Solid State Phenomena Vols. 124-126

Abstract: The effect of heat treatment in superalloy CM247LC on tensile properties at various temperatures has been investigated. In the case of equiaxed poly-crystal specimen, the aged condition (HTA condition) has higher strength than the solution treated and aged condition (HTSA condition) at low temperature. The HTSA condition exhibits higher strength than HTA condition in directionally solidified specimen in all temperature range. Shearing of γ´ particle is a principal deformation process at the low temperature, and cutting of fine secondary γ´ particle plays important role in the early stage of deformation in the HTA poly-crystal specimen. Tangles of short misfit dislocations form at the γ/γ´ interface during the high temperature deformation due to the partial loss of coherency at the interface. The rafting of γ´ increases tensile strength of the HTSA specimen at and above 871°C.

Abstract: Effect of carbon and boron on the grain boundary strengthening of a second generation single crystal superalloy RR2072 was investigated. Single crystal and bicrystal specimen with three kinds of tilt type misorientation angle were grown by Bridgman technique. Creep and stress rupture tests were conducted at 950oC and 1050oC. Rupture life of single crystal of the alloy modified with carbon and boron was comparable to that of the RR2072 which is free from boron and carbon. TCP phase precipitation such as sigma was suppressed in the modified alloy during thermal exposure and creep deformation. Rupture life of the modified alloy bicrystal was superior to that of the RR2072 bicrystal. M23C6 carbide formation and suppression of γ′ band growth is thought to be the role of minor elements for the grain boundary strengthening of a nickel base single crystal superalloy RR2072.

Abstract: Thick alumina coatings were performed on A356-20vol.% SiCp composites by micro-arc oxidation (MAO) process with different processing time. The dry sliding wear tests were performed on A356-20vol.% SiCp composites with and without surface coating. The samples were tested by pin-on-disc wear test equipment with different applied load and sliding velocity. It is revealed that MAO coating improves resistance to wear of A356-20vol.% SiCp composites in the severe wear conditions. On the basis of the observations and analysis of the worn surface, worn subsurface, wear debris and variation of friction coefficient, the role of MAO coating layer is examined.

Abstract: The effects of thermal aging on microstructural evolution and mechanical properties are important in the understanding of the in-service behavior of ferritic/martensitic steels in advanced nuclear power system. Ferritic/martensitic steels have been aged at 600oC for times up to 20,000 hrs. The change of mechanical properties has been examined for these aged materials. The strength and hardness was hardly changed after the thermal aging at 600oC for 20,000 hrs in all specimens. The impact absorbed energy decreased with the aging time. But the decrease of the impact absorbed energy was larger at the early stage of aging in tungsten added steels. This is attributed to the formation of Laves phase. Nitrogen which is known to increase the creep rupture strength had no effect on the degradation of the microstructure and mechanical properties during thermal aging.

Abstract: This paper presents a study of heat treatment on the quasi-static and dynamic compressive properties of the open cell aluminum alloy foams in as-fabricated (F), age-hardened (A) and T6-strengthened (T6) conditions. Although the strain rate and heat treatment of foams are different, all exhibit similar deformation behavior in the subsequent deformation. The yield stress of foams at different strain rates are improved by heat treatment, all exhibit some strain rate sensitivity. However, the densification strain of foams is not sensitive to heat treatment.

Abstract: In this work, the effect of mischmetal (48wt%Ce–27wt%La–14wt%Nd–6wt%Pr) additions on the microstructural characteristics of an Cu-7wt%P brazing filler metal (BCuP-2) were investigated. The volume fraction of primary Cu was considerably increased with increasing mischmetal addition in the BCuP-2. Also, the eutectic structure was changed from lamella to fibrous morphology and the pseudo-eutectic phase (Cu3P) was formed around the primary Cu with the mischmetal addition above 0.03wt%. Compound containing a small amount of mischmetal was formed in the primary Cu phase and this compound acts as the nucleation sites of primary Cu. The fraction of primary Cu was increased by about 1-2wt% due to the mischmetal rich phase. The mischmetal rich phase also formed in the pseudo-eutectic phase and this enhanced the formation of pseudo-eutectic phase. The addition of mischmetal changes the eutectic phase lamella to fibrous by suppressing both the solid-liquid interfacial energy and the surface energy of the solid. The mischmetal additions showed a very strong influence on the nucleation process of primary Cu and the subsequent formation of Cu3P phase as well as the eutectic structure change.

Abstract: This study examines for keyhole behavior by observing the laser-induced plasma and investigates the relation between keyhole behavior and formation of weld defect. Laser-induced plasma has been accompanied with the vaporizing pressure of zinc ejecting from keyhole to surface of primer coated plate. This dynamic behavior of plasma was very unstable and it was closely related to the unstable motion of keyhole during laser welding. As a result of observing the composition of porosity, much of Zn element was found from inner surface of it. But Zn was not found from the dimple structure fractured at the weld metal. therefore we can prove that the major cause of porosity is the vaporization of primer in lap position. Mechanism of porosity-formation is that the primer vaporized from the lap position accelerates dynamic behavior of the key hole and the bubble separated from the key hole is trapped in the solidification boundary and remains as porosity.

Abstract: Solid State diffusion bonding is obtained by applying heat, well below the melting temperature of the metals, a static pressure which does not cause a macroscopic plastic deformation in the material, and a time required to form a metallurgical bond with atomic diffusion process. This process is used for aluminum alloys, high strength steels and titanium alloys in the aerospace industry to produce complex and inaccessible joints without localized distortion. Ability to diffusion bond titanium alloys is strongly needed to promote the use of superplastic forming technology. In the present work, the solid state diffusion bonding was carried out using specimens in Ti-6Al-4V and Ti-15V-3Cr-3Sn-3Al. The microstructure of the bonded interface indicates the diffusion bonding process is successful for both alloys. It is also shown that the diffusion bonding of a superplastic Ti-6Al-4V alloy is possible at the optimum superplastic condition so that two processes can be performed simultaneously. The structural integrity of diffusion bonding was evaluated with a hydraulic test of diffusion bonded part.

Abstract: RF sputtering process was applied to produce thin hydroxyapatite(HAp, Ca10(PO4)6(OH)2) films on Ti-6Al-4V alloy substrates. The effects of different heat treatment conditions on the bonding strength between HAp thin films and Ti-6Al-4V alloy substrates were studied. Before deposition, the Ti-6Al-4V alloy substrates were heat treated for 1hr at 850°C under 3.0×10-3torr, and after deposition, the HAp thin films were heat treated for 1hr at 400°C, 600°C and 800°C under the atmosphere, and analyzed optical microscope, FESEM, FTIR, XRD, nano-indentor, micro-vickers hardness, respectively. Experimental results represented that the HAp thin films on the heat treated substrates had higher hardness than none-heat treated substrates before the deposition.

Abstract: Equal-channel angular pressing (ECAP) is an efficient method of improving strength of metallic alloys through (sub) grain refinement to, typically, the sub-micrometer level by introducing intensive plastic strain into materials through repetitive pressing. In the present study, the post-ECAP aging effect on strength of age hardenable aluminum alloys (6061, 2024, 7075 Al) was examined and it could be concluded that pre-ECAP solid solution treatment combined with post-ECAP low-temperature aging provided much more significant room-temperature strengthening effect than conventional ECAP processing routes without involving special heat treatments.