P.M. Kelly

The martensitic transformation in ceramics — its role in transformation toughening

This paper reviews the current knowledge and understanding of martensitic transformations in ceramics - the tetragonal to monoclinic transformation in zirconia in particular. This martensitic transformation is the key to transformation toughening in zirconia ceramics. A very considerable body of experimental data on the characteristics of this transformation is now available. In addition, theoretical predictions can be made using the phenomenological theory of martensitic transformations. As the paper will illustrate, the phenomenological theory is capable of explaining all the reported microstructural and crystallographic features of the transformation in zirconia and in some other ceramic systems. Hence the theory, supported by experiment, can be used with considerable confidence to provide the quantitative data that is essential for developing a credible, comprehensive understanding of the transformation toughening process. A critical feature in transformation toughening is the shape strain that accompanies the transformation. This shape strain, or nucleation strain, determines whether or not the stress-induced martensitic transformation can occur at the tip of a potentially dangerous crack. If transformation does take place, then it is the net transformation strain left behind in the transformed region that provides toughening by hindering crack growth. The fracture mechanics based models for transformation toughening, therefore, depend on having a full understanding of the characteristics of the martensitic transformation and, in particular, on being able to specify both these strains. A review of the development of the models for transformation toughening shows that their refinement and improvement over the last couple of decades has been largely a result of the inclusion of more of the characteristics of the stress-induced martensitic transformation. The paper advances an improved model for the stress-induced martensitic transformation and the strains resulting from the transformation. This model, which separates the nucleation strain from the subsequent net transformation strain, is shown to be superior to any of the constitutive models currently available

Crystallography of Mg17Al12 precipitates in AZ91D alloy

Kikuchi diffraction was used to accurately determine the orientation relationship (OR) between Mg17Al12 precipitates and matrix in an AZ91D alloy. For both continuous and discontinuous precipitations, the Burgers OR and the Potter OR were equally observed. The lattice parameter of Mg17Al12 associated with the former is bigger than that of the latter

Boron solubility in Fe-Cr-B cast irons

Boron solubility in the as-cast and solution treated martensite of Fe-Cr-B cast irons, containing approximately 1.35 wt.% of boron, 12 wt.% of chromium, as well as other alloying elements, has been investigated using conventional microanalysis. The significant microstructural variations after tempering at 750 degreesC for 0.5-4 h, compared with the original as-cast and solution treated microstructures, indicated that the matrix consisted of boron and carbon supersaturated solid solutions. The boron solubility detected by electron microprobe was between 0.185-0.515 wt.% for the as-cast martensite and 0.015-0.0589 wt.% for the solution treated martensite, much higher than the accepted value of 0.005 wt.% in pure iron. These remarkable increases are thought to be associated with some metallic alloying element addition, such as chromium, vanadium and molybdenum, which have atomic diameters larger than iron, and expand the iron lattice to sufficiently allow boron atoms to occupy the interstitial sites in iron lattice

Edge-to-edge matching - The fundamentals

The basis of the present authors’ edge-to-edge matching model for understanding the crystallography of partially coherent precipitates is the minimization of the energy of the interface between the two phases. For relatively simple crystal structures, this energy minimization occurs when close-packed, or relatively close-packed, rows of atoms match across the interface. Hence, the fundamental principle behind edge-to-edge matching is that the directions in each phase that correspond to the “edges” of the planes that meet in the interface should be close-packed, or relatively close-packed, rows of atoms. A few of the recently reported examples of what is termed “edge-to-edge matching” appear to ignore this fundamental principle. By comparing theoretical predictions with available experimental data, this article will explore the validity of this critical atom-row coincidence condition, in situations where the two phases have simple crystal structures and in those where the precipitate has a more complex structure.

Morphology and crystallography of Mg24Y5 precipitate in Mg-Y alloy

Orientation relationships between Mg24Y5 precipitates and matrix in a Mg-Y alloy were accurately determined using Kikuchi line diffraction. The Burgers relationship with habit planes of {10 (1) over bar0}(H) and {31 (4) over bar0}(H) were observed for all precipitates. Compared with the Mg17Al12 precipitate in AZ91, the precipitation hardening effect in this alloy was significantly increased

Determination of carbon content in bainitic ferrite and carbon distribution in austenite by using CBKLDP

Convergent beam Kikuchi line diffraction patterns taken with a 10nm-diameter electron beam have been used to determine the lattice parameter and hence the carbon concentration in both ferrite and austenite. The experimental results show that bainitic ferrite is supersaturated in carbon and that, during ageing of austenite prior to the precipitation of cementite, the original carbon distribution across a grain becomes very nonuniform with distinct regions of both carbon enrichment and carbon depletion.

Accurate orientation relationship between ferrite and austenite in low carbon martensite and granular bainite

Convergent beam Kikuchi diffraction was used to accurately determine the orientation relationships (ORs) between austenite and martensite, and between austenite and granular bainite in two Fe-Ni-Mn-C alloys. Both martensite and granular bainite have the same crystallographic characteristics with the OR: (111)(A)parallel to(101)(F), [1 (1) over bar0](A) 2.5degrees +/- 2degrees from [1 (1) over bar(1) over bar](B).

ACCURATE ORIENTATION RELATIONSHIPS BETWEEN FERRITE AND CEMENTITE IN PEARLITE

In most previous work, the interlamellar interface between pearlitic ferrite and cementite is considered to have a low energy and dislocations have been observed at the interfacial boundary, in both plain carbon steels and high manganese steels. Here, four new orientation relationships (ORs) between pearlitic ferrite and cementite have been determined using the more accurate CBKLDP technique. In eutectoid steel the pearlite obeys the New-2, New-3 and New-4 ORs; in hypoeutectoid steel only the Isaichev OR is observed; in hypereutectoid steel the New-5 OR is followed. The two widely accepted ORs, the Pitsch-Petch OR and the Bagaryatskii OR have never been observed. They probably do not exist.

Surface alloying of AZ91D alloy by diffusion coating

A new technique of surface modification by diffusion coating for AZ91D alloy was developed. A 1.0-2.0-mm alloy layer, which has hardness four to five times higher than the substrate metal, was formed after the treatment. Consequent solution treatment and aging could further improve the hardness of the alloy layer. Microstructure and chemical composition were investigated using optical microscope and electron probe.

Crystallography of spheroidite and tempered martensite

Abstract Convergent beam Kikuchi line diffraction patterns, which have an accuracy more than an order of magnitude better than the more widely used conventional selected area diffraction, have been used to determine the orientation relationship between cementite and the ferrite matrix in spheroidite and tempered martensite. It has been shown that when pearlite spheroidized in plain carbon eutectoid steel, the original orientation relationship between cementite and ferrite is maintained in some cases. In other pearlite colonies in the later stages of the spheroidization the cementite particles rotate into a different orientation relationship with respect to the surrounding ferrite. In twinned martensite tempered in the range 450–650°C, the Isaichev orientation relationship between ferrite and cementite is obeyed, until recrystallization of the ferrite occurs. After full recrystallization the strict orientation relationship is destroyed, but the cementite particles still maintain a pseudo orientation relationship, in which close packed planes are approximately parallel, while the angle between corresponding close packed directions varies widely.