This paper describes the metal injection molding of titanium powders using a high performance binder. A commercial titanium and a titanium hydride powders were processed by the injection molding. Acrylate, atactic-polypropylene and wax were used as the binder. The sintered specimen which was debinded in air had less defects than that debinded in argon gas. The densification process during sintering followed by using a thermal dilatometer clearly showed that densification behavior and the microstructure of sintered compacts were strongly affected by the sintering atmosphere. An acceleration of densification rate during sintring was observed in the case of titanium hydride powder compacts. The compacts with highest density were obtained by sintering in vacuum. On the other hand, the compacts sintered in nitrogen gas showed many defects such as cracks due to expansion. Compression tests were carried out on titanium and titanium hydride powder compacts sintered at 1300°C for 1hr in vacuum. Also, the microstructure of these compacts was observed. These compacts possess a higher strength but lower ductility than conventional wrought titanium.
Indentation tests using a steel ball were carried out on the (001), (011) and (111) faces of αCu-Al single crystals in order to elucidate the mechanism of plastic deformation produced in the surface layer. The indentation configuration produced by the indentor and the slip trace distribution were studied in detail. In addition, the dislocation density around the indentation on the (111) and (211) faces was revealed by removing successively the thin surface layers and developing etch pits on the exposed surfaces. The results obtained are summarized as follows: (1) The slip tends to occur in the ‹110› direction on the (001) and (011) crystal faces. (2) In the case of indentation on the (111) face, the slip occurs on two sets of planes arranged in truncated triangular pyramids, one diverging and the other converging into the material. (3) The distributions of the dislocation density and the range of plastic deformation on the (111) and (211) faces were clarified.
αCu-Al single crystals were scratched with a steel ball in various directions in order to clarify the mechanism of the deformation observed in abrasive wear of the finished surface. Microscopic observations of the slip traces and of the scratched track were made on the (111) face scratched in the ,  and  directions. After the (111) face had been scratched, the face was electropolished to remove a thin layer of suitable thickness in order to observe the distribution of the etch pits and slip traces on the exposed surface. This in turn permitted the determination of the distribution of the dislocation density on the cross-section perpendicular to the (111) crystal face. The conclusions are summarized as follows: (1) The slip width and slip depth increase in proportion to the load. (2) The scratched track width and the slip depth are wider for scratches in the  direction than for scratches in the  direction. (3) The active slip systems in the scratch can be explained by taking into consideration the stresses produced in the material. (4) Beyond a certain depth, the effect of the frictional stress produced by the scratch disappears, and only the effect of the normal stress remains.
A recently established statistical theory of fracture location combined with a competing risk theory was used to derive the distribution functions of fracture location, defect size and defect orientation. In this report, a new theory was applied to non-linear elastic bodies obeying a non-symmetric constitutive equation. Practically, a square cross-sectioned beam loaded on a three point bending apparatus was analyzed to study three types of competing defects; inner defects, surface defects, edge defects as fracture origins. From these results, the effect of non-linearity of the constitutive equation on the distribution function of fracture location was discussed, and a new estimating method of the constitutive equation derived from fracture location data was suggested.
Fatigue tests were conducted using solid cylindrical specimens of pure copper with circumferential blunt notches subjected to combined axial-torsional loadings. The behavior of crack growth was observed by a plastic replication technique, and the property of fatigue life was investigated. The cracking at notch root was identified as an intergranular type. The dominant morphology of crack growth was found to be the coalescence of distributed small-cracks, while some difference was observed depending on the stress multiaxiality. The fatigue life was correlated with the equivalent stress of the Mises type and the maximum shear stress. The fatigue life for the same value of each stress parameter became longer as increasing shear component in the stress state at notch root. An analytical procedure for the crack growth at notch root was established by using a model based on the competition between the coalescence growth and the propagation of a dominant crack. Characteristics of intergranular cracking at the notch root and its dependence on the stress state were very well simulated by the present analytical model. Fatigue lives in several test conditions were statistically estimated by a simulation of the Monte Carlo type. The prediction with the simulated scatter-bands almost coincided with the experimental results.
The aim of the present study was to clarify the residual stress of TiC coated steels by the X-ray diffraction measurements and to make clear the effect of TiC coating on fatigue strength. TiC layers were coated on three kinds of steels by a chemical vapor deposition method (CVD) under several coating conditions. Fatigue tests were conducted by using the plain specimens (flat plate type) and stress concentration type specimens (cantilever flat plate type). As the results, it was confirmed that a large compressive stress was observed in the CVD layer of TiC, whereas tensile stress or smaller compressive stress occurred in the substrate region adjacent to the interface. It was also found that the fatigue strength of TiC coated steels become lower than that of the specimens without TiC coating, but the quenching and tempering treatment after TiC coating improved the fatigue strength. It was concluded that the hardness of substrate and the residual stress in the top part of substrate and the TiC layer were the main factors affecting fatigue strength of TiC coated steels.
Ion-nitriding treatment has been brought into greater use as a surface hardening technique, for it offers such potential advantages as saving energy, clean procedure and so forth. Though the fundamental mechanical properties of ion-nitrided specimens have been investigated from various points of view, there exist few papers on their fatigue behavior under repeated impact load. In the present study, a series of low cycle impact fatigue tests were carried out on the ion-nitrided SM50 steel specimens by using the Matsumura type impact fatigue testing machine. The results obtained at four impact energy levels of 0.2, 0.3, 0.4 and 0.5 kgf·m indicated that the surface hardened layer of the ion-nitrided specimen behaved in a brittle manner, that is, surface cracks initiated at the first impact regardless of the impact energy level. It was concluded that such brittle cracks shortened the fatigue lives of the ion-nitrided specimens, when compared with the results for the annealed SM50 specimens. Furthermore, the above mentioned fatigue properties were discussed in relation to the fracture mode in monotonic tensile tests.
Fatigue crack growth properties under freely corroding and cathodic protection in 3%NaCl aqueous solution were studied under very-low cycling for HT80 high tensile and SUS304 stainless steels. The crack growth rate in 3%NaCl solution was accelerated as compared with that in air. The acceleration was dependent on material, environment, ΔK-level and frequency, and the following three types of acceleration was observed. (1) The CF type acceleration was inherent to corrosion fatigue in free corroding and the rate was accelerated about twice at frequencies lower than 0.3Hz independently of material and ΔK level. (2) The HE(IG) type was related to the intergranular cracking due to hydrogen embrittlement and appeared in the range of ΔK from 4 to 20MPa·m1/2 and at frequencies from 10-3 to 102Hz only for HT80 steel. The maximum acceleration rate became about five times at ΔK=10MPa·m1/2 and f=0.3Hz. (3) The HE(TG) type was related to the transgranular and brittle fracture due to hydrogen embrittlement and the rate became constant at frequencies lower than 0.03Hz. This type was also dependent on material, environment and ΔK level. The maximum acceleration rate was nearly thirty and ten times for TH80 and SUS304 steels, respectively.
Service failure and many accidents induced by stress corrosion cracking (SCC) frequently occur in the structures made of duplex stainless steel. In this paper, the duplex phase effect on susceptibility of SCC in duplex stainless steel weld metal was discussed analytically by an electro-chemical computer simulation method. The following properties became evident after examining the analytical results. Susceptibility of SCC became lowest under the condition of α content being nearly 30%, which was closely connected with the transition of weld metal solidification mode. As for the duplex phase effect on susceptibility of SCC, in the case that α content being less than 30%, α phase was found to play an important role of preventing SCC initiation by the so-called keying effect which is caused by the potential deviation toward noble potential. On the other hand, γ phase was simultaneously subjected cathodic protection by α phase. High resistance for SCC of duplex stainless steel was confirmed to be based on the interaction of these phenomena.
Various roles of oxide in high-temperature, high-cycle fatigue lives of engineering steels in air were classified and discussed to find out the dominant role for each steel and test temperature. The materials examined were 9 kinds of steels such as carbon steels, low alloy steels and austenitic and martensitic stainless steels. As for the effect of oxidation on high-cycle fatigue, the following three roles of oxide were pointed out: (1) Oxide at the specimen surface prevents crack formation from the surface, and the fish-eye fracture occurs. (2) The propagation process at the early stage of cracks is delayed by the oxide formed on the crack surface and thus the fatigue life is prolonged. (3) Notches acting as crack initiation sites are formed by penetrating oxide from the surface. At temperatures of 300-400°C for carbon and low alloy steels and 700-800°C for austenitic stainless steels, fish-eye fracture occurred in the high-cycle region above about 107 cycles. The oxidation effect (1) is dominant in these conditions. At 600°C for low alloy steels, the fatigue strength decreased drastically in the high-cycle region and a shell-like-pattern was formed at the crack initiation site of fracture surface. The oxidation effect (3) plays an important role in such a condition.
Low cycle fatigue (LCF) and fatigue crack propagation (FCP) tests under creep conditions were carried out on HK40, HP, and their modified alloys containing a small amount of Zr, Nb, and/or Ti. The failure mechanism was established based on the microscopic observations by the scanning and transmission electron microscopies. Also, the mechanical factors affecting the LCF life were discussed, considering the crack propagation law. The main results obtained are as follows; (1) The addition of Nb, Ti and/or Zr improves the LCF life and the FCP resistance under creep conditions. In particular, the effect of Zr is marked. (2) Cracks propagate along the intensive sub-boundaries, on which coarse M23C6 carbides precipitate during the LCF test. (3) Zirconium, Nb and Ti form fine and dispersed secondary carbides (MC) in the matrix of material. These fine carbides lead to a uniform dislocation structure without intensive sub-boundaries and then result in the dispersed precipitation of coarse M23C6 carbides. Accordingly, the addition of these elements improves LCF and FCP properties. (4) From a mechanical viewpoint, the LCF life can be determined by the fatigue and creep crack propagation laws and the strain energy densities which are obtainable based on J-integral analysis on a cracked body.
Elevated temperature fatigue tests were conducted on P/M HIP'ed superalloy, MERL76. The defects, such as preciptitates on the prior particle boundaries (PPBs), gas-porosities and nonmentallic inclusions, were found at fatigue origins. The relation between defect size, ai, and fatigue life, Nf, was independent of defect types and was formulated as Nf·Δσmeq=h·a1-m/2i Δσeq=Δσ√1+2Eg(n)/n+1·Δεp/Δσ where m: the exponent in crack propagation law, da/dN=CΔKm, h: a material constant, E: Young's modulus, g(n): a function of cyclic strain hardening exponent, n, and Δεp: plastic strain range. The relation between threshold stress, Δσth, and defect size, ai, was also expressed as ΔKth=2/π·Δσth√π(ai+a0) where ΔKth: threshold stress intensity factor for long cracks. The value, a0, was about 0.1mm, which coincided with the size estimated by an experimental relationship between grain size and a0 on steels. The usage of Ar-treated powders and the hot-work after HIP'ing improved fatigue life since they prevented PPB failure. Furthermore, the usage of fine powders is preferable because it can reduce the size of pores and inclusions.
An analytical prediction method of brittle and ductile creep rupture lives was proposed for 1CrMoV steel multiple-lug rotor steeples which have a tendency of degradation in creep ductility after long-term service. Rupture time was evaluated as a sum of crack initiation life and crack growth life with consideration of creep ductility and load-carrying factors of multiple-lugs in the steady creep state. This method was experimentally verified by creep rupture tests of notched thick plate specimens and interrupted creep tests and microstructure observation of notched thick plate specimens and blade-root and rotor-steeple models at 600°C. Two kinds of 1CrMoV steel in creep ductility were tested. The main conclusions are as follows. 1) An estimation method of load-carrying factors of multiple-lugs in the steady creep state was proposed and a uniforming tendency of load-carrying factors was recognized in the present models. 2) An estimation method of creep stress at the lug root with consideration of the stress-dependent creep strain rate was proposed for damage analysis and its validity was verified. 3) Creep rupture life of 1CrMoV rotor steeples can be evaluated in a practically reasonable accuracy as a sum of creep crack initiation life and creep crack growth life with consideration of creep ductility and load carrying factors of multiple-lugs in the steady creep state.
In order to evaluate the thermal shock resistance of epoxy resin, the thermal shock test of quenching notched disk specimens in low temperature liquid was developed and the theoretical analysis was made based on the theory of elasticity and linear fracture mechanics. The thermal shock test was carried out on a very brittle epoxy resin by changing notch length and cooling time. The results obtained were summarized as follows: (1) By using this thermal shock test, the critical temperature difference which corresponded to the minimum energy necessary to propagate crack, was clearly obtained. (2) The critical temperature difference was represented by the dimensionless stress intensity factors K*I. The distribution of K*I increased with an increase of notch length, and showed a maximum peak. K*I obtained by the experiment agreed well with the theoretical one. (3) For the thermal shock resistance of epoxy resin, the usefulness of the minimum critical temperature difference calculated from the thermal shock fracture toughness was clearly shown. (4) From both of the theoretical and experimental results, the suitable cooling time for this test was found to be the time when the dimensionless stress intensity factor becomes maximum, and it was 280sec for the resin used.
An ultrasonic experiment has been done to find the effect of ultrasonic waves on the accumulative creep damage which occurs in polycrystalline pure copper during high-temperature tensile loading. The longitudinal wave velocity was sensitive to the intergranular creeping process controlled by grain boundary cavitation and subsequent microcracking. The velocity decreased slowly and linearly up to about 60% of the time to rupture, when the steady creep shifted to the accelerated creep. It then decreased with increasing rate until the eventual failure. Measurements of porosity and photomicrographic observations revealed that the first period is associated with the nucleation and growth of cavities and the second corresponds to cracking perpendicular to the stress. A nondestructive technique for predicting the remaining life time of high-temperature components was also suggested.
Wear experiments of a carbon steel in wet air and in dry air were conducted by using a pin-on-disk wear test rig to obtain the relationship between the wear rate and contact load. The severe and mild worn-surfaces of pin specimens were observed with a SEM before and after etching. A digital measurement system including a surface profilometer and a personal computer made it possible to analyze the three-dimensional topographies of pin specimens in severe and mild wear regimes, especially to investigate the shape of each asperity by dividing the worn surface into small areas. These experimental results were useful in evaluating the condition for T1-transition from severe to mild wear in wet air. In the T1-transition and mild wear regimes, the readhesion of wear debris to the lower part of the worn surface resulted in the generation of large flat-surface. One of the most important conditions for the T1-transition was that the mean contact pressure at the flat-surfaces fell rapidly to the contact pressure in the mild wear regime owing to the wear of asperities in the severe wear regime.