Search

The effect of cooling condition on the intergranular corrosion resistance of UNS S32506 duplex stainless steel was investigated by means of Huey test and microstructural observation. It was found that quenching at a temperature below 800 °C after air cooling at a fixed rate from solutionization temperature enhances the intergranular corrosion rate. No precipitation has been found at all cooling conditions. Formation of Cr-depletion zone on the γ phase side of the α/γ interface due to local chemical change accompanying α-γ phase transformation during air cooling is responsible for the intergranular corrosion degradation of the UNS S32506 duplex stainless steel.

The Kissinger-type formula, dX/dt=A(1–X)exp(–Ed/RT), which was thought to be applicable only in detrapping-controlled desorption has proved to be applicable also in the case of diffusion-controlled desorption under certain conditions including existence of effective diffusion coefficient or assumption of Oriani’s local equilibrium and pre-exposure of hydrogen before temperature ramp. Analysis indicated that the constant A depends on the specimen geometry and the pre-exponential factor of effective diffusion coefficient. Numerical verification has been carried out by comparing the simulation result of Kissingertype formula with that using the rigid McNabb-Foster model for typical specimen geometry including plate, cylinder and sphere. The merit of using this formula lies in its simplicity in numerical simulation of the thermal desorption spectrum with an accuracy as high as that based on the M-F model. Two different derivative forms of the Kissinger-type formula, i.e. the Choo-Lee plot d[ln (φ/Tp 2 )]/d(1/Tp)=–Ed/R and the Lee-Lee plot d[ln(φ/Tp)]/d(1/Tp)=–Ed/R, for determination of desorption activation energy have also been discussed and the analytical result showed that the Choo-Lee plot is a proper method to determine desorption activation energy while the Lee-Lee plot is not correct. The limit of application of the formula and the Choo-Lee plot in thermal desorption analysis is discussed in a semi-quantitative manner.

The Kissinger’s formula has been analyzed theoretically and numerically to be applicable in simulation of not only the detrapping-controlled thermal desorption but also the diffusion-controlled thermal desorption provided that a sufficient pre-exposure before thermal desorption is carried out in the diffusion-controlled desorption. The desorption activation energy or the binding energy can be evaluated by a single thermal desorption spectrum from a single type of trap site. In the case of detrapping-controlled desorption the constant parameter A in the Kissinger’s formula approaches the pre-exponential factor of detrap parameter, p0, in the McNabb–Foster model as specimen size becomes smaller. In the case of diffusion-controlled desorption where local equilibrium may be maintained and an effective diffusivity can be expressed, the A value changes according to A = α2D0 where D0 is the pre-exponential factor of effective diffusivity and α is a geometrical parameter of specimen.

Development of primary recrystallization was studied in a steel containing 0.4 vol.% of fine TiC precipitations with an average size of 12 nm. After sufficiently large cold strains, the recrystallization developed readily upon annealing at temperatures above 600 °C. An increase in the cold strain as well as the annealing temperature resulted in the acceleration of recrystallization kinetics. However, a certain amount of cold worked microstructures of about 15 vol.% remained unrecrystallized even after annealing at a rather high temperature of 700 °C. The unrecrystallized portions were composed of grains with the 〈0 0 1〉 crystallographic direction parallel to the compression axis. Both the low stored energies in these grains and the pinning of recrystallizing grain boundaries by the dispersed carbides were discussed as crucial factors that resulted in the incomplete recrystallization.

The (Nb)/Nb3Si eutectic alloys in the Nb–Si binary and Nb–Ti–Si ternary systems were unidirectionally solidified in an optical floating zone melting furnace. The effect of solidification rates on microstructures of the alloys is investigated. The coupled growth of (Nb) and Nb3Si is observed in the binary alloy solidified at 10 mm/h. The ternary alloys exhibit the development of discontinuous (Nb) particles within the Nb3Si matrix. The unidirectionally solidified alloys show improved fracture toughness and enhanced high temperature compressive strength as compared with the arc-melted counterparts. The role of an aligned eutectic microstructure in toughening is discussed from the existing models based on the crack bridging and the crack deflection.

Mechanical properties of E2 1 based Co 3 AlC single crystals and single-phase polycrystalline Co 3 AlC and Fe 3 AlC alloys have been evaluated by compression tests performed in a wide temperature range from room temperature to 1373 K. Effect of the ordering of C on the phase stability and mechanical properties has been investigated and discussed with the anti-phase boundary (APB) formation. Temperature dependence of critical resolved shear stress (CRSS) has been determined to consider the compressive mechanical behavior of Co 3 AlC single crystal. Yield strength anomaly appears in Co 3 AlC single crystal as we have expected from the similarity between E2 1 ′ and L1 2 . Schmid law is only applicable in a high temperature range between 1173 and 1273 K. Crystallographic analysis and microstructure observation were conducted using transmission electron microscopy (TEM). It was found that disordering of C atoms could be induced by the irradiation damage of electron beam after high-resolution transmission electron microscopy (HRTEM) observation.

Phase equilibria in Co–Al–C ternary and Co–Ni–Al–C quaternary systems were investigated to establish the basis for designing new class of α(Co) based heat resistant alloys strengthened by the E2 1 type intermetallic compound Co 3 AlC. Phase stability of E2 1 (Co, Ni) 3 AlC was examined from the viewpoint of magnetic properties such as Curie temperature and saturation magnetization. The possibility of two-phase separation is indicated between E2 1 (E2 1 ′) (Co,Ni) 3 AlC and E2 1 (L1 2 ) (Ni,Co) 3 Al(C) in the Co–Ni–Al–C quaternary system, where we denote E2 1 ′ as standing for the ordered crystal structure of (Co,Ni) 3 AlC 0.5 formed by the extra ordering of carbon atoms. Phase diagrams information was determined by means of electron probe microanalysis and microstructural observation for the T–Al–C (T: Co, Ni, Rh, Ir) and T–Al–B (T: Rh, Ir) systems to examine the phase equilibria in each alloy system focusing on the existence of E2 1 T 3 AlC and T 3 AlB. The existence of E2 1 Ir 3 AlB (E2 1 ′ Ir 3 AlB 0.5 ) phase has been revealed in the Ir–Al–B system by diffraction analysis of transmission electron microscopy and electron probe microanalysis.

The crystallographic structures of the [0 1 1]/54.7° lath boundary and the intralath and interlath cementite precipitates in a 0.2C steel tempered at 400 °C have been observed by high resolution transmission electron microscopy and discussed in terms of the O-lattice model. The [0 1 1]/54.7° lath boundary which is composed of one Kurdjumov–Sachs variant ( α 1) and one Nishiyama–Wasserman variant ( α 2) is likely to facet on the (0 12 7) α 1 //(5 19 14) α 2 irrational plane. The O-lattice analysis indicated that the facet coincides with a plane with a high density of O-points. Intralath cementite obeys the Isaichev orientation relationship (OR) with ferrite and has a habit plane of (1 0 1) θ //(1 2 1) α . Interlath cementite precipitating on the [0 1 1]/54.7° boundary maintains a strict Bagaryatskii OR with the ferrite on one side of the boundary and has a (1 0 0) θ habit plane.

Thermal desorption spectrometry study of the hydrogen trapping capability of a tempered Fe–0.2C martensite indicated that dislocations in the martensitic substructure are the primary traps and new grain boundaries resulting from recovery and recrystallization of the ferrite matrix are the secondary traps for hydrogen. Cementite has a negligible effect on hydrogen trapping.

Hydrogen absorption of incoherent TiC particles that were once reported to be strong hydrogen traps in iron at room temperature was investigated by means of thermal desorption spectrometry (TDS). The results indicated that incoherent TiC particles in iron do not trap hydrogen at all at room temperature even they are cathodically charged for a long time. Only at high temperatures and in atmosphere containing hydrogen source, incoherent TiC particles can trap hydrogen. The origin of hydrogen trapped by incoherent TiC particles was justified to be water vapor in the atmosphere during heat treatment.

A new method has been developed to determine the activation energy for hydrogen desorption from steels by means of thermal desorption spectrometry (TDS). This method directly fits the Kissinger’s reaction kinetic formula dX/dt=A(1-X)exp(-Ed/RT) to experimentally measured thermal desorption spectrum and best fit yields the activation energy (Ed) and the value of constant A. It has been proven that this new method is applicable to precise measurement of the activation energy for hydrogen desorption from incoherent TiC particle, coherent TiC precipitate, grain boundary and dislocation in 0.05C-0.20Ti-2.0Ni and 0.42C-0.30Ti steels.

Nanoindentation technique was applied to evaluate nanohardness distribution in a submicron scale for two kinds of martensitic steels: Fe-0.4C binary steel and Fe-0.05C-0.22Ti steel with a stoichiometric composition of TiC. AFM images showed that Fe-C steel includes relatively coarse cementite particles with about 100~200 nm in diameter and a couple of hundreds nanometer in average spacing, while high-resolution TEM observation showed that the Fe-C-Ti steel has fine TiC precipitates with 5 nm in diameter and 15 nm for average spacing. Nanoindentation results revealed that the standard deviation was much higher for the Fe-C than that for the Fe-C-Ti. Since the typical indent size was a couple of hundreds nanometer, which was about two orders larger than the size of the TiC and comparable to the cementite size, the small distribution of nanohardness of the Fe-C-Ti was attributed to the homogeneous microstructure in sub-micron scale, while the inhomogeneity of cementite particles in the Fe-C steel leaded to large nanohardness.

The effect of atmosphere in heat treatment on the hydrogen trapping of incoherent TiC particles in iron has been studied in order to clarify the origin of hydrogen trapped by incoherent TiC particles. The hydrogen trapped by incoherent TiC particles in iron after austenitizing and tempering treatments in air, in a nonprotective argon atmosphere, and in an ultrahigh vacuum (UHV) was identified and quantitatively measured by thermal-desorption spectrometry (TDS). Results showed that incoherent TiC particles in iron do not trap hydrogen at ambient temperature by a cathodic-charging method. It was justified that incoherent TiC particles trap hydrogen during high-temperature heat treatment in nonprotective atmospheres. The amount of hydrogen trapped by incoherent TiC particles decreases with increasing heat-treatment temperature, which is well explained by the equilibrium concentration of hydrogen trapped by incoherent TiC particles in iron under an atmosphere containing water vapor. The hydrogen is supplied through water-vapor oxidation of iron at high temperatures. According to this model, a binding energy between hydrogen and incoherent TiC of 53 kJ/mol was obtained. The energy barrier for hydrogen to jump into incoherent TiC was determined to range from 21 to 35 kJ/mol, which is too high for hydrogen to be trapped by incoherent TiC at low temperatures.

In spite of the significance of NaCl-type transition-metal carbides in steels, their crystallography and morphology have not been understood on an atomic scale. High-resolution electron microscopy was employed in the present work to examine the crystallography and morphology of TiC particles that precipitated in the quenched and tempered 0.05 wt% C–0.20 wt% Ti–2.0 wt% Ni steel. Plate-like TiC precipitates with a thickness ranging from a couple of atomic layers to 20 nm were observed in steels by tempering at 550, 600 and 800°C. It was found that the Baker–Nutting orientation relationship is always satisfied with the ferrite matrix within about 5° for both the nanosized and the coarse TiC particles. The habit plane of the TiC precipitate is the (100) ferrite plane. A moderate tendency for the faceting of lateral interfaces on the {001} and {011} ferrite planes was found. The presence of interfacial misfit dislocations was revealed by examining the excess lattice fringes terminating at the interfaces betwee...

The effect of tempering on hydrogen trapping in 0.42C-0.30Ti steel was studied by means of a hardness test, hydrogen thermal desorption spectrometry and high resolution transmission electron microscopy. In addition to the coarse undissolved TiC particles, fine TiC particles with a volume fraction up to 0.22 % precipitated during tempering at temperatures above 500°C. Coherent TiC square platelets with a diagonal length of about 2 nm and a thickness of less than 1 nm precipitated at 550 and 600°C, acting as reversible hydrogen traps. Tempering at 700°C caused the fine TiC particles to lose their coherency with the matrix, resulting in a very limited increase in reversibly trapped hydrogen content compared to the samples tempered below 500°C without TiC precipitation. On the other hand, the coarse undissolved TiC particles with an average diameter of 2 μm, acting as irreversible traps, greatly enhanced the irreversibly trapped hydrogen content when tempered at 500°C and were accompanied by a decrease in hydrogen desorption rate peak temperature. Comparison of both types of TiC particles with the same volume fraction indicated that the fine coherent TiC platelets were more effective in trapping hydrogen than the coarse incoherent undissolved TiC particles.

A new class of heat resisting ferritic steels is investigated in the Fe–Cr–Nb and Fe–Cr–Nb–Ni systems, in which the major strengthener is the Fe2Nb Laves phase. Ferritic steels strengthened by Laves phase are expected to show excellent high temperature strength, while the brittleness of Laves phase may lower the toughness of the alloy. The α-Fe/Fe2Nb two-phase microstructure is selected to improve mechanical properties through changing volume fraction and morphology of Laves phase. Effects of Nb and Ni contents on the microstructure and mechanical properties of the alloys, fixed at 10 at.% Cr, have been systematically investigated. Mechanical properties were evaluated by tensile tests conducted at room temperature, 873 and 973 K. The tensile test revealed that the room temperature ductility decreases with increasing Nb content. The alloys with 1.0–1.5 at.% Nb are found to exhibit a good balance between room temperature ductility and high temperature strength.

Composite coatings composed of titanium nitride, TiN, and diboride, TiB2, were reactively produced by the electrothermally exploded powder spray technique, in which feedstock powder was prepared from titanium and boron nitride particles. The microstructure of the coating was composed of titanium-ceramic particles the size of which were on the order of several nanometers to a few hundred nanometers. Such reactive thermal spraying brought base-metal saturation into a coating layer at the early stages of coating formation. The ceramic composite spray using feedstock of TiN and TiB2 particles preferentially brought a new phase of cubic titanium boronitride together with TiN and TiB2 into a coating. On comparing such a coating to one produced by the conventional method, the reactive thermal spray coating was richer in TiN and TiB2 due to the excess nitrogen in the feedstock.

Dependence of frequency of annealing twinning events on annealing temperature and composition, especially boron content, in polycrystalline Ni 3 Al alloys has been investigated by means of optical and transmission electron microscopy, and also by evaluation of coherent twin boundary energy via counting the wrong atomic bonds across twin boundaries. Microstructural observation found that after homogenization at temperatures from 1000 to 1360°C both binary and boron-doped Ni 3 Al alloys without prestrain show annealing twins only on the Al-rich side with respect to stoichiometry below 1200°C. Above 1200°C annealing twinning may also take place on the Ni-rich side of stoichiometry in the binary alloy system but no twins were found in the y + y' two-phase field. In contrast to plenty of annealing twins in binary alloys, addition of boron decreases greatly the number of twins. 0.2 at% addition significantly decreases the frequency of twinning events and no twins appear above 1200°C. 0.5 at% boron almost cleans out all the twins from the microstructure. Annealing twinning is supposed to be completed by two processes including sweeping of 1/3(211) superpartial dislocations on successive (111) planes and simultaneous diffusion of boron atoms into the energetically favorable octahedral interstice that are surrounded by six Ni atoms. Boron acts as a strong obstacle to the movement of the superpartial dislocations and greatly increases difficulty in annealing twinning.

Plastic deformation features of Co3AlC0.5 and a Co3AlC0.5-strengthened cobalt-based alloy were studied by means of transmission electron microscopy. It was found that {111} slip system is activated and dissociation of superlattice dislocations occurs in Co3AlC0.5 in a manner similar to the L12 structure. However, in contrast to the L12 structure, motion of dislocations is hindered by the pre-existing thermally grown-in anti-phase boundaries formed by wrong site occupation of carbon atoms, a unique type of crystal defects in Co3AlC0.5. On the other hand, deformation of the α(Co) alloy containing fine Co3AlC0.5 precipitates is characterized by localized slip of 1/6 Shockley partial dislocations cutting through the fine Co3AlC0.5 particles, leaving behind a highly disordered band region.

The crystallography of the lamellar structure composed of the tetragonal C11b phase and the hexagonal C40 phase which is similar to the well-known TiAl/Ti3Al lamellae was investigated by means of transmission electron microscopy in a MoSi2–15mol%TaSi2 pseudo-binary alloy after homogenized at 1400 °C for 168 h. It was found that the orientation relationship between C11b and C40 phases obeys the coincidence of the equivalent hexagonal atomic planes for the two phases. Three distinct variants of this orientation relationship were identified by electron diffraction. 1 3 1 1 0 Dislocations prevail in the C11b/C40 interfaces to accommodate the lattice misfit between the two phases. On the other hand, networks of 1 3 1 1 0 and 1 3 1 1 3 dislocations are frequently observed in C40 phase. Significance of the 1 3 1 1 3 c-component dislocations for improvement of the plasticity of C40 phase has been discussed.

The phase stability of molybdenum disilicide (MoSi 2 , C1 1 b structure) relative to other phases, C40 and C54 phases, in the pseudo-binary systems of MoSi 2 and other types of disilicides of transition metals including Cr, V, Nb, Ta and Ti was investigated by establishing the MoSi 2 -TSi 2 (T=Cr, V, Nb, Ta, Ti) pseudo-binary phase diagrams. It was found that V, Nb, Ta and Ti which substitute for Mo in MoSi 2 strongly stabilize the C40 phase while Cr only shows a weak C40 structure-stabilizing effect. The phase stability was also discussed on the basis of geometrical change in the three phases when composition varies. Change in lattice parameter of each phase indicated that phase stability of C1 1 b , C40 and C54 structures greatly depend on the relative stacking spacing of the equivalent hexagonal atomic plane for the three structures. Based on the results of phase diagram investigation, the present work attempted to design a C11 b /C40 lamellar microstructure, Existence of the equivalent hexagonal stacking atomic plane among C1 1 b , C40 and C54 phases makes it possible to design a coherent C 11 b /C40 two-phase microstructure. Assuming the equivalent atomic planes in C 11 b and C40 phases coincide with each other, high lattice coherency between C 11 b and C40 phases is available in MoSi 2 -CrSi 2 system with a lattice misfit of less than 0.5% and in the other four systems with a misfit of 1.3 to 2.7%, A lamellar microstructure was observed in MoSi 2 -TaSi 2 and MoSi 2 -NbSi 2 systems but no lamellar microstructures were obtained in MoSi 2 -CrSi 2 , MoSi 2 -VSi 2 and MoSi 2 -TiSi 2 systems.

Nano-precipitates of alloy carbides TiC, NbC and VC which have the same NaCl-type crystal structure in tempered martensite have been characterized by means of high-resolution transmission electron microscope (HRTEM) and correlated to the hydrogen trapping property. The examination of whether the amount of hydrogen absorbed by the TiC particles depends on their surface area or volume indicate that the coherent and semi-coherent TiC particles trap hydrogen at the precipitate/matrix interface at ambient temperature while the incoherent TiC particles trap hydrogen inside themselves only at high temperatures. Coherent and semi-coherent NbC and VC particles also demonstrate a surface area dependence of hydrogen trapping capacity with NbC > TiC ≫ VC. Contrary to TiC, incoherent NbC and VC particles are unable to trap hydrogen.

Targeting to develop E21 Co3AlC based heat resistant alloys, phase stability of E21 Co3AlC and (Co, Ni)3AlC has been investigated together with the magnetic properties of E21’ Co3AlC0.5 which is formed by the extra ordering of carbon atoms accompanying anti-phase boundary (APB). The correlation of ferromagnetism with APB in E21’ Co3AlC0.5 was evaluated using single crystals by high-resolution transmission electron microscopy (HRTEM) and vibrating sample magnetometer. Anti-phase domain (APD) size affects the ferromagnetism: for instance, the saturation magnetization becomes larger as the APD size is smaller. Local atomic configuration at APB was clearly observed by HRTEM image.

C11b/C40 fully lamellar microstructures, similar to the well-known TiAl/Ti3Al lamellae, were obtained in Ta- and Nb-added MoSi2 polycrystalline alloys in a previous work. In the present study, the crystallography of the lamellar structure is investigated in a MoSi2-15mol%TaSi2 pseudo-binary alloy after homogenized at 1400°C for 168h, in order to provide some useful parameters for microstructural control to improve mechanical properties. The orientation relationship between C11b and C40 phases and its three distinct variants were identified. Coherency of the lamellar interface is analyzed in comparison with the TiAl/Ti3Al lamellae. Approach to modify the C11b/C40 lamellar microstructure to increase its coherency is discussed based on the results obtained.

Intermetallic compound Co3A1Cx, or called ic-phase, has been reported to assume the E21, or Perovskite structure. In the present work its crystal structure is critically reinvestigated in the two-phase alloys containing κ-phase in the Co primary solid solution matrix, α(Co), using conventional electron diffraction. It is shown that the crystal structure of κ-phase is a derivative of E21 being a cubic structure composed of eight E21 sub-unit cells in a half of which the body center sites are not occupied by carbon atoms. As a result, its space group is Fm3m and the chemical formula should be Co3AIC0.5. It is also found that the lattice parameter of the phase is about twice as large as that of α(Co). Orientation relationship of the κ-phase with the matrix α(Co) is found to be similar to the case for the γ' phase with the fcc γmatrix in a Ni-base superalloy since three orthogonal axes of κ-phase are parallel to those of α(Co). Lattice misfit under this orientation relationship between the two phases is found to be about 2.5%.