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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’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.

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.

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.

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.

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.