Effect of stress during aging on the precipitation of θ′ in Al-4 Wt pct Cu

Experiments have been made on Al-4 wt pct Cu single crystals to determine the influence of stress, applied during aging, on precipitation. Without stress, θ platelets precipitate on all (100) planes. Transmission electron microscopy showed a marked effect of aging stress on the microstructure. A compressive stress of approximately 48 MPa (～ 7,000 psi) parallel to [001] favored precipitation on (010) and (100) and inhibited precipitation on (001). When a tensile stress of similar magnitude was applied parallel to [001], precipitation was favored on (001) and inhibited on (010) and (100).     

Precipitation hardening in Cu-Zn-Be alloys

Brasses containing 20 and 30 pct Zn and up to 1.15 pct Be have been examined to provide tensile test data on the age-hardening response of these ternary alloys. It was found that yield strengths of 1100 Mpa were attainable with cold working prior to aging at 300°C. The mode of precipitation, as determined by electron micros copy and electron diffraction, agrees substantially with recent work on the binary Cu-Be system. Precipitation commences as GP zones on the (001)_α planes. The observed precipitate phase, after aging times of up to seven days at 400°C, is CuBe with an orientation relationship of the type (301)_ppt ∥(113)_α [010]_ppt ∥[110]_α.     

Fracture of single crystals of the nickel-base superalloy PWA 1480E in helium at 22 °C

The fracture behavior of single crystals of the PWA 1480E nickel-base superalloy was studied using both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. Notched single crystals with seven different crystal growth orientations near [100], [110], [111], [013], [112], [123], and [223] were tensile tested at 22 °C in a helium atmosphere at 34 MPa. Gamma prime particles were orderly and closely aligned with the cube edges along the [100], [010], and [001] directions of theγ matrix. The cuboid morphology of theγ’ precipitate was not influenced by the crystal growth orientation. The specimen with the [110] orientation was the strongest, while the crystal with the [100] orientation was the weakest. A stereoscopic technique, combined with the use of planary’ morphologies, was applied to identify the cleavage plane orientation. All specimens failed predominately by {lll}-type cleavage which originated from combined slip on various {111} planes. In most cases, deformation was found to occur inhomogeneously in intense slip bands lying on {111} planes and aligned parallel to the different slip directions. Both SEM and TEM studies indicated that {lll}-type slip was the controlling factor during cleavage fracture of single crystals of the PWA 1480E nickel-base superalloy. Formerly Graduate Student, Auburn University     

Growth crystallography and lamellar to rod transition in directionally solidified Nb-Nb2C eutectic composites

The crystallographic relationship displayed by the niobium and niobium carbide <Nb₂C> phases in an aligned eutectic sample with a lamellar carbide morphology is lamellar interface ∥ {110}_NB ∥ (001)_{Nb 2C} growth direction ∥<112>_NB ∥ [010]Nb₂C or [1-20]_{Nb 2}C and for the rod-like carbide morphology rod interface (major axis) ∥{110}_Nb ∥ (001)_{Nb 2}C growth direction 11(H2)Nb II l010]Nb.,c or [210]NB₂C. The transition in morphology of the carbide phase is discussed in terms of the relative volume fraction of the phases, growth rate, and orientation relationships. The carbide morphology is influenced by the growth rate and carbon content. For constant growth rate increasing the volume fraction of the carbide phase favors the lamellar morphology. At low growth rates the lamellar morphology is favored, and at high growth rates the rod-like morphology is favored. Growth crystallography has no direct influence on the transition in carbide morphology.     

Studies of Orientations of β″ Precipitates in Al-Mg-Si-(Cu) Alloys by Electron Diffraction and Transition Matrix Analysis

Transition matrix, combine with high-resolution transmission electron microscopy (HRTEM) and stereographic projection, was used in this article to predict and characterize the orientations and electron diffraction patterns of β″ precipitates in Al-Mg-Si-(Cu) alloys. It was found that the β″ phases as the main strengthening precipitates had 12 variants with Al matrix, and the orientation relationships could be expressed as (010) _β″//{100}_Al, [001] _β″//〈310〉_Al, and [100] _β″//〈230〉_Al. Further, based on the C-centered monoclinic structure of β″ precipitate, a new diffraction patterns model under the [001]_Al zone axis could be established, which was in good agreement with the experiment date. Further, it could be used to explain reasonably some issues (for example, the “cross-shaped” diffraction streaks phenomenon). Simultaneously, we also found that the β″ precipitates had only three different zone axes, [010] _β″, [304] _β″, and [ [`1]06 ]_b^￠￠ , \left[ {\bar{1}06} \right]_{{\beta^{\prime \prime } }} , parallel to the [001]_Al direction when they were precipitated from the Al matrix, and the β″ precipitate might deviate 1.6 deg from its original orientation because of the lattice relaxation.     

On the effect of stress on nucleation and growth of precipitates in an Al-Cu-Mg-Ag alloy

A study has been made of the effect of an externally applied tensile stress on Ω and Θ′ precipitate nucleation and growth in an Al-Cu-Mg-Ag alloy and a binary Al-Cu alloy which was used as a model system. Both solutionized and solutionized and aged conditions were studied. The mechanical properties have been measured and the microstructures have been characterized by transmission electron microscopy (TEM). The volume fraction and number density, as well as the precipitate size, have been experimentally determined. It was found that for as-solutionized samples aged under stress, precipitation occurs preferentially parallel to the stress axis. A threshold stress has to be exceeded before this effect can be observed. The critical stress for influencing the precipitate habit plane is between 120 and 140 MPa for Ω and between 16 and 19 MPa for Θ′ for the aging temperature of 160 °C. The major effect of the applied stress is on the nucleation process. The results are discussed in terms of the role of the lattice misfit between the matrix and the precipitate nucleus.     

Microstructural investigation on the precipitation of α″ nitrides and α″ → γ′ nitride transformation in ion-nitrided pure iron

The precipitated characteristics of α″-Fe₁₆N₂ nitrides in the diffusion layer of ion-nitrided pure iron were investigated with transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM). Three sets of α″ nitrides, whose habit planes are (100)_α,(010)_α,(001)_α, respectively, do not precipitate simultaneously from the diffusion layer, which is different from the normal homogeneous precipitation in Fe-N alloys. Unlike the typical disc-shaped morphology reported widely, the α″ nitrides in the diffusion layer appear as ribbonlike slices. They grow on {001}_α matrix planes with a parallel orientation relationship, and the direction of their length is parallel to the <110>_α direction. The interface between the α″ nitride and α matrix and a 7 deg [111]/(112) low-angle-tilt grain boundary in the α″ nitride were examined with HREM. The distributions of dislocations at the interface and the grain boundary were investigated. During microstructural examination, it was observed that a γ′-Fe₄N nitride could grow on an α″ nitride directly. The orientation relationship during the α″ → γ′ nitride transformation was determined as to be (001)_γ//(110)_α,[110]_γ//[111]_α.     

Isothermal Decomposition of Ferrite in a High-Nitrogen,Nickel-Free Duplex Stainless Steel

The formation and crystallography of second phases during isothermal decomposition of ferrite (α) in a high-nitrogen, nickel-free duplex stainless steel was examined by means of transmission electron microscopy (TEM). At an early stage of aging, the decomposition of α started along the α/γ phase boundaries where sigma (σ) phase and secondary austenite (γ ₂) precipitated in the form of an alternating lamellar structure. The combined analyses based on the simulation of diffraction patterns and stereographic projection have shown that most of the σ phase was related to the γ ₂ by the following relation: (111)_g ||(001)_s (111)_{\gamma } \parallel (001)_{\sigma } and [10[`1]]<sub>g</sub> ||[110]<sub>s</sub> . [10\bar{1}]_{\gamma } \parallel [110]_{\sigma } . The intergranular and intragranular precipitation of Cr<sub>2</sub>N with trigonal structure were identified, and the orientation relationships (ORs) with α and γ matrix could be expressed as ( 110 )<sub>a</sub> ||( 0001 )<sub>\textCr2 \textN</sub> \left( {110} \right)_{\alpha } \parallel \left( {0001} \right)_{{{\text{Cr}}_{2} {\text{N}}}} , [ [`1]11 ]_a ||[[`1]100]<sub>\textCr2 \textN</sub>  ; (111)<sub>g</sub> ||(0001)<sub>\textCr2 \textN</sub> \left[ {\bar{1}11} \right]_{\alpha } \parallel [\bar{1}100]_{{{\text{Cr}}_{2} {\text{N}}}} \,;\,(111)_{\gamma } \parallel (0001)_{{{\text{Cr}}_{2} {\text{N}}}} , and [ [`1]10 ]_g ||[ [`1]100 ]_{\textCr2 \textN} , \left[ {\bar{1}10} \right]_{\gamma } \parallel \left[ {\bar{1}100} \right]_{{{\text{Cr}}_{2} {\text{N}}}} , respectively. The precipitation of intermetallic χ phase was also observed inside the α matrix, and they obeyed the cube-on-cube OR with the α matrix. Prolonged aging changed both the structure of matrix and the distribution of second phases. The γ ₂, formed by decomposition of α, became unstable because of the depletion of mainly N accompanied by the formation of Cr₂N, and it transformed into martensite after subsequent cooling. As a result, the microstructure of the decomposed α region was composed of three kinds of precipitates (intermetallic σ,χ, and Cr₂N) embedded in lath martensite.     

Microstructure Evolution and Analysis of A [011] Orientation, Single-Crystal, Nickel-Based Superalloy During Tensile Creep

By means of the elastic?Cplastic finite-element method (FEM) for calculating the distribution features of the von Mises stress and strain energy density, the influences of the applied stress on the von Mises stress of the ????/?? phases and the rafting of the ???? phase for the [011] orientation, single-crystal, nickel-based superalloy are investigated. The results show that, after being fully heat treated, the microstructure of the [011] orientation, single-crystal, nickel-based superalloy consists of the cuboidal ???? phase embedded coherently in the ?? matrix, and the cuboidal ???? phase on (100) plane is regularly arranged along a 45?deg angle relative to the [011] orientation. Compared with the matrix channel of [010] orientation, the bigger von Mises stress is produced within the [001] matrix channel when the tensile stress is applied along the [011] orientation. Under the action of the larger principal stress component, the bigger expanding lattice strain occurs on the (001) plane of the cuboidal ???? phase along the [010] direction, which may trap the Al, Ti atoms with a bigger atomic radius for promoting the directional growth of the ???? phase into the stripe-like rafted structure along the [001] orientation. The changes of the interatomic potential energy, misfit stress, and interfacial energy during the tensile creep are thought to be the driving forces of promoting the elements?? diffusion and directional growth of the ???? phase.     

The microstructural details of β-Sn precipitation in a 5Sn-95Pb solder alloy

Solders of Pb-rich compositions, such as 5Sn-95Pb, are commonly used in electronic packaging applications, and this demanding use necessitates that the microstructure-property-processing relationships of the solder be understood fully. In this study, the microstructure of 5Sn-95Pb solder was characterized using a variety of metallographic techniques. The effect of cooling rate on the precipitation of β-Sn from supersaturated α-Pb was determined. On slow cooling, β-Sn precipitates discontinuously with resultant β-Sn lamella alternating with the α-Pb. Upon rapid cooling, the β-Sn precipitates with a nominally homogeneous distribution. These discrete precipitates were found to have a platelike shape with a (111)_Pb habit plane and an orientation relationship of (111)_Pb‖(010)_Sn and [011]_Pb‖ [001]_Sn. Regions of the β-Sn precipitates that curved away from this habit plane formed ledges. Upon heating, the precipitates were found to dissolvevia a ledge mechanism. Prolonged aging of both the fast and slow cooled 5Sn-95Pb resulted in a coarsening of the β-Sn precipitates with a resultant decrease in strength. Furthermore, the strength of the aged alloy was observed to be independent of cooling rate. D.R. FREAR, formerly Graduate Research Assistant, Lawrence Berkeley Laboratory