Materials Science Forum Vols. 783-786

Abstract: A lot of technical processes require metallic materials which are able to withstand very high temperatures under extreme conditions. Examples are applications in glass industry, space technology and crystal growing. Application temperatures are in the range from 1100°C to 2300°C. Besides the extremely high temperature the materials are often influenced simultaneously by high mechanical loading and chemical attack. Due to their outstanding chemical stability, corrosion resistance and high mechanical strength the platinum group metals, in particular platinum, rhodium and iridium, are therefore ideal materials for high temperature use under extreme conditions. These metals are widely used in spite of their high prices. High temperature applications require high melting point metals, commonly strengthened by solid solution or oxide dispersion hardening. This paper reports e. g. on the development of oxide dispersion hardened platinum and platinum alloys manufactured by fusion technique. Furthermore the paper presents a comprehensive review of studies on platinum materials which facilitate the design of equipment used for high temperature applications under extreme conditions. Stress-rupture strength and creep behavior have been investigated in a temperature range between 1200°C and 2300°C. The results of the investigations can supply a basis to optimize materials selection for high temperature applications under extreme conditions.

Abstract: In order to understand the composition of the Nb-Mo bcc phase suitable to introduce B2-NiAl phase for the improvement of oxidation resistance as an Al reservoir layer for maintaining Al₂O₃ surface layer, a NiAl-Nb-Mo section of the quaternary Al-Nb-Ni-Mo phase diagram is studied. It is found that the ternary τ₂ phase appears in wide composition range of alloys, both the hardness and cracking tendency decreases with increasing the Mo concentration of the alloys.

Abstract: Ultra-high-temperature ceramics (UHTC) such as ZrB₂ and HfB₂ with SiC nanofiller are useful for propulsion and thermal protection systems. ZrB₂ and HfB₂ with 10-20 wt% SiC were prepared using ultra-sonication, rotary evaporation, and spark plasma heat treatment to high temperatures (~2,000°C) and pressures (50-60 MPa). We used positron annihilation lifetime spectroscopy (PALS) to study the nanoporosity, SEM for particle size distribution, and microhardness tester for Vickers hardness. The PALS studies were performed using a ²²Na positron source and the positron lifetime spectra were analyzed to three components using POSFIT program. The first and second components are related to positrons annihilating in bulk and in vacancy clusters, respectively; and the third component to positronium annihilation in nanopores within the granules. The PALS results indicate that HfB₂ has larger vacancy clusters and nanopores with lesser concentrations compared to ZrB₂ and SiC. The SEM observations showed that HfB₂ has larger particles compared to ZrB₂ and SiC showed wide range of size distribution. The Vickers-Hardness Number (VHN) is measured for spark plasma heat treated composites using a microhardness tester and the results indicate that 10wt%SiC composite has higher hardness compared to 20wt%SiC in both ZrB₂-SiC and HfB₂-SiC composites. HfB₂-SiC composites seem to be more brittle compared to ZrB₂-SiC composites. This may be due to larger size and smoother surface of HfB₂ particles (600 nm) compared to ZrB₂ particles (240 nm).

Abstract: Nickel base alloys due to their high performances have been widely used in biomass and coal fired power plants. They can undertake plastic deformation with different strain rates such as those typically seen during creep and fatigue at elevated temperatures. In this study, the mechanical behaviors of Alloy 617 with strain rates from 10^-2/s down to 10^-6/s at temperatures of 650°C and 700°C have been studied using tensile tests. Furthermore, the microstructures have been investigated using electron backscatter detection and electron channeling contrast imaging. At relatively high strain rate, the alloy shows higher fracture strains at these temperatures. The microstructure investigation shows that it is caused by twinning induced plasticity due to DSA. The fracture strain reaches the highest value at a strain rate of 10^-4/s and then it decreases dramatically. At strain rate of 10^-6/s, the fracture strain at high temperature is now smaller than that at room temperature, and the strength also decreases with further decreasing strain rate. Dynamic recrystallization can also be observed usually combined with crack initiation and propagation. This is a new type of observation and the mechanisms involved are discussed.

Abstract: By means of creep properties measurement and microstructure observation, an investigation has been made into the damage and fracture mechanism of a nickel-based single crystal superalloy during creep at moderate temperature. Results show that the deformation mechanism of the alloy in the latter stage of creep is that the primary-secondary slipping systems are alternately activated, and the micro-crack is firstly initiated on the γ′/γ phases interface in the intersection regions of two slip systems. As creep goes on, the micro-crack is propagated along the γ′/γ interface, which is perpendicular to stress axis, to form the square-like cleavage plane on the (001) plane. Thereinto, the propagation of the cracks on (001) plane is intersected with {111} cleavage plane which is secondly activated, which may terminate the propagation of the crack to form the cleavage plane with square-like feature on (001) plane along the <110> directions. Due to the multi-cracks may be propagated on different cross-section of the alloy during creep, and the tearing edge or secondary cleavage plane are formed along the direction with bigger shearing stress at the crack tip, which makes the multi-cracks connected each other until the occurrence of creep fracture, this is thought to be the main reason of the creep fracture having the uneven and multi-level cleavage characteristics.

Abstract: The changes in the γ’ solvus temperature and the volume fraction of Co-Al-W based alloys with fcc / L1₂ two-phase microstructures upon alloying with quaternary elements have been investigated. All investigated quaternary elements, except for Fe and Re, increase the γ’ solvus temperatures of Co-Al-W based alloys with varying efficiencies depending on quaternary element. On the other hand, the variation of the γ’ volume fraction with alloying depends on the alloying element. Of the investigated quaternary elements, Ta is found to be the most effective in increasing the γ’ solvus temperature of Co-Al-W based alloys. The lattice mismatch significantly increase upon alloying with Ta of 4at.%, which destroys the coherent cuboidal structure.

Abstract: This work presents an artificial intelligence based design of a series of novel advanced high performance steels for ambient and high temperature applications, following the principle of the materials genome initiative, using an integrated thermodynamics/kinetics based model in combination with a genetic algorithm optimization routine. Novel steel compositions and associated key heat treatment parameters are designed both for applications at the room temperature (ultra-high strength maraging stainless steel) and at high temperatures (ferritic, martensitic and austenitic creep resistant steels). The strength of existing high end alloys of aforementioned four types are calculated according to the corresponding design criteria. The model validation studies suggest that the newly designed alloys have great potential in outperforming existing grades.

Abstract: Nb-base refractory intermetallic materials have potential interest for high temperature applications thanks to their low density and high temperature strength. While advanced intermetallics in monolithic form have limited prospects for providing the required balance of properties for use at high temperatures, two-phase or multicomponent intermetallic systems composed of a ductile, Nb-base refractory phase in equilibrium with one or more silicide intermetallics show promise for further development as structural materials. In the present paper, Nb-base refractory alloys based on Nb-35Ti-15Al (at.%) were doped with small amount of Si (1 and 2 at% of silicon) addition to improve its high temperature strength by keeping an acceptable ductility at room temperature. The samples were prepared by arc-melting starting from pure elements (99.99%). The silicon addition effects on the microstructural features were investigated by using X Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) techniques. Its effects on the mechanical properties were assessed by compression tests at ambient and high temperatures. Compression tests show the beneficial effect of the Si addition on strength.

Abstract: Beta-type Ti-29Nb-13Ta-4.6Zr (TNTZ) was recently developed as a representative biomedical Ti alloy. As-solutionized TNTZ has a low Young’s modulus less than 60 GPa close to that of cortical bone along with very low cytotoxicity and good bone biocompatibility. Solution treatment and aging (STA) is a typical heat treatment for improving the mechanical properties of beta-type titanium alloys. However, STA also drastically increases the Young’s modulus. Therefore, this study investigated the effects of surface modification, micro-shot peening, on the mechanical properties of TNTZ subjected to severe thermomechanical treatment in order to maintain a relatively low Young’s modulus. The bone contact characteristics of TNTZ samples subjected to surface modification and cancellous bone were also compared. The Vickers hardness of cold-swaged TNTZ (TNTZ_SW) subjected to micro-shot peening was significantly increased within 20 mm from the very edge of the specimen surface. The fatigue strength of TNTZ_SW subjected to micro-shot peening increased especially in the high cycle fatigue life region. The fatigue limit was around 400 MPa. The bone formations on TNTZ_SW subjected to micro-shot peening and TNTZ_SW with the mirror surface as comparison material were almost identical to each other. However, the relative bone contact ratio of TNTZ_SW subjected to micro-shot peening was better than that of TNTZ_SW with the mirror surface.

Abstract: Porous titanium alloys are widely used as implant materials due to their mechanical behavior similar to that of bone. In addition, fatigue properties of implant materials are especially important since medical implants mostly exposed to cyclic compressive loading conditions. In this study, porous Ti-6Al-4V alloy has been produced via sintering at 12000C for 2 hours employing magnesium space holder technique. Porosity of the produced foams were measured according to Archimedes’ principle and calculated to be in the range of 51 ± 1 vol.%. Mechanical properties of the foams were characterized by monotonic compressive and compression-compression mode fatigue tests. The compressive strength and elastic modulus of the foams were determined to be 167 ± 18 MPa and 12 ± 1 GPa respectively. Fatigue tests conducted with a frequency of 5 Hz and a constant stress ratio (σ_min/σ_max) of 0.1 revealed that porous Ti-6Al-4V alloys have a fatigue limit of approximately 135 MPa. Furthermore fracture surfaces of the foams were characterized by field emission scanning electron microscopy (FESEM).