Tadashi Maki

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

Abstract The morphology and crystallography of lath martensite in Fe-C alloys containing various carbon contents from 0.0026 to 0.61% were studied by analyzing electron back scattered diffraction patterns in scanning electron microscopy and Kikuchi diffraction patterns in transmission electron microscopy. As carbon content increases, the sizes of both packet and block decrease. In low carbon steels (0.0026–0.38%C), a block which is observed as having different contrasts under optical microscopy contains two groups (sub-blocks) of laths which are of two K-S variants with a misorientation of about 10 degrees. On the other hand, in the high carbon alloy (0.61%C), a block consists of laths of a single K-S variant.

Variant selection in heterogeneous nucleation on defects in diffusional phase transformation and precipitation

In heterogeneous nucleation on lattice defects in the matrix during diffusional phase transformation and precipitation reactions, the variant of product phase with the specific orientation relationship is strongly selected by the nature of the defects. For dislocations, effective accommodation of the transformation strain by the strain field of dislocations occurs, leading to the variant selection in which the direction of the maximum misfit is nearly parallel to the Burgers vector of the dislocations. For high-angle grain boundaries and subgrain boundaries, precipitates tend to select the variant with a low-energy interface (often the parallel close packed planes) inclined at the smallest angle to the grain boundary plane. On the other hand, precipitates that nucleate on the incoherent inclusions embedded in the matrix do not hold any specific orientation relationship with respect to the matrix. It is important to produce many variants locally for the effective refinement of microstructures that improves the mechanical properties of steel and titanium alloys.

Crystallography of grain boundary α precipitates in a β titanium alloy

The crystallography of α(hcp) precipitates formed on the β(bcc) matrix grain boundaries has been studied with transmission electron microscopy (TEM) in a Ti-15V-3Cr-3Sn-3Al alloy. The α precipitates have a near-Burgers orientation relationship with respect to at least one of the adjacent β grains. Among the possible 12 variants in this orientation relationship, the variant that [11•20]α is parallel to the 〈111〉β closest to the grain boundary plane tends to be preferred by the α precipitates. Additionally, further variant selections are made so as to minimize the deviation of orientation relationship with respect to the “opposite“ β grain from the Burgers one. Such rules in variant selection often result in the formation of precipitates with a single variant at a planar grain boundary. Prior small deformation of β matrix changes the variant of α precipitates at the deformed portion of grain boundary. It is considered that the stress field of dislocations in the slip bands intersecting with the boundary strongly affects the variants of α precipitates. Discussion of these results is based upon a classical nucleation theory.

Thermoelastic martensite in an ausaged FeNiTiCo alloy

A thermoelastic martensite which is related to a shape memory effect has been observed mainly in non-ferrous alloys such as Ti-Ni, Cu-Zn, Cu-Al-Ni. For ferrous alloys, only two alloys have been found to exhibit the thermoelastic martensite, i.e., one is an ordered Fe- about 25at%Pt (fcc in equilibrium bct)(1) and the other is an Fe- about 30at%Pd (fcc in equilibrium fct)(2). Thermoelastic martensite is characterized by a small thermal hysteresis and a reversible movement of martensite interface during forward and reverse transformation. Two of the present authors have studied the transformation characteristics of thin plate martensite in Fe-NiC alloys and shown that the interface of thin plate martensite in Fe-Ni-C alloys is essentially mobile and the transformation is somewhat reversible, although the martensite is not typically thermoelastic per se because of a fairly large thermal hysteresis. These results strongly suggest that there is a possibility that, even in ferrous alloys other than Fe-Pt and Fe-Pd, the thin plate ferrous martensite would become thermoelastic by an introduction of suitable factors for reducing its thermal hysteresis.

Mechanism of dynamic continuous recrystallization during superplastic deformation in a microduplex stainless steel

Microstructural evolution during the cyclic cold-rolling and annealing process in an (α + γ) microduplex stainless steel, which consists of α subgrains and fine γ particles, has been studied in detail with the aim of clarifying the mechanism of dynamic continuous recrystallization. A continuous increase in α subgrain boundary misorientation is obtained by the present processing where grain boundary sliding does not occur and the effect of increasing boundary misorientation with cumulative strain is comparable to those observed in dynamic continuous recrystallization of superplastic aluminium alloys. The increase in boundary misorientation is accounted for by the absorption of dislocations into subgrain boundaries during annealing, dislocations which had operated to accommodate the plastic strain incompatibility of the neighboring phases undergoing slip deformation. The present results show that grain boundary sliding is not indispensable but the difference in accommodation deformation between adjacent subgrains is of great importance for the dynamic continuous recrystallization during superplastic deformation.

Recrystallization and formation of austenite in deformed lath martensitic structure of low carbon steels

The effect of prior deformation on the processes of tempering and austenitizing of lath martensite was studied by using low carbon steels. The recrystallization of as-quenched lath martensite was not observed on tempering while the deformed lath martensite easily recrystallized. The behavior of austenite formation in deformed specimens was different from that in as-quenched specimens because of the recrystallization of deformed lath martensite. The austenitizing behavior (and thus the austenite grain size) in deformed specimens was controlled by the competition of austenite formation with the recrystallization of lath martensite. In the case of as-quenched (non-deformed) lath martensite, the austenite particles were formed preferentially at prior austenite grain boundaries and then formed within the austenite grains mainly along the packet, block, and lath boundaries. On the other hand, in the case of lightly deformed (30 to 50 pct) lath martensite, the recrystallization of the matrix rapidly progressed prior to the formation of austenite, and the austenite particles were formed mainly at the boundaries of fairly fine recrystallized ferrite grains. When the lath martensite was heavily deformed (75 to 84 pct), the austenite formation proceeded almost simultaneously with the recrystallization of lath martensite. In such a situation, very fine austenite grain structure was obtained most effectively.

Crystallography of intragranular ferrite formed on (MnS + V(C, N)) complex precipitate in austenite

Abstract Crystallography of intragranular ferrite formed on incoherent MnS+V(C,N) complex precipitate in austenite was studied in an Fe–Mn–C hypoeutectoid alloy. As transformation temperature decreases, intragranular ferrite morphology changes from idiomorph to acicular ferrite. Idiomorphic ferrite has no specific orientation relationship with austenite although acicular one holds near Kurdjumov–Sachs relationship.

Dynamic strain aging during fatigue deformation in type 304 austenitic stainless steel

Abstract The test conditions (i.e. the test temperature and the strain rate) at which dynamic strain aging takes place during strain-controlled fatigue deformation in type 304 austenitic stainless steel were studied. When the strain rate was low (i.e. 5 × 10−4 − 1.7 × 10−3s−1), the lower critical temperature for the appearance of serrations during fatigue deformation was much lower than that during monotonic tensile deformation. This result was considered to be caused by the longer arrest time of dislocations in fatigue deformation in comparison with that in monotonic tensile deformation. In such an arrest time the dislocations are locked by solute atoms. The longer arrest time in fatigue deformation is considered to be closely related to the cell-shuttling motion of dislocations in the saturated stress stage of the fatigue process.

Atomic structure of interphase boundary of an a precipitate plate in a β Ti[sbnd]Cr alloy

Abstract The atomic structure of the interphase boundary enclosing an α (hcp) precipitate formed from the β (bcc) matrix in a Ti-Cr alloy was examined. All the interfaces of an α lath exhibit continuity of atomic planes between the matrix and the precipitate and thus are considered to be coherent or semicoherent. Such observations imply that there is an atomic site correspondence during transformation. The shear component of the transformation strains arising from the passage of (a/12) [111]β transformation dislocations (structural ledges) on (112)β∥[1100)α planes is accommodated by a set of glissile dislocation loops existing on every sixth (1100)α plane at the side facet and the edge of an α lath. The volumetric component of the transformation strain is accommodated by a set of sessile misfit dislocations (b = (a/2)[110]β = (c/2) [0001]α J on the risers of the growth ledges at the side facet. The presence of sessile dislocations implies that the migration of α-β interfaces is a non-conservative process ...

Atomic structure of interphase boundary enclosing bcc precipitate formed in fcc matrix in a Ni-Cr alloy

AbstractThe atomic structure of the interphase boundaries enclosing body-centered cubic (bcc) lath-shape precipitates formed in the face-centered cubic (fcc) matrix of a Ni-45 mass pct Cr alloy was examined by means of conventional and high-resolution transmission electron microscopy (HRTEM). Growth ledges were observed on the broad faces of the laths. The growth ledge terrace (with the macroscopic habit plane