Tadashi Furuhara

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.

Interphase boundary structures of intragranular proeutectoid α plates in a hypoeutectoid TiCr alloy

Abstract Interphase boundary structure on the board faces of proeutectoid α plates in a retained β matrix has been studied in a Ti-6.62at. %Cr alloy by means of conventional and high resolution transmission electron microscopy. Misfit compensating ledges with b = c /2[0001] α are observed on the broad faces of α plates. Growth ledges, irregular in path, spacing and alignment, are also observed. The apparent habit plane of the broad faces of α plates and the riser directions of the misfit compensating ledges, determined by trace analysis, imply the presence of the structural ledges predicted in the companion modeling study. High-resolution TEM observation has revealed that biatomic structural ledges with ( 1 1 00) α //(2 11 ) β terraces do indeed exist on the broad faces of α plates, with exactly the characteristics predicted. This result provides the first experimental evidence for structural ledges in any b.c.c.:h.c.p. system.

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.

Crystallographic and mechanistic aspects of growth by shear and by diffusional processes

Growth by shear and by diffusional processes, both taking place predominantly by means of ledge mechanisms, are reviewed for the purpose of distinguishing critically between them at the atomic, microscopic, and macroscopic levels. At the atomic level, diffusional growth is described as individual, poorly coordinated, thermally activated jumps occurring in the manner of biased random walk, whereas growth by shear is taken to be tightly coordinated “glide” of atoms to sites in the product phase which are “predestined” to within the radius of a shuffle. Obedience to the invariant plane strain (IPS) surface relief effect and the transformation crystallography prescribed by the phenomenological theory of martensite is shown to be an unsatisfactory means of distinguishing between these two fundamentally different atomic growth mechanisms. In substitutional alloys, continuous differences in compositionand in long-range order (LRO) from the earliest stages of growth onward are concluded to be the most useful phenomenological approach to achieving differentiation. At a more fundamental level, however, the details of interphase boundary structure are the primary determinant of the operative mechanism (when the driving force for growth is sufficient to permit either to occur). In the presence of a stacking sequence change across the boundary, terraces of ledges are immobile irrespective of their structural details during diffusional growth. Kinks on the risers of superledges are probably the primary sites for diffusional transfer of atoms across interphase boundaries. In martensitic transformations, on the other hand, terraces containing edge dislocations in glide orientation or pure screw dislocations are mobile and accomplish the lattice invariant deformation (LID), though probably only after being overrun by a transformation dislocation. Risers associated with transformation dislocations are also mobile and cause the crystal structure change during growth by shear. The successes achieved by the invariant line (IL) component of the phenomenological theory of martensite in predicting precipitate needle growth directions and precipitate plate habit planes (Dahmen and co-workers) are here ascribed to the rate of ledge formation usually being a minimum at an interface containing an IL, primarily because nuclei formed sympathetically at this boundary orientation are likely to have the highest edge energies. Since martensite plate broad faces also contain the IL, the ability of the phenomenological theory to predict the habit plane and the orientation relationships of both precipitate and martensite plates is no longer surprising. The IPS relief effect at a free surface can be generated by precipitate plates when growth ledges are generated predominantly on only one broad face and only one of several crystallographically equivalent Burgers vectors of growth ledges is operative. Both pReferences probably result from larger reductions in transformation strain energy for the particular geometry with which a given plate intercepts the free surface. Precipitate morphology often differs significantly from that of martensite even if precipitates are plate-shaped and can readily differ very greatly. Whereas martensite morphology is determined by the need to minimize shear strain energy, that of precipitates derives from the more flexible base of the interphase boundary orientation-dependence of the reciprocal of the average intergrowth ledge spacing, as modified by both the orientation-dependence of interkink spacing on growth ledge risers and the spacing/ height ratio dependence of diffusion field overlap upon growth kinetics.

Microstructures and mechanical properties of metastable Ti–30Zr–(Cr, Mo) alloys with changeable Young’s modulus for spinal fixation applications

In order to develop a novel alloy with a changeable Youngs modulus for spinal fixation applications, we investigated the microstructures, Youngs moduli, and tensile properties of metastable Ti-30Zr-(Cr, Mo) alloys subjected to solution treatment (ST) and cold rolling (CR). All the alloys comprise a β phase and small athermal ω phase, and they exhibit low Youngs moduli after ST. During CR, deformation-induced phase transformation occurs in all the alloys. The change in Youngs modulus after CR is highly dependent on the type of deformation-induced phase. The increase in Youngs modulus after CR is attributed to the deformation-induced ω phase on {3 3 2} mechanical twinning. Ti-30Zr-3Cr-3Mo (3Cr3Mo), which exhibits excellent tensile properties and a changeable Youngs modulus, shows a smaller springback than Ti-29Nb-13Ta-4.6Zr, a β-type titanium alloy expected to be useful in spinal fixation applications. Thus, 3Cr3Mo is a potential candidate for spinal fixation applications.

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.

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 ...

Computer modeling of partially coherent B.C.C.:H.C.P. boundaries

Abstract A modeling study of partially coherent b.c.c.:h.c.p. coherent boundaries, using a computer-aided graphical technique, has been made for parallel pairs of low index planes in the presence of specified lattice orientation relationships. Three well-known lattice orientation relationships between the b.c.c. and h.c.p. phases, each of which was also slightly perturbed in various ways, were utilized. All of the planar interfaces thus formed which were based upon parallel pairs of low index planes can be described by two arrays of parallel misfit dislocations. The possibility of replacing one array of misfit dislocations with an array of structural ledges was then analyzed. In the presence of near-Burgers orientation relationships, the most probable structural ledges were found to have (1 1 00) h.c.p. //(2 11 ) b.c.c. terraces with risers 2, 4 or 6 atomic layers high. This type of structural ledge has a Burgers vector of 1/12[111] b.c.c. , which lies in the terrace plane, associated with its riser. Thus it can replace a set of a -type misfit dislocations; hence only a single set of c -type misfit dislocations is now necessary on the terrace of structural ledges. This types of structural ledge was also found to step down along the lattice invariant line in order to accommodate simultaneously the misfit normal to the terrace plane. Further, the possibility of structural ledges with (1101) h.c.p. //(110) b.c.c. terrace was also discerned in the presence of near-Potter orientation relationships.

Crystallography and interfacial structure of proeutectoid α grain boundary allotriomorphs in a hypoeutectoid TiCr alloy

Abstract The interphase boundary structure of grain boundary allotriomorphs of proeutectoid α formed at β grain faces in a Ti-6.62 at.% Cr alloy was studied with TEM. Allotriomorphs were shown to have a near-Burgers orientation relationship with respect to one β grain and usually an irrational orientation relationship with respect to the “opposite” β grain. Two types of ledges were observed on both interfaces of α allotriomorphs. One type is growth ledges, with irregular paths and interledge spacings; the other is misfit compensating ledges, lying along a well-defined direction with a uniform spacing. The latter ledge has an effective Burgers vector of either a c-type or (c+a)-type near-edge dislocation. These results provide direct support for the proposal that the interfaces of face-nucleated grain boundary allotriomorphs should be partially coherent during growth with respect to both bounding matrix grains.