Hyokyung Sung

Effect of superplastic forming exposure on tensile and S-N fatigue behavior of Ti64 alloy

The effect of superplastic forming (SPF) on tensile and S (stress)-N (number of cycles to failure) fatigue properties of Ti64 alloy was examined at 298 and 473 K. For simulating the superplastic forming exposure, millannealed Ti64 alloy sheet was heated in a vacuum chamber with a pre-determined temperature profile. For some as-exposed specimens, the α-case formed on the surface during expousre was mechanically removed to understand the effect of α-case on the mechanical properties of Ti64 alloy. It was found that the presence of α-case significantly affected the tensile and the fatigue properties of Ti64 alloy at 298 and 473 K by providing an easy initiation site for both tensile and fatigue fracture. The microstructural change during the SPF exposure was marginal in affecting the S-N fatigue properties of Ti64 alloy. Different testing temperature of 298 and 473 K affected the S-N fatigue behavior of as-received and as-exposed (α-case removed) Ti64 specimens, but not that of as-exposed specimen.

S-N fatigue behavior of Fe25Mn steel and its weld at 298 and 110 K

The S-N fatigue behavior of newly developed Fe25Mn steel, including base metal and butt-welded joint, was investigated at 298 and 110 K, and the results were compared to those of previously reported Fe16Mn2Al and STS304L steels. Fe25Mn steel has quite promising fatigue performance at 298 K and even at 110 K, showing comparable resistance to fatigue to STS304L. The S-N fatigue behavior of Fe25Mn steel was dependent on tensile strength at 298 and 110 K, the trend of which well agreed to that of other austenitic steels. The electron backscatter diffraction and micrographic analyses suggested that transformation induced plasticity and twinning induced plasticity effects did not occur in Fe25Mn steel under fatigue loading at room and cryogenic temperatures. The butt-welded Fe25Mn/Fe25Mn and Fe25Mn/STS304L specimens also showed a satisfactory fatigue behavior which was even comparable to that of STS304L/STS304L specimen at 110 K. The S-N fatigue behavior of Fe25Mn steel and its welds was discussed based on the fractographic and microscopic observations.

Effects of cooling rate and stabilization annealing on fatigue behavior of β-processed Ti-6Al-4V alloys

The effects of stabilization annealing and cooling rate on high cycle fatigue (HCF) and fatigue crack propagation (FCP) behaviors of β-processed Ti64 alloys were examined. After β-process heating above β transus, two different cooling rates of air cooling (β-annealing) and water quenching (β-quenching) were utilized. Selected specimens were then underwent stabilization annealing. The tensile tests, HCF and FCP tests on conducted on the β-processed Ti64 specimens with and without stabilization annealing. No notable microstructural and mechanical changes with stabilization annealing was observed for the β-annealed Ti64 alloys. However, significant effect of stabilization annealing was found on the FCP behavior of β-quenched Ti64 alloys, which appeared to be related to the built-up of residual stress after quenching. The mechanical behavior of β-processed Ti64 alloys with and with stabilization annealing was discussed based on the micrographic examination, including crack growth path and crack nucleation site, and fractographic analysis.

Fatigue crack propagation behavior of Fe25Mn and Fe16Mn2Al steels at room and cryogenic temperatures

The fatigue crack propagation (FCP) behavior of Fe25Mn and Fe16Mn2Al austenitic steels was investigated at 298 and 110 K, and the results were compared with the reported results of Fe24Mn2Cr steel. It was found that the FCP behavior of high-Mn, austenitic steels was largely influenced by the stacking fault energy (SFE) and the grain size. The resistance to FCP of high-Mn steels in this study was enhanced in the near-threshold ΔK regime with decreasing temperature from 298 to 110 K. The improvement for the Fe25Mn and the Fe16Mn2Al specimen was, however, marginal as compared to that of the Fe24Mn2Cr specimen. Other than the change in SFE, the secondary cracking at cryogenic temperature appeared to affect the FCP behavior of high-Mn steels, since the secondary cracks perpendicular to the crack propagating direction could reduce the effective stress intensity factor, decreasing the FCP rates. Sufficiently high stress concentration at grain boundary tended to occur at low temperature for relatively large grain sized Fe24Mn2Cr specimen and cause the secondary cracking, but not for the Fe25Mn and the Fe16Mn2Al specimen.

Effect of stabilization annealing on SCC susceptibility of β-annealed Ti-6Al-4V alloy in 0.6 M NaCl solution

The effect of stabilization annealing on the stress corrosion cracking (SCC) susceptibility of β-annealed Ti-6Al-4V (Ti64) alloy was examined in an aqueous 0.6 M NaCl solution under various applied potentials of +0.1, -0.05 and -0.1 V vs Ecorr, respectively, at a strain rate of 10 -6 s -1. The stabilization annealing substantially improved the resistance to SCC of β-annealed Ti64 alloy in 0.6 M NaCl solution under cathodic applied potentials, while the effect was marginal under an anodic applied potential. It was also noted that the areal fraction between ductile and brittle fracture of β-annealed Ti64 specimens, which were slow strain rate tested in 0.6 M NaCl solution, varied with stabilization annealing and applied potentials. The effect of stabilization annealing on the SCC behavior of β-annealed Ti64 alloy in SCC-causing environment was discussed based on the micrographic and fractographic observation.

Reviews on factors affecting fatigue behavior of high-Mn steels

A variety of factors affect the fatigue behavior of high-Mn steels, which include both extrinsic (i.e., loading type, R ratio, specimen type, surface condition, temperature, and environment) and intrinsic (i.e., chemical composition, grain size, microstructure, stacking fault energy) factors. Very often, the influence of extrinsic factors on the fatigue behavior is even greater than that of intrinsic factors, misleading the interpretation of fatigue data. The metallurgical factors influence the initiation and propagation behaviors of fatigue by altering the characteristics of slip that is prerequisite for fatigue damage accumulation. It is however not easy to separate the effect of each factor since they affect the fatigue behavior of high-Mn steels in complex and synergistic way. In this review, the fatigue data of high-Mn steels are summarized and the factors complicating the interpretation are discussed.

Effect of Cooling Rate on SCC Susceptibility of β-Processed Ti–6Al–4V Alloy in 0.6M NaCl Solution

The effects of cooling rate on the stress corrosion cracking (SCC) susceptibility of β-processed Ti–6Al–4V (Ti64) alloy, including BA/S specimen with furnace cooling and BQ/S specimen with water quenching, were investigated in 0.6M NaCl solution under various applied potentials using a slow strain rate test technique. It was found that the SCC susceptibility of β-processed Ti64 alloy in aqueous NaCl solution decreased with fast cooling rate, which was particularly substantial under an anodic applied potential. The micrographic and fractographic analyses suggested that the enhancement with fast cooling rate was related to the random orientation of acicular α platelets in BQ/S specimen. Based on the experimental results, the effect of cooling rate on the SCC behavior of β-processed Ti64 alloy in aqueous NaCl solution was discussed.