The transformation of hexagonal to tetragonal germanium dioxide was studied in the temperature range 760°to 900°C by X-ray diffraction analysis. Lithium carbonate was used as the minor component added to germanium dioxide. The experimental results obtained are summarized as follows: (1) The hexagonal form transforms to the tetragonal form through an intermediate transition phase. (2) The transition phase is found to be noncrystalline from the evidence of X-ray analysis. (3) The transformation proceeds by a series of consecutive reactions A→B→C in which A, B and C are the hexagonal form, the intermediate phase, and the tetragonal form, respectively.
A study on the pitting resistance of white cast iron containing 1.1-2.8%C, which was cast or forged, was carried out by performing experiments using an Amslar-type wear testing machine under sliding conditions with light oil as a lubricant. The results obtained are summarized as follows: (1) Pitting resistance of white cast iron which contains higher carbon (C>1.8%) is significantly improved by forging. (2) The most hardened layer generated by repeating contact stresses exists in 0.10∼0.20mm below the contact surface. The higher the contact stress is, the more the hardness increment of the hardened layer is. (3) It seems that the propagation of crack is strikingly affected by the shape of large-size carbide and the adhesive strength between carbide and matrix.
The martensitic transformation-induced plasticity (TRIP phenomenon) of pre-deformed austenite in Fe-15% Cr-12% Ni alloy (Ms<room temperature) was investigated by means of tensile test at various temperatures between -196°∼+120°C. As pre-deformation, the reduction of 0∼80% was given by rolling at 200°C. Since the rolling temperature was above Md temperature of the specimen, all specimens consisted of austenitic single phase even after rolling at 200°C. The TRIP was observed in the pre-deformed austenite as well as in the non-predeformed one, and the maximum elongation appeared at a certain temperature between Md and Ms. The Ms temperature decreased with increasing the degree of pre-deformation. Accordingly, as the degree of pre-deformation increases, the range of temperature showing the TRIP phenomenon became narrower and the temperature at which the maximum elongation appeared shifted toward the lower temperature. While the strength of austenite (especially the 0.2% proof stress) at 120°C was markedly increased by rolling, at the same time the elongation became smaller. However, the elongation of pre-deformed austenite was enhanced when the TRIP occurred. For example, the elongation of the specimen given a pre-deformation of 80% was only 5% at 120°C (above Md), but it was increased to 57% by TRIP at -196°C. It was found that the amount of increase in elongation by TRIP, which is the difference between the elongation value at the temperature showing maximum TRIP and that at the temperature of austenitic state, was almost constant (about 50%) independently of the degree of pre-deformation of austenite. Therefore, it is possible to obtain the optimum condition that the alloy shows high strength and ductility by TRIP after pre-deformation of austenite. The yield point and plateau stage similar to Lüders strain were observed in the stress-strain curves of pre-deformed specimens. These phenomena became to appear more distinctly with increasing the degree of pre-deformation and with decreasing the test temperature.
Many studies have been carried out in regard to the grain size dependence of fatigue strength of metals. And it has been made clear that fatigue strength of f. c. c. metals greatly depends on the stacking fault energy γ, and the effect of γ on the crack propagation process is different from that on the crack initiation process. In this work, the grain size dependence of the fatigue strength was mainly examined under the rotating bending fatigue test, and the cyclic work hardening behavior under the push-pull fatigue test. Furthermore, the surface structure due to fatigue deformation was observed by means of optical and electron microscopes. From the viewpoints of both microscopic structure and dynamical behavior of the materials, the grain size dependence of fatigue strength was investigated in copper and α-brass. The results obtained are summarized as follows: (1) The grain size dependence in the crack initiation process in α-brass is very remarkable at 200°C as well as at room temperature. This result suggests that the effect of grain size on fatigue strength must be considered from both viewpoints of the configuration of slip line and ageing effect. (2) The grain size dependence in crack propagation process in α-brass is very small at R.T, but it increases at 200°C. (3) The cyclic work hardening in α-brass is larger in fine grains than in coarse grains. On the other hand, in copper almost opposite result was obtained. From these results the difference in grain size dependence under constant stress fatigue test can be explained.
In the preceding report, the effects of hydrogen upon various characteristics of steel were found by performing the static bending test and delayed fracture test on the specimens in which hydrogen was introduced by acid dipping. In this report, the initial crack-nucleating condition in delayed fracture has been obtained based on the plastic solution of A.P. Green et al. for a sharply V notched specimen and the experimental results of the preceding report. And also the maximum stress to nucleate a crack when hydrogen is introduced has been obtained by using the calculation and the diagrams. Furthermore, the static bending test and delayed fracture test were performed on the high strength hexagon bolts in order to ascertain whether this result can be applied in practice. And it has been found that the crack-nucleating stress obtained is applicable.
Notched specimens of spheroidal graphite cast iron were fatigued under alternating bending stress. The fatigue fracture surface was observed using a two-stage plastic-carbon replica with Cr shadowing. The result obtained are as follows: (1) Ductile-appearing fatigue striations were observed in the ferrite region on the fracture surface of spheroidal cast iron as in the cases of steels, Al alloys, etc. (3) Roughly speaking, the striation spacing was equivalent to the magnitude of macroscopically measured rate of crack growth. But in the range of lower growth rate, there was a tendency of striation spacing being larger than the macroscopic rate. (3) The rate of fatigue crack growth (dl/dn) was closely related to the stress intensity factor K and could be expressed by (dl/dn)=c·K2
This work has been carried out for the purpose of obtaining more detailed information on the fatigue properties of ausformed Mn-Cr-B spring steel. The fatigue strength and the fracture behavior of the ausformed specimens under completely reversed plane bending stress were compared with those of the conventionally heat-treated specimens tempered to the approximately same hardness. The results obtained are as follows: (1) The endurance limit of the ausformed steel increases by about 8 to 9 percent on the average from that of specimens heat-treated normally. The strength of the former at 106 cycles is also higher than that of the latter. (2) The results of fractographic examination of the specimens fractured at higher stress levels show that the fracture surfaces of the conventionally heat treated specimens exhibit the dimple pattern, while the fracture surfaces of the ausformed specimens are mostly occupied by the featureless flat area. This difference in pattern of the fractographs between the conventionally treated specimen and the ausformed specimen suggests that the improvement of the fatigue strength by ausforming is closely related to the decrease in the rate of fatigue crack propagation. (3) The carbides in the ausformed steel are dispersed very densely in comparison with those in the conventionally heat treated specimens, and the endurance limit σw is approximately related to the mean-free path of carbide λ by the equation: σw=σwo+Cλ-1/2 As the results of this experiment, σwo≈8kg/mm2 and C≈0.6(kg/mm2) mm1/2 were obtained.
The authors previously observed that the tufftriding treatment was effective to the improvement of fatigue limit of iron and steel. However, it is a complex task that the designer obtains the fatigue limit of tufftrided steel from the fatigue test. In this paper, the prediction of fatigue limit was discussed from the viewpoints of the increase of hardness due to the tufftriding treatment and the cyclic stress and the existence of residual stress due to the tufftriding treatment which contributes to the improvement of fatigue limit. The following predicting equation was proposed. σw·t=C1(h/a-h)σw·nt+(1+C2)σw·nt where, σw·t: fatigue limit of tufftrided specimen σw·nt: fatigue limit of non-tufftrided specimen h: hardened depth a: distance from the surface to the center of specimen C1: a constant which indicates hardening due to cyclic stress C2: a constant which indicates the degree of improvement of fatigue limit due to residual stress The validity and the applicability of the equation was investigated by the experiment.
Many works have so far been made on the problem of notch strength in engineering materials, but little on cast iron. The author carried out some experiments on the notch strength of FC 25 class cast iron, using plate tensile specimens with notches of various degrees of sharpness, and discussed the experimental results. The finite element method was used to analyze the stress distribution for blunt notch. The results obtained are summarized as follows: (1) The rupture strength of cast iron was decreased by the notch in plate specimen. The criterion that the average stress of the notch section is constant was not valid. (2) The rupture strength decreased with increasing the sharpness of notch, but showed a tendency to become almost constant in the region of sharp notch (ρ<0.5mm). (3) The stress analysis by the finite element method for blunt notch (ρ=10, 15mm) showed that the notched specimen ruptured when the maximum axial stress under the notch reached the tensile strength of smooth specimen. That is to say, the maximum stress criterion was valid. (4) Though the rupture condition for the sharper notch was not analyzed by the method taken in this paper, it was suggested that the rupture strength was influenced by the stress gradient besides the maximum stress. (5) The tendency of saturation of strength in the region of sharp notch is thought to be due to the microscopic notch effect of graphite flakes.
Recently, it has become possible to analyze the stress or strain of a complicatedly shaped body by the help of the finite element method and the computer. In this case, there is no problem for elastic deformation because the stress-strain equations are well defined. But, at present, for plastic deformation, the satisfactory stress-strain equations which represent actual deformation have not been established yet, because the equations are known to change with the strain paths. So, it is necessary to make clear the relation beween stress and strain during plastic deformation. The behaviors of deformation in a single crystal have been made clear by the dislocation theory and crystallography. But the metallic materials are generally polycrystals, and the behaviors of deformation in a polycrystal can not be represented by the average value of those in a single crystal because of the interaction between crystallites and the effect of grain boundary. In this paper, it is assumed that each grain in a polycrystal deforms by slip and that the strain of each grain is equal to the mechanical strain. Then the theoretical stress-strain curve of a face centered cubic metal is derived with consideration for the anisotropy which arises from the difference of work-hardening on each slip system. The results obtained are as follows: (1) According to the dislocation theory, the equation of work-hardening on each slip system is in the form, τck=τ0+ΣjHkj√γj+γ0dγk (2) The stress-strain curve of a polycrystal can be obtained from that of a single crystal by using the above equation with the same coefficients of work-hardening.