Cold Consolidation of Ball-Milled Titanium Powders Using High-Pressure Torsion

Pure Ti (99.5 pct) powders after processing with ball milling (BM) were consolidated to disc-shaped samples with 10-mm diameter and 0.8-mm thickness at room temperature using high-pressure torsion (HPT). A relative density as high as 99.9 pct, high bending and tensile strengths of 2.55 to 3.45 and 1.35 GPa, respectively, and a moderate ductility of 8 pct with an ultrafine grained structure are achieved after cold consolidation with HPT, which exceed those of hot consolidation methods. X-ray diffraction (XRD) analysis showed that a phase transformation occurs from α phase to ω phase during HPT under a pressure of 6 GPa as in bulk pure Ti, whereas no phase transformation is detected after processing with BM alone. It was confirmed that the strength and ductility are improved by a combined application of BM and HPT when compared with other severe plastic deformation methods applied to Ti and Ti-6 pct Al-4 pct V, so that no alloying elements are required for the achievement of a comparable strength and ductility.     

Sensitization and Intergranular Corrosion Behavior of High Nitrogen Type 304LN Stainless Steels for Reprocessing and Waste Management Applications

High nitrogen 304LN stainless steels (SS) intended for chloride and nitric acid environments in spent nuclear fuel reprocessing and waste management applications were evaluated for their sensitization and intergranular corrosion (IGC) resistance. For this purpose, high nitrogen (0.132 pct, 0.193 pct and 0.406 pct) containing, impurity-controlled, vanadium-added 304LN SS alloys were developed. For comparison, 304L SS, which is currently used in reprocessing plants, was also studied. These stainless steels were subjected to heat treatment at 948 K (675 °C) for various durations ranging from 1 to 1000 hours and tested for susceptibility to IGC as per ASTM A262 Practice A and E tests. The degree of sensitization was estimated with the double loop electrochemical potentiokinetic reactivation technique. The increase in nitrogen content resulted in higher hardness and finer grain size. Based on the detailed microstructural and corrosion studies, it was determined that an addition of 0.132 pct and 0.193 pct nitrogen showed better IGC resistance and an additional increase in nitrogen resulted in deterioration resulting from chromium nitride precipitation, which was confirmed by electrochemical phase separation and X-ray diffraction studies. The onset of desensitization was faster for the alloy with 0.132 pct nitrogen as well as 0.406 pct nitrogen because of the lower nitrogen content in the former case and the finer grain size in the latter case. The higher hardness and superior IGC resistance of 0.132 pct and 0.193 pct nitrogen containing Type 304LN SS suggests the suitability of this alloy for nitric acid- and chloride-containing environments of reprocessing and waste management plants.     

Effect of Cold Rolling on as–ECAP Interstitial Free Steel

Ti-stabilized interstitial free steel subjected to eight passes, route B_C room temperature equal channel angular pressing (ECAP) additionally was cold rolled (CR) up to 95 pct thickness reduction. Electron back-scattering diffraction and transmission electron microscopy characterized microstructural refinement and microtexture evolution, whereas the mechanical properties were assessed by uniaxial tensile tests. After 95 pct CR, the average high-angle grain boundary spacing reduces to 0.14 μm, whereas the high-angle boundary fraction increases to ~81 pct. The ECAP negative simple shear texture components rotate by ~15 deg around the transverse direction toward the rolling direction for up to 50 pct CR, with typical rolling textures observed at 95 pct CR. The decrease in boundary spacing produces a ~500 MPa gain in 0.2 pct proof stress, a ~600 MPa increase in ultimate tensile strength (UTS), and a ~4 pct loss in total elongation after 95 pct CR. Similar rates of decrease in work hardening correspond to comparable rates of cross and/or multiple slip events irrespective of the processing regime and substructural refinement. The fracture mode of the tensile samples changes from ductile to brittle type between ECAP and 95 pct CR and is attributed to the reduced work hardening capacity of the latter. The modified Hall–Petch equation shows that the convergence of high-angle boundary spacing values with their low-angle counterparts results in an increased contribution via boundary strengthening to the 0.2 pct proof stress and UTS.     

Effects of Fe Addition on the Snoek-Type Damping Behavior of Surface-Oxidation-Treated Ti-Mo Alloys

Effects of Fe addition on the oxygen diffusion and the Snoek-type relaxation damping behavior of the Ti-15 wt pct Mo alloy were investigated in this study. After surface oxidation treatment, the Ti-15 wt pct Mo-1 wt pct Fe alloy exhibits a higher damping capacity compared to the Ti-15 wt pct Mo alloy. The dual-phase zone and the oxygen-enriched β-phase zone in the surface-oxidation-treated Ti-Mo alloys were determined by electron backscattered diffraction (EBSD) and hardness measurements. Based on the oxygen distributions in both alloys obtained through a diffusion model, the relative damping capacity of different zones contributing to the beam sample damping was estimated to be proportional to the thickness of the oxygen dissolved zones. On the other hand, the substitutional solute of Fe in the Ti-Mo-Fe alloy is considered to affect the oxygen distribution by lengthening the oxygen diffusion zone and increasing the oxygen concentration in this zone. As a result, the addition of Fe in Ti-Mo alloy improves the damping capacity of the surface-oxidation-treated alloys.     

Oxidation and Diffusion in Ti-Al-(Mo, Nb) Intermetallics

The oxidation resistance of Ti-Al intermetallics is superior to many of their counterparts at high temperatures. High-temperature stability of these intermetallics appears to improve with a ternary alloying addition such as Mo, Nb, etc. A detailed analysis of oxidation of the Ti₃Al intermetallics, Ti₃Al-2.9 at. pct Mo and Ti₃Al-1.1 at. pct Nb, in pure oxygen using crystallographic and microscopic techniques is presented here. The alloy containing 1.1 at. pct Nb did not show an improvement in oxidation resistance over the base alloy but that containing 2.9 at. pct Mo showed marked resistance to oxidation. Activation energies of oxidation for both the alloys were comparable with those reported in the literature for similar compositions. TiO₂ and Al₂O₃ were the major phases present in the oxide scales of the oxidized alloys. The crystal orientations from the X-ray diffraction (XRD) patterns and the morphologies from scanning electron microscopy (SEM) were in good agreement and were helpful in further analysis of the oxidation process. The effective diffusion of oxygen in the oxide layers was calculated using the mole fraction of individual oxide and the diffusivity of oxygen in pure oxide. The activation energy for the effective diffusion of oxygen through the oxide layers was determined to be ~24 kJ/mol. The activation energy for the oxidation process was higher than that of the diffusion of oxygen. Hence, it can be concluded that the oxidation process in both the alloys studied is not diffusion controlled.     

The Kinetics of the Nitriding of Ternary Fe-2 at pct Cr-2 at pct Ti Alloy

The mechanism and the kinetics of growth of the nitrided zone of ternary Fe-2 at pct Cr-2 at pct Ti alloy was investigated by performing gaseous nitriding experiments at temperatures of 833 K and 853 K (560 °C and 580 °C) and at nitriding potentials r _N = 0.004 atm^−1/2 and 0.054 atm^−1/2. The microstructure of the nitrided zone was investigated by transmission electron microscopy and the elemental compositional variation with depth was determined by employing electron probe microanalysis. Fine platelet-type mixed Cr_{1 – x} Ti _x N nitride precipitates developed in the nitrided zone. To describe the evolution of the nitrogen concentration depth profile, a numerical model was developed with the following parameters: the surface nitrogen content, the solubility product(s) of the alloying elements and dissolved nitrogen in the ferrite matrix, and a parameter defining the composition of the inner nitride precipitates. These parameters were determined by fitting model-calculated nitrogen depth profiles to the corresponding experimental data. The results obtained demonstrate that the type of nitride formation (i.e., whether Cr and Ti precipitate separately, as CrN and TiN, or jointly, as mixed Cr_{1 – x} Ti _x N) as well as the amounts of mobile and immobile excess nitrogen taken up by the specimen considerably influence the shape and extent of the nitrogen concentration profiles.     

Nanostructured Ti Consolidated via Spark Plasma Sintering

Cryomilled nanocrystalline commercially pure (CP)-Ti powders were spark plasma sintered (SPS) using different process parameters (heating rate, temperature, pressure, and dwell time) to study densification, microstructure, and mechanical behavior. The results were rationalized on the basis of the relevant literature and experimental results, and they reveal a strong dependence on SPS parameters. An interesting finding was that the measured high ductility was accompanied by a moderate strength (yield strength [YS] = 770 MPa, ultimate tensile strength [UTS] = 840 MPa with ~27 pct elongation to failure). The combinations of microstructure and mechanical response were attributed to the multistep processing at different temperature ranges as well as to the presence of interstitial solutes.     

Numerical Simulations of Carbon and Nitrogen Composition-Depth Profiles in Nitrocarburized Austenitic Stainless Steels

Unusual composition-depth profiles have been observed after low-temperature nitrocarburization of austenitic stainless steels. When nitridation is performed after carburization, the carbon concentration in the nitrogen diffusion zone is reduced from ≈10 to ≈2 at. pct. Conversely, the carbon concentration in advance of the nitrogen diffusion zone is as high as 10 at. pct. This has been called a “push” effect of nitrogen on carbon, but this concept is non-physical. The profiles can be better understood from conventional thermodynamic principles, recognizing that (1) diffusion always occurs in response to gradients in chemical potentials and (2) the diffusivity of interstitial solutes in austenite is strongly concentration dependent, increasing dramatically with higher solute concentrations. Parameters from the CALPHAD literature quantitatively indicate that interstitial nitrogen and carbon in austenitic stainless steel mutually increase their chemical potentials. Based on these data, we have conducted numerical simulations of composition-depth profiles that correctly account for the chemical potential gradients and the concentration dependence of the diffusion coefficients for nitrogen and carbon. The simulations predict the “push” effect observed on nitridation after carburization, as well as the corresponding composition-depth profiles for other scenarios, e.g., carburization followed by nitridation or simultaneous nitridation and carburization (nitrocarburization).     

Texture and Formability of One-Step and Two-Step Cold-Rolled and Annealed Interstitial Free High-Strength Steel Sheets

In the current experimental investigation, a comparative study has been carried out to understand the development of texture under different conditions, viz. controlled hot-rolled, one-step cold-rolled (CR) and annealed, and two-step cold-rolled and annealed conditions. Mechanical properties were determined and experimental formability limit diagrams (FLDs) were plotted after both one-step and two-step cold-rolled and annealed conditions. From the tabulated data, it was found that hot band texture of 85 to 90 pct deformation was strong and the main orientations were ( 112 )[ 1[`1]0 ] \left( {112} \right)\left[ {1\bar{1}0} \right] and ( 332 )[ [`1][`1]3 ]. \left( {332} \right)\left[ {\bar{1}\bar{1}3} \right]. One-step cold rolling developed the strong and uniform α-fiber and γ-fiber at 80 pct cold reduction. The strong and uniform new γ-fiber was obtained at the one-step 80 pct CR annealed condition. In addition, the highest drawability was found at the one-step 80 pct CR annealed condition. In two-step cold rolling, orientation ( 223 )[ 1[`1]0 ] \left( {223} \right)\left[ {1\bar{1}0} \right] was the main texture component along with extremely strong γ-fiber. Moreover, batch annealing of two-step cold-rolled steel sheets developed exceptionally strong and uniform γ-fiber, and all mechanical properties were enhanced significantly except yield strength. From FLDs, it is observed that the formability properties of interstitial free (IF) high-strength (HS) steel sheets were excellent at both one-step and two-step cold-rolled and annealed conditions. However, the two-step cold-rolling and annealing process was found to be superior to the one-step process. The data of this investigation may be used at the industrial level to design the entire processing of IF-HS steel sheets.     

Theoretical Estimation of Temperature-Dependent Surface Tension of Liquid Antimony,Boron, and Sulfur

A theoretical calculation of the temperature-dependent surface tension of liquid antimony, boron, and sulfur in the temperature ranges 904 K to 1200 K (631 °C to 927 °C), 2349 K to 3085 K (2076 °C to 2812 °C) and 388 K to 453 K (115 °C to 180 °C), respectively, in the framework of Eyring theoretical consideration is explored. A reasonable agreement between the calculated and measured reported values is detected. The surface tensions of the three elements decrease linearly with temperature, with a change of 5.7 pct, 15 pct, and 16 pct, and the calculated surface tension data are expressed as γ = 388–0.0749 (T-Tm), γ = 1200–0.259 (T-Tm), and γ = 71–0.183 (T-Tm) (mJ/m²) for Sb, B, and S, respectively. Moreover, the surface tension was calculated, at melting point, for liquid metalloids and liquid nonmetals.     

Processing and Characterization of Cu-Al-Ni Shape Memory Alloy Strips Prepared from Elemental Powders <Emphasis Type="Italic">via</Emphasis> a Novel Powder Metallurgy Route

In the present work, Cu-Al-Ni shape memory alloy strips were prepared successfully from premixed elemental Cu, Al, and Ni powders in the ratio 82:14:4 (wt pct) by a novel processing route consisting of preparing powder preforms, sintering, and unsheathed hot rolling of the sintered preforms. Subsequently, the hot rolled strips were homogenized. The as-rolled strips consisted of two phases—α and β′. A postconsolidation homogenization of the hot rolled strips was carried out at 1173 K (900 °C) for different time periods. It has been shown that a homogenization period of 4 hours was sufficient to achieve a single-phase material consisting of only martensitic phase. It also has been shown that the 4-hour homogenized and quenched Cu-Al-Ni shape memory alloy strips primarily consisted of self-accommodated β′ martensite plates, which are necessary for realizing shape memory effect (SME). The finished hot rolled Cu-Al-Ni strips had a fracture strength of 476 MPa, coupled with 2.5 pct elongation. The shape memory tests showed almost 100 pct recovery after 10 thermomechanical cycles in the hot rolled strips at 1 pct and 2 pct prestrain level.     

Influence of Chemical Composition of Mg Alloys on Surface Alloying by Diffusion Coating

A recently developed technique of surface alloying by diffusion-coating has been used to produce coatings on Mg alloys with various Al and Zn contents. The experimental results show that both Al and Zn solutes in the alloy promote the diffusion of alloying elements through grain refinement of the substrate alloys and through reduction of diffusion active energy because of the reduction of melting temperature of the alloys. Therefore, the efficiency of surface alloying increases by diffusion coating. Thick, dense, uniform, and continuous layers of intermetallic compounds, which consist of a τ-phase layer and a β-phase layer, can be produced on the surface of various Mg alloys. The intermetallic compound layers not only have microhardness values that are 4 to 6 times higher than the substrate but also provide effective protection of the Mg alloys from corrosion in 5 pct NaCl solution at room temperature.     

Anomalous Mixing State in Room-Temperature Ionic Liquid-Water Mixtures: <Emphasis Type="Italic">N,N</Emphasis>-diethyl-<Emphasis Type="Italic">N</Emphasis>-methyl-<Emphasis Type="Italic">N</Emphasis>-(2-methoxyethyl) Ammonium Tetrafluoroborate

The mixing states of room-temperature ionic liquid (RTIL) H₂O mixtures (x = 0.0 mol pct to 99.5 mol pct H₂O) were investigated using wide-angle X-ray scattering (WAXS), small-angle X-ray scattering (SAXS), and optical absorption in an ultraviolet and visible (UV-vis) region. The RTIL is N, N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium tetrafluoroborate, [DEME][BF₄]. In a “prepeak” region of the WAXS, the scattered intensities increased at 85 mol pct to 95 mol pct. A medium-range order (MRO) in the liquid structure as observed in network-forming materials developed markedly. In the SAXS experiments, we can detect nanoscale fluctuations relating to polar and nonpolar regions. At 65 mol pct to 85 mol pct, the SAXS intensity increased unexpectedly. Furthermore, entirely different optical absorption spectra in the UV-vis region were observed as a macroscopic property from 90 mol pct to 95 mol pct. We suppose that these anomalies relate to the MRO of the liquid structure. All anomalies probably are induced by an intrinsic property in [DEME][BF₄]-H₂O mixtures.     

Controlled dissolution of colossal quantities of nitrogen in stainless steel

The solubility of nitrogen in austenitic stainless steel was investigated thermogravimetrically by equilibrating thin foils of AISI 304 and AISI 316 in ammonia/hydrogen gas mixtures. Controlled dissolution of colossal amounts of nitrogen under metastable equilibrium conditions was realized, with nitrogen contents as high as corresponding to an occupancy of y _N=0.61 of the interstitial sublattice, i.e., about 38 at. pct N. Associated with the dissolution of these unprecedented nitrogen contents in an austenitic matrix a reversible volume expansion of the austenite lattice occurred for y _N > 0.17. A simplistic model based on a statistical distribution fo the nitride forming elements over the octahedrons constituting the solid state agrees favorably with the experimental data.     

Selective Oxidation and Reactive Wetting of 1.0 Pct Si-0.5 Pct Al and 1.5 Pct Si TRIP-Assisted Steels

Heat treatments were performed using an isothermal bainitic transformation (IBT) temperature compatible with continuous hot-dip galvanizing on two high Al–low Si transformation induced plasticity (TRIP)-assisted steels. Both steels had 0.2 wt pct C and 1.5 wt pct Mn; one had 1.5 wt pct Al and the other had 1 wt pct Al and 0.5 wt pct Si. Two different intercritical annealing (IA) temperatures were used, resulting in intercritical microstructures of 50 pct ferrite (α)-50 pct austenite (γ) and 65 pct α-35 pct γ. Using the IBT temperature of 465 °C, five IBT times were tested: 4, 30, 60, 90, and 120 seconds. Increasing the IBT time resulted in a decrease in the ultimate tensile strength (UTS) and an increase in the uniform elongation, yield strength, and yield point elongation. The uniform elongation was higher when using the 50 pct α-50 pct γ IA temperature when compared to the 65 pct α-35 pct γ IA temperature. The best combinations of strength and ductility and their corresponding heat treatments were as follows: a tensile strength of 895 MPa and uniform elongation of 0.26 for the 1.5 pct Al TRIP steel at the 50 pct γ IA temperature and 90-second IBT time; a tensile strength of 880 MPa and uniform elongation of 0.27 for the 1.5 pct Al TRIP steel at the 50 pct γ IA temperature and 120-second IBT time; and a tensile strength of 1009 MPa and uniform elongation of 0.22 for the 1 pct Al-0.5 pct Si TRIP steel at the 50 pct γ IA temperature and 120-second IBT time.     

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.     

Microstructural Evolution and Bonding Behavior during Transient Liquid-Phase Bonding of a Duplex Stainless Steel using two Different Ni-B-Based Filler Materials

Microstructural evolution and bonding behavior of transient liquid-phase (TLP) bonded joint for a duplex stainless steel using MBF-30 (Ni-4.5Si-3.2B [wt pct]) and MBF-50 (Ni-7.5Si-1.4B-18.5Cr [wt pct]) were investigated. Using MBF-30, the microstructure of the athermally solidified zone was dependent on B diffusion at 1333.15 K (1060 °C). Ni₃B and a supersaturated γ-Ni phase were observed in this zone. BN appeared in the bonding-affected zone. However, using MBF-50, the influences of base metal alloying elements, particularly N and Cr as well as Si in the filler material, on the bond microstructure development were more pronounced at 1448.15 K (1175 °C). BN and (Cr, Ni)₃Si phase were present in the bond centerline. The formation of BN precipitates in the bonding-affected zone was suppressed. A significant deviation in the isothermal solidification rate from the conventional TLP bonding diffusion models was observed in the joints prepared at 1448.15 K (1175 °C) using MBF-50.     

Diffusion of cobalt,chromium, and titanium in Ni3Al

Diffusion studies of cobalt, chromium, and titanium in Ni₃Al (γ′) at temperatures between 1298 and 1573 K have been performed using diffusion couples of (Ni-24.2 at. pct Al/Ni-24.4 at. pct Al-2.91 at. pct Co), (Ni-24.2 at. pct Al/Ni-23.1 at. pct Al-2.84 at. pct Cr), and (Ni-24.2 at. pct Al/Ni-20.9 at. pct Al-3.17 at. pct Ti). The diffusion profiles were measured by an electron probe microanalyzer, and the diffusion coefficients of cobalt, chromium, and tita-nium in γ′ containing 24.2 at. pct Al were determined from those diffusion profiles by Hall’s method. The temperature dependencies of their diffusion coefficients (m[su2]/s) are as follows: ~D_(Co) = (4.2 ± 1.2) × 1O^-3exp {-325 ± 4 (kJ/mol)/RT} ~D_(Cr) = (1.1 ± 0.3) × 10^-1 exp {-366 ± 3 (kJ/mol)/RT} and D_(Ti) = (5.6 ± 3.1) × 10¹ exp {-468 ± 6 (kJ/mol)/RT} The values of activation energy increase in this order: cobalt, chromium, and titanium. These activation energies are closely related to the substitution behavior of cobalt, chromium, and titanium atoms in the Ll₂ lattice sites of γ′; the cobalt atoms occupying the face-centered sites in the Ll₂ structure diffuse with the normal activation energy, whereas the titanium atoms oc-cupying the cubic corner sites diffuse with a larger activation energy that includes the energy due to local disordering caused by the atomic jumps. The chromium atoms which can occupy both sites diffuse with an activation energy similar to that of cobalt atoms.     

Microstructure and Thickness of 55 pct Al-Zn-1.6 pct Si-0.2 pct RE Hot-Dip Coatings: Experiment, Thermodynamic, and First-Principles Study

The microstructure and thickness of 55 pct A1-Zn-1.6 pct Si-0.2 pct RE coatings during continuous hot-dip on Q235 steel were investigated in this work. The experimental results revealed that the intermetallic layer was composed of the Fe₂Al₅, FeAl₃, and α-FeAlSi phases. The results of thermodynamic calculations with Pandat software package (CompuTherm, LLC, Madison, WI) indicated that FeAl₃ and α(β)-FeAlSi phase precipitated during the period of temperature cooling, which was consistent with experimental result. Then, the thickness of intermetallic layer was characterized. It was shown that the thickness of intermetallic layer decreased after 0.2 wt pct RE was added. Finally, a first-principles calculation was performed to interpret the effect mechanism of RE on the thickness of intermetallic layer. The results indicated that La substitution in Fe₂Al₅ and FeAl₃ phases could grab electronic charges from Al atoms and weaken the formation of Fe-Al compounds.