Microstructural Evolution of the 55 Wt Pct Al-Zn Coating During Press Hardening

Press hardening is increasingly being used to produce ultra-high strength steel parts for passenger cars. Al-Si, Zn, and Zn-alloy coatings have been used to provide corrosion protection to press hardening steel grades. The use of coatings has drawbacks such as coating delamination or liquid metal-induced embrittlement. In the present work, the microstructural evolution of Al-Zn coating during press hardening was studied. The 55 wt pct Al-Zn coating can in principle provide both Al barrier protection and Zn cathodic protection to press hardened steel. During the heat treatment associated with the press hardening, the 55 wt pct Al-Zn alloy coating is converted to an intermetallic surface layer of Fe₂Al₅ and a FeAl intermetallic diffusion layer. The Zn is separated from both intermetallic compounds and accumulates at grain boundaries and at the surface. This Zn separation process is beneficial in terms of providing cathodic protection to Al-Zn coated press hardening steel.     

Effects of Route on Microstructural Evolution and Mechanical Properties of Cu-8 Wt Pct Ag Alloy Processed by Equal Channel Angular Pressing

Equal channel angular pressing (ECAP) is applied to investigate the microstructural evolution and mechanical properties of Cu-8 wt pct Ag alloy subjected to one to four passes via four different routes (A, B_A, B_C, and C). It is demonstrated that better mechanical properties, a higher fraction of high-angle boundaries, and a smaller grain size can be obtained most rapidly with route A, whereas the specimen processed by route B_C contains relatively inhomogeneous microstructure and has poor mechanical properties. The ultimate tensile stress (UTS) of the Cu-Ag alloy processed by route B_C saturates after four passes; in contrast, the UTS of the Cu-Ag alloy processed by route A increases continuously in relation to the number of ECAP passes. Based on the experimental results, the strengthening mechanisms of the Cu-8 wt pct Ag alloy processed by different routes as well as the efficiency of different routes in refining the binary alloy are discussed.     

Tempering-Induced Microstructural Changes in the Weld Heat-Affected Zone of 9 to 12 Pct Cr Steels and Their Influence on Sliding Wear

Increasing amount of tribological applications is working under alternating high/low temperature conditions where the material is subjected to temperature fatigue mechanisms such as creep, softening due to annealing, and at the same time must withstand mechanical wear due to sliding contact with pairing bodies. Steam turbine valves, gate valves, valve heads, stems, seats and bushings, and contacting surfaces of the carrier elements are some examples of such applications. The purpose of the present study is to evaluate the potential of X20 and P91 steels as materials for applications operating under combined effect of mechanical wear and alternating high/low temperature conditions. It was focused on how the microstructural changes occurring in the weld zone affect the wear properties of the selected materials. Generally, with longer tempering time and higher tempering temperature, the number of carbide precipitates decreased, while their relative spacing increased. Before tempering, the morphology of the steel matrix (grain size, microstructure homogeneity) governed the wear resistance of both steels, while after tempering wear response was determined by the combination of the number and the size of carbide particles. After tempering, in X20 steel larger number of stable M₂₃C₆ carbides was observed as compared with P91 steel, resulting in lower wear rates. It was observed that for both steels, a similar combination of number density and size distribution of carbide particles provided the highest wear resistance.     

Electrolytic Deposition and Diffusion of Lithium onto Magnesium-9 Wt Pct Yttrium Bulk Alloy in Low-Temperature Molten Salt of Lithium Chloride and Potassium Chloride

The electrolytic deposition and diffusion of lithium onto bulk magnesium-9 wt pct yttrium alloy cathode in molten salt of 47 wt pct lithium chloride and 53 wt pct potassium chloride at 693 K were investigated. Results show that magnesium-yttrium-lithium ternary alloys are formed on the surface of the cathodes, and a penetration depth of 642 μm is acquired after 2 hours of electrolysis at the cathodic current density of 0.06 A·cm⁻². The diffusion of lithium results in a great amount of precipitates in the lithium containing layer. These precipitates are the compound of Mg₄₁Y₅, which arrange along the grain boundaries and hinder the diffusion of lithium, and solid solution of yttrium in magnesium. The grain boundaries and the twins of the magnesium-9 wt pct yttrium substrate also have negative effects on the diffusion of lithium.     

Electropulsing Induced Texture Evolution in the Recrystallization of Fe-3 Pct Si Alloy Strip

Electropulsing induced texture evolution in the primary recrystallization of a Fe-3 pct Si alloy strip was studied using the electron backscattered diffraction technique. The results revealed that the electropulsing strengthened considerably the recrystallization of a cold-rolled Fe-3Si alloy strip. Various textures with high-energy storages, such as α (100)〈110〉, γ (111)〈110〉, γ (111)〈112〉, and G-texture (110)〈001〉, formed after several seconds of electropulsing treatment (EPT), depending on the intensity of electropulsing. The athermal effect of electropulsing is 319 times stronger than the thermal effect of electropulsing for the formation of the G texture. The mechanism of electopulsing induced texture evolution is discussed from the point of view of Gibbs free energy and dislocation dynamics.     

A Study on the Oxidation Behavior of Nb Alloy (Nb-1 pct Zr-0.1 pct C) and Silicide-Coated Nb Alloys

In the current work, silicide coatings were produced on the Nb alloy (Nb-1 pct Zr-0.1 pct C) using the halide activated pack cementation (HAPC) technique. Coating parameters (temperature and time) were optimized to produce a two-layer (Nb₅Si₃ and NbSi₂) coating on the Nb alloy. Subsequently, the oxidation behavior of the Nb alloy (Nb-1 pct Zr-0.1 pct C) and silicide-coated Nb alloy was studied using thermogravimetric analysis (TGA) and isothermal weight gain oxidation experiments. Phase identification and morphological examinations were carried out using X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. TGA showed that the Nb alloy started undergoing accelerated oxidation at and above 773 K (500 °C). Isothermal weight gain experiments carried out on the Nb alloy under air environment at 873 K (600 °C) up to a time period of 16 hours exhibited a linear growth rate law of oxidation. In the case of silicide-based coatings, TGA showed that oxidation resistance of silicide coatings was retained up to 1473 K (1200 °C). Isothermal weight gain experiments on the silicide coatings carried out at 1273 K (1000 °C) in air showed that initially up to 8 hours, the weight of the sample increased, and beyond 8 hours the weight of the sample remained constant. The oxide phases formed on the bare samples and on the coated samples during oxidation were found to be Nb₂O₅ and a mixture of SiO₂ and Nb₂O₅ phases, respectively. SEM showed the formation of nonprotective oxide layer on the bare Nb alloy and a protective (adherent, nonporous) oxide layer on silicide-coated samples. The formation of protective SiO₂ layer on the silicide-coated samples greatly improved the oxidation resistance at higher temperatures.     

Microstructural Evolution and Mechanical Property of AA5050 Alloy Deformed by Accumulative Roll Bonding

In this study, ultrafine-grained AA5050 sheets were fabricated by the accumulative roll bonding (ARB) process. Transmission electron microscope observations showed that at the early stage of ARB, the grain size was reduced in the normal direction and became elongated along the rolling direction. The elongated grains were cut out by dense dislocations, which then tangled and condensed, resulting in the formation of dislocation cells. As the deformation proceeded, the dislocation cells evolved to sub-grain boundaries and then grain boundaries. The ultrafine-grained microstructure was obtained via four ARB cycles. The tensile tests at 473 K and 523 K (200 °C and 250 °C) showed large elongations for strain rates of 1 × 10³ s^?1 and 1 × 10⁴ s^?1.     

Effect of Ti Addition and Microstructural Evolution on Toughening and Strengthening Behavior of as Cast or Annealed Nb–Si–Mo Based Hypoeutectic and Hypereutectic Alloys

Room temperature fracture toughness along with compressive deformation behavior at both room and high temperatures (900 °C, 1000 °C and 1100 °C) has been evaluated for ternary or quaternary hypoeutectic (Nb–12Si–5Mo and Nb–12Si–5Mo–20Ti) and hypereutectic (Nb–19Si–5Mo and Nb–19Si–5Mo–20Ti) Nb-silicide based intermetallic alloys to examine the effects of composition, microstructure, and annealing (100 hours at 1500 °C). On Ti-addition and annealing, the fracture toughness has increased by up to ~ 75 and ~ 63 pct, respectively with ~ 14 MPa√m being recorded for the annealed Nb–12Si–5Mo–20Ti alloy. Toughening is ascribed to formation of non-lamellar eutectic with coarse Nb_ss, which contributes to crack path tortuosity by bridging, arrest, branching and deflection of cracks. The room temperature compressive strengths are found as ~ 2200 to 2400 MPa for as-cast alloys, and ~ 1700 to 2000 MPa after annealing with the strength reduction being higher for the hypoeutectic compositions due to larger Nb_ss content. Further, the compressive ductility has varied from 5.7 to 6.5 pct. The fracture surfaces obtained from room temperature compression tests have revealed evidence of brittle failure with cleavage facets and river patterns in Nb_ss along with its decohesion at non-lamellar eutectic. The compressive yield stress decreases with increase in test temperature, with the hypoeutectic alloys exhibiting higher strength retention indicating the predominant role of solid solution strengthening of Nb_ss. The flow curves obtained from high temperature compression tests show initial work hardening, followed by a steady state regime indicating dynamic recovery involving the formation of low angle grain boundaries in the Nb_ss, as confirmed by electron backscattered diffraction of the annealed Nb–12Si–5Mo alloy compression tested at 1100 °C.

     

Microstructural and Orientation Dependence of the Plastic Deformation Behavior in β-type Ti-15Mo-5Zr-3Al Alloy Single Crystals

The plastic deformation behavior of a β-type Ti-15Mo-5Zr-3Al alloy with a body-centered cubic (bcc) structure, which is a promising material for biomedical applications, was investigated. The orientation dependence of the plastic deformation behavior was examined by using a single crystal. In addition, changes in the mechanical properties depending on the microstructure were examined. The β single phase was maintained even after short-time annealing below 673 K (400 °C). Thus, the variations in the mechanical properties were small. However, an ellipsoidal ω phase and a lath-like α phase were precipitated in long-time annealing at 573 K (300 °C) and 673 K (400 °C), leading to large increases in the yield stress. For the deformation behavior, a dislocation with a Burgers vector parallel to á 111 ñ \left\langle {111} \right\rangle was observed irrespective of the heat-treatment conditions and loading orientations. However, the observed slip plane changed considerably depending on the loading axis, and the yield stress exhibited a strong orientation dependence because of the dislocation core structure effect in the bcc-structured crystals. The physical properties of Mo, which is the main constituent atom in the current alloy, may strongly affect the dislocation core structure and induce the characteristic orientation dependence of the plastic behavior.     

A Comparative Study of Ratcheting Fatigue Behavior of Modified 9Cr–1Mo Steel and Inconel 617 Alloy at Homologous Temperature of 0.42

The modified 9Cr–1Mo steel and Inconel (IN) 617 superalloy are high-temperature alloys of prime importance in several industrial applications. Both of these are used at elevated temperature for various piping and tubing components and undergo asymmetric cyclic loading; therefore, it is highly essential to assess their resistance to accumulation of inelastic strain (ratcheting) under asymmetric stress cycling. In this study, ratcheting behavior of these alloys is compared at the homologous temperature of 0.42, under uniaxial asymmetric stress loading with positive mean stress. Deformation behavior of the tested samples is analyzed by transmission electron microscope. While there is homogeneous deformation in the modified 9Cr–1Mo steel, there is intersection of slip bands and presence of dislocation tangles in the IN 617 alloy. Fracture surface of the modified 9Cr–1Mo steel, tested under asymmetric cyclic loading, reveals typical ductile fracture resulting from tensile loading, whereas there is characteristic fatigue fracture of the IN 617 alloy.     

Microstructural Evolution and Mechanical Behaviors of an Nb-16Si-22Ti-2Al-2Hf Alloy with 2 and 17 at. pct Cr Additions at Room and/or High Temperatures

X-ray diffraction analysis, scanning electron microscopy, and transmission electron microscopy were used to investigate the microstructures and orientation relationships (ORs) of Nb-16Si-22Ti-2Al-2Hf-(2,17)Cr alloys (hereafter referred to as 2Cr and 17Cr alloys, respectively). The mechanical properties of the two alloys at room and/or high temperatures were compared. The 2Cr alloy comprised Nb_SS and (α + β)-Nb₅Si₃ phases, while the 17Cr alloy consisted of Nb_SS, (α + β)-Nb₅Si₃ and Laves Cr₂Nb phases with a C15 structure. The β-Nb₅Si₃ and Laves Cr₂Nb phases exhibited variable ORs with respect to the Nb_SS phase. The Laves Cr₂Nb phase was found to play a negative role on the fracture toughness at room temperature and on the compressive strength at temperatures from 1523 K to 1623 K (1250 °C to 1350 °C). The fracture toughness and the compressive yield strength at 1623 K (1350 °C) both decreased from 14.4 to 10.3 MPa m^1/2 and from 300 to 85 MPa, respectively, when the nominal Cr content increased from 2 to 17 at. pct. Finally, the fracture modes of these typical Nb_SS/Nb₅Si₃ and Nb_SS/Nb₅Si₃/Cr₂Nb microstructures under bending and compression conditions at room and high temperatures were investigated and discussed.