Materials Science Forum Vol. 941

Abstract: The effect of an unconventional thermal treatment method aimed to improve toughness behavior in Al alloys is reported. The method involves solution heat treating and an intermediate warm working step, before final ageing thermal treatment on a AA7050 high resistance aluminum alloy. Results show the possibility to increase fracture toughness behavior without tensile and conductivity (IACS) properties loss by adopting a warm deformation process instead of the standard cold deformation. Moreover, the adoption of an intermediate warm deformation instead of standard cold deformation, allows to reduce material microstructural grain-size heterogeneity.

Abstract: In the present work, the High-Entropy Alloys Al_0.2Co_1.5CrFeNi_1.5Ti, Al_1.3Co_1.4Cr_2.0FeNi_4.0Ti_4.0 and Al_6.0Co_1.2Cr_2.5FeNi_3.5Ti_6.0 were produced by Mechanical Alloying and subsequent Spark Plasma Sintering processes to obtain properly densified bulks. The characterization of the materials was accomplished through X-Ray Diffraction, Scanning Electron Microscopy, microhardness and nanoindentation tests to identify and analyze the acquired microstructures’ features, phases formed, morphology and size of the grains and its average hardness. The results indicate that it was possible to obtain alloys presenting high values of hardness and multi-phase microstructures. The effect of the multiple phases on the microstructures was discussed in terms of its influence on the mechanical properties. A satisfying densification level of the materials was achieved with the selected parameters.

Abstract: High-entropy alloys (HEAs) are currently attracting much interest because they offer unique properties and good ductility at low temperatures. These materials are of interest primarily because they contain five or more principal elements, with each element having a concentration between 5 and 35 at. %, and yet they have very simple structures based on solid solution phases. Superplasticity is defined formally as a tensile elongation without failure of at least 400% and very recent experiments have shown that the HEAs also have a potential for exhibiting superplastic ductilities when testing at elevated temperatures. Since superplasticity requires a very small grain size, typically <10 μm, it is feasible to process HEAs using severe plastic deformation in order to introduce significant grain refinement. The objective of this review is to summarize the recent results showing superplasticity in HEAs and to compare directly the superplastic flow in HEAs and superplasticity in conventional metallic alloys.

Abstract: The effect of the strain asymmetry on low-cycle fatigue properties and microstructure of Fe–15Mn–10Cr–8Ni–4Si (in. wt. %) alloy undergoing the strain-induced ε-martensitic transformation (ε-MT) were investigated at strain ratios, R, of-1, -0.2, 0.2 and 0.5 under total strain-control mode with total strain amplitude of 0.01. At studied strain ratios the clear asymmetry in tension and compression stress providing tensile mean stress was observed in alloy deformed at R of-0.2, 0.2 and 0.5. The mean stress rapidly decreases to ~ 100 cycles and remain almost zero until failure. It was found that strain-induced ε-martensitic transformation and lattice rotation of austenite provide cyclic hardening of the studied alloy leading to the mean stress relaxation and provides the stability in hysteresis loops behavior at studied R. As a consequence, the fatigue life, N_f, of the alloy remains on the level of the alloy deformed by LCF at R, of -1 (N_fR=-1=9200 cycles). The details of the fatigue behavior, deformation mechanisms and microstructure evolution of the studied alloy are discussed.

Abstract: Magnesium alloys have the characteristic with high specific strength and lightweight property, it is widely used for auto mobile industry. Heat-resistant magnesium alloy is focused as a suitable material for weight reduction of the engine and power train parts in automotive field. In this study, microstructure and heat-resistant property in Mg-3mass%Al-1mass%Si (Mg-3%Al-1%Si) alloy with containing large amount of Sn (tin) were investigated. The alloys produced by permanent mold casting were investigated by optical microscope (OM), scanning electron microscopy (SEM) and measuring of bolt load retention at 423K. The heat-resistant property of Mg-3mass % Al-1mass % Si alloy with containing 6-13masss%Sn was higher compared with Sn free alloy and conventional Magnesium alloys (e.g. AZ91 and AM60 alloys).

Abstract: Intermediate annealing on anisotropy and mechanical properties of an Al-0.78Mg-0.90Si-0.65Zn-0.16Cu (wt. %) alloy was investigated by tensile test, optical microscope (OM) and electron back-scattered diffraction (EBSD). The results indicated that alloy sheet displayed various microstructure after without intermediate annealing, intermediate annealing treatment at 350°C for 2 h, 450°C for 2 h and 500°C for 5 min. Compared with the alloy without intermediate annealing, the alloy exhibited equiaxed grain with 35.0µm after annealing at 500°C for 5 min, grain orientation of the alloy was distributed randomly, and plastic strain ratio value of 45° direction was more than 0.6. The anisotropy of the sheet was significantly reduced. Annealing at 500°C for 5 min was suitable for the production process of automobile body sheets.

Abstract: Al-7mass%Si-0.3mass%Mg alloy is widely applied to the automotive components, such as road wheel or suspension frame because of having higher ductility and corrosion resistance. Two-step aging behavior of solution treated Al-7mass%Si-0.3mass%Mg system alloy A356 cast into permanent mold and solution treated was investigated by micro-vickers hardness measurement, optical microscopy (OM) and transmission electron microscopy (TEM). The microstructure of as-cast state was consist of primary crystallized α-Al and secondary crystallized eutectic phases. Al-7mass%Si-0.3mass%Mg alloy after casting, the test specimens were heat treated for different pre-aging temperatures at 273K, 348K and 423K for various times after solution treatment at 813K for 36ks. After pre-aging treatment, the test specimens were heat treated for artificial aging at 523K for various times. The peak hardness increased almost the same value when the pre-aging temperature was 273K. On the other hands, positive effect of the final-aging was occurred after pre-aging at 348K and 423K with significantly increasing hardness in the under-aging region. The fine precipitates were observed in the specimen which was final aging at 523K after pre-aging at 348K and 423K.Such a positive effect is considered due to the influence of precipitated phase mainly such as clusters and /or G.P.zone.The present study aims to investigate the effect of pre-aged temperature on final-aged behavior in A356 system alloy.

Abstract: Blended Elemental Powder Metallurgy is a very attractive method for producing titanium alloys, which can be formed near net shape and have freedom in composition selection. However applications are still limited due to affordability. In this paper, we will discuss a possible cost-effective route, combining vacuum sintering, extrusion, and heat treatment, to produce titanium alloys with similar or better mechanical properties than that of ingot metallurgy titanium alloys. The as-processed material with an oxygen content of 0.34 ± 0.005 wt.% was subjected to heat treatments such as β annealing plus ageing and α+β annealing without ageing to attain a typical lamellar/Widmanstätten/basketweave type structure with a large variation in terms of the microstructural features such as grain size, colony size, inter-lamellar spacing, thickness of grain boundary α, and size of individual lamellar. From mechanical property data attained here, it was apparent that annealing in high α-β region gave a much better combination of mechanical properties: yield strength (860-902 MPa), ultimate tensile strength (1060-1084 MPa) and ductility/plastic strain (11.5-13.6%). The hardness values of heat treated material varied between 346-376 Vickers hardness (36.8-44.5 Rockwell hardness).

Abstract: In this work, both Finite element simulated method and contour method experimental measurement are used to obtain residual stresses of different Titanium welded alloys, the results show that the maximum of the residual stress is mainly related to the internal restraint degree which formed inside of the thickness, the distribution of the residual stress depends primarily on the shape of weld shape. The heating stage plays a major role in relaxing the residual stress in this research. 95% of the residual stress is relieved in the temperature rising period, and about 75% of it is relieved in the temperature rising period when the temperature is above 500°C.

Abstract: Interest in the use of laser technology is growing in the aeronautical sector. The implementation of new Yb: YAG solid laser sources and new optical generations (dynamic focal length; 2in1 fiber: Fiber core and ring core) offers advantages in terms of quality, accuracy, reproducibility and weld dimensions. The LASER beam Yb: YAG of these new sources is generated, no longer from a bar of yttrium-aluminum garnet but from a disk. Moreover, a top-hat shaped power distribution and a top-hat shaped power distribution with a sharply limited recess in the center (ring structure) may be at the focal point using respectively the inner fiber and the coaxial fiber. These technological innovations offer new possibilities for cutting and welding of sheet metal parts. The welding domains of EN AW-6061 aluminum alloy, Commercial Purity Titanium - Grade 2 (T40), AISI 321 stainless steel alloy, nickel based Hastelloy X and Inconel 625 and cobalt based Haynes 188 superalloys are defined according to process parameters such as power density, focal diameter, welding speed and fiber type. Optimal welding parameters are determined for each alloy. The evolution of the microstructures and the mechanical properties of each zone of the welds are explained according to the power density, the heat input energy and the welding speed.