Showing papers by "Hengzhi Fu published in 2013"

Microstructure characterization and mechanical properties of a Laves-phase alloy based on Cr2Nb

Abstract: Microstructure and mechanical properties of a binary Cr 2 Nb-based alloy containing the Laves phase Cr 2 Nb and Cr phase have been studied in the arc-melted condition. Microstructure consisting of fully coupled eutectic was firstly observed in this hypereutectic alloy. The Cr 2 Nb primary phase experienced a faceted to nonfaceted transition as the solidification distance increased. Based on the maximum interface growth temperature criteria, the microstructure evolutions in this alloy were explained successfully by means of the competitive growth between the β-Cr 2 Nb phase and eutectic. Meanwhile, the yield strength and fracture toughness of the samples taken from the bottom of the ingot attained 1986 MPa and 4.3MPa⋅m 1/2 respectively, which were considerably enhanced over the monolithic Cr 2 Nb. The enhancements were explained in terms of the second Cr phase toughening and grain boundary toughening mechanisms. Furthermore, the variation of mechanical properties in different position of arc-melted ingot was proven to result in different eutectic fracture mechanisms.

Longitudinal cross-section microstructure of growth striation in Al2O3/Y3Al5O12/ZrO2 directionally solidified eutectic ceramic prepared by laser floating zone

Abstract: Periodic growth striations with a width of 380 μm are observed in Al 2 O 3 /Y 3 Al 5 O 12 /ZrO 2 ternary directionally solidified eutectic ceramic prepared by laser floating zone It is found that the microstructure of the growth striations exhibits disparate growth characteristics The direct experimental evidence of the formation of the microstructure of the growth striations is obtained through examining the solid–liquid interface The lamellar coarsening of the growth striations is related to the nucleation of Al 2 O 3 particles and their engulfment by the extended Y 3 Al 5 O 12 phase The mechanism accounting for this phenomenon is explained by considering eutectic growth behavior under mutative condition induced by oscillatory convection

Directional solidification of Ni–Ni3Si eutectic in situ composites by electron beam floating zone melting

Abstract: Combining the intermetallic compound with the ductile metal at the eutectic composition is one promising method to improve the ductility of the intermetallic compound. This paper reports the microstructure and the micro-hardness of the Ni–Ni 3 Si eutectic in situ composites prepared by electron beam floating zone melting technique. Ni–Ni 3 Si eutectic in situ composites display regular lamellar eutectic structure at the solidification rate R =0.3–4.0 mm/min. The lamellar spacing is decreased with the increase of the solidification rate. The phase composition of the Ni–Ni 3 Si eutectic in situ composites is also determined by X-ray diffraction. Ni–Ni 3 Si eutectic in situ composites present lower micro-hardness than pure Ni 3 Si, although a small quantity of metastable Ni 31 Si 12 phase is formed during the directional solidification process.

A lateral remelting phenomenon of the primary phase below the temperature of peritectic reaction in directionally solidified Cu-Ge alloys

Abstract: During peritectic solidification, besides the longitudinal remelting of the primary phase at the temperature of peritectic reaction $$\left( {T_{\rm{p}}^K} \right)$$ , a lateral remelting phenomenon of the primary phase below $$T_{\rm{p}}^K$$ is observed under high velocity in directionally solidified Cu–Ge alloys. The lateral remelting occurs continuously along a liquid channel as temperature decreases, and the lateral remelting velocity is larger than that of peritectic transformation. The lateral remelting leads to the morphological change of the primary dendrites, even the fragmentation of dendrite arms. The phenomenon also means that the classical theory calculating the volume fraction of the primary phase during peritectic transformation can need to be modified under some conditions. However, under low velocity, the phenomenon is not so significant. The phenomenon is explained by means of solidification and remelting theory.

Prediction of the solidification path of Al-4.37Cu-27.02Mg ternary eutectic alloy with a unified microsegregation model coupled with Thermo-Calc

Abstract: An extended unified microsegregation model with solid back diffusion effects and five different dendrite morphologies was used to investigate the solidification path of Al-4.37Cu-27.02Mg (wt.%) ternary eutectic alloy at different cooling rates and solid back diffusion coefficients, coupled with Thermo-Calc. It was indicated that the cooling rates (Rf) had no obvious effect on the solidification path which was (L + α) → (L + α + T) → (L + α + β + T); but the solid back diffusion coefficient (Φ) had a great effect on the solidification path, which evolved gradually from (L + α) → (L + α + T) → (L + α + β + T) into (L + α) → (L + α + T) when Φ increased from 0 to 1. The volume fractions of primary α phase (Vα), binary eutectic (V2E) and ternary eutectic (V3E) at each solidification path were calculated. It was shown that V2E decreased with the increase of Rf whereas V3E increased and Vα was almost invariant. The dependence of V2E, V3E and Rf were determined by linear regression analysis given as: V2...

Primary dendrite distribution in directionally solidified Sn–36 at.% Ni peritectic alloy

Abstract: The primary dendrite arm spacing and its distribution at the solid–liquid interface has been examined in directionally solidified Sn–36 at.% Ni peritectic alloys under constant temperature gradient in a range of growth rates (2–200 μm/s). Statistical analysis of the primary dendrite arm spacing on transverse sections has been carried out using the minimum spanning tree and Voronoi polygon. The frequency distribution of the number of nearest neighbors determined by the Voronoi polygon suggested that the arrangement of dendrites at the solid–liquid interface could be visualized as hexagonal tessellation. The primary dendrite arm spacing determined by the conventional area counting method and minimum spanning tree all decreased with increasing growth rate, and a range of primary dendrite spacing was present during solidification. The range first increased with increasing growth rate, but when the growth rate exceeded 20 μm/s, it turned to decrease, which can be attributed to disorder induced by growth rate and interdendritic convection.

Effect of rotating magnetic field on microstructure formation of directionally solidified Sn–1.6Cd peritectic alloy

Abstract: In order to investigate the effect of rotating magnetic field on the microstructure formation of peritectic alloys, directional solidification experiments of Sn–1.6Cd peritectic alloy have been conducted under different rotating magnetic field conditions. The directional solidification microstructure of Sn–1.6Cd peritectic alloy changes from banded structure to axisymmetric isolated banded structure to axisymmetric oscillatory tree-like banded structure and to single primary phase structure when the magnetic Taylor number of forced-melt flow generated by a rotating magnetic field increases from 0 to 91 to 364 and to 1456. The second and third banded structures are observed in a peritectic alloy for the first time. The results indicate that it is possible to control solidification microstructure and prepare a single primary phase structure by using a rotating magnetic field during directional solidification of peritectic alloys. The experiments show that the effect of forced-melt flow on solute distribution transforms from solute buildup to homogenization with an increase in the magnetic Taylor number. The formation mechanisms of those structures are also discussed.

Two-phase separated growth and peritectic reaction during directional solidification of Cu–Ge peritectic alloys

Abstract: During directional solidification of Cu–Ge peritectic alloys, a two-phase separated structure has been observed. With proper growth conditions, the peritectic ζ-Cu5Ge and primary α-Cu phases completely separate and form cylindrical layered structures. It is found that the formation of the separated structure is closely related to double diffusive convection and growth conditions. In the two-phase separated structure, a large trijunction region of peritectic reaction forms around the cylindrical α-Cu phase. During peritectic reaction, the morphological instabilities of ζ-Cu5Ge occur under high pulling velocities and are explained by the constitutional undercooling criterion. A new coupling growth between the ζ-Cu5Ge-phase and the groove of α-Cu phase near the trijunction is observed. Different from peritectic coupling growth, the diffusion coupling is established below the peritectic temperature. This two-phase separated growth process creates new opportunities for the fabrication of functionally layered materials.

Influence of initial solid-liquid interface morphology on further microstructure evolution during directional solidification

Abstract: Preparation of the initial solid–liquid interface on which growth is started is a very critical step in directional solidification experiments. Dedicated experiments concerning preparation of the initial solid–liquid interface morphology and its influence on further directionally solidified microstructure were performed on Cu-20 wt% Sn peritectic alloy in a Bridgman-type furnace. To verify the morphology of the initial solid–liquid interface, steady-state directional dendritic growth was interrupted by thermal stabilization ranging from 0 to 1 h prior to quenching. With thermal stabilization duration increase, the solid–liquid interface morphology degenerated from dendritic to cellular and finally to planar. To verify the influence of the initial state on further solidification microstructure, directional solidification experiments were performed at a low pulling rate of 1 μm/s with different initial solid–liquid interface morphologies. The initial state affects solute redistribution and formation of peritectic coupled growth structure in the subsequent directional solidification process.

Influences of travelling magnetic field on the dendritic structures of Sn-1.8Cd peritectic alloy during directional solidification

Abstract: The effects of melt flow driven by a travelling magnetic field (TMF) on solidification structures of Sn–1.8 wt.% Cd peritectic alloy have been investigated numerically and experimentally. Numerical results indicate that the flow velocity at the solid–liquid interface under a downward TMF is smaller than that under an upward TMF. The experimental results show that the growth directions of dendrites are chaotic, and several crotches among the dendrites are observed at the solid–liquid interface in the case of no field. It is concluded from TMF results that the ordered growth of dendrites at two different directions occurs, and only one crotch among the dendrites appears at the solid–liquid interface. The location of the crotch gradually approaches the interface center with increasing magnetic field intensity (B≤10.3 mT). Moreover, the growth of high-order branches occurs at the crotch under a downward TMF. A simple model is established for explanation and it well corresponds to the experimental results.

Uniformity analysis of magnetic field in an electromagnetic cold crucible used for directional solidification

Abstract: Purpose – The purpose of this paper is to introduce a numerical calculation method to study the uniformity of the magnetic field in a cold crucible used for directional solidification (DS) and provide information for designing a cold crucible that can induce a uniform magnetic fieldDesign/methodology/approach – To obtain the characteristics of the magnetic field in a cold crucible and its influence on the directional solidification processing, based on experimental verification, 3‐D finite element (FE) models with different crucible configuration‐elements and power parameters were established to study the uniformity of the magnetic field in a cold crucible In addition, different TiAl ingots were directionally solidified with different cold crucibles, and the solid/liquid (S/L) interfaced were examined to investigate the effect of the magnetic field on the macrostructure of those ingotsFindings – The uniformity of the magnetic field in a given domain can be quantitatively analyzed by statistical methods

Electromagnetic Cold Crucible Technology Applied for Producing Big-Sized γ-TiAl Based Ingots with Directional Growth Structure

Abstract: Developing high performance aero-engines usually depends on the development of new candidate materials with charming properties such as relatively light-weight, good high temperature strength and environmental resistance. Fortunately, γ-TiAl alloys were followed into this catalogue that they offer a significant potentiality for weight savings. Recently, in order to improve their low room temperature plasticity, a motive technology called electromagnetic cold crucible directional solidification technology for preparation γ-TiAl alloy ingots with directional growth structure was put forward by our group. By the approach, TiAl ingots with controllable lamellar structures and less impurity contaminations are obtained. Typically, Ti-46Al-0.5W-0.5Si and Ti-47Al-2Cr-2Nb were performed with respect to compositions and solidification parameters. As a result, Ti-46Al-0.5W-0.5Si DS-samples exhibit good combination of mechanical properties at room temperature in TS of 500MPa and EL of 2% in average. Meanwhile TS of DS Ti-47Al-2Cr-2Nb reaches 650 MPa and EL exceeds 3%.

Microstructure Investigation of Directionally Solidified NiAl-Cr(Mo)-xDy (x = 0, 0.1 wt.%) Hypereutectic Alloys at Different Withdrawal Rates

Abstract: The microstructures of directionally solidified Ni-31Al-32Cr-6Mo (at.%)-xDy (x = 0, 0.1 wt.%) hypereutectic alloys were studied at different withdrawal rates. The results show that the microstructure changes from the planar eutectic to the cellular eutectic and the volume fraction of the primary Cr(Mo) dendrites decreases for the Dy-free alloy with the withdrawal rate varying from 6 μm/s to 30 μm/s. The addition of 0.1 wt.% Dy promotes the planar-to-cellular transition. Moreover, the white Dy-containing phase does not form in the alloy for the planar interface growth (6 μm/s), but it can occur in the boundary of eutectic cells for the cellular interface growth (30 μm/s). A sketchy model of the planar and cellular growth is supposed to interpret it.

Effect of peritectic reaction on the migration of secondary dendrite arms in the presence of tertiary dendrites: analysis of a directionally solidified Sn–36 at.%Ni peritectic alloy

Abstract: Directional solidification experiments have been performed on Sn–36 at.%Ni peritectic alloy in a constant temperature gradient at different growth velocities. Experimental result shows that a “sawtooth” morphology forms on secondary dendrite arms during the migration of secondary dendrites in the presence of tertiary dendrite arms. A theoretic model is therefore proposed to describe the formation of this “sawtooth” morphology in the peritectic solidification with tertiary dendrite arms taken into consideration. The migration of secondary dendrite arms is caused by remelting/solidification at the hot/cold sides of a liquid pool between secondary dendrite arms, which is a form of temperature gradient zone melting. And, the “sawtooth” morphology is ascribed to the difference in remelting velocity at the hot side of liquid pool during the migration of secondary dendrite arms due to the presence of tertiary dendrite arms. In addition, the proceeding of peritectic reaction can accelerate the formation of “sawtooth” morphology.

Finite Element Analysis of Fretting Wear for Nuclear Inconel 690 Alloy

Abstract: Finite element method to analyze the fretting wear characteristics based on Hertz theory, Coulomb friction law and a modified Archard wear equation, has been applied to a cylinder-on-flat configuration for typical steam generator tube material Inconel 690 alloy and typical anti-vibration bar material Inconel 600 alloy (Cr plating) in the nuclear power plant, under gross slip and partial slip conditions. The evolutions of contact profile, surface contact variables with increase in wear cycles are predicted. The slip regime is predicted to have significant effects on the fretting wear behavior. Under the gross slip regime, it is found that the peak contact pressure occurs at the center of the contact scar, and the actual relative slip is slightly smaller than the applied value. The contact width increases, and the peak pressure decreases gradually with increase in wear cycles. Whereas under the partial slip regime, the peak contact pressure occurs at the stick-slip boundary, the actual relative slip is much smaller than the applied value, and no relative slip occurs in the stick zone. The contact width increases gradually, and the peak pressure increases rapidly with increase in wear cycles.

Secondary dendrite arm migration caused by temperature gradient zone melting in the directionally solidified Sn-40 at.% Mn peritectic alloy

Abstract: Sn–40 at.% Mn peritectic alloys were directionally solidified at different growth rates (1–100 μm/s) under a steep temperature gradient (40 K/mm). The migration of secondary dendrite arm was observed in this peritectic alloy in which both the primary phase and the peritectic phase are intermetallic compounds with nil solubility. This migration is caused by coupling remelting/solidification at the hot/cold sides of the liquid pool between two adjacent secondary dendrite arms by temperature gradient zone melting. Its novel feature is that the remelting temperature of primary phase is a little higher than the solidification temperature of peritectic phase. Analytical solutions based on the assumption that the solubility of both primary and peritectic phases are nil have been proposed to describe this migration. It has also been found that the migration of secondary dendrite arm is most obvious at intermediate growth rates under steep temperature gradient in the directionally solidified Sn–40 at.% Mn peritectic alloy.