Closure-affected fatigue crack propagation behaviors of powder metallurgy-processed Al-Li alloys in various environments

The environment-affected fatigue crack propagation (FCP) behavior of rapid solidification-processed (RSP) Al-2.6Li-1.0Cu-0.5Mg-0.5Zr (RSP 644-B) and mechanically alloyed (MA) Al-4.0Mg-1.5Li-1.1C-0.8O₂ (MA 905-XL) were examined in air, in vacuum, and in an aqueous 3.5 pct NaCl solution at R=0.1 and a sinusoidal frequency of 20 Hz. The emphasis was placed on the effect of environment-sensitive crack closure on the FCP behavior of fine-grain-sized powder metallurgy (P/M)-processed Al-Li alloys. The present study suggests that closure is extremely sensitive to environmental factors and significantly alters the environment-affected da/dN-ΔK relationships for both alloys. In the submicron grain-sized MA 905-XL, for example, increased corrosion product-induced closure in aqueous NaCl appeared to overwhelm the detrimental environmental effects in low- and intermediate-ΔK regimes. The environment-sensitive closure contribution alone, however, cannot completely explain the FCP behavior of P/M-processed Al-Li alloys. The intrinsic environmental effects also need to be considered for further understanding of this behavior.     

Amorphization of Ti1−x Mn x

Three amorphous Ti_1−x Mn _x alloy powders, withx = 0.4, 0.5, and 0.6, were prepared by mechanical alloying (MA) of the elemental powders in a high-energy ball mill. The amorphous powders were characterized by X-ray diffraction (XRD) and high-resolution transmission elec- tron microscopy (HRTEM). The crystallization temperatures for these alloys detected by dif- ferential scanning calorimetry (DSC) varied from 769 to 830 K. The calculated enthalpies of mixing in these amorphous phases are relatively small compared with those for other Ti-base binary alloys. The criteria for solid-state amorphization reaction are examined. It is suggested that the kinetics of nucleation and growth favors the formation of the amorphous phases and the supply of atoms for nucleation and growth is predominantly through the defective regions induced by MA. Formerly Graduate Student, National Tsing Hua University     

Microstructure development in finely atomized droplets of copper-iron alloys

The solidification behavior of Cu_(100-X)FeX (X = 15, 20, 30, and 40) alloys was investigated by gas atomization technology. The effects of the size and composition of the atomized droplet on the microstructure development during cooling through the metastable miscibility gap have been discussed. A smaller atomized droplet achieves a finer dispersed microstructure. Alloys of composition close to the critical composition of the alloy system are relatively easy to undercool into the miscibility gap. The forces acting on the Fe-rich sphere during the liquid-liquid phase transformation were analyzed. The formation of an Fe-poor layer on the powder surface is the result of the common action of the Fe-rich sphere’s Marangoni migration and the repulsive interaction between the cellular solid/liquid interface and the solidified Fe-rich sphere. The Fe-rich spheres in the center part of the powder are entrapped between the equiaxed grains of Cu-rich phase and finally distributed at the grain boundaries and triple junctions.     

Thermally assisted and mechanically driven solid-state reactions for formation of amorphous AI33Ta67 alloy powders

The rod milling technique using the mechanical alloying (MA) process has been employed for preparing amorphous Al₃₃Ta₆₇ alloy starting from elemental Al and Ta powders. X-ray diffraction (XRD), differential thermal analysis (DTA), differential scanning calorimetry (DSC), optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) are utilized to follow the progress of amorphization. The results show that during the first few kiloseconds of MA time, layered composite particles of Al and Ta are intermixed and form an amorphous phase upon heating to 685 K by DTA. This process is called thermally assisted solid-state amorphization (TASSA). During the early stage of milling, the number of layers of the composite particles increases. This leads to an increase in the heat formation of amorphous Al₃₃Ta₆₇ alloyvia the TASSA process, ΔH _TASSA ^a . After 360 ks (100 h) of the MA time, all Al atoms emigrate to Ta lattices to form a solid solution phase and the powder particles have no more layered structure. At this stage of milling, the value of ΔH _TASSA ^a becomes zero. This solid solution phase is not stable against the shear forces that are generated by the rods and transforms completely to an amorphous phase upon milling for 720 ks (200 hours). This phase transformation is attributed to the accumulation of several lattice imperfections, such as point and lattice defects, which raise the free energy from the more stable phase (solid solution) to a less stable phase (amorphous). After 1440 ks (400 hours) of MA time, a homogeneous amorphous phase is formed. The amorphization process in this case is attributed to a mechanical driven solid-state amorphization (MDSSA). The heat of formation of the amorphous phase formedvia the MDSSA process, ΔH _MDSSA ^a , has been calculated. Moreover, the crystallization characteristics indexed by the crystallization temperature, and the enthalphy of crystallization, of the amorphous phases formed by TASSA and MDSSA processes are investigated as a function of MA time. The role of amorphizationvia each process has been discussed. Formerly lecturer of Materials Science, Department of Mining and Petroleum Engineering, Faculty of Engineering, AI-Azhar University, Nasr City 11884, Cairo, Egypt     

Thermodynamic Basis for Glass Formation in Cu-Zr Rich Ternary Systems and Their Synthesis by Mechanical Alloying

Topological factors such as mismatch entropy and configurational entropy, along with thermodynamic entity such as enthalpy of chemical mixing, are found to control glass formation in metallic systems. Taking both these factors into consideration, a parameter called P _HS was proposed to correlate glass forming ability successfully in the Cu-Zr-Ti system. The parameter P _HS (=?H _chem × ?S _σ /k _B ) is a product of enthalpy of chemical mixing and mismatch entropy. Our study indicates that the more negative is the P_HS value within the configurational entropy (?S _config/R) range of 0.9 to 1.0, the higher is the stability of glassy phase resulting in a larger diameter of bulk metallic glass rods. Observed theoretical predictions are supported by experimental results in which the compositions with high negative P _HS resulted in easy amorphous phase formation in comparison with less negative P _HS compositions by mechanical alloying. This criterion was extended to Cu-Zr-Al and Cu-Zr-Ag systems as well, thus establishing a strong correlation between P _HS and the glass forming ability of alloys. The role of size effect, probability of atomic arrangements, and heat of formation among constituent elements in obtaining a larger dimension bulk metallic glasses was addressed in this study.     

Structure, Properties, and Glass Forming Ability of Melt-Spun Fe-Zr-B-Cu Alloys with Different Zr/B Ratios

Melt-spun ribbons of Fe_99–x–y Zr _x B _y Cu₁ alloys with x + y = 11 and x + y = 13 were prepared under similar experimental conditions and characterized for structure and soft magnetic properties. Substitution of Zr by B changes the structure of as-spun ribbons from completely amorphous to cellular bcc solid solution coexisting with the amorphous phase at intercellular regions and then to completely dendritic solid solution. The glass forming ability (GFA) of the Fe-Zr-B-Cu system, evaluated from thermodynamic properties such as enthalpy of mixing and mismatch entropy, is found to be in good agreement with the experimental observations. Annealing of all ribbons leads to the precipitation of nanocrystalline bcc α-Fe phase from both amorphous phase and already existing bcc solid solution. A window of alloy compositions that exhibit the best combination of soft magnetic properties (high saturation magnetization and low coercivity) was identified.     

Bulk amorphous metallic alloys: Synthesis by fluxing techniques and properties

Bulk amorphous alloys having dimensions of at least 1 cm in diameter have been prepared in the Pd-Ni-P, Pd-Cu-P, Pd-Cu-Ni-P, and Pd-Ni-Fe-P systems using a fluxing and water-quenching technique. The compositions for bulk glass formation have been determined in these systems. For these bulk metallic glasses, the difference between the crystallization temperature (T _x) and the glass transition temperature (T _g, ΔT=T _x−T _g) ranges from 60 to 110 K. These large values of ΔT open the possibility for the fabrication of amorphous near-net-shaped components using techniques such as injection molding. The thermal, elastic, and magnetic properties of these alloys have been studied, and we have found that bulk amorphous Pd₄₀Ni_22.5Fe_17.5P₂₀ has spin glass behavior for temperatures below 30 K. This article is based on a presentation made in the “Structure and Properties of Bulk Amorphous Alloys” Symposium as part of the 1997 Annual Meeting of TMS at Orlando, Florida, February 10–11, 1997, under the auspices of the TMS-EMPMD/SMD Alloy Phases and MDMD Solidification Committees, the ASM-MSD Thermodynamics and Phase Equilibria, and Atomic Transport Committees, and sponsorship by the Lawrence Livermore National Laboratory and the Los Alamos National Laboratory.     

Preparation and thermal stability of Zr-Ti-Al-Ni-Cu amorphous powders by mechanical alloying technique

This study examined the amorphization feasibility of Zr_70−x−y Ti _x Al _y Ni₁₀Cu₂₀ alloy powders by the mechanical alloying (MA) technique. According to the results, after 5 to 7 hours of milling, the mechanically alloyed powders were amorphous basically in the ranges of 0 to 12.5 at. pct Ti and 2.5 to 17.5 at. pct Al. These ranges are larger than those of bulk amorphous alloys prepared by a squeeze mold casting technique. Most of the amorphous mechanically alloyed powders exhibited a wide supercooled liquid region of more than 60 K before crystallization. The glass-transition and crystallization temperatures of mechanically alloyed samples were different from those prepared by squeeze casting. It is suspected that different thermal properties arise from the introduction of impurities during the MA process. The amorphization behavior of Zr₅₀Ti_7.5Al_12.5Ni₁₀Cu₂₀ was examined in detail. The X-ray diffraction and extended X-ray absorption fine structure (EXAFS) results show the fully amorphous powders formed after 5 hours of milling. A kinetically modified thermodynamic phase transformation process was observed for the glass-transition behavior in the Zr₅₀Ti_7.5Al_12.5Ni₁₀Cu₂₀ amorphous powder.     

Formation and stability of fe-rich precipitates in dilute Zr(Fe) single-crystal alloys

The formation and stability of Fe-rich precipitates in two α-Zr(Fe) single-crystal alloys with nominal compositions I, 50 parts per million by atom (ppma) Fe, and II, 650 ppma Fe, have been investigated. Optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were used to examine the characteristics of Fe-rich precipitates. The SEM and TEM micrographs showed that in as-grown alloy II, Zr₂Fe precipitates were located at “stringers. ”Precipitates were not observed in as-grown alloy I. Annealing treatments below 700 °C, for alloy I, and 820 °C, for alloy II, resulted in the diffusion of excess Fe (above the α-phase solution limit) to the free surface with the subsequent formation of Zr₃Fe precipitates in both alloys. Dissolution of Zr₃Fe surface precipitates of alloy I (annealing above the solvus) left precipitate-like features on the surfaces. Zr₂Fe precipitates in as-grown alloy II were readily dissolved by β-phase annealing.     

The Influence of 3d and 4d Transition Metals on the Glass Forming Ability of Ternary FeCo-Based Alloys

Thermodynamic modeling was used to determine enthalpies of formation and other thermodynamic parameters describing glass forming ability of Fe-Co-TM (TM = V, Nb, Cr, Mo) alloys. FeCo-based alloys are considered as candidates for applications as high magnetic flux density materials due to their high magnetic saturation and low magnetic anisotropy. Nevertheless, mechanical properties, especially the lack of ductility, are their main weakness. Therefore, further optimization by vitrification, further heat treatment and alloying should be considered. As the most crucial step is the synthesis of amorphous precursors, discussion is concentrated on the effect of transition metal substitution on the glass forming ability. The highest glass forming ability was reported for Fe-Co-Nb alloys. It can be also noted that the driving force for vitrification can be improved by substitution of Fe by other transition elements, as glass forming ability parameter ∆P_HS reaches the lowest values for Fe-less compositions.

     

Iron-rich intermetallic phases and their role in casting defect formation in hypoeutectic Al-Si alloys

Iron is the most common and detrimental impurity in aluminum casting alloys and has long been associated with an increase in casting defects. While the negative effects of iron are clear, the mechanism involved is not fully understood. It is generally believed to be associated with the formation of Fe-rich intermetallic phases. Many factors, including alloy composition, melt superheating, Sr modification, cooling rate, and oxide bifilms, could play a role. In the present investigation, the interactions between iron and each individual element commonly present in aluminum casting alloys, were investigated using a combination of thermal analysis and interrupted quenching tests. The Fe-rich intermetallic phases were characterized using optical microscope, scanning electron microscope, and electron probe microanalysis (EPMA), and the results were compared with the predictions by Thermocalc. It was found that increasing the iron content changes the precipitation sequence of the β phase, leading to the precipitation of coarse binary β platelets at a higher temperature. In contrast, manganese, silicon, and strontium appear to suppress the coarse binary β platelets, and Mn further promotes the formation of a more compact and less harmful α phase. They are therefore expected to reduce the negative effects of the β phase. While reported in the literature, no effect of P on the amount of β platelets was observed. Finally, attempts are made to correlate the Fe-rich intermetallic phases to the formation of casting defects. The role of the β phase as a nucleation site for eutectic Si and the role of the oxide bifilms and AlP as a heterogeneous substrate of Fe intermetallics are also discussed.     

A rationalization of stress-strain behavior of two-ductile phase alloys

An extensive literature review indicated that the law of mixture rule can at times account for stress-strain behavior of two-ductile phase alloys in terms of the stress-strain behavior of component phases. In the present investigation, various factors which can contribute to the stress-strain behavior of two-ductile phase alloys are considered, using Ti-Mn alloys as the model system. Particular attention is focused on the effect of elastic, elasto-plastic, and plastic interactions between the phases on the stress-strain behavior. It is shown that the law of mixture cannot adequately explain the stress-strain behavior. The following equation is proposed to describe the stress-strain behavior of two-ductile phase alloys: P_α-β = f_αP_α ^c + f_βP_β ^c + I_α-β ^p, where P_α-β is a given stress-strain property, fα and f/gb are respective volume fractions of α and β-phases, P_α ^c and P_β ^c are corrected properties of α and β-phases, and I_α-β ^p is the interaction term. It is found that for α- β Ti-Mn alloys, for 0.2 pct yield strength, I_α-β ^p is positive, negative, or zero depending on the microstructure; but I_α-β ^p is always positive for the ultimate tensile strength and strain hardening rates and its magnitude depended on the microstructure. The reasons for the nature or sign of the interaction parameter for a given property are discussed in detail.     

Effects of Phosphorus and Carbon Contents on Amorphous Forming Ability in Fe-based Amorphous Alloys Used for Thermal Spray Coatings

Cost-effective Fe-based amorphous alloys used for thermal spray coatings were developed by varying contents of P and C, and their microstructure, hardness, and corrosion resistance were analyzed. In order to achieve chemical compositions having high amorphous forming ability, thermodynamically calculated phase diagrams of Fe-Al-P-C-B five-component system were used, from which compositions of super-cooled liquid having the lowest driving force of formation of crystalline phases were obtained. The thermodynamic calculation results showed that only phases of Fe₃P and Fe₃C were formed in the Fe₇₈Al₂P_(18.3?x)C _x B_1.7 alloy system. Considering driving force curves of Fe₃P and Fe₃C, the carbon contents were selected to be 6.90 and 7.47 at. pct, when the thermodynamic calculation temperatures were 697 K (414 °C) and 715 K (442 °C), respectively. According to the microstructural analysis of suction-cast alloys, the Fe₇₈Al₂P_10.83C_7.47B_1.7 alloy showed a fully amorphous microstructure, whereas the Fe₇₈Al₂P_11.40C_6.9B_1.7 and Fe₇₈Al₂P_10.3C_8.0B_1.7 alloys contained Fe₃P and Fe₃C phases. This Fe₇₈Al₂P_10.83C_7.47B_1.7 alloy showed the better hardness and corrosion resistance than those of conventional thermal spray coating alloys, and its production cost could be lowered using cheaper alloying elements, thereby leading to the practical application to amorphous thermal spray coatings.     

Amorphization reaction of Ni-Ta powders during mechanical alloying

This study examined the amorphization behavior of Ni _x Ta_100−x alloy powders synthesized by mechanically alloying (MA) mixtures of pure crystalline Ni and Ta powders with a SPEX high energy ball mill. According to the results, after 20 hours of milling, the mechanically alloyed powders were amorphous for the composition range between Ni₁₀Ta₉₀ and Ni₈₀Ta₂₀. A supersaturated nickel solid solution formed for Ni₉₀Ta₁₀, as well. X-ray diffraction analysis reveals two different types of amorphization reactions. Through an intermediate solid solution and by direct formation of amorphous phase. The thermal stability of the amorphous powders was also investigated by differential thermal analysis. As the results demonstrated, the crystallization temperature of amorphous Ni-Ta powders increased with increasing Ta content. In addition, the activation energy of amorphous Ni-Ta powders reached a maximum near the eutectic composition.     

Phase equilibria in prototype Nb-Pd-Hf-Al alloys

The phase equilibria in two prototype alloys with nominal compositions 60Nb-20Pd-10Hf-10Al and 40Nb-30Pd-15Hf-15Al (in at. pct) are investigated using scanning electron microscopy and X-ray diffraction. The alloys were heat treated at 1200 °C and 1500 °C for 200 hours each. The phase analysis revealed that the alloys were, for the most part, in the three-phase equilibrium between (Nb), Pd₂HfAl, and Pd₃Hf. The compositions of these three phases along with other observed phases such as PdAl and (α-Hf) provide important data for establishing the Nb-Pd-Hf-Al quaternary phase diagram. A preliminary Nb-Pd-Hf-Al phase diagram, with pertinent tie-tetrahedra, was constructed based on the available composition data. The lattice parameters of (Nb), Pd₂HfAl, Pd₃Hf, and the coefficient of thermal expansion of Pd₂HfAl were measured, and models were developed to predict the composition dependence of the mean atomic volumes/lattice parameters of (Nb) and Pd₂HfAl and the temperature dependence of the lattice parameter of the (Nb) phase. The validity of the models was confirmed by good agreement between predicted and experimental values.     

Solidification of Aluminum-Copper B206 Alloys with Iron and Silicon Additions

Solidification of B206 aluminum alloys with additions of iron and silicon was studied to investigate their combined effect on the formation and precipitation of intermetallics, particularly Fe-rich phases. Iron is precipitated mainly by either β(CuFe) or α(MnFe) phases, or both depending of the iron and silicon content, as well as the cooling rate. It was found that in alloys having up to 0.3 wt pct Fe, the precipitation of β(CuFe) phase can be largely suppressed if the ratio Si/Fe is close to 1 and the cooling rate is moderately high. The low mobility of the large facets of the β(CuFe) platelets is likely the cause limiting the amount of this phase, especially when the iron atoms have the possibility to be captured by another phase, in this case, the α(MnFe) phase.     

Electrochemical Preparation of Mg-Li-Zn-Mn Alloys by Codeposition

The electrochemical codeposition of Mg-Li-Zn-Mn alloys on a molybdenum electrode in LiCl-KCl-MgCl₂-ZnCl₂-MnCl₂ melts at 943 K (670 °C) was investigated. Preparation of the alloys by electrolysis was proven feasible in LiCl-KCl-MgCl₂-ZnCl₂-MnCl₂ melts from cyclic voltammograms and chronopotentiometry measurements. X–ray diffraction (XRD) indicated that Mg-Li-Zn-Mn alloys with different phases were prepared via galvanostatic electrolysis. The microstructure of typical α + Mg₇Zn₃ phase of Mg-Li-Zn-Mn alloys was characterized by an optical microscope and scanning electronic microscopy). The analysis by energy dispersive spectrometry showed that the addition of ZnCl₂ leads to the formation of intermetallic Mg₇Zn₃ distributed in grain boundaries, whereas Mn mainly existed on polygon particles. The results of inductively coupled plasma analysis showed that the chemical compositions of alloys were consistent with the phase structures of XRD patterns.     

Solidification of Nb-bearing superalloys: Part II. Pseudoternary solidification surfaces

Equilibrium distribution coefficients and pseudoternary solidification surfaces for experimental superalloys containing systematic variations in Fe, Nb, Si, and C were determined using quenching experiments and microstructural characterization techniques. In agreement with previous results, the distribution coefficient, k, for Nb and Si was less than unity, while the “solvent” elements (Fe, Ni, and Cr) exhibited little tendency for segregation (k ≈ 1). The current data were combined with previous results to show that an interactive effect between k _Nb and nominal Fe content exists, where the value of k _Nb decreases from 0.54 to 0.25 as the Fe content is increased from ≈2 wt pct to ≈47 wt pct. This behavior is the major factor contributing to formation of relatively high amounts of eutectic-type constituents observed in Fe-rich alloys. Pseudoternary γ-Nb-C solidification surfaces, modeled after the liquidus projection in the Ni-Nb-C ternary system, were proposed. The Nb compositions, which partially define the diagrams, were verified by comparison of calculated amounts of eutectic-type constituents (via the Scheil equation) and those measured experimentally, and good agreement was found. The corresponding C contents needed to fully define the diagrams were estimated from knowledge of the primary solidification path and k values for Nb and C.     

Formation and Stability of Equiatomic and Nonequiatomic Nanocrystalline CuNiCoZnAlTi High-Entropy Alloys by Mechanical Alloying

Nanocrystalline equiatomic high-entropy alloys (HEAs) have been synthesized by mechanical alloying in the Cu-Ni-Co-Zn-Al-Ti system from the binary CuNi alloy to the hexanary CuNiCoZnAlTi alloy. An attempt also has been made to find the influence of nonequiatomic compositions on the HEA formation by varying the Cu content up to 50 at. pct (Cu _x NiCoZnAlTi; x = 0, 8.33, 33.33, 49.98 at. pct). The phase formation and stability of mechanically alloyed powder at an elevated temperature (1073 K [800 °C] for 1 hour) were studied. The nanocrystalline equiatomic Cu-Ni-Co-Zn-Al-Ti alloys have a face-centered cubic (fcc) structure up to quinary compositions and have a body-centered cubic (bcc) structure in a hexanary alloy. In nonequiatomic alloys, bcc is the dominating phase in the alloys containing 0 and 8.33 at. pct of Cu, and the fcc phase was observed in alloys with 33.33 and 49.98 at. pct of Cu. The Vicker’s bulk hardness and compressive strength of the equiatomic nanocrystalline hexanary CuNiCoZnAlTi HEA after hot isostatic pressing is 8.79 GPa, and the compressive strength is 2.76 GPa. The hardness of these HEAs is higher than most commercial hard facing alloys (e.g., Stellite, which is 4.94 GPa).