Papers by Author: Tadashi Maki

Abstract: The microstructural development of cold-rolled lath martensite structure in the low carbon steels and ultra-low carbon steels are studied and compared. In low carbon steel of as-quenched specimens, very thin austenite films exist at boundaries of adjacent laths, but do not exist in ultra-low carbon steel. After cold rolling for the low carbon steel, the lamellar dislocation cells, irregularly bent laths and kinked laths regions are frequently observed and, in some instances, the disappearance of initial lath boundaries is observed. The existence of retained austenite films suggests that the lath boundaries rarely disappear during cold-rolling in the low carbon steel.

Abstract: 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.

Abstract: The formation of ultrafine-grained structure in steels by various thermomechanical processings is reviewed from a metallurgical point of view. In the recent new type TMCP, ultrafine ferrite grains with a grain size of about 1μm are obtained when the austenite is heavily deformed at lower temperatures. In this case, dynamic phenomena such as dynamic recrystallization become prominent in the process. In the aging after heavy cold rolling of supersaturated matrix phase in two-phase alloys, the competition between the recovery or recrystallization of matrix phase and the precipitation of second phase occurs, resulting in various types of two-phase structures including microduplex structure. Microduplex structure is also obtained by annealing after heavy cold rolling of coarse two-phase structure in duplex stainless steel and high carbon steel. Recently, various severe plastic deformation processings, in which very large plastic strain over 4 is applied to the materials, have been developed to produce ultrafine grained materials with nanocrystalline and/or submicrocrystalline structures.

Abstract: Microstructures formed by degenerate pearlite transformation in an Fe-0.38mass%C alloy were studied by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Degenerate pearlite which contains fine cementite particles even at the growth front was observed with other structures such as proeutectoid ferrite, lamellar pearlite and bainite in a temperature range between 773K and 923K. As the isothermal transformation temperature is lowered, a fraction of the degenerate pearlite increases. The degenerate pearlite consists of ‘block’ (a region in which ferrite orientations are nearly the same) and ‘colony’ (a region containing cementite particles of nearly the same orientation), both of which are similar to those in lamellar pearlite. Block boundaries within an austenite grain are generally of high-angle type and their misorientations deviate largely from intervariant relationships for the K-S orientation relationship. In contrast, colony boundaries are of low-angle type. Cementite films are formed along those ferrite boundaries in the degenerate pearlite, presumably formed by encounter of the blocks or colonies.

Abstract: The microstructure change by warm deformation in high-carbon steels with different initial ferrite (α) + cementite (θ) duplex microstructures has been examined. Three kinds of initial structures, i.e., pearlite, α+spheroidized θ and tempered martensite, were prepared using Fe-0.8C-2Mn and Fe-1.0C-1.4Cr alloys and compressed by 30-75% at 973K at a strain rate of 5x10-4 s-1. Equiaxed fine α grains, approximately 2μm in diameter and mostly bounded by high-angle boundaries, are formed with spheroidized θ by dynamic recrystallization during compression of the pearlite by 75%. When the (α+θ) duplex structure containing spheroidized θ was deformed, the original α grains become elongated and only subgrains are formed within them by dynamic recovery. For the tempered martensite, equiaxed α grains similar to those in the deformed pearlite were obtained after 50% compression. This indicates that the critical strain needed for the completion of dynamic recrystallization of α is smaller for the tempered martensite than for the other structures.

Abstract: Fe-Ni-Mn maraging alloys show discontinuous coarsening of fct θ-NiMn precipitates along prior austenite grain boundaries (PAGBs) during isothermal aging. Heavy cold rolling of a solution annealed Fe-10Ni-7Mn (wt. %) maraging alloy and subsequent aging treatment at 773 K led to the formation of a nanostructured material. Hardness measurement and transmission electron microscopy were used to study the aging behaviour and microstructural evolution of the nanostructured alloy. At the early stage of aging, an ultrafine grained structure was observed with fine matrix precipitates. However, at later stages of aging, severe coarsening of precipitates was found. At that stage, a lamellar microstructure composed of finely precipitated ferrite laths and ultrafine ferrite grains were identified. Coarse fct θ-NiMn precipitates were identified at the ultrafine ferritic grain boundaries. Development of the coarsening reaction was found to increase the ultrafine grained ferritic regions with prolonged aging.

Abstract: Microstructures of the plasma nitrided ferritic Fe-M (M = 0.5, 0.7, 1.0mass%Ti, 1.0mass%V, 1.0mass%Al) binary alloys were studied mainly by TEM. Metastable clusters are uniformly formed in the Ti added alloys. Near the surface of the specimen, transition from the clusters to the equilibrium B1-type TiN is observed. The microstructure formed in the 1%V alloy is similar to that of the Ti added alloy. In contrast, no cluster is formed in the 1%Al alloy and B1-type AlN is observed to nucleate on dislocations.