Materials Science Forum Vol. 802

Abstract: The objective of this work is to evaluate creep behavior of a maraging steel (300 grade) solution annealed before and after superficial treatment of plasma nitriding. Creep tests were conducted on a standard creep machine at stress range of 200 to 500 MPa at 550°C. Samples with a gage length of 18.5 mm and a diameter of 3.0 mm were used for all tests. Creep parameters are determined and a comparative analysis is established with the results gotten from the alloy with and without plasma nitriding. Maraging 300 steel plasma nitrided has showed a similar creep behavior compared with the same alloy without superficial treatment, with creep rate and stress exponent results very close to the material only solution annealed. This result can be associated with the strong impact of reversion of martensite to austenite and overaging at this temperature and time of exposure that minimizes the benefits of a superficial treatment.

Abstract: Ti-13Nb-13Zr alloy produced via powder metallurgy was submitted to heat treatment under various conditions and the effects on microstructure and elastic modulus were investigated. Heat treatment was performed using temperatures above and below α/β transus combined with different cooling rates – furnace cooling and water quenching. Microstructure and phases were analyzed employing scanning electron microscopy and X-ray diffraction. Elastic Modulus was determined using a dynamic mechanical analyzer (DMA). The results indicated that α phase precipitation and elastic modulus values increased after heat treatment performed using temperature below α/β transus. However, when it was performed above α/β transus and using higher cooling rate, a decrease in elastic modulus was observed despite higher α phase precipitation, indicating that the microstructural modifications observed via SEM, due to the presence of martensitic α phase, influenced on elastic modulus values.

Abstract: Titanium alloys are widely used in machine building, aircraft manufacturing, medicine, motors, chemistry, and biomedicine due to their high strength-to-weight ratio, elasticity, corrosion resistance, and biocompatibility. In particular, Ti-6Al-4V containing (α + β) structure plays a very important role in aerospace industry in the manufacturing of components such as disks and blades for aircrafts turbines and structural forgings. However, one of the major factors limiting the life of titanium alloys in service is their degradation due to gaseous environments, in particular, to environments containing oxygen at elevated temperatures during long-term use. The sensitivity of titanium alloys to high-temperature exposure is a well-known phenomenon. When titanium alloys are heated to temperatures above approximately 800oC, oxygen, hydrogen and nitrogen can penetrate into them. The penetration of these elements increases hardness and brittleness while decreasing the toughness of the alloy. Laser surface nitriding is a technique used to modify the near-surface microstructure and/or composition by melting the surface using a high-power laser beam with reactive gas as a shrouding environment, forming a nitride layer on the surface of Ti–6Al–4V to improve the alloy’s tribological and mechanical properties. The results of laser gas nitriding of Ti–6Al–4V showed a significant increase of microhardness and enhanced erosion resistance significantly compared with untreated Ti–6Al–4V, since the nitride layer acts as a diffusion barrier for inward oxygen diffusion into the alloy, reducing the contribution of oxygen dissolution in the substrate to the total mass gain. Other important technique that was developed for the beneficial modification of surface sensitive properties is Nitrogen Plasma Immersion Ion Implantation N-PIII. A sample is immersed in plasma and subjected to negative high-voltage pulses. In the electrical field, the ions are accelerated to high energies and incorporated into the sample. Enhancing of the hardness and wear process of the materials due to the N-enriched layer caused by diffusion of N in the sample at PIII process can be expected. Both techniques provide an improvement in the creep resistance. The objective of this work was evaluating the creep resistance of the Ti-6Al-4V alloy with superficial treatments of laser nitriding and Nitrogen Plasma Immersion Ion Implantation N-PIII in creep test of Ti-6Al-4V alloy. It was used Ti-6Al-4V alloy as cylindrical bars under forged and annealing of 190 oC by 6 hours condition and cooled by air. The Ti-6Al-4V alloy after the superficial treatment of laser nitriding and N-PIII was submitted to creep tests at 600 oC in the stress of 250 MPa and 319 MPa, under constant load mode. The creep parameters are determined and a comparative analysis is established with the results gotten from the alloy with both treatments. The laser nitrided has showed an improved creep behavior compared with the same alloy with N-PIII coating, with a reduction in the creep rate and increasing the creep lifetime.

Abstract: In this paper, we study the behavior of Pure Mg and AZ91 Mg alloy processed by Spark Plasma Sintering (SPS) under hot compression. On Pure Mg, SPS it was performed at 400OC. For AZ91 the powder was first homogenized for 50 minutes at 420OC and sintered at 470OC. The produced samples were able to produce a solid cylinder capable of being further processed with extrusion process and called preform. The same samples were submitted to a hot compression test at 330 and 380OC where they supported without breaking a strain of 0.5. We report a difference on the behavior of both materials after the yield point. For pure Mg, the work hardening was bigger than the so-called dynamic softening, and then the curve starts to rise after the yield point. On the AZ91 the effect is the opposite.

Abstract: Titanium and its alloys are largely used for many industrial applications due to their high mechanical and corrosion resistance and low specific mass. Ti-6Al-4V alloy is the most used in aerospace industry and it is applied for the manufacturing of aircraft blades and steam turbines. This alloy has high affinity with oxygen which constrains its application at high temperatures due to creep resistance reduction. Methods to increase creep resistance of Ti-6Al-4V alloys includes the use of metallic coatings and its combination with ceramic thermal barrier coating deposition on the material surface. The aim of this work is to compare the creep behavior of Ti-6Al-4V alloy in three different conditions: uncoated, metallic coated, metallic + ceramic coated specimens. The metallic coating layer (CoNiCrAlY) and the ceramic coating (ZrO₂ + 8wt%Y₂O₃) are both applied by plasma spray deposition technique. The specimens were submitted to constant load creep tests at 600oC and stress conditions of 125, 250 and 319MPa. Specimens of metallic + ceramic coating have presented higher creep resistance, longer lifetime and lower stationary creep rate when compared to uncoated and metallic coated specimens.

Abstract: For the sintered materials, the mechanical properties are strongly dependent on the density of the final product. A substantial reduction of the porosity can be achieved using additives in the powder mixture which promote the formation of a liquid phase during sintering. Boron is a potential liquid phase promoter in ferrous alloys, when sintering is carried out using hydrogen or argon atmospheres. These atmospheres, however, are costly, and the use of nitrogen containing low content of hydrogen could be beneficial. In this study the effects of 10 to 50% hydrogen in nitrogen atmosphere on the microstructure and mechanical properties of a Fe-0.3%C-0.1%B alloy sintered at 1120 and 1250°C were investigated. Boron addition increased the sintered densities, but lowered the transverse rupture strength and hardness in relation to the control alloy (Fe-0.3%C). No significant differences were observed among the samples sintered in different atmospheres for each alloy. Nitrogen containing up to 50% hydrogen atmosphere is not suitable to sinter Fe-C alloy containing boron since it lowers the mechanical properties with the formation of fragile boron nitride precipitates at the grain boundaries and lower perlite fraction.

Abstract: High temperature sintering, i.e. at temperatures above 1150°C is a well-known concept in industry. For example in the metal injection molding (MIM) process sintering temperatures employed are higher than 1250°C for ferrous alloys [1]. The advantages of this technology respect to conventional sintering are many: an increase in the homogeneity and in density, a better pores morphology, the elimination of some reducible oxides. All these lead to better mechanical properties and corrosion resistance which means better performance [2, 3, 4, 5].

Abstract: Spark Plasma Sintering (SPS) of LaFeSi alloy powders was conducted to prepare magnetocaloric La-Fe-Si-based uniform microstructures. Two electrically insulating discs made of alumina were interposed between the punches and powder sample inhibiting the flow of electric current across the powder. This approaching aiming at improving the sample temperature distribution by deviating the electric current throughout the graphite die, since the electric current induces overheating by in situ Joule effect on powder. The LaFeSi powder with particles under 150 µm was obtained by mechanical milling of particles from hydrogenated and decrypted casting ingot. The characterizations of sintered samples were performed by Scanning Electron Microscopy (SEM), Archimedes principle, Vicker’s hardness and microhardness. The uniformity of the microstructure was evaluated by checking the evidence of position on the Vicker’s microhardness by means of ANOVA statistics.

Abstract: Studies show that porous titanium alloys improve osseointegration at the implant-bone interface, since they induce new bone tissue formation inside the pores providing a better mechanical stability. In this work, porous Ti-35Nb samples were manufactured by powder metallurgy for orthopedic implant application. The titanium and niobium powders were mixing with a pore former additive and then uniaxially and cold-isostatically compacted. The samples sintering were performed at 1200oC and 1300oC. Samples characterization was performed by Scanning Electron Microscopy with Energy Dispersive X-Ray (SEM/EDS), X-Ray Diffractometry (XRD) and Optical Microscopy (OM). Moreover, ultrasound test and Quantitative Analysis by Optical Metallography (QAOM) were conducted to obtain the modulus of elasticity and total porosity, respectively. The results indicated that the used processing parameters made it possible to obtain homogeneous microstructures throughout the length of the sample.

Abstract: Several studies about porous biomaterials indicate that surgical implants success is directly linked to its surface morphology and structural characteristics. Porous implants improve osseointegration at the implant-bone interface, since they induce new bone tissue formation inside the pores providing a better mechanical stability. One of the most important parameters is the size of the pores. However, if the connectivity among the pores is not large enough for good blood irrigation, the osteocytes cells cannot reach the pores, no matter its size. The aim of this work is to analyze the porous structure of titanium samples fabricated by powder metallurgy, by characterizing pores and connections separately. This kind of structure characterization is important to improve the design of porous biomaterials. To accomplish it, a numerical code which converts 3D images into a pore-throat network structure was adopted. With this code, parameters such as size, frequency and quantity of pores and their connections can be determined. To acquire the 3D images of the samples, X-ray microtomography was used. Two samples were analyzed with distinct pore morphological types. The main result showed marked differences between the structures related to the connections radius, which suggests the one with better blood permeability.