Elevated Temperature Hardness Research Articles

Solid solution softening or hardening induced by minor substitutional additions in a Hf20Nb31Ta31Ti18 refractory high entropy alloy

The effect of minor additions of substitutional elements such as Al, Cu, Mn, and Fe on the room-temperature (RT) and elevated-temperature hardness of a single bcc phase Hf20Nb31Ta31Ti18 refractory high entropy alloy is studied here. Interestingly, 2.5 at. % nominal addition of Fe hardened the base Hf20Nb31Ta31Ti18 alloy in the temperature range from RT to 800 °C, while the same nominal content of addition of Al, Cu, and Mn softened the base alloy from RT to 1000 °C. Regardless of solid solution hardening or solid solution softening, the hardness variation with temperature essentially showed the same three-stage pattern for all studied alloys here: a temperature-dependent decrease in hardness below 300 °C/400 °C, followed by a temperature-independent hardness plateau between 300/400 and 800 °C, and finally a temperature-dependent decrease in hardness at temperatures higher than 800 °C. The mechanism for solid solution hardening or softening in bcc-structured refractory high entropy alloys is discussed, together with their temperature dependence.

The manufacture and characterization of WC-(Al)CoCrCuFeNi cemented carbides with nominally high entropy alloy binders

Recently, there has been increasing interest in cemented tungsten carbide hardmetals and titanium carbonitride cermets with binders of multi-component alloys (≥4 elements) or high entropy alloys (≥5 principal elements in equimolar ratios). Property improvements have been reported, such as increased ambient and elevated temperature hardness, as well as greater oxidation resistance.This study has thoroughly investigated model cemented carbides manufactured using coarse WC with a binder content of 20 wt% (32–37 vol%) from three different (Al)CoCrCuFeNi high entropy alloys (HEAs) and at different carbon levels (low, medium and high).Binder alloys were manufactured by both planetary ball milling of elemental powder mixtures and gas atomizing. Sintering was performed in vacuum for 2 h at different temperatures between 1200 °C and 1500 °C. Post-HIP treatments were also applied in some cases as all systems were difficult to densify without residual porosity.Detailed analyses were performed on the as-manufactured binder alloys, sintered binder alloys (without WC) and the actual sintered cemented carbides (WC + HEA). Various analysis methods were used to examine the materials. These included thermogravimetry analysis (TGA) and differential scanning calorimetry (DSC) to determine the melting behaviour; X-Ray Diffraction (XRD), Electron Backscatter Diffraction (EBSD) and Energy-Dispersive X-Ray Spectroscopy (EDS) to identify the type, crystal structure and exact composition of the phases present; and light optical microscopy (LOM) and scanning electron microscopy (SEM) for microstructural characterization. Additionally, the hardness and Palmqvist indentation toughness of each composition were also measured.2-Phase WC-HEA microstructures could not be obtained using the investigated high entropy alloys. Several solid solution binder alloys and numerous carbide phases were present after sintering, formed by segregation and reaction. The type and quantity of the phases depend on the carbon balance. For the compositions containing aluminium, it was found that aluminium forms oxides and intermetallic phases during sintering. The paper presents these findings in detail.

Structure and properties of cast Al–Si based alloy with Zr–V–Ti additions and its evaluation of high temperature performance

A hypoeutectic Al–Si based alloy with Cu, Mg and additions of Zr, V and Ti, was evaluated to determine its suitability for applications at higher temperatures. The as-cast alloy consists of α-Al matrix, Al–Si eutectic and Al2Cu, Al5Mg9Si8Cu2 and Al14Mg4FeSi6/Al5FeSi, intermetallic phases, plus two distinct Zr–V–Ti-based phases. Solutionizing treatment leads to dissolution of Al5Mg8Si6Cu2 and Al2Cu and the partial dissolution of Zr–V–Ti-based intermetallics. Integration of aging kinetics with mechanical properties data allowed to establish different aging parameters leading to T6 peak-aging and T7 over-aging conditions corresponding to hardness of 96 and 68 HRF respectively. T7 condition enabled higher hardness retention than T6 condition; the alloy softening was observed at 260°C versus 240°C after T6 treatment. TEM characterization of the alloy in the T6 condition revealed GP zones, and rod-shaped trialuminide Al3(Zr, V, Ti) with average size of 230nm as well as S-phase (Al2CuMg) precipitates. After T6 treatment followed by isothermal annealing at 475°C, the alloy showed hardness retention improvement by approximately 30%. Subsequent TEM analysis revealed a higher density of the Al3(Zr, V, Ti) rod-shaped precipitates along with formation of new lath shaped Q′-phase AlSiCuMg precipitates. Two over-aging reactions between 250–350°C and 440–460°C were associated with the stability of Cu–Mg and Al-Zr–V–Ti based nanoprecipitates respectively. Temperature of 250°C was established as the transition temperature above which degradation of mechanical properties was observed from UTS of approximately 311MPa at room temperature to 208MPa at 250°C. The results indicate that additions of Zr, V and Ti improve yield strength and cyclic yield strength at room temperature but lower ductility, while slightly improve elevated temperature hardness with almost no effect on tensile strength.

Invited Article: Indenter materials for high temperature nanoindentation

As nanoindentation at high temperatures becomes increasingly popular, a review of indenter materials for usage at high temperatures is instructive for identifying appropriate indenter-sample materials combinations to prevent indenter loss or failure due to chemical reactions or wear during indentation. This is an important consideration for nanoindentation as extremely small volumes of reacted indenter material will have a significant effect on measurements. The high temperature hardness, elastic modulus, thermal properties, and chemical reactivities of diamond, boron carbide, silicon carbide, tungsten carbide, cubic boron nitride, and sapphire are discussed. Diamond and boron carbide show the best elevated temperature hardness, while tungsten carbide demonstrates the lowest chemical reactivity with the widest array of elements.

Reactively Synthesized Nanostructured PM Aluminium Composite - Microstructure Stability and Elevated Temperature Hardness Response

Reactive synthesis by mechanical alloying and subsequent hot consolidation is an appropriate way to produce an Al-Al 3 Ti-Al 2 O 3 nanocomposite from readily available and inexpensive aluminium powder and nanosized TiO 2 . This processing route provides a fine grained and thermally stable microstructure with a controllable amount of dispersoids. The respective material shows a high hardness at room and elevated temperature and does not suffer from softening during long-term annealing at 250°C. It is therefore considered to be a promising light-weight material for elevated temperature applications.

Microstructure and properties of laser-clad high-temperature wear-resistant alloys

A 2-kW CO 2 laser with a powder feeder was used to produce alloy coatings with high temperature-wear resistance on the surface of steel substrates. To analyze the microstructure and microchemical composition of the laser-clad layers, a scanning electron microscope (SEM) equipped with an energy dispersive X-ray microanalysis system was employed. X-ray diffraction techniques were applied to characterize the phases formed during the cladding process. The results show that the microstructure of the cladding alloy consists mainly of many dispersed particles (W 2C, (W,Ti)C 1− x , WC), a lamellar eutectic carbide M 12C, and an (f.c.c) matrix. Hardness tested at room and high temperature showed that the laser-clad zone has a moderate room temperature hardness and relatively higher elevated temperature hardness. The application of the laser-clad layer to a hot tool was very successful, and its operational life span was prolonged 1 to 4 times.

The effect of hydrogen on the strength and superplastic deformation of beta-titanium alloys

The effect of hydrogen on mechanical properties and superplastic deformation of two commercial β titanium alloys, Ti-4Al-7Mo-10V-2Fe-1Zr (Ti-471021) and Ti-10V-2Fe-3Al (Ti-1023), was studied in the range of hydrogen concentration up to 1.3 wt%. The elevated temperature hardness of Ti-471021 alloy increased with hydrogen concentration. The stress levels during the superplastic deformation in both alloys increased with increasing concentrations of hydrogen in the β-phase region. X-ray diffraction results and examination of the microstructure with TEM revealed that no hydrides had formed up to hydrogen concentrations of 1.3 wt%. The increase in flow stress was mainly due to the solid solution strengthening by hydrogen during the superplastic deformation.

高濃度浸炭用鋼のプラズマ浸炭および疲労特性

Super carburizing is one of the carburizing processes where surface carbon content is increased up to 2 to 4%, extremely higher than that of conventional one. This is characterized by its carbide precipitation in the case and is expected to show superior fatigue durabilities.At first the effect of plasma super carburizing conditions on the carbide morphology was studied using 0.18%C-0.5%Si-2.25%Cr-0.4%Mo steel. Low temperature carburizing at 1143K was confirmed to be effective to obtain finely and uniformly dispersed carbides. Furthermore, the optimum condition was selected with taking other case properties into account.The bending and contact fatigue tests were carried out as a next step, and it was revealed that plasma super carburized steel exhibited superior fatigue durabilities to those of conventionaly carburized JIS-SCM 420. The superiority was considered due to its higer surface hardness, finer grain, intergranular oxidation free case and higher elevated temperature hardness.

The effect of molybdenum on γ′ coarsening and on elevated-temperature hardness in some experimental nickel-base superalloys

The coarsening of γ′ and the elevated-temperature hardness have been studied as a function of molybdenum content, time, and temperature in experimental wrought nickel-base superalloys. The alloys were selected from a systematic series containing 3, 4 1/2, and 6 wt pct Al and 1 wt pct Al plus 3 1/2 wt pct Ti. Each of the aluminum (plus titanium) series consisted of four alloys containing 0, 2, 5, and 8 wt pct Mo. The alloys were solution-treated plus aged up to 112 h at 1700°F (925°C) and up to 1000 h at 1400°F (760°C). Molybdenum retards the coarsening of γ′ on aging; this retarding effect is most pronounced in alloys containing 6 wt pct Al. The coarsening of γ′ particles follows Ostwald ripening kinetics. Hardness testingin vacuo at temperatures up to 1750°F (955°C) shows that molybdenum also increases the elevated-temperature hardness significantly. The relation of elevated-temperature hardness to the volume fraction of γ′ is considered, and the influence of aluminum and titanium contents is discussed.