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Correlation between the Microstructure and Mechanical Properties of W-Bearing Ti-6Al-4V Alloys
Godwin Kwame Ahiale,계인석,권영삼,오용준 대한금속·재료학회 2021 대한금속·재료학회지 Vol.59 No.6
W-containing Ti-6Al-4V alloys (W=0, 1, and 5 wt%) were fabricated by the powder injection molding process, and the corresponding effects of tungsten content on the mechanical properties and microstructure of the alloys were investigated. The alloy powders were sintered at 1200 °C and then hot-isostatically-pressed at 900 °C. The fabricated alloys were subjected to microstructural and chemical analyses, and tensile and nano-indentation tests. The yield strength and tensile strength proportionally increased as the W content was increased from 0 wt% to 5 wt%. Ductility was not affected by the addition of up to 5 wt% W due to its complete dissolution in the matrix. Higher W addition induced finer α/β lamellar microstructures and increased the β to α phase ratio. Moreover, the added W dissolved preferentially in the β phase by solid solution hardening, increasing the hardness of the β phase, which originally was significantly softer than the α phase. For the alloys containing up to 5 wt% W, the strengthening without ductility loss was attributed to the finer α/β lamellae and the volume increase in the β phase hardened by W. These results suggest that adding W to Ti-6Al-4V alloy is a promising method for developing Ti alloys with both high strength and toughness.
Godwin Kwame Ahiale,오용준,Won-Doo Choi,Kwang-Bok Lee,Jae-Gyu Jung,Soo Woo Nam 대한금속·재료학회 2013 METALS AND MATERIALS International Vol.19 No.5
This study presents the microstructure and high cycle fatigue performance of lap shear joints of dual phase steel (DP590) welded using gas metal arc welding (GMAW) and plasma arc welding (PAW) processes. High cycle fatigue tests were conducted on single and double lap joints under a load ratio of 0.1 and a frequency of 20 Hz. In order to establish a basis for comparison, both weldments were fabricated to have the same weld depth in the plate thickness. The PAW specimens exhibited a higher fatigue life, a gentle S-N slope, and a higher fatigue limit than the GMAW specimens. The improvement in the fatigue life of the PAW specimens was primarily attributed to the geometry effect that exhibited lower and wider beads resulting in a lower stress concentration at the weld toe where cracks initiate and propagate. Furthermore,the microstructural constituents in the heat-affected zone (HAZ) of the PAW specimens contributed to the improvement. The higher volume fraction of acicular ferrite in the HAZ beneath the weld toe enhanced the PAW specimen’s resistance to fatigue crack growth. The double lap joints displayed a higher fatigue life than the single lap joints without changing the S-N slope.
Low-Cycle Fatigue Behavior of Reduced Activation Ferritic-Martensitic Steel at Elevated Temperatures
Godwin Kwame Ahiale,Won‑Doo Choi,Seungyon Cho,Yi‑Hyun Park,Young‑Bum Chun,Yong‑Jun Oh 대한금속·재료학회 2023 METALS AND MATERIALS International Vol.29 No.1
Changes in the low-cycle fatigue (LCF) behavior of reduced activation ferritic-martensitic (RAFM) steel containing trace amount of Zr—namely, Advanced Reduced Activation Alloy (ARAA) were presented in the temperature range of 573 to 873 K. The fatigue lifetime decreased significantly as the temperature increased from 573 to 823 K, but it was rather similar between 823 and 873 K, where the tensile properties deteriorated most rapidly. The decrease in lifetime at elevated temperatures is mainly due to the disappearance of the initial tempered martensitic structure during cycling resulting in rapid cyclic softening and local oxidation at surface slip bands which led to earlier crack initiation. The fatigue lifetime and cyclic softening behavior of ARAA were compared with those of the other conventional RAFM steels.
Godwin Kwame Ahiale,Doo-Hyun Kim,Won-Jon Yang,Jong-Hoon Lee,Yong-Jun Oh 대한금속·재료학회 2018 METALS AND MATERIALS International Vol.24 No.4
We investigated the change of precipitation behavior and impact resistance as a function of depth from the surface of thickblock of SAF 2507 super duplex stainless steel with the thickness (T) of 200 mm after water quenching from 1050 °C. Theamount of detrimental sigma phase increased smoothly until the depth of 0.25T, followed by a rapid increase from 0.25T tothe center. However, the impact strength decreased signifi cantly with only 1.3% of area fraction of sigma phase as the depthincreased past 0.1T. Based on fractography analysis for the samples at such small depth ranges, the distance between thesigma phase particles aff ected the relative amount of initiating brittle cracks in front of the notch and was one of the crucialfactors that dramatically reduced impact resistance with depth.
Ahiale, Godwin Kwame,Yoo, Ji Sung,Lee, Sunghak,Park, Dong Yong,Oh, Yong-Jun Elsevier 2018 Materials science & engineering. properties, micro Vol.725 No.-
<P><B>Abstract</B></P> <P>We present the evolution of crystallographic orientation and microstructure during thermomechanical fatigue (TMF) of heat-resistant cast austenitic stainless steel at peak temperatures reaching 950 °C using the electron backscatter diffraction (EBSD) method. Higher restraints or peak temperatures induced larger crystal misorientation by geometrically necessary dislocations (GNDs), forming dislocation walls or subgrains in the grains. Networked carbide clusters in the microstructure locally amplified the misorientation in the adjacent matrix and initiated fatigue cracks. The mean value of cumulative misorientations over a specific distance in the matrix was linearly proportional to the cyclic plastic strain.</P>