Toshio Osada

Creep mechanisms of a new Ni‐Co‐base disc superalloy at an intermediate temperature

The microstructures of a new Ni‐Co‐base disc superalloy, TMW‐4M3, before and after the creep test at 725 °C/630 MPa have been systematically investigated by transmission electron microscopy (TEM). The crept microstructures were marked as three different deformation stages (I, II and III) corresponding to the gradually increased strain. At stage I, stacking fault (SF) shearing was the main deformation mechanism. The SF was extrinsic and lay on {111} plane. However, deformation microtwinning became the dominant mode at stage II and III. The average spacing of deformation twins decreased from 109 ± 15 nm at stage II to 76 ± 12 nm at stage III, whereas the twin thickness did not change significantly. The influence of stacking fault energy (SFE) of γ matrix on the deformation mechanism is discussed. It is suggested that lower SFE in TMW‐4M3 is partly responsible for the enhanced creep resistance.

A Novel Design Approach for Self-Crack-Healing Structural Ceramics with 3D Networks of Healing Activator

Self-crack-healing by oxidation of a pre-incorporated healing agent is an essential property of high-temperature structural ceramics for components with stringent safety requirements, such as turbine blades in aircraft engines. Here, we report a new approach for a self-healing design containing a 3D network of a healing activator, based on insight gained by clarifying the healing mechanism. We demonstrate that addition of a small amount of an activator, typically doped MnO localised on the fracture path, selected by appropriate thermodynamic calculation significantly accelerates healing by >6,000 times and significantly lowers the required reaction temperature. The activator on the fracture path exhibits rapid fracture-gap filling by generation of mobile supercooled melts, thus enabling efficient oxygen delivery to the healing agent. Furthermore, the activator promotes crystallisation of the melts and forms a mechanically strong healing oxide. We also clarified that the healing mechanism could be divided to the initial oxidation and additional two stages. Based on bone healing, we here named these stages as inflammation, repair, and remodelling stages, respectively. Our design strategy can be applied to develop new lightweight, self-healing ceramics suitable for use in high- or low-pressure turbine blades in aircraft engines.

Improvement of the Contact Strength of Al2O3/SiC by a Combination of Shot Peening and Crack-Healing

Al2O3/SiC composite ceramics with high crack-healing ability were subjected to shot peening (SP) using zirconium oxide shots with several peening pressures and shot diameters. Specimens subjected to SP were heat-treated in air to heal the surface cracks induced by SP. The residual stress, the apparent fracture toughness, and the Weibull distribution of the contact strength were investigated, revealing that the combination of SP and crack-healing is effective for increasing the contact strength and decreasing the scatter of the contact strength of Al2O3/SiC.

Self-crack-healing behavior in ceramic matrix composites

Abstract: Self-crack-healing is one of the most valuable phenomena to overcome the decrease in reliability of ceramics caused by cracking. This is because self-healing is triggered by crack initiation and gives complete strength recovery through the passive oxidation of SiC. Thus, self-healing ceramics are candidates for high-temperature materials for gas turbine components, i.e., turbine blades and stator vanes. Nano-composite and multi-composite concepts for self-healing ceramics are described. The design concept is based on ‘damage management’ rather than ‘damage prevention’. The kinetics of self-crack-healing of ceramic/SiC composites including the effects of temperature and oxygen partial pressure is introduced. The time for crack-healing with low oxygen partial pressure can be estimated. Furthermore, the effects of oxygen partial pressure on self-healing under tensile stress are discussed. The design stress of ceramic turbine blades with self-crack-healing is considered.

Effect of Solution Temperature on the Microstructure and Mechanical Properties of a Newly Developed Superalloy TMW-4M3

The influence of solution temperature on the microstructure and mechanical properties of TMW-4M3 superalloy has been investigated. Comparisons of mechanical properties have also been made between the heat-treated TMW-4M3 variants and the commercial U720Li. The key microstructural variables examined were grain size and the volume fraction and size of the strengthening γ′ precipitates that control the mechanical properties of these alloys. By increasing the solution temperature from 1373 K to 1393 K (1100 °C to 1120 °C), the volume fraction of primary gamma prime dropped from 16.9 pct to 14.5 pct, whereas the average grain size increased from 8.7 μm to 10.6 μm. Compared with an alloy with a smaller grain size, the alloy with a larger grain size exhibited superior resistances to creep and fatigue crack growth without the expense of reduced tensile strength and low-cycle fatigue resistance. This suggested that a higher solution temperature may benefit TMW-4M3 in terms of superior overall properties. The greater overall properties of TMW-4M3 variants than that of commercial U720Li were also demonstrated experimentally. The possible explanations for the improvement of mechanical properties were discussed.

New Technology for Increasing Through-Life Reliability of Ceramics Components Using Self-Crack-Healing Ability

Structural ceramics are superior to metallic materials in terms of their high-temperature strengths and critical heat proof temperatures. However, compared to metallic materials, ceramics exhibit lower fracture toughness, so they are more sensitive to flaws such as pores and cracks. The shortness considerably decreases the component reliability. To overcome the shortness, in this study, special attention is paid to structural ceramics with self-crack-healing ability. There are several advantages for using a material with self-crack-healing ability. (1) After an efficient machine operation, the materials are able to self-heal the cracks introduced by the machining. (2) The materials are able to self-heal the cracks introduced during service and recover the strength completely at healing temperature. However, ways of organizing the available knowledge to increase the through-life reliability of ceramics components have not been extensively studied. The authors propose a new concept and the corresponding flowchart. This new concept is a promising technique for increasing the through-life reliability of ceramics components with excellent self-crack-healing ability.

Structural ceramics with self-healing properties

Abstract: Structural ceramics are leading candidate materials for high-temperature applications. However, ceramics exhibit low fracture toughness and are very sensitive to flaws such as pores and cracks. Thus they have low reliability. To overcome the problem, a new concept of self-crack-healing was developed by the authors. A material design with self-crack-healing ability, high strength and high fracture toughness is firstly discussed. Basic self-crack-healing behavior as a function of healing temperature, oxygen partial pressure, crack depth are then introduced in detail. Finally, self-crack-healing behavior during service and strength recovery behavior is described.

Improvement in Fatigue Limit by Shot Peening for High-Strength Steel Containing Crack-Like Surface Defect: Influence of Surface Crack Aspect Ratio

The effect of shot peening (SP) on the bending fatigue limit of high-strength steel (SUP9A) containing a semi-elliptical surface slit was investigated. SP was conducted on specimens containing a semi-elliptical surface slit with an aspect ratio (a/c) of 0.4, where a was the crack depth (a = 0.1, 0.2, and 0.3 mm) and c was half the surface length of the crack. Bending fatigue tests were carried out under a stress ratio R equal to 0. The results showed that the fatigue limit of the shot-peened specimens with slits having a depth of less than 0.2 mm was almost the same as that of the shot-peened specimens without slits. Meanwhile, some of the specimens fractured at the surface in areas other than the slit. Thus, the maximum depth of the slit that could be rendered harmless by SP was 0.2 mm. The maximum depths of cracks with various aspect ratios that could be rendered harmless by SP were predicted, assuming that the cracks were arrested when the apparent stress intensity factor at the slit tip was less than the threshold stress intensity factor of the material. The estimated values were in good agreement with the experimental values. A harmless crack assessment diagram was proposed based on this estimation method.Copyright

Modeling of Overload Effect on Fatigue Crack Growth Threshold Using Finite Element Method

A model of overload effect for hardening elastic-plastic solids is proposed to evaluate the stress intensity factor for compressive residual stress Krs at fatigue crack-tip fields. The residual stress rs introduced at the tip in SUS316 by overload Kov = 6, 15, 30 and 45 MPam 1/2 can be estimated using Finite Element Method (FEM). The Krs values as a function of fatigue crack growth length a were calculated from the rs according to Dugdale model. It was found that the calculated Krs decreased significantly with increasing a and reached to maximum value of |Krs|. Therefore, the maximum stress intensity factor Kmax will decrease apparently because of the action of Krs. As a result, effective stress intensity factor range given by Keff = Kmax+Krs decreased with increasing a. Defining the fatigue crack cannot grow when Keff = Kth, the apparent fatigue crack growth threshold   Kth can be estimated. Then, we can obtain the theoretical equation as Kth=0.30Kov+4.10. The equation showed in good agreement with experimental results.

Strength recovery of machined alumina by self crack healing

In this study, the effect of crack-heal on cracks propagated during machining was studied systematically. From the obtained results, we proposed a new method for increase in reliability of machined alumina by using the crack-healing treatment.