The mechanism of the bainite transformation in steels is reviewed, beginning with a summary of the early research and finishing with an assessment of the transformation in the context of the other reactions which occur as austenite is cooled to temperatures where it is no longer the stable phase. The review includes a detailed account of the microstructure, chemistry, and crystallography of bainitic ferrite and of the variety of carbide precipitation reactions associated with the bainite transformation. This is followed by an assessment of the thermodynamic and kinetic characteristics of the reaction and by a consideration of the reverse transformation from bainite to austenite. It is argued that there are useful mechanistic distinctions to be made between the coherent growth of ferrite initially supersaturated with carbon (bainite), coherent growth of Widmanstatten ferrite under paraequilibrium conditions, and incoherent growth of ferrite under local equilibrium or paraequilibrium conditions. The nature of the so-called acicular ferrite is also discussed.

Bainite in Steels

Amorphous semiconductors, being intrinsically metastable in nature, exhibit a wide variety of changes in their physical properties, particularly when photoinduced using bandgap illumination. This article reviews the photoinduced phenomena exhibited by amorphous semiconductors such as amorphous hydrogenated silicon (and other tetrahedrally coordinated materials) and chalcogenide glasses. Features exhibited in common by all types of amorphous semiconductors, whether in the experimentally observed photoinduced metastability or the theoretical models used to account for such behaviour, are stressed.

Photoinduced effects and metastability in amorphous semiconductors and insulators

The direct room-temperature synthesis (mechanochemical synthesis or mechanosynthesis—MS) of most metal carbides by milling metal—carbon powders (size some tens of micrometers) mixtures is described. The particle size of the carbides obtained is of the order of 20 nm. Stable, metastable, mixed, and new carbides can be formed. Moreover, metal—carbon alloys can be obtained, such as in the Fe—C system. The synthesized compounds have been characterized by X-ray diffraction and Mossbauer spectroscopy for iron-containing systems. Carbides of the following elements were obtained: Ti, V, Cr, Mn, Fe, Co, Ni, Zr, Nb, Mo, Hf, Ta, W, Re, Al and Si.

Room-Temperature Mechanosynthesis of Carbides by Grinding of Elemental Powders

The microstructure of cobalt-based hardfacing alloys deposited by manual metal arc (MMA) welding, tungsten inert gas (TIG) welding, and laser cladding has been investigated as part of a study attempting to establish the relationship between microstructure and abrasive wear properties. For typical deposition conditions, the differences in freezing rates associated with the three processes are found to give rise to large differences in microstructure. The MMA process is found to lead to the largest degree of dilution of the hardfacing deposit; the TIG and laser deposits exhibited much lower levels of mixing with the base plate. For the deposition conditions used in this study and for the alloys examined, the scale of the microstructure decreases in the order MMA, TIG, and laser cladding, leading to an increase in the deposit hardness in the same order. It is found that with alumina as an abrasive, the wear rate persistently is higher with the MMA deposits (which have the coarsest microstructure with the lowest starting hardness), the weight loss being approximately linear with time. The laser and TIG deposits, which have more refined microstructures and slightly higher carbon concentrations, both are found to exhibit significantly lower wear rates. Initially, the TIG samples are the most resistant to abrasion, even though their microstructure compares with that of the laser samples; this is a consequence of their higher ductility associated with a lower rate of strain hardening. The laser samples, which contain a lower matrix iron concentration, strain harden more rapidly; consequently, they exhibit an initial decrease in wear rate. With the much harder silicon carbide abrasive, all samples show similar wear rates which do not decrease with time. The wear data are found to correlate with scanning and transmission electron microscopy observations, and it is possible to rationalize the interaction among microstructure, abrasive, and alloy deposition processes.

Comparison of the microstructures and abrasive wear properties of stellite hardfacing alloys deposited by arc welding and laser cladding

Most metal carbides or slicides may be synthesized at room temperature, by ball milling mixtures of elemental powders for some tens of hours with a vibratory mill. Both stable and metastable compounds containing a high density of defects can be obtained. In general, phases stable at low temperatures are synthesized. This observation allows us to confirm recent estimations for the maximum temperature (∼600 K) attained in these powders during mechanical alloying. Exceptions are found for some MSi2 suicides with M = titanium, iron or molybdenum for which both low- and high-temperature phases are formed.

Mechanically driven syntheses of carbides and silcides

M7C2, carbides prepared in various conditions have been observed by electron microscopy and diffraction. Examinations of the first-order Laue zone with crystals having the c axis parallel to the electron beam demonstrate that the unit cell describing the structure of these carbides is the orthorhombic cell proposed by Fruchart and Rouault. This cell can be considered as an ordered cell of an average hexagonal cell and can have three orientations at 120° from each other. It contains metal atom tetrahedra whose local environment in prismatic interstices containing carbon atoms is typical and is never destroyed, whatever the conditions of preparation may be. Thus, in primary carbides present in cast irons and steels, the state of crystallization is such that a long-range order of the environment is established : ortho-rhombic domains are separated by {1I00} twins and antiphase boundaries. In Fe7C3 carbides crystallized from amorphous iron-carbon alloys, the local environments are only short...

Crystalline defects in M7C3 carbides

Various types of M7C3 carbides and M'7B3 borides have been studied by means of X-ray diffraction and electron microscopy. We confirm the three orientation variants of the orthorhombic cell proposed by Fruchart and Rouault for the M7C3 carbides and the hexagonal cell proposed by Aronsson and coworkers for the M'7B3 borides. These two cells, which have many similarities, differ from each other by the local environment of the prismatic interstices around each metal atom tetrahedron. We point out the following facts. The defects which are present in M7C3 carbides are twins or antiphase boundaries between the orientation variants of the orthorhombic cell. They can be ordered or disordered but they always conserve the typical local environment of the orthorhombic cell. Depending on the state of crystallization, this typical local environment can be short- or long-range ordered. It is probably because it is not possible to create defects without disturbing the local environment of the hexagonal cell that we observe very few defects in M'7B3 borides.

Crystallography of M7C3 carbides and M'7B3 borides: The difference in the local order in carbides and borides to explain the nature and density of defects in these compounds☆

E. Bauer-Grosse

Papers

Crystalline defects in M7C3 carbides

Crystallography of M7C3 carbides and M'7B3 borides: The difference in the local order in carbides and borides to explain the nature and density of defects in these compounds☆