Materials Science Forum Vol. 941

Abstract: Silver alloy is one of semi-precious alloys for dental prosthesis, which has been applied remarkably in Japanese dental field. Ag-20Pd-17.7Cu-12Au alloy (G12), which was newly developed for commercial dental silver alloy, shows the unique hardening mechanism after a simple solution treatment (ST) at relatively high temperature. However, the relationship between ST at various temperature and the mechanical strength of G12 has not been investigated fully. Therefore, the relationship between ST and mechanical properties of G12 after ST at various temperatures was investigated systematically in this study. G12 subjected to ST at relatively high temperature of 1173 K is composed of three kinds of phases with Ag-rich α₂ phase, Cu-rich α₁ phase and β phase of Pd-Cu intermetallic compound. Although nominal melting point of G12 was around 1233 K, the high Cu concentration area like α₁ phase was partially dissolved and then the α₁, α₂ and partial β phases re-precipitated during cooling. On the other hand, the microstructure of G12 aged at 673 K after ST at 1023 K (STA) was mainly composed of two kinds of phases with α₂ and α₁ phases with a small amount of β phase, which was a typical microstructure of STA. The Vickers hardness of G12 subjected to ST at 1173 K was identical to that of the same alloy subjected to STA although the tensile strength, fatigue limit and ductility deteriorated to some degrees. The corrosion potential of G12 changed drastically with the temperature of ST. The potential may have been increased by the decrease in the volume fraction of the α₁ phase with relatively high Cu content.

Abstract: The objective of this study is the evaluation of high entropy alloys for aeroengines applications up to 1000°C. Al_xNbMoTiV alloys with 10 and 20 at.% Al have been produced by arc melting. As-cast microstructure and phase transformations during heat treatments have been investigated through SEM, DRX and TEM, revealing the possibility of homogenization at 1400°C and the stability of the BCC structure at 1000°C and 800°C for both alloys. Mechanical properties have been evaluated through micro-hardness and compression tests up to 800°C. It appears that, although both alloys show a similar microstructure and hardness evolution with heat treatment, the Alloy containing 10 at.% of Al show a higher yield strength at room temperature and 800°C, related to the brittle character of the alloy containing 20 at.% of Al.

Abstract: Most of the studies on phase transformation in metallic materials have focused on transformations during cooling processes due to the easiness of the conservation of the product phase. However, for phase transformation happening during heating processes, the experimental investigations have been indirect if the product high temperature phase could not be preserved to the convenient observation temperature, for example the room temperature. The high density Electric Current Pulse (ECP) treatment allows the phase transformation during heating process and the preservation of the high temperature phase to the room temperature, offering possibilities for direct experimental examinations. Thus, in the present work, a cold-rolled Cu–40%Zn alloy was ECP treated and the microstructure of the product phase and the transformation orientation relationship were investigated. Results show that during the ECP treatment, the high temperature beta phase with BCC structure formed in the parent alpha phase with FCC structure. Especially, two kinds of orientation relationships could be detected between the parent alpha phase and the product beta precipitates. The one is the Kurdjumov-Sachs orientation relationship (K-S OR), and the other is the Nishiyama-Wasserman (N-W). In addition, the amount of beta precipitates obeying the K-S OR is more than that of precipitates obeying the N-W OR. The results of this work provide new fundamental information on phase transformation of metallic materials.

Abstract: We measured the thermal expansion coefficients of 18R-LPSO phase and α-Mg phase in a Mg₉₇Zn₁Y₂ alloy polycrystal. This was achieved by using a Gandolfi camera, which was attached on a high precision diffractometer at SPring-8 BL40XU beamline. By using this system, fine diffraction data were obtained from a Mg₉₇Zn₁Y₂ polycrystal at 6 different temperatures between 90 and 450 K. We succeeded to determine cell parameters of 18R-LPSO phase and α-Mg phase separately in the Mg₉₇Zn₁Y₂ alloy polycrystal. The thermal expansion coefficients were determined from the refined cell parameters. The differences of the thermal expansion coefficients of 18R-LPSO phase and α-Mg phase in the Mg₉₇Zn₁Y₂ alloy were much smaller than those of single-phase 18R-LPSO and α-Mg polycrystals.

Abstract: The effect of crystal orientation on damping capacity is studied on hot rolled AZ31 magnesium alloy. AZ31 magnesium alloys which was machined out parallel to rolled direction and perpendicular to rolled direction were investigated about the relationship between the initial texture and damping capacity. The specimens are annealed at 623 K and time ranging from 30 to 180 minutes. After heat treatment recrystallization is occurred. Then, average grain size and damping capacity are increased with increasing annealing time. Two types of specimens show different main component of texture and damping capacity respectively. It is found that initial texture was affected on damping capacity.

Abstract: Metastable β-Ti alloy Ti-35.3Nb-7.3Zr-5.7Ta-0.7O (wt. %) shows properties desirable for use as an implant material. However, very large grains (with the size of 0.5 – 3 mm) negatively affect the strength and fatigue resistance. Combination of cold-swaging and recrystallization annealing is used to refine the microstructure. Microstructure after cold swaging and after annealing is studied using scanning electron microscope by electron back-scatter diffraction measurements. Grain size and texture is determined and homogeneity of deformation inside the grains is discussed. Microhardness maps are measured in the cold-swaged samples. It is found that microhardness and thus also the deformation is higher in the center of each rod however the grains retain their original size. Annealing leads to recrystallization that is complete or nearly complete at the temperatures of 1000°C and more. Annealing at 1000°C for 15 min yields grain size of around 100 μm. This grain size is believed to increase the fatigue performance to satisfactory values.

Abstract: The equimolar Cr, Mn, Fe, Co and Ni alloy, first produced in 2004, was unexpectedly found to be single-phase. Consequently, a new concept of materials was developed: high entropy alloys (HEA) forming a single solid-solution with a near equiatomic composition of the constituting elements. In this study, an equimolar CoCrFeMnNi HEA was modified by the addition of 5 at% of either Al, Cu or Zr. The cold-rolled alloys were annealed for 30 minutes at high temperature to investigate the recrystallization kinetics. The evolution of the grain boundary and the grain size were investigated, from the as-cast to the recrystallized state. Results show that the recrystallized single phase FCC structures exhibits different twin grains density, grain size and recrystallization temperatures as a function of the at.% of modifier alloying elements added. In comparison to the equimolar CoCrFeMnNi, the addition of modifier elements increases significantly the recrystallization temperature after cold deformation. The sluggish diffusion (typical of HEA alloys), the presence of a solute in solid solution as well as the low twin boundary energy are responsible for the lower driving force for recrystallization.

Abstract: The wrought Mg-containing high-silicon aluminum alloy has become more attractive as an ideal structural material, because it has moderate strength and the ductility, high wear and corrosion resistance, low thermal expansion coefficient and low cost. However, as structural material, the fatigue properties and the fatigue fracture behavior of it should be paid much more attention to, especially the effect of Si content on the fatigue properties of wrought Mg-containing Al-Si alloy. In this paper, the wrought Al-(1.44%, 4.92%, 6.61%, 8.81% and 12.4%)Si-0.7%Mg alloy were prepared through DCC and homogenization treatment, and then hot-rolled and cold-rolled into 1.3mm sheets. The microstructure and fatigue fracture morphology of Al-(1.44~12.4)Si-0.7Mg-T4 alloy sheet after fatigue test were investigated by LSCM and SEM. The results showed that the size of Si particles in Al-(1.44~12.4)Si-0.7Mg alloy sheets was approximately the same, but the number of Si particles increased as the Si content increased. The size of recrystallization grain in Al-(1.44~12.4)Si-0.7Mg alloy sheets decreased from 47μm to 10μm when Si content increased from 1.44% to 12.4%, which indicates that increasing of Si content can refine the grain of Al-(1.44~12.4)Si-0.7Mg alloy sheets. With the increasing of Si content the propagation area of fatigue fracture surface of Al-(1.44~12.4)Si-0.7Mg alloy sheets in T4 temper became rougher, and crack propagation became more difficult. Moreover, dimples in the fast fracture area became larger in amount, smaller in size and more uniform in distribution.

Abstract: The main purpose of the investigation is to develop reliable methods to repair IN 792 superalloy components by Laser Beam Welding (LBW). The jet engine structural components made of nickel-base superalloys are subjected to high stresses and severe environmental, therefore crack may occurs during in-service life. Considering the high cost of this components they are often repaired by welding. Welds must be made: (i) by altering as little as possible the original microstructure; (ii) without introducing in the molten (MZ) and heat affected (HAZ) zones relevant residual stresses; (iii) without producing cracks in MZ and HAZ; (iv) without massive chemical segregation; (v) without elemental diffusion changing the composition of g and g’ phases.LBW has been used to realize seams on 2mm-thick plates of directionally solidified (DS) IN792 superalloy. An Yb fiber laser has been employed in present experiments and the molten pool was shielded from oxidation through a patented equipment. A grid of samples has been prepared by varying the pass speed v from 1 to 2.0 m/min with pre-heating temperature of 200 °C. The microstructural changes occurring in the microstructure of molten zone (MZ) and heat affected zone (HAZ) have been investigated by optical and scanning electron microscopy (SEM) observations and energy dispersion spectroscopy (EDS).

Abstract: A heat treatable A7N01 (Al-Zn-Mg) aluminum alloy mainly used to fabricate high-speed train body structures was double-sided welded by MIG welding. The natural aging behavior of the joint was systematically studied by local and global mechanical property testing method. Based on TEM observation and EPMA analysis, the key factors that lead to different natural aging behavior among various regions of the joint were discussed. The global tensile strength of the joints increased obviously after natural aging. All the joint samples fractured in the weld zone, which demonstrated the weld zone was the weakest zone of the joints. And the strength of the global welds were depended on the microstructure and natural aging ability of the weld zone. Moreover, the increase of the tensile property of the three local regions in the weld zone after post natural aging decreased as Middle>lower>upper. The above phenomenon was also further confirmed by microhardness measurement. The age hardening behavior is greatly influenced by the concentration of strengthening precipitates forming element Zn. The Zn content in the weld zone determined by EPMA showed that element Zn was dispersed unevenly, which is attributed to the double V-groove design used in this work and Zn vaporization in the molten pool during welding. The Zn content in the middle layer metals is higher than that of the upper and lower layers, resulting in the remarkable hardening response for the middle layer of the weld zone during natural aging process. Additionally, the solution zone containing the same Zn content as the base metal and undergoing sufficient solid solution during welding was found to possess the high hardening ability after natural aging process.