Kou Hongchao

Isothermal Oxidation Behavior of Haynes 230 Alloy in Air at 1100 °C

Abstract The isothermal oxidation behavior of Haynes 230 alloy (Ni-Cr-W-Mo alloy) in air at 1100 °C was investigated by means of SAXD and SEM (EDAX). The predominant phase in the oxide scales were Cr 2 O 3 and (Ni, Mn, Cr) 3 O 4 . Multilayer was observed with increasing of oxidation time. The outer (Ni, Mn, Cr) 3 O 4 scale may reduce the volatilization of inner Cr 2 O 3 scale and its oxidation behavior almost followed parabolic law. Due to preferential diffusion of manganese and chromium, alloy composition beneath the oxide scale changed, which led to subsurface degradation such as intergranular oxidation, void formation, and no carbide zone formation.

Correlations between Shear Bands and Plasticity in Ti-Based Bulk Metallic Glass

Abstract The compressive plasticity of Ti 40 Zr 25 Ni 8 Cu 9 Be 18 bulk metallic glass with different specimen aspect ratio was investigated. The specimens with aspect ratios A >1 exhibit a poor plasticity, contrariwise, showing an excellent plasticity. The maximum elongation before failure is up to about 86%. The difference in the plasticity is a result of shear bands behavior due to the geometrical constraints. The results show a linear increase in the density (spacing) of shear bands with increasing of stress, and the hardness decreases linearly with increasing of plastic strain. The results are consistent with a strain-induced local dilatation, and the free volume in the glass increases after deformation relative to the undeformed glass.

Study on Microstructure Characterization of YBCO Bulk Prepared by Directional Seeded Infiltration-Growth

Abstract The seeded infiltration-growth process is an alternative to replace conventional melt processing techniques for the preparation of bulk YBa2Cu3O7-s (Y-123) with finely dispersed small size of Y2BaCuO5 (Y-211) particles. The directional solidification is also a quite effective technique to produce well-oriented cylinder grains. Well-oriented single domain Y-123 bulks can be fabricated by combining seeded infiltration-growth and directional solidification technique (DSIG). The infiltration-growth process is able to control the size and distribution of Y-211 particles in the final Y-123 superconductor and can fabricate net-shape bulk. Y-211 particles and a liquid phase precursor were used as starting materials in the present investigation. The main parameters (temperature gradient, configuration and maximum processing temperature) of the directional seeded infiltration-growth process were discussed in the present paper. The result shows that Y-211 particle sizes upon using 035 liquid phase composition are smaller than those upon using other two liquid phase compositions.

Influence of 1.0 wt% Li on Precipitates in Al-Zn-Mg-Cu Alloy

Abstract Microstructure control always plays a key role in enhancing properties of high-strength Al alloys. Attempts to improve the microstructure of 7000 series alloys by addition of 1.0 wt% Li have been made for a long time, but unsystematically. This article compares the microstructural features of 1.0 wt% Li-containing Al-Zn-Mg-Cu alloy with those of Li-free Al-Zn-Mg-Cu alloy by using differential scanning calorimetric (DSC) techniques, Vickers microhardness and transmission electron microscopy (TEM). The results show the dominance of Guinier Preston (GP) zones, η′or ηphases in 1.0 wt% Li-containing Al-Zn-Mg-Cu alloy, and confirm the capability of Li to retard the rate of precipitates growth and coarsening.

High-Temperature Oxidation Behavior of Ti-22Al-27(Nb, Zr) Alloys

Abstract The oxidation behavior of Ti-22Al-(27- x )Nb- x Zr ( x =0, 1, 6, at%) was evaluated over a temperature range of 650∼800°C in air. The phase composition and the morphology of the formed scales were investigated by X-ray diffraction and scanning electron microscopy. The results show that the alloys containing Zr exhibit better oxidation resistance than Ti-22Al-27Nb alloy. The main oxidation product is TiO 2 for all the alloys oxidized at 650°C for 100 h. A mixture of TiO 2 , Al 2 O 3 and AlNbO 4 is formed for Ti-22Al-27Nb and Ti-22Al-26Nb-1Zr oxidized at 800°C for 100 h; however, ZrO 2 is also traced in Ti-22Al-21Nb-6Zr. The improved oxidation resistance for Ti-22Al-26Nb-1Zr can be ascribed to the refinement of oxides. While in Ti-22Al-21Nb-6Zr alloy, although formation of Al 2 O 3 is obvious at 800°C, ZrO 2 in the scale can provide sites for rapid oxygen transport, leading to the high mass gain.

Crystal Plasticity Finite Element Study of Incompatible Deformation Behavior in Two Phase Microstructure in Near β Titanium Alloy

Abstract The incompatibility of plastic deformation in a near β titanium alloy with (α + β) two phases microstructure was analyzed based on the crystal plasticity finite element (CPFE) model. The bicrystal simulation results show that the plastic deformation in the (α + β) two phases microstructure is incompatible, and the deformation behavior of α phase is significantly affected by the crystal orientation and loading direction. In addition, through coupling with three-dimensional (3D) Voronoi tessellation (VT) geometric model, the effect of globular primary α phase in the β matrix on the macroscopic mechanical response was discussed. Based on the CPFE-VT model, it is found that the incompatibility of plastic deformation between the α and β phases can result in a linear reduction of the macroscopic strength with the increase of the volume fraction of α phase. Furthermore, the distribution of primary α phase also plays an important role in the stress-strain response. The proposed model for near β titanium alloy can yield important informations for the microstructure controlling and the performance prediction.

Phase Evolution of Diffusion Bonding Interface between High Nb Containing TiAl Alloy and Ni-Cr-W Superalloy

Abstract The diffusion bonding between high Nb containing TiAl alloy and Ni-Cr-W superalloy was carried out under the condition of 1000 °C-50 MPa-60 min. The microstructure and the composition of diffusion bonding interface were characterized by optical microscope (OM), scanning electron microscope (SEM), energy disperse spectroscopy (EDS) and X-Ray diffraction (XRD). The phase evolutions of bonds with and without Ti/Cu interlayers were analyzed. The results indicate that the interfacial phases of high Nb containing TiAl alloy/Ni-Cr-W superalloy joint consist of γ -TiAl, Ni 2 AlTi, Ni 3 Ti, (NiCr)ss (ss represents solid solution), γ phase, while the interfacial phases of joint with Ti/Cu interlayers consist of Ti 3 Al, Tiss, Ti 2 Ni, rich Cu-NiAlTi, α 2 -Wss, Ni 2 AlTi, (NiCr)ss, γ phase. Besides, Cr and W atoms are concentrated in the zone near Ni-Cr-W superalloy because their diffusion is difficult. Ti/Cu foils can promote diffusion and reaction; therefore the micro-cracks in the bonded joint will be avoided efficiently.

Simulation of Microstructure for Hot Pack-Forging of a High Nb Containing TiAl Alloy

Abstract The Deform-3D software was employed to describe the microstructure evolution of a high Nb containing TiAl billet (Ti-42Al-8Nb-0.2W-0.1Y) during its hot pack-forging. To obtain the simulation parameters, hot compression tests were performed at temperatures of 1100∼1250 °C and strain rates of 0.001∼0.5 s−1. Dynamic recrystallization model was developed by using an undirected method, and the recrystallized volume fraction was described by an Avrami-type equation. The constitutive model of the flow stress of the alloy was established based on Cingara hardening equation and the dynamic recrystallization model. The simulated results indicate an inhomogeneous distribution of grain structure occurs due to friction, heat loss and single direction upsetting operation in forging process. The microstructure evolution of the present alloy during pack-forging is validated by comparing the simulated results with that of the experimental data. The result indicates that the implemented microstructure module can be effectively used to predict the microstructure evolution of the high Nb containing TiAl alloy during pack-forging.

PHASE SELECTION AND THE SOLIDIFICATION CHARACTERISTICS OF TiAl BASE ALLOYS IN THE NONEQUILIBRIUM SOLIDIFICATION

The Al content and the β stabilizers in TiAl base alloys were investigated,which have an important effect on the solidification characteristics and phase selection.TiAl base alloys with single β-solidifying have the characteristics of low Al content and narrow crystallization temperature range.It is attributed to forming a homogeneous fine-grained microstructure.Dendrite morphology in TiAl base alloys is investigated by SEM-BSE in the nonequilibrium solidification conditions.The relationships between undercooling and nucleation of two phases are investigated systematically by using the classical nucleation theory in the undercooled Ti48A12Cr2Nb(atomic fraction,%) peritectic alloy.By calculating the interfacial critical nucleation work and steady state nucleation rates of β phase(bcc) and α phase(hcp),it is obtained that β phase is always prior to nucleate from the Ti48A12Cr2Nb undercooled melts at the cooling rate about 15 K/s in the achieved undercooling range.Due to narrow crystallization temperature range,microstructures of TiAl base alloys with high Al content as the hyper-peritectic solidification path take on the gradual solidification style and finer dendrite morphology.It is conducive to reducing and eliminating shrinkage porosity and hot cracking caused by mushy solidification.The β stabilizers have an important effect on solidification path depending on its Al equivalent and will change the solidification path of TiAl alloy from hyper-peritectic solidification to hypo-peritectic solidification.The overmuch Al content in TiAl base alloys leads to the formation of large number of massive 7 phase impacting on strength and ductility of alloys.

Phase Transformation during the Continuous Cooling in Near α Titanium Alloy Ti60

Abstract The phase transformation mechanism of near α titanium alloy Ti60 was investigated during the continuous cooling from β phase field, at the cooling rate from 80 °C·s−1 to 0.1 °C·s−1. The initial and final temperatures and kinetics of the β→α′ and β→α diffusion type transformations were analyzed by a dilatometric method. Over the cooling rate of 50 °C·s−1, the original β phase rapidly transformed to α′ martensite at 910 °C in the form of acicular platelets without any β phase left. A great amount of dislocations and (10 1) stacking faults were found inside the marten site. Under cooling rate of 5 °C·s−1, α phase nucleates along the grain boundary as well as inside the grain, while at the cooling rate below 1 °C·s−1, the nucleation occurred only along the grain boundary. A small amount of retained β phase was observed along the α platelet boundary under TEM, while the ordering transformation of α phase occurred inside the platelet, resulting in forming of α2 phase. Therefore, the cooling rate determines the alloy phase transformation mechanism and the inner structure of product phase.