The effect of solute Copper (Cu) on yield strength was investigated in dislocation-strengthened iron such as martensitic steels and cold-rolled steels having various dislocation densities. Yield strength of the Cu bearing martensitic steels increases with increasing the solute Cu content. However, the increment of yield strength by Cu solid solution is smaller in the martensitic steels than in the ferritic steels. Dislocation density of martensitic steels increases with increasing Cu content, and yield strength is also enhanced depending on the dislocation density. The increment of yield strength can be reasonably explained by dislocation strengthening mechanism based on Bailey-Hirsch relationship. In the cold-rolled Cu bearing ferritic steels, the strengthening by Cu solid solution is significant but it tends to disappear with increasing the dislocation density. These facts indicate that yield strength of the steels with high dislocation density is determined by dislocation strengthening and the contribution of Cu solid solution disappears due to the high density of dislocations.
Yield strength of particle dispersion strengthened ferritic steel is discussed in consideration of the size-distribution of particles in some imaginary models with different type of size-distribution and real materials of an Fe-C alloy and Fe-oxide alloys. Estimation of the mean particle spacing (λ) using the models has revealed that λ strongly depends on the type of size-distribution: ideal distribution, nominal distribution and log-nominal distribution, and that the influence of size-distribution on λ becomes more significant with increasing size and volume fraction of particles. This means that the size-distribution has to be considered for the correct estimation of the increment in yield strength (Δσ) depending on particle dispersion strengthening, especially in high strengthened materials. The Δσ measured for the real materials was found to be proportional to λ-1 of the kind of particles by making an appropriate correction of the size-distribution of particles. Consequently, the increment of yield strength due to particle dispersion strengthening was described as Δσ [MPa]=64/λ [μm] in the iron-base ferritic alloys.
Interaction between the microscopic and macroscopic behaviours of pearlite steel was investigated using the homogenization method and elastic-plastic finite element method. Attention was focused upon the microscopic yielding under macroscopically elastic loading state on the tension side. Tangent of macroscopically elastic line at the initial stage of the stress-strain diagram was precisely examined to find that microscopic yielding occurs in the microstructure although macroscopic state is elastic. The zone where microscopic yielding is achieved does not expand throughout the unloading process to the macroscopically-zero stress level, but it expands when the macroscopic stress level exceeds a threshold level in the inverse loading process. The absolute value of this threshold level is equal to the threshold level above which microscopic yielding appears in the initial loading on the tension side. Cyclic loading test was applied to specimens in a laboratory and increase in specimen temperature was measured to check the qualitative validity of the numerical investigation.
Effect of bainite volume fraction on tensile properties of ferrite-bainite steels was investigated experimentally and analytically. Ferrite-bainite steels with different bainite volume fractions having the similar mechanical properties for each constituent phase were prepared and the tensile properties were investigated. Lower yield ratio (YR) and higher n-value were obtained for the steels with bainite volume fraction of 16 to 35%, while the ferrite or bainite single phase steels exhibited relatively higher YR and lower n-value. In order to investigate microscopic deformation behavior of ferrite-bainite steels, an axisymmetric unit cell model based on a regular array of second-phase particles arranged on a BCC lattice was applied. Finite element analyses were carried out to investigate the macroscopic and microscopic response of unit cells with morphological features based on the actual steels. Macroscopic deformation behavior of ferrite-bainite steels calculated by the unit cell model showed good agreement with experimental results. In addition, the effect of bainite volume fraction on YR and n-value was well simulated by the unit cell model. Microscopic investigation on the unit cells revealed that significant strain concentration existed in the ferrite phase near the ferrite/bainite boundary for ferrite-16% and 35% bainite steel, which had higher experimental n-value. However, further increasing of bainite volume fraction caused plastic deformation of bainite phase and the strain concentration was decreased, resulting in lower level of n-value for ferrite-58% bainite steel. Effect of bainite morphology on strain hardening behavior of ferrite-bainite steels was also discussed based on analytical study using the unit cell model.
Pearlite is composed of ferrite and cementite with different strengths. Even in the ferrite matrix, stress must be different from grain to grain depending on the crystal orientation which is called block stress. The ferrite block stress and phase stress in two pearlite steels with a strong ‹110› fiber texture or a weak texture were studied by using a time of flight method of in situ neutron diffraction during tensile deformation. It is found that the texture influences the block stress particularly in the transverse direction.
Small angle neutron scattering (SANS) and insitu neutron diffraction during tension test were performed for three austenitic stainless steels with different concentrations of nitrogen. The existence of nano-sized cluster composed of nitrogen and alloy element atoms is suggested by SANS. The cluster seems to make dislocation line planar and hinder cross slip. Because of such a characteristic dislocation motion, the stress relaxation at the grain boundaries becomes difficult leading to enlarge the intergranular stress in nitrogen bearing steels. The insitu neutron diffraction reveals clearly that the intergranular stress increases with increasing of nitrogen concentration.
Tensile tests were carried out for fine austenitic stainless steel wires with different grain sizes and wire diameters, and then the effect of wire diameter on the grain size dependence of tensile properties was discussed in terms of the behavior of dislocation slip near the material surface. 0.2% proof stress and total elongation of fine wires depended on not only grain size but also its diameter: these properties were strongly influenced by the ratio of wire diameter (D) to grain size (d), D/d, which represents the number of grains existing along a diameter direction. It was found that the tensile properties of fine wires were markedly deteriorated when the D/d became smaller than 5. This was due to the occurrence of local necking which is just like the deformation of single crystal. In the grains facing the wire surface, dislocations can easily pass out from the material surface and this leads to easier deformation in comparison with the grains within the wire where the dislocation movement is interrupted by grain boundaries. In the case when D/d is sufficiently large, because the volume fraction of the grains facing the surface is very small: thus, the whole deformation of wire is not affected by the wire diameter.
The effect of a fine lamellar structure due to deformation twin on the yield strength of fcc ferrous alloys has been investigated by using cold-rolled Fe-36mass%Ni alloy and 310S stainless steels. Microstructural evolution during cold-rolling of both alloys has been also examined by using transmission electron microscopy. In cold-rolled Fe-36mass%Ni alloy, a kind of layered cell structure was observed, but no deformation twin was found. On the other hand, in a 310S steel, a fine lamellar structure of twin-matrix (T-M) was markedly developed with cold-rolling. The cold-rolled 310S steels exhibited yield strengths about 2 times higher than those of the rolled Fe-Ni alloys in spite that the yield strength of as-annealed 310S is lower than that of as-annealed Fe-Ni alloy. This indicates that the occurrence of fine T-M lamellae during cold-rolling enhances the evolution of fine-grained structure, which causes the marked increase in the yield strength of the rolled specimens.
Mechanical milling (MM) process is applied to SUS316L austenitic stainless steel powder and a nano-duplex structure formation behavior is investigated. A nano grain structure with BCC phase forms in surface region of the powder milled for 720 ks. Those nano grains have the grain size of less than 20 nm. The (α+γ) nano-duplex structure, whose grain size is approximately 100 nm, forms in the inner region of the milling powder. The α grain in the surface region is an aggregated structure and consists of equiaxed grains, while those α grains in the inner region disperse homogeneously and indicate more spherical and smooth shape compared with the γ grains around them. TEM/EDS examination revealed that there occurs two kinds of BCC transformation. Those α nano grains in the surface region are considered to form by increase of grain boundary energy. On the other hand, the spherical α nano grains in the inner region form by increase of the grain boundary energy as well as the chemical free energy. Such an increase of chemical free energy is assumed to be caused by the existence of a huge number of vacancies which are introduced by the MM treatment. Although the austenite phase in the SUS316L steel is meta-stable at the room temperature, it hardly transforms to martensite phase even by a heavy cold rolling. Thus, these two kinds of α transformation are extraordinary phenomena and are considered quite different from these conventional strain induced martensitic transformation. A (γ+σ) nano-duplex structure material obtained by sintering (α+γ) nano-duplex powder has a good mechanical property.