Papers by Keyword: Alloy Design

Abstract: In cold forming for automotive lightweight design, advanced high strength steels (AHSS) lead to limited formability, high springback and press forces, low stretch flangeability, multiple operations for complex geometries and large scrap rates. Two sets of AHSS, namely ferritic-martensitic dual-phase (DP) steel and martensitic-bainitic complex-phase (CP) steel with some amounts of retained austenite (RA), were designed for the hot-forming route, which eliminates the above drawbacks and guarantees higher performance in the body-in-white (BIW). Design of four DP and four CP alloys was accomplished using JMatPro6.0 thermodynamic software and available literature. The alloys were manufactured in the laboratory in cold-rolled gauge of ~1.5 mm and subjected to hot-forming cycles including hot deformation (up to 20% strain), using a dilatometer and a Gleeble 3800 machine. The thermal cycles of the DP alloys included an intercritical reheating whereas in-situ austempering or slow continuous cooling followed by supercritical reheating was used for the CP alloys. The results showed that yield strength (YS) of 605MPa & 695MPa, ultimate tensile strength (UTS) of 1097MPa & 1242MPa with a total elongation (TE) of 12.6% & 14.1% can be achieved in the best performing DP alloys with a martensite content of 65% & 60 vol.%. The best CP alloys with austempering achieved YS of 673MPa & 699MPa, UTS of 983MPa & 1026MPa and TE of 9.2% & 13.6% with RA of 4%-12 vol.%. The continuously-cooled alloys achieved even better properties. Higher bendability at 1.0 mm gauge in the critical direction was achieved in the CP alloys (90^o&107^o) than in the DP alloys (73^o&76^o).

Abstract: Intermetallic TiAl alloys based on the γ-TiAl phase are already used as engineering light-weight high-temperature materials in aircraft and automotive engines. Thereby, they partly substitute the twice as heavy Ni-base superalloys. Present applications are, for example, blades in the low-pressure turbine of advanced aero-engines, turbine wheels for turbocharger systems of car diesel engines as well as engine parts used in racing cars. All these applications require balanced mechanical properties, i.e. certain ductility at room temperature as well as defined creep strength at elevated temperatures. The first part of this paper reviews the alloy design strategy, which was used for the development of a β-solidifying γ-TiAl-based alloy, the so-called “TNM alloy”, which exhibits an excellent hot-deformability. In the meantime, the TNM alloy with the nominal composition of Ti-43.5Al-4Nb-1Mo-0.1B (in atomic percent, at.%) is introduced in a particular eco-friendly and fuel-saving aero-engine, which is powering a medium-range aircraft since the beginning of 2016. In the second part of this work the microstructural parameters are highlighted, which influence the failure strain at room temperature and creep strength at elevated temperatures. It will be shown how the creep resistance can be improved by tailoring phase fractions as well as the spatial arrangement of the microstructural constituents.

Abstract: High life expectancy of cast components and good material performance at dynamic load are a prerequisite to cater for future trends in wind energy generators. To remain competitive in this ever evolving sector challenges reside in alloy development. In this work fractional factorial design has been applied to ferritic ductile iron with varying contents of silicon (1.6‑2 wt%), nickel (0‑1 wt%), cobalt (0‑3 wt%) and copper (0‑0.2 wt%). The minimum criteria the new alloy should meet were a minimum yield strength of 240 MPa and an impact work of minimal 8 J at a temperature of -20 °C for wall thicknesses of 60‑200 mm. To obtain these mechanical properties thick-walled castings with additional insulation were produced to achieve a higher thermic module. They provided the material for test specimens to perform static tensile tests, Charpy impact tests at varying temperatures and a microstructure analysis. With these results, a sweet spot plot has been created. That way, an optimum alloy composition could be found and has been proven by validation experiment.The optimum alloy for thick-walled castings is composed of Si = 1.6 wt%, Cu = 0.2 wt%, Ni = 0 wt% and Co = 0 wt%. It offers an enhancement in yield strength and acceptable impact work at low temperatures for massive castings in as cast state. The heat treated, full ferritic material could even improve these results.

Abstract: High silicon grades of ductile cast iron are known to be highly advantageous in regard to technically relevant properties and economic efficiency. In particular, the outstanding mechanical properties lead to an increasing demand since 2011, the year of incorporation to the EN 1563 standard. However, low impact resistance and spontaneous failure are concerns that limit the application, especially at lower temperatures. Silicon serves as a solid solution strengthener. By the addition of cobalt, aluminum and nickel as additional solid solution strengthener, an improvement in mechanical properties compared to only silicon could be obtained. Previous studies showed that the addition of 1.5 wt.% Ni to an EN-GJS-500-14 grade with 3.8 wt.% Si resulted in a tensile strength of 650 MPa at 15 % elongation. In the present study, silicon was substituted stepwise by nickel and aluminum, simultaneously aiming at the retention of the mechanical properties of the EN-GJS-500-14 grade. By decreasing the silicon content to 3.3 wt.% Si at 1.1 wt.% Ni and 0.2 wt.% Al, EN-500-14 was obtained. Even though, the presence of pearlite in the matrix was observed, this substitution of silicon led to an increase in Charpy-V-notch toughness by 4 Joule at room temperature. For further alloy design of high silicon ductile cast iron for simultaneously substituting silicon and improving the mechanical properties and notch toughness, the restrictions for pearlite formation must be complied.

Abstract: The molecular orbital approach to alloy design is reviewed in this paper. This approachis based on the electronic structure calculations by the DV-Xα cluster method. New alloyingparameters are obtained for the first time by the calculations of titanium alloys and used for theprediction of phase stability and alloy properties. For example, it is shown that any titanium alloycan be classified into either the α, or α+β, or β type from the alloy composition by using the newalloying parameters. The corrosion resistance is also treatable along this approach. This theoreticalapproach is useful for the practical design of biomedical titanium alloys.

Abstract: Al containing intermetallic phases have been evaluated in various bcc and fcc steels. Attractive application options have been derived for hot working tools steels with respect to a reduction of resource critical alloying elements and in cold formable steels by the combined density reduction and strength increase.

Abstract: The microstructures, phase composition and hardness of the AlCrFeNi_xMo_0.2 high entropy alloy (x=0.5, 0.8, 1.2 and 1.5, the x values refer to molar ratio) were reported. When the value of x was smaller than 1.2, the alloys consisted of BCC and B2 structures. The BCC and B2 phases were identified to be (Cr, αFe) solid solution and NiAl intermetallic compound, respectively. With the increase of x from 0.5 to 1.2, the microstructure transformed from dendrite/inter-dendrite to eutectic microstructures. When the x was equal to 1.5, besides BCC and B2 phases, another CrFe2.32MoNi phases formed and Net-like (Cr, αFe) phases distributed in the NiAl intermetallic compound matrix. The hardness first decreased then increased with the increase of Ni content. Generally, Ni element is a FCC stabilizer. However, in AlCrFeNi_xMo_0.2 alloys, Ni element promoted the formation of B2 and CrFe2.32MoNi phases. The influence mechanism of Ni element was discussed systematically.

Abstract: A series of CoFeNi₂W_0.5Ta_x (x = 0-0.6) high entropy alloys (HEAs) were synthesized by arc melting to investigate the alloying effect of Ta element on the microstructure and mechanical properties of the CoFeNi₂W_0.5 alloy system. Phase constitution, microstructure and mechanical properties of the alloys were analyzed by X-ray diffraction (XRD), scanning electron microscopes (SEM), Vickers hardness and compressive test. It was found that when x = 0, the alloy consists of a single-phase face-centered cubic (FCC) solid solution structure and exhibit excellent ductility, the compressive plastic elongation of which can reach 80% without fracture. While with increasing Ta content, the brittle Co₂Ta-type Laves phase appears which leads to a decrease of the plastic strain and an increase of the yield strength, and the Vickers hardness shows an obvious increase from HV 179.5 to HV 753.2.

Abstract: The phase equilibria of new-type Co-based superalloys which include the system of Co-Al-W were studied by CALPHAD method. It was shown that γ+γ' two-phase regions were existed in the calculated phase diagrams. The compositions of new-type Co-based superalloys which can obtain γ+γ' microstructures were predicted due to the calculated phase diagram. The mole fractions of the constituent phases of Co-Al-W-Ni-Cr alloys were calculated. The temperature of γ' phase began to precipitate at about 1050°C.

Abstract: To solve the heavy vehicle leaf spring material issues, developed heavy vehicle leaf spring with a new material. This article focuses on technical solutions and new materials, materials testing trial and material properties, structure and toughening mechanism of tissue material analysis, leaf spring using new materials bench and road tests. The test results show that the new material with high strength, high ductility and excellent manufacturability. And fewer leaf spring meet material requirements, such as the high stress, high-performance and high reliability fatigue. The development of leaf spring using new materials has high fatigue properties, and it uses for liberation series replacement truck leaf spring.