Advanced Materials Research Vols. 15-17

Abstract: The affect of boron (B) on the microstructure and creep behavior of a Ti-15Al-33Nb (at%) alloy was investigated. In addition to the normal constituent phases present in the monolithic alloy, the B-modified alloy contained borides enriched in titanium and niobium. These borides were present in the form of needles/laths up to 50 μm long and 10 μm wide which took up 5-9% of the volume. Constant load, tensile-creep experiments were performed in the stress range of 150-340 MPa and the temperature range of 650-710°C, in both air and vacuum environments. An addition of 0.5 at% B did not improve the creep resistance of the monolithic alloy, while the addition of 5 at% B significantly improved the creep resistance.

Abstract: For optimum fabrication and usage of Cu films, an understanding of the relationship between processing and microstructure is required. The existence of twins is another significant factor for texture development in Cu films. Texture character and strength in the Cu film is dependent on the twin boundary development that is a function of processing conditions and film thickness. In this study, determination of grain growth and texture in the sputtered and electroplated Cu films during annealing was performed for films of 100, 480 and 850 nm in thickness deposited on a Ta(25 nm)/Si wafer. The texture was measured by X-ray pole figure. The effect of film thickness on the annealing texture in the sputtered and electroplated Cu films is examined and discussed.

Abstract: Rail steel at crossing areas bears much higher loads over any other section of a regular railway. Mn-containing casting steel is normally used for its high load-carrying capability and reduced wear rate. However, since Mn-containing casting steel tends to have casting defects, the cost of manufacturing defect-free Mn-containing casting steel becomes quite expensive. Therefore, through the use of welding, this study investigates the possibility of resurfacing Mn-containing rail steel using a CH-90 electrode as an alternative to completely replacing it. In this study, a series of experimental build up weldings was made and their microstructures, chemical compositions, work-hardening index and friction coefficients were investigated. The results showed that both microstructures and chemical compositions from the build up weld section were similar to that of Mn-containing casting steel, showing an austenitic microstructure with approximately 13% Mn. The friction coefficients measured closely to one another as well (mu of Mn-containing steel = 0.847 and mu of the resurfaced weld metal = 0.831). The work-hardening index of the build up weld metal was 30% higher than that of Mn-containing casting steel. This difference could be attributed to the residual stresses in the build up weld metal which indicates that the hardening speed of the build up weld metal is faster than that of Mn-containing casting steel by impact. Therefore, build up welding, using a Mn-alloyed steel electrode on rail steel, could be a safe and economic alternative to the high cost of Mn-containing casting rail steel replacement.

Abstract: Melts dispersion technique is a new process to produce solder powders in oil surroundings by the formation of molten metal’s droplets. The entire viscosity of the fluid after uniform mixture of melts and oil mainly affected the shape and size of the solder powder. The size of solder powder was increased and the irregular shape was altered into the spherical shape with increasing the fluid viscosity. The effect of alloying elements, such as Mg, Al and Ti, on Sn-Zn solder for the oxidation resistance was investigated using the solder paste mixed solder powder fabricated by melts dispersion technique at optimum conditions and RMA type flux. The oxygen concentration was reduced by the addition of alloying elements, especially, by Ti adding.

Abstract: The interaction between Cu/Sn-Ag and Sn-Ag/Ni interfacial reactions has been studied during isothermal aging at 150°C for up to 1000h using a Cu/Sn-3.5Ag/ENIG sandwich solder joint. A typical scallop-type Cu-Sn intermetallic compound (IMC) layer formed at the upper Sn-Ag/Cu interface after reflowing. On the other hand, a (Cu,Ni)6Sn5 IMC layer was observed at the Sn-Ag/ENIG interface. The Cu in the (Cu,Ni)6Sn5 IMC layer formed on the Ni side has to be contributed from the dissolution of the opposite Cu metal pad or Cu-Sn IMC layer. When the dissolved Cu arrived at the interface of the Ni pad, the (Cu,Ni)6Sn5 IMC layer formed on the Ni interface, preventing the Ni pad from reacting with the solder. Although a long isothermal aging treatment at 150°C was performed, any Ni was not detected in the Cu-Sn IMC layer formed on the Cu side. Compared to the single Sn-Ag/ENIG solder joint, the formation of the (Cu,Ni)6Sn5 IMC layer of the Cu/Sn-Ag/ENIG sandwich joint retarded effectively the consumption of the Ni from the electroless Ni-P layer.

Abstract: The welding continuous cooling transformation (WCCT) behavior of eutectoid carbon steel was investigated in different peak temperatures and in the undeformed and deformed conditions. The corresponding WCCT and welding continuous cooling compression transformation (WCCCT) diagrams were constructed by means of dilatometric and metallographic analyses in addition to hardness measurements. It was found that the higher austenitizing temperature slightly accelerates pearlitic transformation, i.e., it shifts the WCCT diagram to shorter times. Furthermore, heavy hot deformation of austenite could strongly promote the formation of pearlite, that is, the WCCCT diagram moved toward the top left corner compared to the WCCT diagram, while martensite start temperature was lowered slightly, which is a characteristic of a displacive transformation mechanism.

Abstract: A basic requirement for the production of large power plant components from ferritic/martensitic 9-12% Cr steels is good weldability. Weldments in these steels are often reported as the weak spots. In this work the weldability of a creep resistant 9% Cr steel is discussed. Different methods are utilized to characterize the microstructural evolution during different welding cycles and the following post weld heat treatment, as well as the resulting mechanical properties. Heat affected zone (HAZ) simulation using a thermo-mechanical testing device GLEEBLE 1500 is performed to study the microstructural evolution and changes in the mechanical properties in the different parts of the HAZ. Specimens exposed to peak temperatures higher than 1150°C showed a minimum of impact toughness after post weld heat treatment (PWHT). In situ X-ray diffraction experiments with synchrotron radiation are used to observe phase transformations during heating to elevated temperatures, where delta ferrite formation was observed at temperatures higher than 1250°C.

Abstract: A commercial RF-sputtering deposition rig was employed to deposit H-free diamond-like carbon (DLC) coatings. The influence of alloying elements such as Ti and Si on the structure, mechanical and tribological properties of the coatings was investigated. The coating was observed in cross section and in plan view with SEM, TEM and AFM. Because of the highly-ionized plasma generated by the RF-powered glow discharge, ion bombardment suppresses the formation of a columnar structure regardless of the composition of the coatings. The method produces featureless microstructures and smooth surfaces. TEM investigations confirm that no crystalline phases form in the coatings regardless of the presence of considerable concentrations of Ti and Si. Tribological tests were performed with a high-temperature tribometer in a ball-on-disk configuration, using coated disks and different materials for the ball countepart. At ambient T the sliding friction coefficient decreases as the concentration of alloying elements increases. Nevertheless, high-T tribotests with a constant thermal load showed that the presence of alloying elements decreases the thermal stability of the coatings. For each coating a temperature exists above which a sudden increase of friction coefficient is observed, with subsequent detachment and failure of the coatings. The mechanism of disruption of the self-lubrication effects is identified and the influence of the alloying elements on the thermal degradation of tribological performance of the coatings is discussed.

Abstract: The Zincalume hot-dip coating process is a well-established technique for excellent corrosion protection of steel products. This paper describes the study of two intermetallics based on the Fe- Al-Si-Zn alloy system, α-AlFeSi and α-AlFeSi(+Zn), pertinent to the Zincalume process. These intermetallics are difficult to characterize in-situ due to their formation at high temperature and because they form as very thin intermetallic layers on a steel substrate, which inhibits accurate quantitative analysis. Controlled magneto-ball milling and hot-pressing have been employed in an attempt to synthesise these intermetallic compounds. Magneto-ball milling, under a He atmosphere, was conducted in shearing mode to enable controlled milling of elemental powders, namely Zn and Al, without the excessive cold-welding often associated with milling of ductile powders with high coefficients of surface friction. XRD analysis indicated that uniform mixtures of highly reactive fine-structured powders were produced, as indicated by diffuse elemental peaks of low intensity. Hot-pressing was utilized to sinter the powders into compact intermetallic compounds without sintering aids. The intermetallics were characterized by DTA, XRD and elemental contrast mapping performed on an SEM with EDS.