Barry C. Muddle

Characterisation of strengthening precipitate phases in a Mg–Y–Nd alloy

Abstract Strengthening precipitate phases in a Mg–Y–Nd based alloy (WE54), aged at 250°C, have been characterised using transmission electron microscopy. Precipitation at 250°C involves formation of three separate metastable phases, {11 2 0} α platelets of an as yet unidentified phase, and the phases designated β′ and β1, preceding formation of the equilibrium phase β. All three phases β′, β1 and β are present in significant fractions in peak-aged samples. The β′ phase has a base-centred orthorhombic structure, with a potential point group of mmm. The β1 phase has an f.c.c. structure (space group Fm 3 m , a=0.74±0.01 nm), which renders it isomorphous with a family of intermetallic compounds of the general form Mg3X, where X represents Nd, Ce, La, Pr, Dy and Sm. The equilibrium phase β has an f.c.c. structure (space group, F 4 3 m , a=2.2±0.1 nm), which makes it isomorphous with Mg5Gd. The formation of β1 phase is shown to generate significant shear strain energy, and a mechanism of shear strain energy accommodation is proposed, involving nucleation in association with β′ phase. With prolonged ageing at 250°C, the β1 phase transforms in situ to the equilibrium β phase.

The precipitate Ω phase in Al-Cu-Mg-Ag alloys

The combined addition of small concentrations of Ag and Mg to Al-Cu alloys promotes precipitation of a phase, designated Ω, that forms as thin, hexagonal-shaped plates on matrix {111}α planes. The structure, morphology and composition of this phase have been examined in two quaternary alloys using transmission electron microscopy, electron microdiffraction and energy dispersive X-ray spectroscopy. Electron microdiffraction patterns from the precipitate phase may be indexed in accordance with an orthorhombic structure (a = 0.496 nm, b = 0.859 nm, c = 0.848 nm) and the orientation relationship between precipitate and matrix lattices is such that (001)Ω∥(111)α and [010]Ω∥[101]α. The morphology of the precipitate phase is consistent with the intersection point group (2m) defined by symmetry elements common to the two lattices in the observed orientation relationship. The plate shape parallel to (111)α is determined by a pinacoid parallel to the common 2-fold axis, [010]Ω. The hexagonal form in this plane is defined by four equivalent prism facets with a common direction perpendicular to [010]Ω and the truncation of this prism section by a second pinacoid normal to this 2-fold axis. The six facets thus defined are equivalent geometrically and may be associated with the hexagonal lattice on which the orthorhombic structure of the precipitate phase is based. Energy dispersive X-ray microanalysis indicates that the Ag in quaternary alloys partitions to the Ω phase during ageing and there is evidence that it may segregate at the interface with the matrix. On the other hand, Ag does not partition to the phase θ′ when it coexists with Ω in these alloys or is present alone in the ternary alloy Al-Cu-Ag. There is also evidence to suggest that Mg associates itself with the Ω phase, but the observations are not conclusive.

Size-dependent modifications of the Raman spectrum of rutile TiO2

Crystallite-size-dependent variations in the Raman spectrum of rutile TiO2 have been characterized and compared with those of the well-investigated rutile SnO2. For an average crystallite size below ∼25nm, the Raman spectrum of rutile TiO2 nanocrystals displays an additional low-frequency, possibly surface vibrational, mode at ∼105cm−1. The disorder-activated, high-frequency surface modes seen in the Raman spectrum of rutile SnO2 nanocrystals are absent. The Eg and A1g vibrational modes of rutile TiO2 show systematic redshifts, broadening, and intensity reductions with decreasing crystallite size, which are consistent with phonon confinement behavior. A phonon confinement model provides reasonable crystallite size quantification, as in the case of rutile SnO2 and RuO2.

Precipitation in magnesium alloy WE54 during isothermal ageing at 250 C

There has been a rapid growth in interest in the development of higher strength, creep-resistant magnesium alloys for the automotive and aerospace industries. All three precipitate phases have been described to form as plates on {l_brace}1{bar 1}00{r_brace}{sub {alpha}} planes of the magnesium matrix phase, and it has been proven difficult to distinguish them on the basis of morphology. These prismatic precipitate plates have been suggested to play an important role in strengthening such magnesium alloys. However, there has been little detailed characterization of these phases and there is a lack of experimental evidence to support the reported precipitation sequence. The role of the metastable intermediate precipitate phase(s) in the nucleation of successive intermediate or equilibrium precipitate phases remains to be established. It is thus the purpose of the present paper to report experimental observations of the precipitation sequence in a WE54 alloy during isothermal ageing at 250 C, with particular attention given to the role of intermediate precipitate phase(s) in nucleating successive metastable and equilibrium precipitation products.

Precipitation hardening of Mg-Ca(-Zn) alloys

Binary Mg-1Ca alloy exhibits a moderate precipitation-hardening response during isothermal ageing at 200 °C. The ternary addition of a small concentration of Zn (1wt%) to the binary alloy leads to a substantial increase in peak hardness and an accelerated rate of ageing. The maximum hardness achievable (63 HV) is comparable with that of ZC63, Mg-8Al-lRE and Mg-1.3RE alloys. This significant increase in maximum hardness appears to be associated with a refined distribution of precipitates. The primary intermetallic phase in as-cast Mg-1Ca-1Zn alloy is a ternary phase with an atomic composition of 69.4 ± 1.4 at.%Mg, 27.0 ± 0.3 at.%Ca, and 3.6 ± 1.0 at%Zn. This phase has a hexagonal crystal structure (point group 6/mmm), with lattice parameters of approximately a = 0.61 ± 0.01 nm, c = 1.02 ± 0.01 nm, which appears isomorphous with Mg2Ca (space group P63/mmc). Two solid state precipitate phases have been observed in as-cast microstructure of Mg-1Ca-1Zn alloy, a hcp phase with a = 0.623 nm, c = 1.012 nm, and a hcp phase with a = 0.556 nm, c = 1.042 nm. These two precipitate phases have a thin plate shape, and have identical orientation relationships with respect to the matrix phase: (0001)P // (0001)m, [2110]P // [1010]m.

Effects of cold work on precipitation in Al-Cu-Mg-(Ag) and Al-Cu-Li-(Mg-Ag) alloys

A study has been made of the effects of cold work prior to aging on precipitation hardening in selected Al-Cu-Mg-(Ag) and Al-Cu-Li-(Mg-Ag) alloys. General aging characteristics have been determined by differential scanning calorimetry, and response to hardening has been correlated with microstructure using transmission electron microscopy (TEM), selected area electron dif-fraction (SAED), and quantitative stereology. Particular attention has been given to the phases Ω andT1 that form on the {111 }α planes, although information on the precipitates θ′,S′ (orS), and δ′ is also reported. Although Ω andT1, have similar morphologies and habit planes, their response to cold work prior to aging is different. Deformation promotesT1 formation at the expense of the δ′ phase in Al-Cu-Li alloys and at the expense of δ′, θ′, andS′ in Al-Cu-Li-Mg-Ag alloys. On the other hand, in Al-Cu-Mg-Ag alloys, deformation assists precipitation of θ′ at the expense of Ω phase, and some decrease is recorded in the hardening response. Prior cold work is also found to reduce the response during natural aging in most alloys. These results are discussed in terms of the role of particular alloying additions.

Precipitate stability in AlCuMgAg alloys aged at high temperatures

Abstract A study has been made of the thermal stability of the Ω phase in AlCuMgAg alloys aged at high temperatures (200 to 350°C). This phase, which precipitates as thin plates on the {111}α planes, has been shown to be replaced by the equilibrium precipitate θ (Al2Cu) after prolonged ageing (e.g. 2400 h at 250°C). Measurements have been made of the thickening behaviour of the Ω plates and the various orientations and morphologies of the θ phase have been characterised. Whilst there is some evidence for the direct allotropic transformation of Ω to θ, it is concluded that a gradual dissolution/re-precipitation mechanism dominates the changes to microstructure at these high temperatures. Although magnesium and silver are known to segregate to the Ω phase, they were not detected in association with θ. Rather they were found to partition to sites of the S phase (Al2CuMg) which forms as a minor precipitate under these ageing conditions.

Characterisation of precipitate phases in magnesium alloys using electron microdiffraction

Abstract Recent results of the characterisation of the structure, morphology and orientation of fine-scale, strengthening precipitate phases in selected magnesium alloys using transmission electron microscopy and microdiffraction are reviewed. The strengthening precipitate phases in Mg–Y–Nd alloys, aged to maximum hardness at 250°C, have been found to include two metastable precipitate phases β′ and β1, and the equilibrium precipitate β. The β′ phase has a globular form, a base-centred orthorhombic structure (potential point group of mmm), and an orientation relationship such that (100)β′//(1210)α, [001]β′//[0001]α. The β1 phase has an f.c.c. structure (space group Fm 3 m), and an orientation relationship that may be described by ( 1 12) β 1 //(1 1 00) α , (100)β1//[0001]α, and forms as plates parallel to {1 1 00} α . The β phase has an f.c.c. structure (space group F 4 3m ) and also forms as plates on {1 1 00} α, with an orientation relationship with the matrix phase that is identical to that observed for β1 phase. Precipitates in Mg–Al alloys, aged isothermally at 200°C, invariably have the b.c.c. structure of the equilibrium precipitate phase β (Mg17Al12). Three orientation relationships have been observed between β and the matrix phase. Most precipitates have an irrational orientation relationship that approximates to the Burgers relationship, (001)β//(0001)α, [1 1 1] β //[2 1 1 0] α , and a faceted lath morphology with habit plane parallel to (0001)α. A minor fraction of precipitates posses an orientation relationship that is of the form (1 1 0) β //(1 1 00) α , [111] β //[0001] α , and have a prismatic rod morphology. The long axes of these rods are parallel to [0001]α, and their faceted surfaces are parallel to {1 1 00} α . A few precipitates are observed to have an orientation relationship such that (1 1 0) β ∼//(1 1 00) α , [11 5 ] β ∼//[0001] α and a rod shape, with their long axes apparently inclined with respect to [0001]α.

Analysis and characterisation of ultra-fine ferrite produced during a new steel strip rolling process

The designing of processing routes that minimize the final ferrite grain size is essential for the development of high strength steels with improved toughness and ductility. In this paper, a novel procedure for producing ultra-fine ferrite is investigated. This method is attractive in terms of its relative simplicity and ability to refine the ferrite grain size in relatively low cost steels. In an earlier paper, it was stated that the high level of ferrite grain refinement occurring during this process was likely to be the result of a strain-induced transformation mechanism. Thus, it has been termed the SITR (strain-induced transformation rolling) process. In the present paper, detailed characterization of the fine ferrite produced using this technique has been performed with the aim of providing a deeper insight into the important factors giving rise to its generation.

Nonlinear size dependence of anatase TiO2 lattice parameters

We characterized the size dependence of anatase TiO2 lattice parameters using Rietveld analysis of angle-dispersive synchrotron x-ray diffraction data obtained on a suite of nanocrystalline samples. The refined crystal structure and microstructure data suggest that for crystallites with size less than ∼10nm, the lattice parameters vary nonlinearly. Small lattice expansion, associated with possible increased Ti vacancy and lattice strain, at reduced crystallite size observed in our samples is in contrast to the lattice contraction behavior reported for “pure” anatase nanocrystals. The nonlinear, composition-dependent variation of anatase unit cell volume contrasts with the linear expansion behavior of rutile lattice at finite sizes.