S.I. Kwun

The effects of process control agents on mechanical alloying mechanisms in the TiAl system

Abstract During the mechanical alloying (MA) of ductile materials, it is often found that welding becomes so dominant that a very fine layered structure or homogeneously alloyed powders cannot be obtained. It is often desired to add an appropriate amount of process control agent (PCA) to powders in order to retain the equilibrium state between welding and fracturing processes. The present work investigated the effects of PCAs on MA mechanisms in the TiAl system. It was found that the amount of PCA and the energy transfer from ball to powder during MA influenced the mechanism of MA. As the amount of PCA and/or energy transferred from ball to powder increased, the mechanism of MA changed from a substitutional diffusion to a penetration of metallic atoms into interstitial sites. The penetration of metallic atoms seems to play an important role in the formation of the metastable f.c.c. phase, for which the lattice parameter is about 4.2 A in the TiAl system.

Magnetic nondestructive evaluation of thermally degraded 2.25Cr–1Mo steel

Abstract Aging was performed to understand the microstructural degradation in 2.25Cr–1Mo steel. Microstructural parameters (mean equivalent carbide size, number of carbides per unit area), mechanical properties (UTS, Vickers hardness) and magnetic properties (coercivity, remanence) were measured to investigate the relationship among these parameters. The magnetic coercivity and remanence were observed to decrease rapidly in the initial 1000 h of aging time and then decrease slowly thereafter. Linear correlations between mechanical and magnetic properties were found.

Microstructural Characterization of Creep Damaged 11Cr-3.5W-3Co Steel

The effects of the precipitate and martensite lath on the softening behavior have been investigated for 11Cr-3.5W-3Co steel during creep at 700. During creep, the precipitate on the PAG (prior austenite grain) boundaries and martensite lath boundaries coarsened. The recovery of dislocation density and an increase of martensite lath width took place. It is shown that the inverse of the size of the precipitates and the inverse of the square root of the martensite lath width have a linear relation with the Vickers hardness, which corresponds to the Hall-Petch relation and particle looping mechanism.

Ultrasonic Evaluation of Cyclically Deformed Microstructures of Cu and Cu-35Zn Alloy

Ultrasonic nondestructive evaluation (NDE) technique has been applied to investigate the cyclically deformed microstructures of a Cu and Cu-35Zn alloy. These materials, which had different stacking fault energies, were cyclically deformed in order to examine the ultrasonic reaction with different dislocation substructures. The observation of a dislocation structure using TEM and the measurement of the ultrasonic NDE parameters were performed after various fatigue deformation in order to clarify the relationship between them. The ultrasonic velocity was observed to decrease with increasing fatigue life fraction in both materials, which was attributed to the increasing dislocation density, resulted from the cyclic deformation. The increasing rate of ultrasonic attenuation in Cu with a cell structure that evolved during cyclic deformation was higher than that in the Cu-35Zn alloy, which had a planar array. This suggests that the dislocation cell structure is more sensitive to the change in the ultrasonic parameters than the planar array structure formed during cyclic deformation.

Evaluation of Thermal Degradation of 2.25Cr-1Mo Steel by High Frequency Ultrasonic Attenuation Measurement

It was attempted to assess nondestructively the degree of isothermal degradation of 2.25Cr-1Mo steel by using high frequency longitudinal ultrasonic wave. Microstructural parameter (mean size of carbides), mechanical property (Vickers hardness) and ultrasonic attenuation coefficient were measured for the 2.25Cr-1Mo steel isothermally degraded at 630°C for up to 4800 hours in order to find the correlation among these parameters. The ultrasonic attenuation coefficients at high frequencies (over 35MHz) were observed to increase rapidly in the initial 1000 hours of degradation time and then slowly thereafter, while the ones at low frequencies showed no noticeable increase. Ultrasonic attenuation at high frequencies increased as a function of mean size of carbides. Ultrasonic attenuation coefficient was found to have a linear correlation with the hardness, and suggested accordingly as a potential nondestructive evaluation parameter for assessing the mechanical strength reduction of the isothermally degraded 2.25Cr-1Mo steel.

Fatigue Crack Growth Characteristics in Dissimilar Weld Metal Joint

The fatigue crack growth (FCG) in dissimilar weld metal joints between SA 508 Cl.3 low-alloy steel and AISI 316L stainless steel (SS) was investigated. The dissimilar weld metal joint was made after buttering alloy 82 on the SA 508 Cl.3 side by gas tungsten arc welding (GTAW). Alloy 82 welding consumable was selected to join these two metals. The fatigue crack growth rate (FCGR) in each material in the dissimilar weld metal joint increased in the order: weldment, AISI 316L SS and SA 508 Cl.3, at the same stress intensity factor range, /K. As the crack propagated across the AISI 316L SS and heat affected zone (HAZ) into the weldment or across the SA 508 Cl.3 and HAZ, into the weldment, the FCGR in the HAZ region did not change or decrease, in spite of the increase in /K. The retardation in the FCGR in the HAZ region was discussed in terms of the welding residual stress.

Strain-Induced Martensitic Phase Transformation by Low-Cycle Fatigue in AISI 316L Stainless Steel

The evolution of the strain-induced α′ martensite during the cyclic deformation of austenitic AISI 316L stainless steel (SS) was investigated. The low-cycle fatigue (LCF) test was conducted at various strain amplitudes in air. The amount of α′ martensite was determined for the fatigue-failed specimens as well as for the specimens interrupted after a specific number of cycles. The volume fraction of α′ martensite increased with increasing strain amplitude and number of fatigue cycles. The cyclic hardening behavior was discussed in terms of the dislocation density and strain-induced α′ martensite transformation.

Precipitation Behavior of γ" in Severely Plastic Deformed Ni-Base Alloys

The precipitation behaviors of γ″(Ni3Nb) in four Ni-base alloys were investigated. The four alloys were forged Ni20Cr20Fe5Nb alloy, mechanically alloyed Ni20Cr20Fe5Nb alloy, IN 718 alloy and ECAPed(equal channel angular pressing) IN 718 alloy. Aging treatment was employed at either 600°C or 720°C for 20 hrs. The TEM observation and hardness test were performed to identify the formation of γ″. The precipitation of γ″ was noticed after aging at 600°C for 20 hrs in the mechanically alloyed Ni20Cr20Fe5Nb alloy and ECAPed IN 718 alloy, while it was observed after aging at 720°C for 20 hrs in the forged Ni20Cr20Fe5Nb alloy and IN 718 alloy before ECAP. It seemed that the lower aging temperature for γ″ precipitation in the mechanically alloyed Ni20Cr20Fe5Nb alloy and ECAPed IN 718 alloy than in the forged Ni20Cr20Fe5Nb alloy and IN 718 alloy before ECAP appeared to be due to the severe plastic deformation which occurred during mechanical alloying or ECAP.

Ultrasonic Evaluation of Creep Damage of High Temperature Metallic Materials with Different Microstructures

The creep damage levels of two metallic materials (PM1000 and IN738LC Ni based superalloy), which had different microstructures and creep damage mechanism, were evaluated by ultrasonic velocity and attenuation measurement. In case of PM1000 alloy, the ultrasonic velocity decreased with increasing creep time, which was mainly attributed to the decrease of Youngs modulus due to the increasing volume fraction of creep cavity. Ultrasonic attenuation coefficient increased with increasing creep time in IN738LC alloy, while ultrasonic velocity showed no distinct trend. This increase of ultrasonic attenuation was attributed to the directional coarsening (rafting) of γ′ precipitates. A linear correlation was found between attenuation coefficient and mean length of γ′ precipitates. Introduction It is increasingly important to evaluate the level of creep damage for securing the reliability of the equipments. It is desirable to establish the process evaluating the creep damage nondestructively, as it is difficult or sometimes impossible to remove part of the component for analysis [1-5]. In order to evaluate the damage to structure nondestructively using in-situ monitoring, ultrasonic [1,2], magnetic [3,4] and electrical resistivity [5] methods have been employed. In this investigation, the creep damage levels of two selected commercial Ni based superalloys (PM1000 and IN738LC alloy), which had differently evolved microstructures during creep (cavity formation/rafting of γ′ precipitates), were evaluated by ultrasonic velocity and attenuation measurement. Experimental Procedure Material. The PM1000 Ni based dispersion strengthened superalloy (Ni: balance, Cr: 20, Al: 0.3, Ti: 0.5 and Y2O3: 0.6 in weight %) was selected under the consideration that the major creep damage is grain boundary cavity formation not involving with complicated carbide or second phase formation [6]. The IN738LC precipitation strengthened Ni based superalloy (Ni: balance, , Cr: 16, Co: 8.6, Mo: 1.75, Ta: 1.8, Al: 3.1, Ti: 3.1, W: 2.7, C: 0.1, Si: 0.09, Mn: 0.03, Nb: 0.82 and Fe: 0.3 in weight %) shows superior high temperature strength due to large amount (volume fraction: approximately 40 %) of cuboidal γ′ precipitates (Ni3(Al,Ti)) [7]. This alloy was selected under the consideration that the most dominant microstructural change during creep deformation is rafting of γ′ precipitates rather than creep cavity formation [8]. Preparation of artificially creep damaged specimens and microstructural analysis. In case of PM1000 alloy, the creep temperature and stress ranges were determined from the previous results [6,8] so that the intergranular fracture dominated the creep fracture. The temperature was set at 1000°C, stress range was 110-123 MPa and the rupture time ranged 2-2000 hours. In case of IN738LC alloy, the creep temperatures and stress ranges were determined from the Larson-Miller parameter so that the rupture time ranged 10-1000 hours. Creep test was interrupted after some fraction of life time for microstructural examination and ultrasonic measurement. The amount of creep cavity in PM1000 alloy was estimated according to the Eq. (1) after measuring the density of Key Engineering Materials Online: 2004-08-15 ISSN: 1662-9795, Vols. 270-273, pp 120-125 doi:10.4028/www.scientific.net/KEM.270-273.120

The Low Cycle Fatigue Behavior of Equal-Channel Angular Pressed Al 5052 Alloy

The low cycle fatigue(LCF) test was performed to characterize the influences of the equal channel angular pressing(ECAP) and subsequent annealing of Al 5052 alloy. In the present research, one group of Al 5052 alloy specimens was directly subjected to ECAP, while another was subjected to ECAP and subsequent annealing. It was found that the tensile strength of the Al 5052 alloy increased, while its elongation decreased, with increasing number of ECAP passes. The LCF test was conducted at constant total strain amplitudes of 0.5%, 0.7%, 0.9% and 1.1%. Only cyclic hardening was observed as the number of fatigue cycles increased at all strain amplitudes in the specimen without ECAP. However, the ECAPed specimens showed a slight amount of cyclic hardening in the beginning and then saturation until fracture.