Goro Miyamoto

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.

Crystallographic Analysis of Proeutectoid Ferrite/Austenite Interface and Interphase Precipitation of Vanadium Carbide in Medium-Carbon Steel

To clarify the mechanism of interphase precipitation of vanadium carbide (VC) in a medium-carbon steel, orientation relationships (ORs) and plane orientations of ferrite/austenite interfaces were investigated. It was found that a large part of grain boundary ferrite holds near-K-S OR with at least one side of austenite adjacent to grain boundary regardless of V addition. By the V addition, a fraction of grain boundary ferrite holding near the K-S OR with both sides of austenite is decreased remarkably. Furthermore, only non-K-S ferrite/austenite interfaces migrate dominantly in the V-added alloy in contrast to the V-free alloy. Ferrite/austenite interface orientations are not fixed crystallographically but are randomly distributed in terms of ferrite and austenite orientations. Those results do not agree with the ledge mechanism originally proposed by Honeycombe. Thus, it is proposed that the ledge mechanism is extended to the non-K-S interface, which partially consists of coherent and less-mobile interfaces.

Incomplete transformation of upper bainite in Nb bearing low carbon steels

Abstract Kinetics and microstructure of bainite transformation in Fe–(0·15 or 0·05)C–0·2Si–1·5Mn (mass%) alloys with Nb addition of 0·03 mass%. Bainite transformation occurs at temperatures below 873 K. At 853 K, transformation rapidly proceeds by formation of bainitic ferrite without carbide precipitation, but transformation stasis appears for a certain period in the Nb added alloys leaving untransformed austenite film between neighbouring bainitic ferrites. On the other band, the Nb free alloys do not show such a stasis until the transformation is completed. By further holding, the transformation in the Nb added alloy restarts by forming the mixture of dislocation free ferrite with cementite precipitation in the austenite films. In contrast, bainite transformation accompanying cementite precipitation occurs in both Nb free and Nb added alloys at 773 K, resulting in no difference in transformation kinetics. It is proposed that the incomplete transformation is caused by suppression of ferrite nucleation at interphase boundaries between pre-existing bainitic ferrite and austenite due to Nb segregation.

Precipitation of nanosized nitrides in plasma nitrided Fe–M (M = Al, Cr, Ti, V) alloys

Abstract Nanosized alloy nitrides or alloying element (M)–Nitrogen(N) cluster formed in plasma nitriding of Fe–M binary alloys were investigated by means of high resolution TEM. Specimen surface of the Fe–Cr alloys was hardened by the formation of disk shaped CrN of NaCl structure, which is ∼2 nm thick and 10 nm in diameter. In nitrided Fe–Al specimen, two kinds of AlN nitrides of metastable NaCl type and stable wurtzite type were formed near the specimen surface. Both of these were much larger than CrN. Contrasting to these specimens, in nitrided Fe–Ti and Fe–V alloys, high density M–N clusters of few nanometres diameter, many of which were monolayered lying on {001}α, were formed, leading to larger hardness increase than that in Fe–Cr and Fe–Al alloys. The formation of M–N clusters in nitriding is explained by the thermodynamical analysis of phase separation between M/N poor and M/N rich bcc phases.

Surface Hardening and Nitride Precipitation in the Nitriding of Fe-M1-M2 Ternary Alloys Containing Al, V, or Cr

Nitride precipitation and resultant surface hardening in nitrided Fe-M1-M2 ternary alloys containing Cr, Al, or V were investigated using transmission electron microscopy and three-dimensional atom probe tomography. The (Al, Cr) and (Cr, V) mixed nitrides are formed by the co-precipitation of these elements during the nitriding of Fe-Al-Cr or Fe-Cr-V alloys. However, the precipitation of V nitrides precedes Al nitride precipitation during the nitriding of the Fe-Al-V alloy, which results in two-step hardening behavior. The addition of Cr or V to the Fe-Al alloy accelerates the precipitation kinetics of Al nitrides by promoting the nucleation of Al nitrides, which leads to substantial surface hardening.

Formation of Ultrafine Grained Ferrite by Warm Deformation of Tempered Lath Martensite in Low Alloy Steels

Microstructure change during warm deformation of tempered lath martensite in Fe-2mass%Mn-C alloys with different carbon contents in the range between 0.1 and 0.8mass%C was investigated. Specimens of the alloys after being quenched and tempered at 923K for 0.3ks were compressed by 50% with a strain rate varying from 10-3 to 10-4s-1 at 923K. EBSD analysis of the deformed microstructures has revealed that fine equiaxed ferrite (α) grains surrounded by high-angle boundaries are formed by dynamic recrystallization (DRX). As carbon content increases, the DRX α grain size decreases. This could be attributed to the change in volume fraction of the cementite (θ) phase as boundary dragging particles. The sub-micron θ particles can suppress the coarsening of the DRX α grains by exerting a pinning effect on grain boundary migration. Furthermore, the fraction of recrystallized region increases by increasing carbon content, presumably due to a decrease in the martensite block width as an initial α grain size and a larger volume fraction of hard second phase (θ) particles. Both of these should increase inhomogeneous plastic deformation which promotes the recrystallization. It seems that continuous DRX is responsible for the formation of ultrafine α grains in the tempered lath martensite.

Crystallography and Interphase Boundary of Martensite and Bainite in Steels

Grain refinements in lath martensite and bainite structures are crucial for strengthening and toughening of high-strength structural steels. Clearly, crystallography of transformation plays an important role in determining the “grain” sizes in these structures. In the present study, crystallography and intrinsic boundary structure of martensite and bainite are described. Furthermore, various extrinsic factors affecting variant selection and growth kinetics, such as elastic/plastic strain and alloying effects on interphase boundary migration, are discussed.

Grain Refinement by Cyclic Displacive Forward/Reverse Transformation in Fe-High-Ni Alloys

A novel method for grain refinement of martensite structures was proposed, in which transformation strain is accumulated by cyclic displacive forward and reverse transformations. This method can refine martensite structures in an Fe-18Ni alloy because a high density of austenite dislocations is introduced by a displacive reverse transformation in addition to an inheritance of dislocations in body-centered cubic martensite into austenite during cyclic transformation. The addition of a small amount of carbon accelerates structure refinement significantly, which results in the formation of ultra-fine-grained structures after ten cycles.

Erratum to: Grain Refinement by Cyclic Displacive Forward/Reverse Transformation in Fe-High-Ni Alloys

TADACHIKA CHIBA is with the Department of Physical Science and Engineering, Nagoya Institute of Technology, Gokisocho, Showaku, Nagoya, Aichi, 466-8555 Japan. Contact e-mail: chiba.tadachika@nitech.ac.jp HASSAN SHIRAZI is with the School of Metallurgy and Materials Engineering, University of Tehran, 14395-731 Tehran, Iran. GORO MIYAMOTO and TADASHI FURUHARA are with the Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japan. The online version of the original article can be found under doi: 10.1007/s11661-017-4152-4. Article published online June 21, 2017

Three-dimensional observations of morphology of low-angle boundaries in ultra-low carbon lath martensite

The lath martensite structure contains hierarchical substructures, such as blocks, packets and prior austenite grains. Generally, high-angle grain boundaries in the lath martensite structure, i.e. block boundaries, are correlated to mechanical properties. On the other hand, low-angle grain boundaries play an important role in morphological development. However, it is difficult to understand their nature because of the difficulty associated with the characterization of the complex morphologies by two-dimensional techniques. This study aims to identify the morphologies of low-angle boundaries in ultra-low carbon lath martensite. A serial-sectioning method and electron backscatter diffraction analysis are utilized to reconstruct three-dimensional objects and analyse their grain boundaries. A packet comprizes two low-angle grain boundaries - sub-block and fine packet boundaries. Sub-blocks exhibit porous morphology, with two large sub-blocks predominantly occupying a block. Several fine packets with different habit planes from the surrounding regions are observed. Fine packets are present in blocks, which frequently share a close-packed direction with the neighbouring fine packets. In addition, fine packets are in contact with the sub-block boundaries.