Kunihiro Yamada

Hot-pressed Ti-Al targets for synthesizing Ti1-xAlxN films by the arc ion plating method

Abstract Ti and Al powders were mixed with identical atomic ratios and hot-pressed at 650, 800, 950, 1100 and 1250°C, respectively, for the use of targets in the arc ion plating (AIP) method. As the sintering temperatures increased, the target density monotonously increased and the porosity correspondingly decreased. Chemical analysis by X-ray diffraction indicated that pure Ti and Al phases decreased and intermetallics such as TiAl3 and Ti3Al phases were formed at 800–950°C. Over 950°C, the amount of the intermetallics decreased and finally TiAl phase was formed. Next, the Ti-Al targets were arc-discharged in nitrogen plasma and Ti1−xAlxN films were deposited on mirror-polished cemented carbide substrates. The targets with lower density, sintered at 650 and 800°C, did not easily trigger and keep stable arc-discharge during the deposition. As a result, synthesized Ti1−xAlxN films had a larger number and size of droplets which were not only circular but were also irregular shapes. Although microstructures of Ti–Al targets were quite different, all films mostly consisted of Ti0.5Al0.5N with the NaCl structure and similar hardness with approximately 3000 HV. Further, all Ti0.5Al0.5N films had the same columnar cross-sectional structures and the growth rate was approximately 17 μm/h.

An effect of the second phase morphology on the tensile fracture characteristics of carbon steels

Abstract The fracture behavior was studied in the two phase alloy with martensitk and ferritic structure. The variation of the phase connectivity of martensitic structure makes a considerable difference in ductility of the bulk materials at room temperature. The incipient crack in the connected martensitk structure triggers cleavage cracks in the ferrite grains, while the extension of the crack in the unconnected martensitic structure is impeded by the extensive plastic flows in the ferrite. Such a marked difference in the fracture behavior can be explained from the interpretation of slip characters within the ferrite grains.

Hydrogen induced cracking in high strength steel

Initiation of quasicleavage (QC) cracks in delayed fracture of hydrogen charged, quenched and tempered AISI 4340 steel was studied by means of a fractographic technique. The fatal crack leading to the delayed fracture of an unnotched specimen is found not to be caused by intergranular cracking as is commonly found in notched specimens, but by QC cracking. An SEM fractographic examination has revealed that QC cracks, which initiate around non-metallic inclusions and under the effect of hydrogen alone, grow mainly along the rolling direction of the specimen. Furthermore, the tensile component of stress causes a QC crack to grow in the plane of maximum tensile stress. The condition under which brittle fracture finally occurs in delayed fracture can be evaluated by the classical Griffith criterion from two parameters: the QC crack length and the nominal applied stress.MST/3264

ROLE OF OXIDATION IN MICRO-FRACTURE PROCESS OF AISI 316 STAINLESS STEEL IN HIGH TEMPERATURE LOW CYCLE FATIGUE

ABSTRACT An effect of oxidation on the initiation and early stage of propagation of fatigue crack was studied on AISI 316 stainless steel at the temperature of 700°C with 3 cpm in frequency. The parameter ♀ “the crack density” (Kunio, Iwamoto and Kanazawa, 1969) was introduced to evaluate the characteristic feature of fracture process involving multiplication of microcracks at the early stage of fatigue in air and vacuum. The transition in crack growth mode from intergranular to transgranular was well characterized by this parameter. The role of oxidation in the transition of fracture mode towards depth direction was discussed based on the results of metallographic and fractographic analyses of the micro-fracture process depending on the environments imposed.

Critical Behavior of Crack Growth in High Strength Steel Subjected to Hydrogen Environment.

A critical behavior of crack growth in delayed fracture in high strength steel was studied with fine grained (18 μm) and coarse grained (105 μm) specimens which had been heat-treated to obtain the same level of yield stress. The specimens were cathodically charged with hydrogen under sustained load conditions. SEM fractographic examination revealed that the essential process of delayed fracture, in which a formation of quasi-cleavage (QC) crack triggers the intergranular (IG) crack after subcritical propagation into a critical size, is almost the same irrespective of the grain size. Experimental analysis showed that the KQ value of QC crack tip corresponds to the value of KIH and the critical behavior of delayed fracture would be affected not only by the critical condition for IG crack propagation but also by the resistance of transgranular crack propagation.

Delayed Fracture of High Strength Steel having Thin Thickness.

A study of hydrogen embrittlement on high strength steels was carried out with thin walled hollow and thin plate specimens, of which stress state would be plane stress condition, to compare them with solid round bar specimens which had been subjected to plane strain condition. The specimens were cathodically charged with hydrogen under sustained load conditions. The delayed fracture life of thin walled hollow and thin plate specimens lasts longer compared with the solid bar specimens. SEM fractographic examination showed that the essential process of delayed fracture, in which a formation of quasi-cleavage (QC) crack triggers the intergranular (IG) crack after subcritical propagation into a critical size, is unchangeable irrespective of the stress condition. Consequently, total life of the hydrogen charged steel is mainly determined by the period of crack initiation and subcritical growth of the QC crack.

Influence of Oxidation on Low-Cycle Fatigue Crack Initiation and Propagation at High Temperature.

The initiation and propagation behavior of small cracks of SUS 316 stainless steels under low-cycle fatigue loading at 700°C have been studied with a special emphasis on the role of oxidation both in the surface and subsurface. Intergranular cracking in air is due to preferential oxidation attack on the grain boundaries, while in vacuum the most frequent mode of cracking is transgranular. Also, coalescence of cracks in the surface plays an important role in the early stage of crack propagation both in air and vacuum, i. e., irrespective of the oxidation attack. Consequetly, the difference between fatigue life in air and that in vacuum could not be associated with the preferential intergranular cracking in the surface but with the relatively high rate of crack propagation in the depth direction in air compared with that in vacuum.

Small crack growth behavior in long life regime of aluminum alloy 2017.

It is generally known that the fatigue phenomenon of non-ferrous metals such as aluminum alloys are somewhat different from those of carbon steels. For example, it has been said that the knee point of S-N curve and non-propagating cracks do not appear in these metals. However, the exact meaning of these phenomenon still seems ambiguous growth behavior at the apparent fatigue limit (defined for N=2×107) and fatigue strength was investigated using aluminum alloy 2017 (aged and annealed). In annealed and aged aluminum alloys, non-propagating cracks can exist after repetition of (25)×107 stress cycles. This suggests the possibility of existence of fatigue limit even in aluminum alloys.

Critical behavior of nonpropagating crack in steel

A study has been made on the critical condition for the nonpropagation of a crack in Stage II growth at the endurance limit of pearlitic steel with special emphasis on the closure behavior of the crack. It is found from fatigue experiments and measurements of crack opening displacement that a tip of the nonpropagating crack (NPC) is closed at the stress level of the endurance limit. It is also found that the tip of the NPC opens again under the original endurance limit when the NPC is vacuum-annealed. Therefore, it turns out that the crack which has propagated in Stage II growth stops its propagation by the closure effect resulting mainly from the local residual compressive stress at the crack tip. Further experimental evidence implies that not only the crack closure due to residual compressive stress (plasticity induced), but also the closure associated with the oxidation during the reduced crack propagation, is responsible for the existence of NPC at the endurance limit.

Relationship between the critical non-propagating crack length and the allowable defect size in steel.

耐久限度における停留き裂の限界寸法と初期久陥を模した微小穴の限界寸法を比較検討した結果,限界小穴寸法は同平滑材の耐久限度レベルの高低を支配するいわゆる限界停留き裂寸法とは一致しないことがわかった.しかしながら,限度小穴寸法は,応力除去燃鈍によって残留応力を除去した疲労予き裂のうち,耐久限度レベルを低下させない予き裂の最大寸法とほぼ一致することが認められた.