The recognition of the potential for enhanced fracture toughness that can be derived from controlled, stress-activated tetragonal (t) to monoclinic (m) transformation in ZrO2-based ceramics ushered in a new era in the development of the mechanical properties of engineering ceramics and provided a major impetus for broader-ranging research into the toughening mechanisms available to enhance the fracture properties of brittle-matrix materials. ZrO2-based systems have remained a major focal point for research as developments in understanding of the crystallography of the t→m transformation have led to more-complete descriptions of the origins of transformation toughening and definition of the features required of a transformation-toughening system. In parallel, there have been significant advances in the design and control of microstructure required to optimize mechanical properties in materials developed commercially. This review concentrates on the science of the t→m transformation in ZrO2 and its application in the modeling of transformation-toughening behavior, while also summarizing the microstructural control needed to use the benefits in ZrO2-toughened ceramics.

Transformation Toughening in Zirconia‐Containing Ceramics

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

The morphology and crystallography of lath martensite in Fe-C alloys

Steels for bearings

A structural phase transition between the cubic (space group, Fm3m) and tetragonal (space group, P42/nmc) phases in a zirconia–ceria solid solution (Zr1−xCexO2) has been observed by Raman spectroscopy. The cubic–tetragonal (c–t″) phase boundary in compositionally homogeneous samples exists at a composition X0 (0.8 < X0 < 0.9) at room temperature, where t″ is defined as a tetragonal phase whose axial ratio c/a equals unity. The axial ratio c/a decreases with an increase of ceria concentration and becomes 1 at a composition X′0 (0.65 < X′0 < 0.7) at room temperature. The sample with a composition between X0 and X′0 is t″ ZrO2. By Raman scattering measurements at high temperatures, the tetragonal (t″) → cubic and cubic → tetragonal phase transitions occur above 400°C in Zr0.2 Ce0.8O2 solid solution.

Raman Scattering Study of Cubic–Tetragonal Phase Transition in Zr1−xCexO2 Solid Solution

The kinetics and metallography of isothermal transformations in four hypereutectoid steels (0.85 to 1.80 wt pct C) have been studied in the temperature range between 623 and 333 K. Isothermal transformation diagrams for each steel were constructed by means of dilatometry and microscopy. It was found that the C curve, the reverse in kinetics, which has been called “Swing Back”, appeared at the temperature near the in each steel. This paper focuses on the swing back phenomena appearing in four steels. The nose temperatures of C curves nearin 0.85, 1.10, 1.45, and 1.80 wt pct C steels were determined to be about 520, 440, 400, and 375 K, respectively. It was clarified that the C curves near in 1.45 and 1.80 wt pct C steels were associated with the formations of lower bainite with midrib (LBm), thin-plate isothermal martensite (TIM), and leaf-like isothermal martensite (LIM), but those in 0.85 and 1.10 wt pct C steels were related to the formation of LBm. On the basis of the kinetics and metalography, the temperatureJcarbon-contentJtransformation diagram was constructed in the hypereutectoid range of steel.

Swing back in kinetics near M s in hypereutectoid steels

The isothermal transformations in five hypereutectoid steels (0.85 to 1.80 wt pct C) have been studied in the temperature range between 623 and 333 K. Two types of lower bainite and a thin plate isothermal martensite were observed. One of the lower bainites was the conventional lower bainite (CLB) formed at the high temperature range of 623 to 473 K, and the other was the newly named “lower bainite with midrib” (LBm) formed at the lower temperature range of 473 to 423 K. The thin plate isothermal martensite (TIM) was also observed below 373 K. This paper brought LBm into focus. Arrhenius plots (transformation ratevs l/T) for each steel revealed an abrupt change in kinetics at the temperature range between 483 and 443 K. This change was considered to correspond to the transition from CLB to LBm. The following two-stage process for the LBm formation is proposed: at the first stage a TIM is formed, which constitutes a midrib of LBm, and secondly the bainitic decomposition of austenite at TIM/austenite interfaces takes place. That is, an LBm is a composite of isothermal martensite and lower bainite.

M. Oka

Papers

Swing back in kinetics near M s in hypereutectoid steels

Lower bainite with midrib in hypereutectoid steels

Couplings of Thin-plate Martensites in an Fe–Ni–C Alloy

Structural study on the tetragonal to monoclinic transformation in arc-melted ZrO2-2mol.%Y2O3—I. Experimental observations

{332} Deformation twins in a Ti-15.5 pct V alloy