Tensile, Fatigue, and Creep Properties of Aluminum Heat Exchanger Tube Alloys for Temperatures from 293 K to 573 K (20 °C to 300 °C)

Since automotive heat exchangers are operated at varying temperatures and under varying pressures, both static and dynamic mechanical properties should be known at different temperatures. Tubes are the most critical part of the most heat exchangers made from aluminum brazing sheet. We present tensile test, stress amplitude-fatigue life, and creep–rupture data of six AA3XXX series tube alloys after simulated brazing for temperatures ranging from 293 K to 573 K (20 °C to 300 °C). While correlations between several mechanical properties are strong, ranking of alloys according to one property cannot be safely deduced from the known ranking according to another property. The relative reduction in creep strength with increasing temperature is very similar for all six alloys, but the general trends are also strong with respect to tensile and fatigue properties; an exception is one alloy that exhibits strong Mg-Si precipitation activity during fatigue testing at elevated temperatures. Interrupted fatigue tests indicated that the crack growth time is negligible compared to the crack initiation time. Fatigue lifetimes are reduced by creep processes for temperatures above approximately 423 K (150 °C). When mechanical properties were measured at several temperatures, interpolation to other temperatures within the same temperature range was possible in most cases, using simple and well-established equations.     

Low Temperature Sensitization on the Orthogonal Surfaces of Prior Deformed AISI 304LN and Aged at 673 K to 873 K (400 °C to 600 °C)

This study investigates the sensitization behavior of AISI 304LN deformed previously by cold rolling (CR) to 5 pct to 25 pct reduction in thickness and aged at 673 K to 873 K (400 °C to 600 °C). The emphasis was on revealing the degree of sensitization (DOS) resulting from low-temperature sensitization (LTS) on the orthogonal surfaces and correlation thereof with the changes in various metallurgical properties caused by cold rolling. It was found that the DOS differs among the orthogonal surfaces such as the rolling surface (RS), transverse surface (TS), and cross transverse surface (CTS). RS showed lower DOS compared with CTS and TS. The differences in the DOS were attributed to the combined effect of deformation-induced martensite (DIM), grain size, and slip band formation. A deformation of 5 pct was critical to the susceptibility of AISI 304LN to intergranular (IGC) and transgranular corrosion (TGC). The sensitization kinetics was slow or saturated leading to desensitization beyond 5pct deformations at 773 K (500 °C) and 873 K (600 °C). It was, however, uniformly accelerated over 5 to 25 pct deformation when aged at 673 K (400 °C).     

Analysis of the Deformation Behavior in Tension and Tension-Creep of Ti-3Al-2.5V (wt pct) at 296 K and 728 K (23 °C and 455 °C) Using In Situ SEM Experiments

The deformation behavior of a Ti-3Al-2.5V (wt pct) near-α alloy was investigated during in situ deformation inside a scanning electron microscopy (SEM). Two plates with distinct textures were examined. Tensile experiments were performed at 296 K and 728 K (455 °C) (~0.4T _m), while a tensile-creep experiment was performed at 728 K (455 °C) and 180 MPa (σ/σ _ys = 0.72). The active deformation systems were identified in the α phase using electron backscattered diffraction based slip-trace analysis and SEM images of the surface. Prismatic slip deformation was the dominant slip mode observed for all the experiments in both plates, which was supported by a critical resolved shear stress (CRSS) ratio analysis. However, due to the texture of plate 1, which strongly favored the activation of prismatic slip, the percentages of prismatic slip activity for specimens from plate 1 tested at 296 K and 728 K (23 °C and 455 °C) were higher than the specimens from plate 2 under the same testing conditions. T1 twinning was an active deformation mode at both 296 K and 728 K (23 °C and 455 °C), but the extent of twinning activity decreased with increased temperature. T1 twinning was more frequently observed in specimens from plate 2, which exhibited a higher fraction of twinning systems favoring activation at both 296 K and 728 K (23 °C and 455 °C). The tension-creep experiment revealed less slip and more grain boundary sliding than in the higher strain rate tensile experiments. Using a previously demonstrated bootstrapping statistical analysis methodology, the relative CRSS ratios of prismatic, pyramidal 〈a〉, pyramidal 〈c+a〉, and T1 twinning deformation systems compared with basal slip were calculated and discussed in light of similar measurements made on CP Ti and Ti-5Al-2.5Sn (wt pct).     

High-Temperature (1023 K to 1273 K [750 °C to 1000 °C]) Plastic Deformation Behavior of B-Modified Ti-6Al-4V Alloys: Temperature and Strain Rate Effects

A minor addition of B to the Ti-6Al-4V alloy, by ~0.1 wt pct, reduces its as-cast prior β grain size by an order of magnitude, whereas higher B content leads to the presence of in situ formed TiB needles in significant amounts. An experimental investigation into the role played by these microstructural modifications on the high-temperature deformation behavior of Ti-6Al-4V-xB alloys, with x varying between 0 wt pct and 0.55 wt pct, was conducted. Uniaxial compression tests were performed in the temperature range of 1023 K to 1273 K (750 °C to 1000 °C) and in the strain rate range of 10^–3 to 10⁺¹ s^–1. True stress–true strain responses of all alloys exhibit flow softening at lower strain rates and oscillations at higher strain rates. The flow softening is aided by the occurrence of dynamic recrystallization through lath globularization in high temperature (1173 K to 1273 K [900 °C to 1000 °C]) and a lower strain rate (10^–2 to 10^–3 s^–1) regime. The grain size refinement with the B addition to Ti64, despite being marked, had no significant effect on this. Oscillations in the flow curve at a higher strain rate (10⁰ to 10⁺¹ s^–1), however, are associated with microstructural instabilities such as bending of laths, breaking of lath boundaries, generation of cavities, and breakage of TiB needles. The presence of TiB needles affected the instability regime. Microstructural evidence suggests that the matrix cavitation is aided by the easy fracture of TiB needles.     

Limestone Dissolution in Converter Slag at 1873 K (1600 °C)

Decomposition and dissolution of limestone in slag at 1873 K (1600 °C) were studied. The limestone samples were in the shape of cubes (11 mm × 11 mm × 11 mm approximately). The decomposition was carried out both in argon and in slag under argon atmosphere. In order to gain an insight into the phenomenon of slow decomposition, the decomposition process of CaCO₃ was simulated using Comsol. The results showed evidently that the decomposition of calcium carbonate was controlled mostly by heat transfer. It was also found that the decomposition product CaO had very dense structure, whether the sample was decomposed in slag or in argon. The slow decomposition and the dense CaO layer would greatly hinder the dissolution of lime in the slag. The present results clearly indicate that the addition of limestone instead of lime would not be beneficial in the converter process.     

Isothermal Reaction of NiO Powder with Undiluted CH<Subscript>4</Subscript> at 1000 K to 1300 K (727 °C to 1027 °C)

In this study, isothermal reaction behavior of loose NiO powder in a flowing undiluted CH₄ atmosphere at the temperature range 1000 K to 1300 K (727 °C to 1027 °C) is investigated. Thermodynamic analyses at this temperature range revealed that single phase Ni forms at the input \( {{n_{{{\text{CH}}_{ 4} }}^{\text{o}} } \mathord{\left/ {\vphantom {{n_{{{\text{CH}}_{ 4} }}^{\text{o}} } {\left( {n_{{{\text{CH}}_{ 4} }}^{\text{o}} + n_{\text{NiO}}^{\text{o}} } \right)}}} \right. \kern-0pt} {\left( {n_{{{\text{CH}}_{ 4} }}^{\text{o}} + n_{\text{NiO}}^{\text{o}} } \right)}} \) mole fractions (\( X_{{{\text{CH}}_{ 4} }} \)) between ~0.2 and 0.5. It was also predicted that free C co-exists with Ni at \( X_{{{\text{CH}}_{ 4} }} \) values higher than ~0.5. The experiments were carried out as a function of temperature, time, and CH₄ flow rate. Mass measurement, XRD and SEM-EDX were used to characterize the products at various stages of the reaction. At 1200 K and 1300 K (927 °C and 1027 °C), the reaction of NiO with undiluted CH₄ essentially consisted of two successive distinct stages: NiO reduction and pyrolytic C deposition on pre-reduced Ni particles. At 1200 K (927 °C), 1100 K (827 °C), and 1000 K (727 °C), complete oxide reduction was observed within ~7.5, ~17.5, and ~45 minutes, respectively. It was suggested that NiO was essentially reduced to Ni by a CH₄ decomposition product, H₂. Possible reactions leading to NiO reduction were suggested. An attempt was made to describe the NiO reduction kinetics using nucleation-growth and geometrical contraction models. It was observed that the extent of NiO reduction and free C deposition increased with the square root of CH₄ flow rate as predicted by a mass transport theory. A mixed controlling mechanism, partly chemical kinetics and partly external gaseous mass transfer, was responsible for the overall reaction rate. The present study demonstrated that the extent of the reduction can be determined quantitatively using the XRD patterns and also using a formula theoretically derived from the basic XRD data.     

748 K (475 °C) Embrittlement of Duplex Stainless Steel: Effect on Microstructure and Fracture Behavior

22Cr-5Ni duplex stainless steel (DSS) was aged at 748 K (475 °C) and the microstructure development correlated to changes in mechanical properties and fracture behavior. Tensile testing of aged microstructures confirmed the occurrence of 748 K (475 °C) embrittlement, which was accompanied by an increase of strength and hardness and loss of toughness. Aging caused spinodal decomposition of the ferrite phase, consisting of Cr-enriched α″ and Fe-rich α′ and the formation of a large number of R-phase precipitates, with sizes between 50 and 400 nm. Fracture surface analyses revealed a gradual change of the fracture mode from ductile to brittle delamination fracture, associated with slip incompatibility between ferrite and austenite. Ferrite became highly brittle after 255 hours of aging, mainly due to the presence of precipitates, while austenite was ductile and accommodated most plastic strain. The fracture mechanism as a function of 748 K (475 °C) embrittlement is discussed in light of microstructure development.     

Chromia-Assisted Decarburization of W-Rich Ni-Based Alloys in Impure Helium at 1273 K (1000 °C)

The microstructure and surface stability of two experimental W-rich Ni-based alloys have been studied at 1273 K (1000 °C) in an impure-He environment containing only CO and CO₂ as impurities. The alloy Ni-2.3Al-12Cr-12W contained 0.08 wt pct carbon in solution, whereas the second alloy Ni-2.3Al-3Mo-12Cr-12Co-12W contained M₆C carbides at the same carbon level. Both alloys, which were preoxidized with ~2.3 μm Cr₂O₃ layer, were decarburized completely within 50 hours of exposure to the helium gas mixture at 1273 K (1000 °C) via the following chromia-assisted decarburization reaction: Cr₂O₃ (s) + 3C_alloy (s) → 2Cr (s) + 3CO (g). Microstructural observations, bulk carbon analysis, and microprobe measurements confirmed that the carbon in solid solution reacted with the surface chromium oxide resulting in the simultaneous loss of chromia and carbon. The Cr produced by the decomposition of the Cr₂O₃ diffused back into the alloy, whereas CO gas was released and detected by a gas chromatograph. Once the alloy carbon content was reduced to negligible levels, subsequent exposure led to the uninterrupted growth of Cr₂O₃ layer in both alloys. In the preoxidized alloys, chromia-assisted decarburization rates were slower for an alloy containing carbides compared with the alloy with carbon in solid solution only. The formation of Cr₂O₃ is shown to be the rate-limiting step in the chromia-assisted decarburization reaction. Exposure of as-fabricated alloys to the impure-He environment led to the formation of a thin layer of Al₂O₃ (<1 μm) between the substrate and surface Cr₂O₃ oxide that inhibited this decarburization process by acting as a diffusion barrier.     

Intergranular Corrosion Behavior of 304LN Stainless Steel Heat Treated at 623 K (350 °C)

Low temperature sensitization of 304LN stainless steel from the two pipes, differing slightly in chemical composition, has been investigated; specimens were aged at 623 K (350 °C) for 20,000 hours and evaluated for intergranular corrosion and degree of sensitization. The base and heat-affected zone (HAZ) of the 304LN-1 appear resistant to sensitization, while 304LN-2 revealed a “dual” type microstructure at the transverse section and HAZ. The microstructure at 5.0-mm distance from the fusion line indicates qualitatively less sensitization as compared to that at 2.0 mm. The 304LN-2 base alloy shows overall lower degree of sensitization values as compared to the 304LN-1. A similar trend of degree of sensitization was observed in the HAZ where it was higher in the 304LN-1 as compared to the 304LN-2. The weld zone of both the stainless steels suffered from cracking during ASTM A262 practice E, while the parent metals and HAZs did not show such fissures. A mottled image within the ferrite lamella showed spinodal decomposition. The practice E test and transmission electron microscopy results indicate that the interdendritic regions may suffer from failure due to carbide precipitation and due to the evolution of brittle phase from spinodal decomposition.     

Phase Relations in CaO-SiO2-Al2O3-15 mass pct CaF2 System at 1523 K (1250 °C)

CaO-SiO₂-Al₂O₃-CaF₂ is a base system of mold flux for high Al steels. Phase equilibrium in CaO-SiO₂-Al₂O₃-15 mass pct CaF₂ system at 1523 K (1250 °C) was investigated using quenching method followed by X-ray diffraction and Scanning electron microscopy equipped with energy dispersive X-ray spectroscopy. Isothermal section in this system at 1523 K (1250 °C) with Al₂O₃ being less than 25 mass pct and CaO/SiO₂ (mass pct) being between 0.43 and 1.25 was experimentally constructed. The liquidus composition and seven solid-liquid coexistence regions at 1523 K (1250 °C) were determined.     

Experimental Investigation of the Ce-Mg-Mn Isothermal Section at 723 K (450 °C) via Diffusion Couples Technique

The isothermal section of the Ce-Mg-Mn phase diagram at 723 K (450 °C) was established experimentally by means of diffusion couples and key alloys. The phase relationships in the complete composition range were determined based on six solid–solid diffusion couples and twelve annealed key alloys. No ternary compounds were found in the Ce-Mg-Mn system at 723 K (450 °C). X-ray diffraction and energy-dispersive X-ray spectroscopy spot analyses were used for phase identification. EDS line-scans, across the diffusion layers, were performed to determine the binary and ternary homogeneity ranges. Mn was observed in the diffusion couples and key alloys microstructures as either a solute element in the Ce-Mg compounds or as a pure element, because it has no tendency to form intermetallic compounds with either Ce or Mg. The fast at. interdiffusion of Ce and Mg produces several binary compounds (Ce _x Mg _y ) during the diffusion process. Thus, the diffusion layers formed in the ternary diffusion couples were similar to those in the Ce-Mg binary diffusion couples, except that the ternary diffusion couples contain layers of Ce-Mg compounds that dissolve certain amount of Mn. Also, the ternary diffusion couples showed layers containing islands of pure Mn distributed in most diffusion zones. As a result, the phase boundary lines were pointing toward Mn-rich corner, which supports the tendency of Mn to be in equilibrium with all the phases in the system.     

Experimentally Determined Phase Diagram for the Barium Sulfide-Copper(I) Sulfide System Above 873 K (600 °C)

The phase diagram of the barium sulfide-copper(I) sulfide system was investigated above 873 K (600 °C) using a custom-built differential thermal analysis (DTA) apparatus. The melting point of barium sulfide was determined utilizing a floating zone furnace. Four new compounds, Ba₂Cu₁₄S₉, Ba₂Cu₂S₃, Ba₅Cu₄S₇, and Ba₉Cu₂S₁₀, were identified through quench experiments analyzed with wavelength dispersive X-ray spectroscopy (WDS) and energy dispersive X-ray analysis (EDS). A miscibility gap was observed between 72 and 92 mol pct BaS using both DTA experiments and in situ melts observation in a floating zone furnace. A monotectic was observed at 94.5 mol pct BaS and 1288 K (1015 °C).     

Determination of Gibbs Free Energy of Formation of MnV2O4 Solid Solution at 1823 K (1550 °C)

The Gibbs free energy of formation of MnV₂O₄ solid solution saturated with either MnO or V₂O₃ was experimentally determined at 1823 K (1550 °C) by employing a chemical equilibrium technique. The MnV₂O₄ solid solution mixed with MnO or V₂O₃ was brought to equilibrium with either liquid Fe or liquid Cu to measure equilibrium compositions of the liquid metal. Using the available thermodynamic parameters such as Wagner’s interaction parameter for liquid Fe or Gibbs free energy of liquid solution composed of Cu-Mn-V-O, the Gibbs free energy of formation of the MnV₂O₄ solid solution from constituent oxides (MnO and V₂O₃) was then determined to be: MnO(s) + V₂O₃(s) = MnV₂O₄(MnO-satd.): $\Updelta g^\circ_{{\rm f,MnV}_{2}{\rm O}_{4}}$ = ?31.4 ± 2 (kJ/mol); MnO(s) + V₂O₃(s) = MnV₂O₄(V₂O₃-satd.): $\Updelta g^\circ_{{\rm f,MnV}_{2}{\rm O}_{4}}$ = ?37.8 ± 2 (kJ/mol). The $\Updelta g^\circ_{{\rm f,MnV}_{2}{\rm O}_{4}}$ values were independent of the oxygen partial pressure within the range from 2.0 × 10^?12 to 6.3 × 10^?11, employed in the current study.     

Oxidation Behavior of CuZr-Based Glassy Alloys at 400 °C to 500 °C in Dry Air

The oxidation behavior of the Cu_47.5Zr_47.5Al₅ (Cu3) and Cu₄₇Ti₃₄Zr₁₁Ni₈ (Cu4) bulk metallic glasses (BMGs) was studied over the temperature range of 400 °C to 500 °C in dry air. The oxidation kinetics of both alloys generally followed a multistage parabolic-rate law, and the steady-state parabolic-rate constants (k _p values) fluctuated with temperature for the Cu3 BMG, but increased with increasing temperature for the Cu4 BMG. The scales formed on the BMGs were strongly dependent on the temperature and alloy composition, and were composed primarily of tetragonal-ZrO₂ (t-ZrO₂) and minor amounts of Al₂O₃, Cu₂O, and CuO at 400 °C for the Cu3 BMG, while the monoclinic-ZrO₂ (m-ZrO₂) phase is present at T ≥ 425 °C, and the Cu₂O phase is absent at 500 °C. Conversely, the scales formed on the Cu4 BMG consisted exclusively of CuO at 400 °C, while minor amounts of t-ZrO₂, TiO₂, and ZrTiO₄ formed at 425 °C to 450 °C, and TiO was also detected at higher temperatures. It was found that both amorphous Cu3 and Cu4 substrates transformed into different crystalline phases, and were strongly dependent on temperature and duration of time. This article is based on a presentation given in the symposium entitled “Bulk Metallic Glasses IV,” which occurred during the TMS Annual Meeting February 25–March 1, 2007, in Orlando, Florida under the auspices of the TMS/ASM Mechanical Behavior of Materials Committee.     

Improvement of Creep Resistance at 950 °C and 400 MPa in Ru-Containing Single-Crystal Superalloys with a High Level of Co Addition

Microstructural features, including γ′ volume fraction and size, γ-γ′ lattice misfit, γ channel width, and dislocation substructure, are known to significantly influence the creep performance in Ni-base single-crystal superalloys. In this study, the microstructural characteristics of Ru-containing single-crystal superalloys with different levels of Co, Mo, and Ru additions were quantitatively investigated after ruptured and interrupted creep tests conducted at 1223 K (950 °C) and 400 MPa. The creep lifetime was slightly increased with the high level of Co addition and significantly increased with the coadditions of Mo and Ru. A minor effect of Co content on the γ channel width and γ′ volume fraction was found in experimental alloys. The alloy with high levels of Mo and Ru additions was determined to possess a more negative γ-γ′ lattice misfit, and a high density of stacking faults (SFs) was formed in the γ channels during creep. The combined effects of the SFs in the γ matrix serving as the barriers to dislocation movement, as well as the dense interfacial dislocation networks preventing dislocation to shear the γ′ phase, were considered as the main mechanism responsible for the improvement of creep resistance. Results from this study are helpful to understand the effect of microstructural features on creep performance and contribute to the knowledge of physical metallurgy in Ru-containing single-crystal superalloys.