Tetsumori Shinoda

The effects of applied stress on the intergranular phosphorus segregation in a chromium steel

Abstract The mean Auger peak ratio (APR) of phosphorus, averaged over the intergranular fracture surface of each sample, has been measured on smooth and notched types of specimens, which were subjected to stress agings at 773 K for times up to 15 h under given stresses of tension or compression after a long term aging at 773 K under no stress. When the stress level for aging was smaller than a critical value, the mean APR of P of the smooth specimen under tension rapidly increased during the first l h aging and then decreased to around the initial value in 15 h of aging, whereas that of the compressed specimen decreased, nearly by the same magnitude as in the case of tension, during the first l h and then similarly approached to the initial value in 15 h of aging. The mean P-APR in the vicinity of the notch root was significantly small compared with those at positions distant from the notch root in a stress aged notched specimen.

Interfacial reaction and strength of SiC fibres coated with aluminium alloys

Silicon carbide fibres (Nicalon) were coated with pure aluminium and aluminium alloys containing silicon. The coated fibres were annealed to produce an interfacial reaction zone between the coated layer and the fibre. The effect of this reaction zone on the tensile strength of the fibre was investigated. During the early stages of growth the reaction zone of the fibre is thin, and the strength of the fibre is controlled by inherent defects so that the fibre retains its original strength. After the early stages, notches are formed in the reaction zone of the fibre on loading at a small strain and the fibre fractures when a notch extends into the fibre. In this stage the fibre strength is dependent on the thickness of the reaction zone. An alloying addition of 1 or 5 at % Si to the aluminium matrix was found to be effective in retarding the growth rate of the reaction zone.

Compatibility between alumina fibres and aluminium

An investigation is carried out on the interfacial wetting behaviour and reactions between aluminium and alumina fibres (85mass% Al2O3 and 15mass% SiO2). Aluminium is coated onto alumina fibres by a vacuum evaporation technique and the surface of the fully coated fibres and the edge of the partially coated fibres are examined by scanning electron microscope after heat treatments at various temperatures. Within a temperature regime between 943 and 1273 K, occurrence of such interfacial reactions as 4Al(I) + Al2O3(s) → 3Al2O3(g) and 4Al(I) + 3SiO2(s) → 2Al2O3(s) + 3Si(s) are detected. It is found that molten aluminium can cover the alumina fibre surface but it peels off near the edge of the coating film on a partially coated fibre, showing the very weak interface cohesion. This is ascribed to the lack of a stable compound formation at the interface. Results of tensile test show that the strength of the coated fibres is degraded after heat-treating at above the melting point of aluminium. The culprits for the tensile failure of alumina fibres are evaluated by the Weibull distribution theory.

A model for the applied stress effects on the Intergranular Phosphorus Segregation in ferrous materials

Abstract A model has been presented in terms of which the facts observed in a previous paper can be successfully explained. It is based on the assumption that phosphorus atoms migrate along grain boundaries so as to decrease the strain energy in the grain boundaries more stressed under normal traction. The change in the intergranular phosphorus concentration (IPC) due to this solute migration during the first aging of At, at a grain boundry normal to the applied stress, is given by ∼ φχ p 0 D p g VσΔt / s 2 RT , where χ p 0 is the initial value of IPC prior to stress aging, D p p the diffusion coefficient of P along grain boundaries, V the specific volume of the alloy, s half the grain diameter, φ a numerical factor of the order of unity, RT has the usual meaning and σ is an apparent traction at the grain boundary, being more than 4 ∼ 5 times as large as the applied stress.

Theoretical estimates of phosphorus concentration profiles across grain boundaries in Fe-P and Fe Ni-P systems

Abstract The equilibrium “intergranular segregation extent (ISE)” of P in the above two alloy systems has been calculated on the basis of a model presented here. The ISE of P is defined as the monolayer thickness of a P-enriched bulk region adjacent to the grain boundary, where a normalized concentration of P is greater than one-tenth of that of the grain boundary. The model is in principle analogous to Onos discrete lattice approach based on a simple regular solution model under the assumption of the nearest neighbor interaction of atoms. The ISE of P thus calculated is. irrespective of the Ni-concentration of the alloys, ~ one monolayer thick at most whatever magnitudes are chosen for the various parameters adopted within their limited ranges in the present model. This suggests that the equilibrium intergranular segregation of P occurs exclusively at grain boundaries per se in ferrous alloys.

Cluster variation method approach to estimating vacancy properties in B2 type ordered NiAl and NiFeAl alloys

Abstract To compute the vacancy concentration in B2 type ordered NiAl and NiFeAl alloys, a new calculation technique based on the cluster variation method (CVM) was developed. The critical condition for finding the solution is prescribed as the vacancy concentration at which the chemical potential in the alloys vanishes. Using this method together with a set of appropriate interaction parameters for each nearest or second nearest neighbouring atom-atom or atom-vacancy pair, we can easily and definitely calculate the vacancy concentration in these alloy systems, in contrast to the conventional method which is more sophisticated since it deals with an open system with respect to the number of vacancies. Comparisons between the calculated results and available experimental data for the vacancy properties of these alloy systems are made.

The back stress decrease on annealing and hydrogen embrittlement of deformed eutectoid carbon steel

Abstract The back stress increase on work hardening and its subsequent decrease on annealing of a eutectoid steel have been studied. The effects of work hardening and annealing on hydrogen embrittlement of the steel have also been investigated. The hardening due to back stress has been found to be much smaller in spheroidized steels than in pearlitic steel. The softening of the deformed steel induced by annealing is mostly attributed to the decrease in back stress. The back stress decrease was accompanied by a dimensional change without any change in volume which was consistent with the characteristics of the back stress. The resistance to the decrease in back stress increased with an increase in the size or the aspect ratio of the cementite. It was experimentally found that hydrogen embrittlement is sensitive to the presence of the back stress; the decrease in back stress on annealing improved the resistance of the steel to hydrogen embrittlement.

Design of Ni-Al base High Temperature Alloys by TB Electronic Theory

A microscopic tight-binding (TB) electronic theory is used to study the lattice defects (point defects, APB, CSF and dislocations) of high temperature intermetallic compounds, like Ni 3 Al, NiAl and Co 3 Ti in conjunction with their mechanical and thermodynamical properties. The calculated lattice defect properties of the intermetallic compounds are compared with the available experimental results, and they are provided as fundamental data for alloy design. It is shown that overall trends of mechanical and thermodynamical properties can be accounted for within the present TB type of electronic theory.

Extra Contribution of Tm Solutes to the Solute-Dislocation/Surface Interactions in Nickel

The solute-dislocation (S-D) interaction energies for various kinds of transition-metal(TM) and B-subgroup solutes in nickel are calculated using a simple tight-binding electronic theory with lattice relaxations taken into account. As a result, the S-D interaction energy for TM solutes is almost identical with that for B-subgroup solutes when compared at an equal level of size-misfit strain( SMS) in Ni. This is neither more nor less than the answer from the classical elastic theory. However, in the solutions of the present calculation there are some points which can not be derived from the continuum elastic theory. Experiments show that Ni 3 Al is, differently from Ni, more strengthened by TM solutes than by B-subgroup ones. It is discussed that this may be due to the extra contribution of TM solutes to the S-D interaction energy in Ni 3 Al. As another possibility that TM solutes may contribute differently from B-subgroup ones to the macroscopic property of Ni, the (100) surface segregation of solute atoms in Ni is discussed.

Profiles of phosphorus concentration at grain boundaries in ferrous alloys

The CDPs of phosphorus across grain boundaries in the preceding alloys have been calculated on the assumption that v13, the interaction parameter for Fe-P system, is negative in sign. The obtained P-CDPs are essentially the same in type as those in the previous study where v13 was positive. When v13 < 0, the normalized P- concentration (with a variable range of 0–1), n(I)i, at the first bulk layer adjacent to the grain boundary is able to reach a level as high as 0.8 or more, while the successive n1- values at the bulk layers of 2nd, 3rd, etc. never exceed the order of magnitude of 10−2 however the parameters for calculation are chosen in each variable range. If another restriction that e13 < e33, where e13 and e33 are the binding energies for the nearest neighbor pair of Fe-P and that of Fe-Fe respectively, is imposed besides the above condition of v13 < 0, the model used in the present study fails in the P-CDP calculation so far as the Fe-P binary system is concerned.