti 2 AlN/TiAl composites aged at 900 °C for 24 h after being heat treated at 1400 °C for 0.5 h. The in-situ composites consist of γ + α 2 lamellar colonies, equiaxed γ grains and Ti 2 AlN reinforcements. Matrix with nearly fully lamellar structure formed after solution and subsequently aging treatment. With the increase of Ti 2 AlN content, the nearly fully lamellar structure becomes instable for the aged composites. According to TEM study, fine Ti 2 AlN precipitates are found to distribute at the grain boundaries of lamellar colony. Needle-like Ti 3 AlN precipitates arrange in line with growing axis parallel to [001] direction of the γ -TiAl matrix and another needle-like Ti 3 AlN precipitates with lager size distribute at the dislocations.