Soo Woo Nam

Refinement of the lamellar structure in TiAl-BASED intermetallic compound by addition of carbon

Intermetallic compounds based on TiAl are under extensive studies for structural materials in aerospace applications. Recently the effects of interstitial elements on the mechanical properties have been reported, which can be summarized as the enhancement of tensile strength and creep resistance. Appel reported that the creep resistance of TiAl alloy could be increased by carbon addition. Considering the important effect of carbon on the creep resistance of TiAl, it is important to gather more information on the basic role of carbon on the microstructure. In ingot alloys, there is an indication that the details of the fully lamellar structure were affected by carbon or nitrogen. This effect, however, has not been addressed in the intermetallic compound processed by powder metallurgical method. In this respect, the TiAl compound made by EPM (Elemental Powder Method) is of particular interest since this process is a potential alternative to ingot metallurgy for fabricating parts of complex shape. Recently the authors reported that a TiAl-Mn-Mo alloy could be successfully produced by EPM. In the present study, therefore, it was intended to study the effect of carbon on the phase transformation of this alloy of a FL (Fully Lamellar) microstructure.

Improved quench sensitivity in modified aluminum alloy 7175 for thick forging applications

A commercial high-strength aluminum alloy 7175 is modified as an attempt to retain the higher hardening capability in heavy forgings. The effects of alloying modification, primarily Zn:Mg ratio, on the age-hardening response and the quench sensitivity are investigated using differential scanning calorimetry, transmission electron microscopy, and measurements of mechanical properties and electrical conductivity. As a results, the modified alloy (lean composition but with higher Zn:Mg ratio) is less quench-sensitive compared to the conventional alloys (usual alloying but with a lower Zn:Mg ratio). In the fast quench, the higher Zn:Mg ratio tends to accelerate the decomposition rate to form more stable phases, giving a lower strength with higher electrical conductivity. In the slow quench, the higher Zn:Mg ratio promotes homogeneous precipitation, while the lower Zn:Mg ratio leads to the greater amount of heterogeneous η precipitation on the Cr-dispersoids. The enhanced natural aging plays a crucial role in the subsequent artificial aging in the higher Zn:Mg ratio alloy. Results of the mechanical properties and electrical conductivity tests also ensure the microstructural homogeneity of the modified alloy, which is uniquely applicable in thick 7175 forgings.

Assessment of damage and life prediction of austenitic stainless steel under high temperature creep–fatigue interaction condition

Abstract It is understood that grain boundary cavitation is one of the detrimental processes for the degradation of austenitic stainless steels that reduces the creep–fatigue life at high temperatures. A new damage function based on a model for the creep–fatigue life prediction in terms of nucleation and growth of grain boundary cavities is proposed for austenitic stainless steel. This damage function is a combination of the fatigue and creep terms related to the cavitational damage (cavity nucleation and growth) in the life prediction equation and is found to be generally applicable to all the materials in which failure is controlled by the grain boundary cavitational damage. The creep–fatigue data from the present and other investigations are used to check the validity of the proposed damage function, and it is shown that they satisfy the general reliability of damage function. Additionally, using this damage function, one may realize that all the Coffin–Manson plots at the various levels of tensile hold time and temperature under strain controlled creep–fatigue tests can be normalized to make the master curve. Using this master curve, one may easily calculate the expected creep–fatigue life for austenitic stainless steels under tensile hold high temperature low cycle fatigue test conditions to save much of the testing time and effort.

The occurrence of grain boundary serration and its effect on the M23C6 carbide characteristics in an AISI 316 stainless steel

Abstract M23C6 precipitation behaviors at the grain boundaries have been systematically investigated in an AISI 316 stainless steel. It is found that the grain boundary serration occurs at the early stage of aging treatment, before the M23C6 carbides precipitate. The occurrence of grain boundary serration is directly dependent on heat treatment condition, which is responsible for carbide characteristics. Planar carbides (low density) are observed at the serrated grain boundaries while triangular carbides (high density) are observed at the flat grain boundaries. Additionally, grain boundary serration leads to the development of an array of carbide particles. Some of these carbide particles are in parallel orientation with one grain and some with the other grain constituting the boundary. High-resolution transmission electron microscope (HRTEM) investigations reveal the interfacial plane of planar carbide formed at the serrated grain boundary to be (11 1 ). These carbides probably possess low interfacial energy.

The effect of δ-ferrite on fatigue cracks in 304L steels

Abstract Continuous low-cycle fatigue tests of 304L stainless steel at 300 and 600°C and Δ e t =±2.0% in an air environment were conducted, and the effects of the amount and direction of δ -ferrite stringers on fatigue crack initiation and fatigue life were investigated. Fatigue crack initiation was observed at the interface between δ -ferrite and a particulate matrix. The effect of δ -ferrite in 304L stainless steel on fatigue life in low-cycle fatigue was investigated. 304L stainless steel with δ -ferrite perpendicular to the loading axis has a much shorter fatigue life than that with δ -ferrite parallel to the stress axis. These phenomena are thought to be caused by the strain incompatibility at the interface between δ -ferrite and the matrix.

Study of growth rate and failure mode of chemically vapour deposited TiN, TiCxNy and TiC on cemented tungsten carbide

The effects of deposition temperature and mole ratio of CH4 to TiCl4 on the growth rate of titanium compound coatings were investigated. Activation energies of TiN, TiCxNy and TiC deposition reactions of 4.8×104, 1.9×105 and 2.8×105 J mol−1, respectively, were obtained experimentally. The carbon content of TiCxNy deposit was increased as the CH4 flow rate and deposition temperature increased. It was found that TiCxNy grain size was finer than TiC and TiN.The cutting temperatures of TiN-coated and TiC-coated tools were 10% (TiN) and 20% (TiC) lower than that of uncoated tools. Feed force and reaction force of coated tools were 30% and 18% less than those of uncoated tools, respectively. The dominant failure mode of coated tools was due to the microchipping of the cutting edge.

Activation processes of stress relaxation during hold time in 1Cr–Mo–V steel

Abstract A quantitative analysis of activation process during hold time under creep–fatigue interaction conditions has been interpreted for 1Cr–Mo–V steel. The apparent activation energy for stress relaxation at the saturated stage was the same for the lattice diffusion activation energy of iron i.e. 251 kJ mol −1 independent of the total strain range. Analyzing the value of the activation volume for the initial transient relaxation behavior, it is suggested that the rate controlling dislocation mechanism is either cross slip, or overcoming Peierls–Nabarro stress. The trend of increasing activation energy with the relaxed stress was found to be due to the effective stress which was decreased with time and approached to zero.

The normalized coffin- manson plot in terms of a new damage function based on grain boundary cavitation under creep- fatigue condition

A new damage function based on a model for the creep-fatigue life prediction in terms of nucleation and growth of grain boundary cavities is proposed. This damage function is a combination of the terms related to the cavitational damage in the life prediction equation and is generally applicable to the materials in which failure is controlled by the grain boundary cavitational damage. The creep-fatigue data from the present and other investigations are used to check the validity of the proposed function, and it is shown that they satisfy the reliability of damage function. Additionally, using this damage function, one may realize that all the Coffin-Manson plots at the various levels of tensile hold time and temperature under strain-controlled creep-fatigue tests can be normalized to make the master curve.

The effect of yttrium addition on the oxidation resistance of EPM TiAl-based intermetallics

Abstract The effect of yttrium on the oxidation resistance of elemental powder metallurgy (EPM) processed TiAl-based alloys was studied. Y-addition improved the oxidation resistance of the experimental alloys exposed at 800 °C for 350 h. The improved oxidation resistance was attributed to a fine-grained (Y, Al)O-type oxides and the segregation of Y in the grain boundaries and at the interfaces of Al 2 O 3 in the Y-rich layer.

Enhancement of mechanical properties of Al–Mg–Si alloys by means of manganese dispersoids

AbstractThe effects of Mn dispersoids on the enhancement of mechanical properties in Al–Mg–Si(–Mn) alloys have been studied to develop a new high Mn alloy which does not need an aging heat treatment after a shaping process (i.e. extrusion process). By adding Mn to Al–Mg–Si alloys, sphere- or rod shaped Mn dispersoids of a size ranging from 0·05 to 0·5 μm are formed by the use of proper heat treatments. The as extruded alloys containing 1·0 wt-%Mn are measured to have higher tensile properties with good ductility, as compared with those of the commercial Al alloy 6N01 (Al–0·69Mg–0·79Si–0·48Cu–0·27Zn–0·37Mn–0·3Cr– 0·11Ti, wt-%). These phenomena are obtained from the dispersion hardening effect and homogeneous deformation by Mn dispersoid particles acting as obstacles to dislocation movement. Comparing the fatigue crack growth behaviour between the high Mn alloys and the commercial 6N01 alloy in the as forged condition, high Mn alloys are shown to have higher fatigue crack growth resistance and show a more t...