Effect of Heat Treatment on the Specific Heat Capacity of Nickel-Based Alloys

Alloy-718 and Udimet alloy 720 are gamma prime strengthened superalloys with excellent mechanical and thermal properties at elevated temperatures, as well as at cryogenic temperatures. The nickel-based alloys were improved to be resistant to creep and become stronger by changing the heat-treatment conditions. The measurement of the specific heat capacity of a nickel-based alloy is a very useful tool to investigate the effect of heat treatment. The specific heat capacity of nickel-based alloys Alloy-718 and Udimet alloy 720 were measured using a differential scanning calorimeter in the temperature range of 100 – 1000 K. The specific heat capacity of the nickel-based alloys increases monotonically with temperature; however, above 800 K, it is strongly dependent on the heat treatment conditions and it is thought to be influenced by the precipitation phase (γ′, γ′′). Optical and scanning electron microscopies are used to investigate the microstructure of the phases. The microstructures of the precipitates are examined.Paper presented at the Seventh Asian Thermophysical Properties Conference, August 23–28, 2004, Hefei and Huangshan, Anhui, P. R. China.     

Microstructure and properties of a new super-high-strength Al–Zn–Mg–Cu alloy C912

Using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), the microstructure of a new super-high-strength Al–Zn–Mg–Cu alloy (C912) has been investigated. Compared with some other high-strength aluminum alloys, the C912 alloy exhibits higher strength and good stress-corrosion resistance and its specific strength is even higher than some Al–Li alloys. Its potential for use in the Chinese AE100 airplane is discussed.     

Formation of nanocrystalline phases during thermal decomposition of amorphous Ni-P alloys by isothermal annealing

The microstructure and the average grain size were investigated by x-ray diffraction and transmission electron microscopy for nanocrystalline (n) Ni-P alloys with 18, 19, and 22 at.% P. A detailed study of the nanocrystalline states obtained along different heat treatment routes has been performed: (1) a-->ni by isothermal annealing of the melt-quenched amorphous (a) Ni-P alloys; (2) ni-->nii by isothermal annealing of the nanocrystalline ni state; (3) ni-->nii by linear heating of the ni state. The heats evolved during the structural transformations were determined by differential scanning calorimetry. From these studies, a scheme of the structural transformations and their energetics was constructed, which also includes previous results on phases obtained by linear heating of the as-quenched amorphous state of the same alloys. Grain boundary energies also have been estimated. In some cases it was necessary to assume a variation of the specific grain boundary energy during the phase transformation to understand the enthalpy and microstructure changes during the different heat treatments.     

Thermal Conductivity of Magnesium Alloys in the Temperature Range from −125 °C to 400 °C

Magnesium alloys have been widely used in recent years as lightweight structural materials in the manufacturing of automobiles, airplanes, and portable computers. Magnesium alloys have extremely low density (as low as 1738 kg · m^?3) and high rigidity, which makes them suitable for such applications. In this study, the thermal conductivity of two different magnesium alloys made by twin-roll casting was investigated using the laser-flash technique and differential scanning calorimetry for thermal diffusivity and specific heat capacity measurements, respectively. The thermal diffusivity of the magnesium alloys, AZ31 and AZ61, was measured over the temperature range from ?125 °C to 400 °C. The alloys AZ31 and AZ61 are composed of magnesium, aluminum, and zinc. The thermal conductivity gradually increased with temperature. The densities of AZ31 and AZ61 were 1754 kg · m^?3 and 1777 kg · m^?3, respectively. The thermal conductivity of AZ31 was about 25 % higher than that of AZ61, and this is attributed to the amount of precipitation.     

Mn和Zr对Al-Zn-Mg-Cu铝合金各向异性的影响

采用光学显微镜(OM)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)以及室温拉伸、剥落腐蚀、晶间腐蚀等测试方法,研究了微量的Mn和Zr对Al-Zn-Mg-Cu铝合金的组织和性能各向异性的影响。结果表明,在Al-Zn-Mg-Cu-Ti合金中,分别添加微量的Mn和Zr,合金中对应析出细小弥散的Al6Mn和Al3Zr相,这两相均能抑制基体再结晶,促使合金的晶粒纵横比增大。合金的力学性能、抗晶间腐蚀和剥落腐蚀性能提高,但性能各向异性增大。同时,结果显示Zr对合金的组织和性能各向异性的影响显著大于Mn。     

Microstructural changes during annealing of cast Ni–Cr–Hf alloys

Abstract

In order to gain knowledge of the nickel–rich corner of the Ni–Cr–Hf phase diagram, microstructures of cast alloys and their evolution towards equilibrium conditions have been followed by optical and scanning electron microscopy, microprobe analysis, and quantitative metallography. The evolution of as–cast microstructures involves the development of a Ni₅Hf plate–like precipitate in two–phase alloys, and various morphological changes in three–phase alloys. As–cast structures are compared with those obtained by arc melting. Direct processing of the scanning electron microscope image has proved to be a valuable tool for microstructural characterization of the transforming phases.

MST/349     

Microstructural Stability and Properties of New Nickel-Base Superalloys with Varying Aluminium: Niobium Ratio

New nickel-base superalloys with higher temperature capability are required for future, more efficient gas turbine engines. In designing such alloys, careful consideration is required of the elemental concentrations to ensure a suitable balance of properties is obtained. Herein, the phase equilibria and microstructural stability of new nickel-base superalloys with varying Al:Nb ratio are assessed via long-term thermal exposures at 700 °C. The alloys are analyzed using scanning and transmission electron microscopy, X-ray diffraction, differential scanning calorimetry, and Vickers hardness testing, with the results rationalized through mechanical property predictions based on strong- and weak-pair dislocation coupling mechanisms. The alloys are shown to have greater thermal stability than Inconel 718 and exhibit a pronounced hardening effect after thermal exposure. Herein, their ability for controlled age hardening and potential ease of processing is highlighted in the results.     

Structural and Phase Transformations in Alloys during Spark Plasma Sintering of Ti + 23.5 at % Al + 21 at % Nb Powder Mixtures

We have studied the effect of sintering temperature on the structural and phase transformations of alloys produced by the spark plasma sintering of Ti + 23.5 at % Al + 21 at % Nb powder mixtures at temperatures in the range 1100–1550°C. The sintered alloys have been characterized by X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray spectroscopy (elemental X-ray mapping). The alloys sintered at temperatures of 1100 and 1200°C have been shown to have a nonuniform microstructure. According to electron microscopy results, the alloys consist of grains of the α₂ and Nb₂Al phases and small precipitates of the O-phase (intermetallic compound Ti₂AlNb). In addition, there are particles of unreacted niobium and titanium. The alloys sintered at a temperature of 1300°C have a uniform lamellar structure.     

Nanostructural materials formation by mechanical alloying: Morphologic analysis based on transmission and scanning electron microscopic observations

The development of nanostructured materials offers new scientific and technological perspectives due to the specific interesting physical properties of these materials. These properties derive either from their reduced grain size or from the structure and properties of the grain boundaries, which constitute a significant volume fraction. Mechanical alloying, widely used to produce dispersion-strengthened and amorphous alloys, has been employed in recent years to synthesize nanocrystalline metallic, semiconductors, and covalent component-based materials. Based on statistical analysis of transmission and scanning electron microscopic images, the distribution and spatial repartition of the nanostructural material prepared by mechanical alloying and/or attrition are presented for some specific cases.     

Section-dependent microstructure and mechanical properties of rapidly solidified and extruded Al-20Si alloy

The microstructure and mechanical properties of rapidly solidified and extruded Al-20Si alloys were studied by a combination of optical microscopy, scanning electron microscopy, X-ray diffraction, compression testing, and wear testing. The microstructure of the extruded bars showed a homogeneous distribution of different primary Si sizes and shapes embedded in the Al matrix depending on the section considered in the extruded bar. As the section changes from diagonal through perpendicular to parallel, the compressive yield strength increased from 219 through 225 to 263 MPa, respectively. The specific wear was the lowest at all sliding speeds for the parallel section in comparison to the perpendicular and diagonal sections of the extruded bar.     

The oxidation of Fe3Al–(0, 2, 4, 6%)Cr alloys at 1000 °C

Thermomechanically treated Fe₃Al–(0, 2, 4, 6at.%)Cr alloys were isothermally oxidized at 1000 °C in air, and their oxidation characteristics were studied using thermogravimetric analyzer, X-ray diffractometer, scanning electron microscope, electron probe microanalyzer, and TEM/EDS. It was found that Cr decreased the oxidation resistance of Fe₃Al alloys to a certain extent. The oxide scales that formed on the unalloyed Fe₃Al alloys consisted primarily of α-Al₂O₃ containing a small percentage of dissolved iron ions. Less than 1% of dissolved chromium ions was additionally present in the oxide scale formed on the Fe₃Al–Cr alloys. An Al-free, Fe-enriched zone was formed beneath the oxide scale, owing to Al consumption to form the oxide scale. The oxide scale on all alloys had poor adherence.     

Experimental investigation and thermodynamic description of the Mg–Y–Zr system

The Mg–Y–Zr system was studied via experimental investigation and thermodynamic modeling. Four diffusion couples and four key alloys of the Mg–Y–Zr system at 500 °C were prepared. The phase relations of the Mg–Y–Zr system were investigated by means of X-ray diffraction, scanning electron microscopy, and electron probe microanalysis. No ternary compound was found at 500 °C. The solubility of (αZr) in the Mg–Y intermetallics, i.e., Mg₂₄Y₅, Mg₂Y and MgY, was determined to be negligible. The differential scanning calorimetry measurement was performed on the Mg–Y–Zr alloys to obtain the phase transition temperature. The present thermodynamic calculations of the Mg–Y–Zr system matched well with the experimental data. The presently established Mg–Y–Zr phase diagram can offer a better understanding of the recent processing technique of creep-resistant magnesium alloys.     

Role of alloying elements in microstructures of beta titanium alloys with carbon additions

Abstract

The effect of 0.2 wt-%C on the microstructure of beta titanium alloys Ti-15X(Fe, Cr, Mn, Mo, Ni, Co, Cu, and V) has been studied using scanning and transmission electron microscopy. It has been found that coarse eutectic TiC_x tends to be formed in beta titanium alloys containing Fe, Cr, Mo, and Mn, and relatively finer homogeneous TiC_x is formed in alloys containing V, Ta, Co, Ni, or Cu. The volume fraction of TiC_x in alloys containing Cu, Co, and Ni is much less than that in other beta titanium alloys. The oxygen content of the matrix is lower than that of Ti₂C in Cr or alloys containing Mn and higher than that of Ti₂C in alloys containing Mo or Ni. These observations are discussed in terms of the role of phase diagrams and the effect of atomic radius of alloying elements on the dimension of interstitial sites in the host alloy and the sublattice of TiC_x.     

Microstructure and microhardness of high entropy alloys with Zn addition: AlCoFeNiZn and AlCoFeNiMoTiZn

High entropy alloys were designed from equiatomic multicomponent systems using powder metallurgy including mechanical alloying and sintering. The structure and morphology of the resulting alloys were characterized by means of X-ray diffraction, scanning electron microscopy, transmission electron microscopy techniques and their hardness values were also determined in the Vickers scale. The results indicate under the milling conditions used, the AlCoFeNiZn, AlCoFeNiMoTi and AlCoFeNiMoTiZn alloys crystallized forming BCC structures whereas the AlCoFeNi alloy presented two different phases, one with FCC structure and the other one with BCC. The synthesis method resulted in alloys with grain sizes in the nano scale having values between 4.1 and 9.4 nm on the powder form up to 40.1 nm after sintering phenomenon which lead to phase transformations which were more evident in the Mo-containing alloys. In addition, the AlCoFeNiZn and AlCoFeNiMoTiZn alloys did not show Zn traces after sintering as it was suggested by chemical analyses using energy dispersive spectroscopy, suggesting it is lost by evaporation during sintering process. Mo-containing systems exhibited the highest microhardness in both milled and sintered conditions.     

Microstructural characterization and dry sliding wear resistance of MoO2-strengthened γ/NiMo alloys with different primary phases

Two γ/NiMo alloys strengthened by a refractory metal oxide MoO₂ phase were fabricated by a laser melting deposition process. Microstructural transformation of the alloys with different primary phases was identified and the crystal structure of the primary phases was confirmed by both X-ray diffraction and transmission electron microscopy. Meanwhile, room-temperature dry sliding wear behavior and mechanism difference of the alloys were also investigated mainly by scanning electron microscopy. Microstructures of the alloys varied from hypoeutectic to hypereutectic, as the molybdenum content increases. The hypoeutectic alloy was solidified on the basis of a Ni-base solid solution γ primary phase, whereas the hypereutectic alloy was grown based on an intermetallic compound with NiMo primary phase. The γ, NiMo primary phases and MoO₂ strengthening phase were confirmed to have face-centered cubic, orthorhombic and monoclinic structures, respectively. Compared with the hypoeutectic alloy, the hypereutectic alloy exhibited higher wear resistance under the same condition. The predominant wear mechanism of the γ/NiMo alloys transformed from micro-cutting to microcracking.     

Oxide adherence of Fe-20Cr-4Al alloys with small amounts of sulfur and reactive elements (Y,Hf)

Abstract

Oxide adherence of Fe–20Cr–4Al alloys with small amounts of sulfur, yttrium and hafnium was studied in air for 360 ks at 1,373, 1,473 and 1,573K by mass change measurements, X-ray diffraction, scanning electron microscopy and electron probe microanalysis. After oxidation at 1,373K, spalling of oxide scales on 7ppmS, 53ppmS and 1,300ppmS alloys was recognized. However, spalling of oxide scales on the other alloys was not observed. After oxidation at 1,573K, spalling of the oxide scales on the alloys with sulfur increased roughly with increasing contents of sulfur, and spalling of oxide scales on the alloys containing yttrium was scarcely recognized, however, oxide scales on all of the alloys containing hafnium spalled at the entire surface. Oxide adherence on the alloys may relate to morphologies of oxide scales and oxide–alloy interface, size and distribution of chromium sulfide, Y₃Al₅O₁₂ and HfO₂ particles at the oxide–alloy interface and temperature of oxidation.     

Calorimetric techniques for the evaluation of thermal efficiencies of shape memory alloys

The latent heat and entropy changes of NiTi shape memory effect (SME) alloys have been evaluated by three different calorimetric techniques; adiabatic calorimetry, differential scanning calorimetry and a Clapeyron analysis of isothermal stress-strain data. It is found that these techniques provide consistent estimates for the enthalpy and entropy to within 20% for NiTi and noble metal SME alloys. From published thermodynamic data for SME alloys, thermal efficiencies were calculated based on an ideal SME heat engine cycle. It was found that NiTi provides the maximum thermal efficiency with the highest temperature transformation range.     

Capability of mechanical alloying and SPS technique to develop nanostructured high Cr,Al alloyed ODS steels

Abstract

Oxide dispersion strengthened (ODS) Fe alloys were produced by mechanical alloying (MA) with the aim of developing a nanostructured powder. The milled powders were consolidated by spark plasma sintering (SPS). Two prealloyed high chromium stainless steels (Fe–14Cr–5Al–3W) and (Fe–20Cr–5Al+3W) with additions of Y₂O₃ and Ti powders are densified to evaluate the influence of the powder composition on mechanical properties. The microstructure was characterised by scanning electron microscope (SEM) and electron backscattering diffraction (EBSD) was used to analyse grain orientation, grain boundary geometries and distribution grain size. Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) equipped with energy dispersive X-ray spectrometer (EDX) were used to observe the nanostructure of ODS alloys and especially to observe and analyse the nanoprecipitates. Vickers microhardness and tensile tests (in situ and ex situ) have been performed on the ODS alloys developed in this work.     

Characteristics of the continuous coarsening and discontinuous coarsening of spinodally decomposed Cu-Ni-Fe alloy

The spinodal decomposition and coarsening reaction of a 45Cu-30Ni-25Fe alloy aged with and without prior 75% cold rolling have been studied by Vickers hardness tester, X-ray diffraction, optical microscope, and scanning and transmission electron microscopes. Spinodal decomposition took place in the alloys aged in the temperature range of 600–950°C without prior deformation, and then spinodal structure in these alloys would coarsen only continuously. Spinodal decomposition and continuous coarsening reaction of spinodal structure took place in the alloys aged at lower temperature with prior deformation. This process was corresponding to recovery of spinodal structure in the deformed alloys. Discontinuous coarsening reaction of spinodal structure would also take place in the alloys aged at higher temperature with prior deformation. This process was corresponding to recrystallization of spinodal structure in the deformed alloys.     

Structural investigations of mechanical properties of Al based rapidly solidified alloys

In this study, Al based Al–3 wt.%Fe, Al–3 wt.%Cu and Al–3 wt.%Ni alloys were prepared by conventional casting. They were further processed using the melt-spinning technique and characterized by the X-ray diffraction (XRD), scanning electron microscopy (SEM) together with energy dispersive spectroscopy (EDS), transmission electron microscope (TEM), differential scanning calorimetry (DSC) and the Vickers microhardness tester. The rapidly solidified (RS) binary alloys were composed of supersaturated α–Al solid solution and finely dispersed intermetallic phases. Experimental results showed that the mechanical properties of RS alloys were enhanced, which can be attributed to significant changes in the microstructure. RS samples were measured using a microhardness test device. The dependence of microhardness H_V on the solidification rate (V) was analysed. These results showed that with the increasing values of V, the values of H_V increased. The enthalpies of fusion for the same alloys were determined by DSC.