Yasuya Ohmori

Effects of ω-phase precipitation on β → α, α transformations in a metastable β titanium alloy

Abstract Effects of ω phase precipitation on β→α, α′′ transformations in a metastable β titanium alloy have been studied by means of transmission electron microscopy. Isothermal holding at 300°C for 100 min after the solution-treatment at 1000°C yielded homogeneously distributed spherical ω phase particles of ∼10 nm in diameter. With raising the holding temperature, ω phase particles of the same orientation variant precipitated as groups. At temperatures above 350°C, α phase laths nucleated at the ω/β interfaces and grew into both the matrix and ω particles with consuming ω phase particles. Thus, the α/ω Takemoto and the ω/β Burgers relationships were held simultaneously, and the orientation of α laths nucleating at an ω/β interface was determined uniquely. In the specimens quenched directly from β single-phase region, α′′ martensite plates nucleated preferentially at β grain boundaries. The α′′ formation, however, was accelerated by a short time holding at temperatures between 600 and 300°C, but was largely retarded by the progress of ω phase precipitation.

Epsilon carbide precipitation during tempering of plain carbon martensite

AbstractEpsilon carbide precipitation in steel martensite has been investigated by means of transmission electron microscopy. The first stage of tempering initiates with the nucleation of very fine ε-carbide particles on the closely spaced parallel line defects, the morphology being so-called “cross-hatched” ε-carbide needles. The ε-carbide particles which produce the well-defined dif-fraction patterns are related to the martensite matrix with a Pitsch and Schrader orientation relationship.[32] These particles subsequently grow into rods elongated in the direction parallel to the

Bainite transformation and the diffusional migration of bainite/austenite broad interfaces in Fe-9%Ni-C alloys

\langle 100\rangle _\alpha \parallel \langle 11\bar 20\rangle _\varepsilon

CRYSTALLOGRAPHY AND STRUCTURAL EVOLUTION DURING REVERSE TRANSFORMATION IN AN Fe-17Cr- 0.5C TEMPERED MARTENSITE

within the matrix. The final reaction in the first stage is the rearrangement of ε-carbide rods into a disklike morphology. The e-carbide rods elongated in the 〈100〉ε di-rections coalesce on planes in a raftlike manner, as in the case of those formed in the quench-aged low-carbon ferrite, the tetragonality of martensite being completely lost. Although the deviation from hexagonal symmetry about the [0001]ε axis exists, no evidence of orthorhombic η-carbide formation was obtained.

Isothermal decomposition of δ-ferrite in a 25Cr-7Ni-0.14N stainless steel

Morphology of bainite and Widmanstätten ferrite in various steels has been investigated by means of microstructural and surface relief observations. It was shown that upper and lower bainite should be classified by ferrite morphology,i.e., lathlike or platelike, and that the morphology of cementite precipitation cannot be the index for the classification. Widmanstätten ferrite formed in the upper C-nose where ferrite grain-boundary allotriomorphs nucleate exhibits quite similar appearance with bainitic ferrite that forms in the lower C-nose of bainitic reaction. The only difference between them exists in the fact that Widmanstätten ferrite laths grow in the temperature range where primary ferrite forms and often terminate at a grain boundary ferrite but that bainitic ferrite has its own C-curve at temperatures belowBs and nucleates directly at an austenite grain boundary. The mechanisms for their formations are discussed.

Mechanism and crystallography of ferrite precipitation from cementite in an Fe-Cr-C alloy during austenitization

Abstract The processes of bainitic structure formation in Fe-9%Ni-C alloys have been investigated by means of transmission electron and hot stage optical microscopies. The bainite laths formed initially with sharp surface reliefs grow in thickness direction rather slowly with surface rumpling-type reliefs. A pearlitic layer often forms in the carbon-enriched region between the bainitic ferrite laths. The pearlitic ferrite nucleates epitaxially on or grows directly from an adjacent bainitic ferrite lath. Such pearlite colonies always exhibit the Isaichev ferrite/cementite orientation relationship with the [010]θ‖ α‖ γ near invariant line between three phases on the bainite habit plane. In the typical upper bainite with cementite layer between ferrite laths, a cementite/ferrite orientation relationship with the (100)θ≈‖{110}α bainite habit plane was recognised in addition to the (011) α ‖( 1 03) θ habit. The general feature of cementite precipitation in such upper bainite laths is that the near invariant [010]θ‖ α‖ γ direction lies on the bainite habit plane.

An interpretation of the carbon redistribution

The crystallography of pearlite with M{sub 7}C{sub 3} carbide lamellae and the atomic structure at the ferrite/carbide interface have been examined by means of transmission electron microscopy in an Fe-8.2Cr-0.96C alloy. Two orientation relationships with the corresponding habit planes were determined: OR-I: ({bar 2}{bar 5}1){sub b}{parallel}(11{bar 2}0){sub hex}--habit plane, ({bar 1}13){sub b}{parallel}({bar 1}100){sub hex}, ({bar 3}1{bar 1}){sub b} {approx} {parallel}(0001){sub hex}; OR-II: ({bar 2}{bar 5}1){sub b}{parallel}(1{bar 1}00){sub hex}--habit plane, ({bar 1}13){sub b}{parallel}(11{bar 2}0){sub hex} ({bar 3}1{bar 1}){sub b} {approx} {parallel}(0001){sub hex}. Variants of these orientation relationships have been frequently observed. Microscopic steps at the ferrite/carbide interfaces accommodate the curvature of the habit planes. Each of these orientation relationships provides a small misorientation between the {l_brace}110{r_brace}{sub b} close packed planes of ferrite and the coincident planes of M{sub 7}C{sub 3} carbide, and the atomic planes are perfectly matched through the interface. The orientation relationship between the parent austenite and M{sub 7}C{sub 3} carbide was also deduced assuming the ferrite/austenite orientation relationship so far obtained.

Phase stability and decomposition processes in TiAl based intermetallics

The mechanism and the crystallography of austenite and d-ferrite formation from tempered mar- tensite at temperatures of 900-12008C have been studied by means of transmission electron microscopy in an Fe-17Cr-0.55C alloy. It was found that austenite nucleates within ferrite at low angle, high angle and twin-related lath boundaries as well as at high angle equiaxed grain boundaries in contact with M23C6 grain/lath boundary carbides. The austenite grains are in a cube-cube relationship with the M23C6 carbide particles and bear the Kurdjumov-Sachs orientation relationship with at least one of the adjacent ferrite grains. They are often in the Kurdjumov-Sachs relationship with both ferrite laths separated by a high angle boundary as far as the laths had formed from the same austenite. The {111}A close packed plane of g precipitate is parallel to the {110}F plane most parallel to the grain boundary. The close packed planes of some austenite grains nucleating at the high angle lath boundaries are parallel to the close packed planes of both ferrite laths. These crystallographic features often result in a single variant of austenite orientation at a grain boundary. After nucleation, the austenite grains grow by the migration of both semicoherent and incoherent interfaces. These results demonstrate that a specific orientation relationship is preferred for the austenite nucleation, but is not necessary for the subsequent growth. The kinetics of austenite growth are controlled by chromium diAusion. The d-ferrite particles precipitate at high temperatures as a non-equi- librium phase. No rational orientation relationship between d-ferrite and retained austenite was found. The experimental results are discussed qualitatively in terms of the thermodynamic predictions using the soft- ware ThermoCalc, assuming local equilibrium at the moving interfaces. 7 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.