Li Chunzhi

Determination of structure of Al20Cu2Mn3 phase in AlCuMn alloys

Abstract The structure of Al20Cu2Mn3 (T phase) which is major constituent phase in AlCuMn alloys has been determined using convergent beam electron diffraction (CBD), selected area electron diffraction (SAD) and geometric figure-forming method (GFFM). It is found that the T phase has Bbmm space group and lattice parameters of a = 2.41 nm, b = 1.25 nm, c = 0.78 nm, and takes the shape of rod with axis along [010] direction.

Microstructure of SiCp-reinforced A356 cast Al metal-matrix composite

The microstructure of SiCp-reinforced A356 cast Al metal-matrix composite (MMC) has been investigated by means of transmission electron microscopy and energy-dispersive X-ray analysis. It is found that the MMC contains β-SiC, (β + γ2) SiC composite, eutectic Si, GP zones of Si sequence, a new J phase and the amorphous phase AlxSi9Ca2. The γ2-SiC is lathlike with a triclinic crystal structure having a=0.308 nm, b=0.305 nm, c=1.262 nm, α= 93.8 °, β = 90.0 °, and γ = 60.0 °. The J phase is bulk-like with a C-face-centred orthorhombic crystal structure having a = 0.680 nm, b = 1.170 nm and c = 0.826 nm.

Transmission electron microscopy on the microstructure of 7050 aluminium alloy in the T74 condition

The microstructure of 7050 aluminium alloy in the T74 condition has been investigated by transmission electron microscopy. It was found that the alloy contains the superlattice Al3Zr phase, η′ phase and Al7Cu2Fe constituent phase. The η′ phase is proposed to have an orthorhombic crystal structure witha=0.492 nm,b=0.852 nm andc=0.701 nm. The orientation relationship between the matrix and η′ phase is [11−2]m//[100]η′; [1−]m//[010]η′;[−1−1−1]m//[001]η′. The phases on the small-angle grain boundary are found to be mainly η′ phase and Cu/Si-rich phase, whereas on the large-angle grain boundary there is only η phase.

On the η′ precipitate phase in 7050 aluminium alloy

Abstract The η′ precipitate phase in 7050 aluminium alloy has been investigated by means of transmission electron microscopy and dynamical diffraction simulation. It is found that the η′ phase has a ⊂-face-centered orthorhombic crystal structure with a=0.492 nm, b=0.852 nm, c=0.701 nm. The orientation relationship between the η′ phase and the matrix is [1 1 −2]m//[1 0 0]η′, [1 −1 0]m//[0 1 0]η′, [−1 −1 −1]m//[0 0 1]η′. A crystal structure model of the η′ phase has been developed and validated by means of dynamical diffraction simulation.

Study of the transformation from T2- to R-phase in Al-Li-Cu-Mg alloy

Following the discovery of an icosahedral phase exibiting five-fold symmetry in rapidly solidified A1-Mn alloys [1], similar phases have been reported in other systems including A1-Li -Cu alloys [2-5]. In the A1-Li -Cu system, the icosahedral Tz-phase (A16CuLi~) is major coarse phase and has a great influence on the properties. The experimental results suggest that the T2-phase forms either through solid-state precipitation or during solidification and is stable [6, 7]. However, recent studies using transmission electron microscopy (TEM) show that the T2-phase becomes thermally unstable and recrystallizes at a temperature near 400 °C [8]. In the rapidly solidified A1-Mn alloy, the icosahedral A16Mn phase is also unstable and becomes a crystal A16Mn at temperatures higher than 380 °C [2]. The purpose of this study was to investigate the Tz-phase transformation in A I L i C u M g alloy by TEM. There are speculations but no experimental evidence for transformation from T2to R-phase. Through observing the microstructures of as-cast and homogenized alloys, we found that Tz-phase decomposes at elevated temperature and transforms into the stable R-phase A15CuLi3. The chemical composition (wt %) of the experimental alloy was 2.55Li, 1.29Cu, 0.93Mg, 0.13Zr and balance A1. The ingot was obtained by semicontinuously direct chill casting, and two pieces chosen from this ingot were homogenized at 525 and 540 °C for 20 h, respectively. Differential scanning calorimetry (DSC) was used to determine the melting and forming temperatures of phases, and a sample of 30.7 mg was heated from 100 to 580 °C at a rate of 20 °C min -1 in aluminium containers. The specimens for TEM observation were prepared by two-jet electropolishing in an electrolyte containing one part HNO3 and three parts CH3OH (by volume). The TEM observation was performed in an H-800 and the accelerating voltage was 200 kV. The structure of an unknown phase was determined using selected-area electron diffraction (SAD). The tyical morphology of as-cast alloy is shown in Fig. la. The microstructures consist of icosahedral T-phase and aluminium solid-solution phase, which form binary eutectic. Fig. lb is the SAD pattern indexed from one spherulite of the eutectic in 0~(A1), and Fig. lc are the sequences of SAD patterns from T 2 dendrite, and obviously exhibit a five-fold symmetry. After 525 °C homogenization for 20 h the microstructure of the alloy changed greatly. It was found that the T2-phase disappeared and there were two kinds of other phases (Fig. 2). The first kind of Figure I (a) The microstructure of the binary eutectics in as-cast alloy, (b) the SAD pattern indexed from one spherulite o:(A1) in the eutectics and (c) the sequence of SAD patterns from T 2 dendrite.

A transmission-electron-microscopy study of a face-centred-cubic phase in an Al-Li-Cu-Mg-Zr alloy

Icosahedral T2 phases can form either by solid-state precipitation or during solidification in Al-Li-Cu-Mg alloys. The T2 phase forming during solidification can transform to an R phase at high annealing temperatures. The T2 phase forming by solid-state precipitation coexists with the Y phase, which has a face-centred cubic (f.c.c.) structure with lattice parameter a≈2.0 nm and can form microtwins with the twin plane of (111). The orientation relationships between the C phase and the T2 phase are: i¯5∥Y〈0 1 1〉, Y〈1 1 3〉; i¯3∥Y〈1 1 1〉, Y〈1 2 3〉, Y〈1 1 5〉; Y〈2 3 5〉; i2∥Y〈0 1 1〉, Y〈1 1 1〉, Y〈1 1 2〉, Y〈1 1 3〉, Y〈1 1 5〉.

STUDY OF THE NEW FRANK KASPER PHASES IN AL-LI-CU-MG ALLOYS

A new group of Frank-Kasper phases has been observed in Al Li-Cu-Mg alloys using high resolution electron microscopy (HREM). The Y phase, which has a face-centre cubic structure with a = 2.0 nm, occurs in grain boundaries during precipitation heat treatment of supersaturated solid solution. Considerable amount of intrinsic faults, extrinsic faults and microtwins were observed in the Y phase. A new domain denominated as D intergrowth with Y phase along {111} planes has been found. By combination of Y phase and D domain, new domains can be constructed.

A new phase transition of SiC particulates in cast Al metal matrix composites

Abstract A pseudo-martensite phase transition from β- to γ2-SiC has been discovered in cast Al metal matrix composites, by means of transmission electron microscopy and energy-dispersive X-ray analysis. γ2-SiC is lath-like with a triclinic crystal structure with a = 0.308 nm, b = 0.305 nm, c = 1.262 nm, α = 93.8°, β=90.0° and γ=60.0°. The phase transition is attributed to the effect of the difference in thermal expansion coefficient between β-SiC and the Al matrix.

High resolution electron microscopy study of defects in an f.c.c. phase formed by ingot casting in AlLiCuMg alloys

An fc.c. phase designated Y has been studied by means of high resolution electron microscopy (HREM) and transmission electron microscopy in Al-Li-Cu-Mg alloys. HREM observation and electron diffraction patterns show the intergrowth of another Franker-Kasper domain (designated D) with the Y phase. The D domain can grow on both the (001) and the (111) planes of the Y phase. The orientation relationships between the D domain and the Y phase are [110]Y//[100]D, (111BAR)Y//(001)D and [110]Y//[100]D, (001)Y//(010)D.

INVESTIGATIONS ON THE INTERGROWTH OF THE O-PHASE AND Z-PHASE IN AL-LI-CU-MG ALLOYS

acad sinica,inst met res,atom imaging solids lab,shenyang,peoples r china.;wang, sc (reprint author), inst aeronaut mat,met phys lab,beijing 100095,peoples r china