P. R. Howell

The formation of α-Al2O3 from θ-Al2O3: The relevance of a “critical size” and: Diffusional nucleation or “synchro-shear”?

The coarsening of θ-Al2O3 crystals to a critical size is often interpreted as the first step in the shear nucleation of α-Al2O3. The existence of this so-called critical size has also been used to explain the observation that α-Al2O3 “nuclei” are generally twice as large as the crystals in the θ-Al2O3 matrix. This paper discusses the important issues in the nucleation of α-Al2O3 from θ-Al2O3. A few key experiments are also presented to clarify the nucleation process. It is concluded that a critical θ-Al2O3 crystal size is not a prerequisite for α-Al2O3 nucleation, but is primarily a result of the incubation time required to produce α-Al2O3 nuclei by diffusional nucleation. It is proposed that the large observed α-Al2O3 crystal size also does not result from a shear nucleation event in a critical size θ-Al2O3 crystal, but is due to the intrinsically low α-Al2O3 nucleation density, together with rapid growth of α-Al2O3 after nucleation.w

On the nucleation ofθ′ and T1 on Al3Zr precipitates in Al-Li-Cu-Zr alloys

Two quaternary Al-Li-Cu-Zr alloys have been investigated using electron microscopy. Ageing at 190° C resulted in the nucleation ofθ′ precipitates on the Al3Zr/matrix interface in addition to heterogeneous nucleation on matrix dislocations. In the majority of cases, the broad, coherent face of theθ′ plans ms in contact with the Al3Zr precipitates. Similar evidence showed that nucleation of T1 precipitates occurred on the Al3Zr, but to a lesser extent thanθ′. Solid-solid nucleation theory has been used to account for the Al3Zr acting as a nucleation substrate.

A combined single crystal X-ray diffraction and electron diffraction study of the T2phase in Al-Li-Cu alloys

Both single crystal X-ray diffraction techniques and convergent beam electron diffraction have been employed to examine the structure of theT2 (Al6CuLi3) phase in three particular Al-Li-Cu alloys. It is shown thatT2 displays icosahedral symmetry both in a high purity laboratory melt and in two “impure” alloys which had been processed industrially. Possible reasons for the five-fold symmetry ofT2 are discussed.

A preliminary study of precipitation in Ti4+-doped polycrystalline alumina

Precipitation processes in two titanium-doped aluminas (0.14 and 0.60 cation % titanium) have been examined using a variety of analytical electron microscopy techniques. The results strongly suggest that, for an ageing temperature of 1573 K, rutile is the only precipitate formed in the 0.14 cation % titanium material. However, evidence for the precipitation of both rutile and Β-Al2TiO5 (the latter at triple junctions) was obtained in the 0.60 cation % titanium sample.

Banding and the nature of large, irregular pearlite nodules in a hot-rolled low-alloy plate steel: A second report

The microstructure of a hot-rolled low-carbon plate steel has been examined using a combination of light microscopy and scanning electron microscopy. It has been found that in the hot-rolled condition, the microstructure consists of alternate bands of ferrite and pearlite, together with relatively large, irregular pearlite nodules. These large nodules were found to be comprised of pearlite, intragranularly nucleated ferrite (both Widmanstätten and idiomorphic), together with carbide-deficient and/or carbide-free regions. It is argued that the carbide-deficient and carbide-free regions form as a result of the premature initiation of the pearlite reaction, i.e. pearlite forms prior to the body of the austenite grains attaining the eutectoid composition. In order to model the formation of the banded structure, specimens were reaustenitized at 1050 °C for 10 min and furnace cooled. This heat-treatment cycle produced an austenite grain size which was less than the chemical banding wavelength. A model for the decomposition of austenite, under these conditions, is presented.

Nucleation ofδ′ (Al3Li) onβ′ (Al3Zr) in Al-Li-Zr and Al-Li-Cu-Zr alloys

En utilisant la theorie classique de germination, on montre que β peut agir comme substrat de germination pour δ (comme ceci a ete observe experimentalement). La barriere denergie peut etre reduite si δ germe sur une face β/δ de bas indice. Lefficacite de β comme substrat de germination augmente comme le rayon de β augmente

Microstructural development in non-oriented lamination steels

Two low carbon steels, in the temper-rolled condition, have been examined using a combination of light microscopy, scanning electron microscopy and transmission electron microscopy. The thermomechanical processing fragments the pearlite colonies and both cavities and cracks were found to be associated with cementite particles. It is suggested that these cracks and cavities arise during cold rolling due to the interaction of dislocation pile-ups with cementite. Subsequent growth occurred during continuous annealing.

A microanalytical study of the early stages of recrystallization in the nickel-base superalloy APK1

Analytical electron microscopy has been used to investigate the composition of various microstructural features in a powder-produced nickel-base superalloy. Specifically, it is shown that in the early stages of recrystallization the strain-free grains are depleted in theγ′-forming elements (i.e. titanium and aluminium). This observation is consistent with the hypothesis that recrystallization in this alloy is initiated in the immediate vicinity of large intergranularγ′ particles. It is also shown that the composition ofγ′ can be written as (Ni, Co)3 (Al, Ti, Cr).

Microstructural development in non-oriented lamination steels: Part 2 Isothermal transformation studies

The isothermal decomposition of austenite in two commercial low carbon (0.04 w/o) steels has been examined using scanning electron microscopy and transmission electron microscopy. Particular emphasis has been placed on analysing the pearlite reaction and the development of massive films of cementite at pro-eutectoid ferrite/pearlite interfaces. Similarly, grain boundary precipitation of cementite has been investigated. The results strongly support the contention that films of cementite at ferrite/pearlite interfaces form predominantly by a coarsening process. In addition, it is shown that grain boundary precipitation of cementite can occur from super-saturated ferrite or from the decomposition of austenite. Examination of the early stages of the pearlite reaction has provided evidence that multiple nucleation of cementite can be a necessary precursor to the development of a pearlite colony.

A microstructural study of a melt-spun ultra high-strength alloy steel

Melt spinning in a controlled atmosphere has been used to produce ribbons of an ultra high-strength alloy steel. The microstructure of these ribbons has been investigated using both optical and transmission electron microscopy. Comparison of the microstructure with that produced by conventional solid-state quenching shows that the martensitic structure is refined in the melt-spun ribbon and the formation of alloy carbides suppressed. These factors lead to a considerable increase in hardness of the meltspun ribbon with respect to that of the solid-state quenched material. The occurrence of interlath austenite is discussed in terms of the crystallography of the martensite transformation.