Materials Science Forum Vol. 755

Abstract: The thermoelectric phenomenon can be used for a wide application spectrum. Typically, the Seebeck effect of metallic materials is used for temperature measurement in common thermocouples. However, there is also a high potential for adapting the effect in nondestructive testing due to a high sensitivity of the thermoelectric properties to a variety of material characteristics.Different studies point out an influence of plastic deformations on the thermoelectric behavior of metals, but a detailed and quantitative analysis and description is not provided yet. There is an increasing interest in detecting the changes of properties by nondestructive characterization of plastically deformed materials. Especially in metal forming, the knowledge about the correlation between the degree of deformation and the thermoelectric behavior can help to check formed metal parts. On that account, the influence of plastic deformations on the thermoelectric behavior, in particular the Seebeck coefficient, of four steel alloys is introduced in this paper.An apparatus based on the integral measuring method was built up to measure the relative Seebeck coefficient to a reference material at different temperature gradients and for several degrees of plastic deformation. Well defined values of plastic deformation are realized by cold rolling. With this compressive forming technology a logarithmic deformation φ up to 2.11 was set for all steel alloys. Besides a high degree of deformation, a uniform strain over the sample thickness can be obtained by rolling.With increasing plastic deformation a significant change of the relative Seebeck coefficient can be observed in a defined temperature range for all investigated steels. The plastic deformation is accompanied by an increase of dislocation density into the lattice structure of the metal. These line defects provoke the formation of new scattering centers and thus the electron motion is disturbed. By the combination of metallurgical examinations and measurements of micro hardness, the relation between the thermoelectric behavior and the plastic deformation of steels can be clearly illustrated.

Abstract: This paper presents results of the microstructure characterization and evaluation of physical and mechanical properties of Al/SiC_p composites intentionally containing the Al₄C₃ phase prepared by pressureless infiltration, as well as results of treating the composites by the hybrid system chemical vapor deposition (HYSYCVD) route using Na₂SiF₆ as solid precursor. It also contains thermodynamic feasibility calculations (using the HSC 6.1^TM program) of possible reactions between the SiF_{x (x=1-4)} gas species (produced during the thermal decomposition of Na₂SiF₆), Al₄C₃ and N₂ in the atmosphere. The composites were fabricated using an L4 Taguchi experimental design, infiltrating SiC_p porous preforms at 1100 and 1200 °C in ultra high purity (UHP) argon and nitrogen (ArN₂) atmosphere for 90 and 120 min, with the Al-6Si-16Mg (wt. %) alloy. The specimens were characterized by XRD, SEM and EDS. Results show that the composites exhibit a surface hardness between 63.95 and 86.5 (HR30T) and Young´s modulus up to 118.2 GPa. Application of a post-processing stage by HYSYCVD led to the elimination of Al₄C₃ and the formation of a coating of stable phases on the composites surface. The experimental results confirm the thermodynamic feasibility of the proposed chemical reactions.

Abstract: In this study, Ag nanoparticles were synthesized using two chemical reduction agents; ethylene glycol and sodium borohydride. Different particle size distributions were obtained and characterized by transmission electron microscopy. Ag nanoparticles concentrations of 1, 2 and 4 parts per million (gmL^-1) were prepared and studied by ultraviolet-visible spectroscopy (UV-Vis) and atomic absorption spectrophotometry (AAS). In the UV-Vis results a characteristic band at 420 nm were observed. However, when the concentration of silver decreased, a change in band intensity was detected. Atomic absorption spectrophotometry measurements from different solutions of Ag nanoparticles showed a linear behavior similar to the silver standard solution in the concentration range 1 to 4 mgL^-1. However, up 4 mgL^-1 concentrations, the slope of the calibration curve is increases when the concentration of Ag nanoparticles is increased too.

Abstract: In this study ductile cast iron plates were welded by tungsten inert gas process (TIG). The objective was to evaluate the effect of filler metals; current welding and post weld heat treatment (PWHT) on the microstructure and corrosion resistance of joints. Metallographic examination of welded plates showed three different zones: fusion zone with a dendritic structure, heat affected zone with mixture of martensite and ledeburite structure and ferritic-pearlitic structure in base metal. Microstructure of welded area changes drastically to acicular ausferrite when the PWHT was applied. Corrosion rates values of 4.5 mpy were obtained in plates welded with E-FeNiCI filler metal and current of 125 and 150 A; this value is approximately half of the ones obtained in the ductile cast iron (10.5 mpy). These results are associated with: a good chemical compatibility of the filler material and ductile cast iron, increases of Ni content, refinement of the structure and a higher dissolution of graphite nodules that mitigate graphitic corrosion mechanism. The PWHT plates showed a slightly decrease in corrosion rates. In particular, plates welded with E-FeNiCI filler material and 125 A of current intensities exhibited the best corrosion resistance (3.75 mpy). This phenomenon can be attributed to homogenization of nickel in the weld bead forming passive layers that increase the corrosion resistance.

Abstract: This work presents the degradation mechanism of the platinum modified aluminide diffusion coating of the GTD 111 SC Ni-base superalloy turbine blades after 16000 h of exposition at different thermal cycles (critical heating temperatures reported ~1000°C and 1120°C). The initial coating condition and the evolution of degradation were characterized applying conventional microscopy and backscatter scanning electron microscopy. The initial microstructure condition consisted of a two phase coating (intermetallics PtAl₂ dispersed in a matrix β-(Ni,Pt)Al). The major microstructure degradation was associated with: intermediate and interdiffusion zones growing, partial transformation of β-(Ni,Pt)Al to γ´-Ni₃Al, and the dissolution of the intermetallic PtAl₂ resulting in a more brittle single phase β-(Ni,Pt)Al coating. The degradation facilitates spallation and crack initiation, resulting in the loss of the coating and by consequence the blade failure.

Abstract: Plasma Transferred Arc (PTA) process is increasingly used in applications where enhancement of wear, corrosion and heat resistance of metals surface is required. The shape of weld bead geometry affected by the PTA welding process parameters is an indication of the quality of the weld. PTA is a versatile method of depositing high-quality metallurgically fused deposits on relatively low cost surfaces. The overlay deposited is an alloy that is hard and more corrosion resistant than counterparts laid down by Gas Tungsten Arc Welding (GTAW) or Oxy Fuel Welding (OFW) processes. Weld deposits are characterized by very low levels of inclusions, oxides, and discontinuities. This process produces smooth deposits that significantly reduce the amount of post weld machining required. Metal-Mechanic industry continuously requires recovering tool steel components subjected to severe wear. The steel known as D2 is considered to be a high carbon, high chromium cold work tool steel. In this research, weld beads were deposited on D2 steel by using PTA process with different parameters as welding current and travel speed using base nickel filler metal. In order to evaluate the metallurgical features on the weld beads/substrate interface a microstructural characterization was performed by using Scanning Electron Microscopy (SEM) and to evaluate the mechanical properties was conducted the wear test.

Abstract: In this study, FeAl₂ and Fe₂Al₅ intermetallic alloys were prepared by conventional casting technique. In order to study their structural stability the alloys were subjected to high-energy ball milling process for 1, 2.5, 5 and 10 h. The structural and chemical characterizations were conducted by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and differential scanning calorimetry. After 10 h of milling, the experimental results indicated a phase transformation from FeAl₂-triclinic phase to Fe₂Al₅-ortorrombic structure. This phase transformation is characterized by a change from low to high symmetry systems.

Abstract: In this work there was studied the structural, textural and effect that has treat thermal on the removal of Cr (VI) of nanoMg/Al hydrotalcite (NHT) synthesized by the sol-gel method with a ratio of Mg/Al=3. These present the characterization of the nanoMg/Al hydrotalcite before and after Cr (VI) removal, as well as the kinetic studies and of balance. In agreement with the results, when the NHT are thermal treated to 350 °C the hydrotalcite crystalline structure remains but the textural properties are improved and structural. The NHT synthesized by the sol-gel method present a good capacity of removal with values of 91 mg of Cr (VI) removed / g of NHT. The above mentioned capacity of removal gets improved after thermal treating the material, reaching values of removal of 124 mg of Cr (VI) removed / g of NHT. The NHT with the Cr (VI) absorbed show decrease of the crystalline structure, as well as a saturation of the pores that annuls the porosity due to the fact that the Cr (VI) lodges both at the pores and at the intersheets. The time that him takes the HLM to remove the Cr (VI) of the watery solution is alone of 2 minutes, with which it is possible to conclude that the NHT have an excellent aptitude to remove Cr (VI) of watery solutions in an interval of very short time.

Abstract: Plates, 6.3 mm thick, of 2205 duplex stainless steel (DSS) were gas metal arc welded (GMAW) under the application of an axial magnetic field (0 to 15 mT) with an ER-2209 filler wire using a gas mixture of 98%Ar + 2%O₂. Microstructural characterization of the welds revealed that electromagnetic stirring (EMS) increases the content of austenite in both weld center and high temperature heat affected zone (HTHAZ). It induced a grain refining effect during freezing of the ferritic matrix which in turn enabled more sites for nucleation of austenite. This mechanism of solidification was reflected in an increase in the mechanical strength of the welds. Besides, the extent of the HTHAZ was reduced and its microhardness increased when applying the external magnetic field. It is believed that the vibration induced in this region favored the regeneration of austenite in the ferritized HTHAZ enhancing the balance of phases.