On the precipitation-hardening behavior of the Al-Mg-Si-Cu alloy AA6111

The precipitation-hardening behavior of aluminum alloy AA6111 during artificial aging and the influence of prior natural aging on the aging behavior were investigated. The evolution of microstructure was studied using quantitative transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The evolution of the relative volume fraction of precipitates for the solution-treated alloy was determined using isothermal calorimetry and a new analysis based on the DSC technique. Quantitative TEM was also used to obtain the rate of precipitation of microscopically resolvable phases during aging at 180 °C. Three types of precipitates, i.e., unresolved Guinier-Preston (GP) zones, β″, and Q′, were found to form during aging at 180 °C. The evolution of yield strength was related to the evolution of microstructure. It was found that the high hardening rate during artificial aging for the solution-treated alloy is due to the rapid precipitation of the β″ phase. Natural aging prior to artificial aging was found to decrease the rate of precipitation of β″. The slow hardening rate for the naturally aged alloy was attributed to the slower nucleation and growth of β″ phase.     

The precipitation of the Q phase in an AA6111 Alloy

The precipitation behavior of the quaternary Q phase in an Al 6111 alloy has been studied by analytical electron microscopy. The transformation strain associated with the Q phase has been determined from high resolution electron microscopy and electron diffraction. The habit plane of the Q laths is shown to be fully coherent with the Al matrix. The transformation strain is used to explain the pattern of heterogeneous precipitation of the Q phase at dislocations and grain boundaries. The crystal structure and composition of the Q phase, whether it forms in the matrix or at grain boundaries, appears similar to that formed directly from the melt in a quaternary Al-Cu-Mg-Si alloy.     

On the precipitation-hardening behavior of the Al−Mg−Si−Cu alloy AA6111

The precipitation-hardening behavior of aluminum alloy AA6111 during artificial aging and the influence of prior natural aging on the aging behavior were investigated. The evolution of microstructure was studied using quantitative transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The evolution of the relative volume fraction of precipitates for the solution-treated alloy was determined using isothermal calorimetry and a new analysis based on the DSC technique. Quantitative TEM was also used to obtain the rate of precipitation of microscopically resolvable phases during aging at 180 °C. Three types of precipitates, i.e., unresolved Guinier-Preston (GP) zones, β″, and Q′, were found to form during aging at 180 °C. The evolution of yield strength was related to the evolution of microstructure. It was found that the high hardening rate during artificial aging for the solution-treated alloy is due to the rapid precipitation of the β″ phase. Natural aging prior to artificial aging was found to decrease the rate of precipitation of β″. The slow hardening rate for the naturally aged alloy was attributed to the slower nucleation and growth of β″ phase. S. ESMAEILI, formerly Ph.D. Student, Department of Metals and Materials Engineering, University of British Columbia, Vancouver, BC, Canada V6T 1Z4     

Effects of Cu content and preaging on precipitation characteristics in aluminum alloy 6022

Effects of Cu content and preaging treatments on precipitation sequence and artificial aging response in aluminum alloy 6022 were investigated using transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and hardness tests. It was found that Cu induces the formation of Q and its precursor metastable phases and has a beneficial effect on the kinetics of artificial aging. For the alloy with 0.07 wt pct Cu, the precipitation sequence is GP zones → needlelike β″ → rodlike β′ + lathlike Q′ → β + Si. On the other hand, the precipitation sequence in the alloy with 0.91 wt pct Cu is GP zones → needlelike β′ → lathlike Q′→Q+Si. For the artificial aging condition of 20 minutes at 175 °C, which is the typical automotive paint bake condition, suitable preaging treatments were found to significantly reduce the detrimental effect of the natural aging on artificial aging response.     

Thermal Stability in Nanocrystalline Fe-30?Wt?Pct Ni Alloy Induced by Surface Mechanical Attrition Treatment

By means of surface mechanical attrition treatment (SMAT), a nanocrystalline surface layer is produced in Fe-30 wt pct Ni alloy, accompanying the formation of the strain-induced martensite. The thermal stability of nanocrystalline martensite and parent phase austenite in Fe-30 wt pct Ni alloy is studied by X-ray diffraction (XRD) and transmission electron microscope (TEM). The grain growth kinetics parameters, time exponent, n, and activation energy, Q, for both martensite and austenite, are determined, respectively. The TEM observations indicate that abnormal grain growth occurs during annealing at high temperatures.     

The pre-bainitic transformation in CuZnAlMn alloy

The behaviour of the pre-bainitic transformation in the CuZnAlMn alloy was investigated by using internal friction (Q⁻¹) measurements and TEM. The results show that there always exists an internal friction peak associated with the segregation of solute atoms before the formation of orthorhombic 9R bainite and that the 9R bainite nucleates martensitically in depleted regions of solute atoms in the B₂ phase. The transformation processes mentioned above were also confirmed in isothermal internal friction and TEM experiments.     

Mechanical behavior of a cryomilled near-nanostructured Al-Mg-Sc alloy

In the present study, the mechanical behavior of a cryomilled Al-7.5 pct Mg-0.3 pct Sc alloy was investigated at temperatures in the range of 298 to 648 K. The grain size of the as-extruded alloy was determined to be approximately 200 nm by transmission electron microscope (TEM) and X-ray diffraction (XRD) analysis. The data indicate that as a result of cryomilling, a supersaturated solid solution with high thermal stability was formed in the Al-Mg-Sc alloy. The high strength at room temperature was primarily attributed to three types of strengthening: grain size effect, solid solution hardening, and Orowan strengthening. The elevated temperature mechanical behavior of the Al-Mg-Sc alloy exhibits the following: (a) a strain-rate sensitivity, m, of less than 0.2; and (b) an activation energy, Q, that increases from 139 to 193 kJ/mol with increasing applied stress. An analysis of the experimental data at elevated temperatures shows that despite the fine-grained structure of the alloy, the deformation characteristics are not consistent with those arising from a superplastic deformation process that incorporates a threshold stress. On the other hand, the analysis suggests that the deformation characteristics agree with those associated with the transition in the creep behavior of Al-based solid solution alloys from that for the intermediate-stress region, where m=0.33 and Q=Q _D (Q _D is the activation energy for self-diffusion in Al), to that of the high-stress region, where m<0.2 and Q>Q _D .     

Pressureless Infiltration and Resulting Mechanical Properties of Al-AlN Preforms Fabricated by Selective Laser Sintering and Partial Nitridation

A novel manufacturing process has recently been developed for the fabrication of intricate Al-AlN composite parts. The process involves green shape formation by selective laser sintering, preform development by nitridation, and net shape forming by pressureless infiltration. The infiltration atmosphere has an important influence on the final fabrication and mechanical properties. This work presents a detailed investigation on the infiltration of Al-AlN preforms with AA 6061 at various temperatures above its liquidus under nitrogen, vacuum, and argon. The green shapes are formed by selective laser sintering of a premix of AA 6061-2Mg-1Sn-3Nylon (wt pct) powders. They are then partially nitrided to create a rigid, 2- to 3-μm-thick AlN skeleton for subsequent infiltration. Nitrogen infiltration results in the highest density (2.4 gcm⁻³) and best tensile properties (UTS: 214 MPa; elongation: 2.5 pct), while argon infiltration gives the lowest density. Fractographs confirmed the difference in density arising from the use of different atmospheres where small pores are evident on the fracture surfaces of both argon and vacuum-infiltrated samples. The molten AA 6061 infiltrant reacts with nitrogen during infiltration leading to a 5-μm-thick AlN skeleton compared to the original 2- to 3-μm-thick skeleton in both argon and vacuum-infiltrated samples. Transmission electron microscope (TEM) examination revealed inclusions of Mg₂Si and Mg₂Si _x Sn_1−x in both nitrogen- and argon-infiltrated samples but not in vacuum-infiltrated samples. Vacuum infiltration is slower than nitrogen and argon infiltration. The mechanisms that affect each infiltration process are discussed. Infiltration under nitrogen is preferred.     

Microstructural studies of a Cu-Zn-Al shape-memory alloy with manganese and zirconium addition

Mn and Zr were added to improve the shape-memory characteristics of a Cu-Zn-Al shape-memory alloy (SMA). The microstructure of a Cu-19.0Zn-13.1Al-1.1Mn-0.3Zr (at. pct) alloy was examined using a transmission electron microscope (TEM). The structure of the parent phase and martensite phase are DO₃ (or L2₁) and M18R₁, respectively. Two kinds of Zr-rich precipitates formed in the alloy. Energy-dispersive X-ray spectroscopy (EDXS) analysis with a TEM indicates that the two precipitates are all new phases and have the compositions of Cu_50.2Zr_24.6Al_17.3Zn_7.9 (at. pct) (Z ₁ phase) and Cu_57.4Zr_20.4Zn_10.3Al_11.9 (at. pct) (Z ₂ phase), respectively. The volume ratio of Z ₁ phase in the alloy is about 70 pct of the total precipitate volume. The structure of Z ₁ phase was studied in detail using TEM electron diffraction analyses. The lattice parameter of fcc Z ₁ phase is a=1.24 nm, and the space group of the phase is F432 (No. 209). The Z ₁ phase possesses an incoherent interface with the parent-phase matrix. The lattice correspondence of the Z ₁ phase and parent-phase matrix is as follows:

Dehydrogenation of nanocrystalline TiH2 and consequent consolidation to form dense Ti

Dehydrogenation of nanocrystalline TiH₂, produced by pulverization of commercially available powder, has been examined in detail by a combination of thermal analysis, X-ray diffraction (XRD), and transmission electron microscopy (TEM). The dehydrogenation to form Ti occurs as a two-step process involving the formation of an intermediate phase, TiH. In-situ experiments on dehydrogenation inside a transmission electron microscope reveal the possibility of a powder-metallurgy process for consolidation of Ti components by vacuum annealing of nanocrystalline TiH₂ at ∼0.5T _m , where T _m is the melting point of Ti. Near-full densification of Ti has been achieved by sintering nanocrystalline TiH₂ under vacuum at ∼0.5T _m .     

Microstructural Evolution and Stress Corrosion Cracking Behavior of Al-5083

The fine scale microstructure of Al-5083 (H-131) sensitized at 448 K (175 °C) for 1, 10, 25, 50, 100, 240, 500, and 1000 hours has been investigated using transmission electron microscopy (TEM) to study the evolution of the β phase (Al₃Mg₂) at grain boundaries and on pre-existing intragranular particles. In fully sensitized Al-5083, the β phase (Al₃Mg₂) forms heterogeneously both at grain boundaries and on pre-existing particles, which are enriched in manganese. TEM observations showed that the grain boundary precipitation of the β phase initially occurs between 0 to 1 hour of aging at 448 K (175 °C), and that the β phase grows with a ribbonlike morphology. The grain boundary planes are covered by the β phase after 240 hours of aging. The contribution of microstructure, defects, and environment on the stress corrosion cracking (SCC) behavior is discussed.     

Role of Mg in the stress corrosion cracking of an Al-Mg alloy

The corrosion and stress corrosion cracking (SCC) susceptibility of an Al-Mg alloy, AA5083, has been shown to depend on the precipitation of the Mg-rich β phase, (Al₃Mg₂), but not the enrichment of elemental Mg at grain boundaries to an enrichment ratio of 1.4. These results were determined by measuring the progress of Mg enrichment at grain boundaries, for increasing thermal-treatment times, using auger electron spectroscopy (AES) of grain boundaries exposed by fracture within the spectrometer and by analytical electron microscopy (AEM) of thin foils. The progress of the β phase precipitation was followed by AEM and scanning electron microscopy (SEM), for the same thermal-treatment times. The lack of a Mg-segregation effect on SCC was demonstrated by results obtained with X-ray photoelectron spectroscopy (XPS) analysis of Mg-implanted Al following in-situ electrochemical tests and SCC tests, while the dominance of β phase precipitation was demonstrated by electrochemical analysis and SCC testing. Crack-growth tests of alloy AA5083 demonstrated faster cracking at potentials anodic to the open circuit potential (OCP) with no increase at potentials cathodic to the OCP.     

On the nature of the Fe-bearing particles influencing hard anodizing behavior of AA 7075 extrusion products

The deleterious effects of Fe-bearing constituent particles on the fracture toughness of wrought Al alloys have been known. Recent studies have shown that the presence of Fe-bearing constituent particles is also detrimental to the nature and growth of the hard anodic oxide coating formed on such materials. The present study, using a combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron probe microanalysis (EPMA), was made to examine the influence of the nature of the Fe-bearing particles on the hard anodizing behavior of AA 7075 extrusion products containing varying amounts of Si, Mn, and Fe impurities. It was found that, in the alloy containing 0.25 wt pct Si, 0.27 wt pct Mn, and 0.25 wt pct Fe, the Fe-bearing constituent particles are based on the Al₁₂(FeMn)₃Si phase (bcc with a=1.260 nm). These particles survive the hard anodizing treatment, add resistance to the electrical path, causing a rapid rise in the bath voltage with time, and cause a nonuniform growth of the anodic oxide film. In the materials containing 0.05 wt pct Si, 0.04 wt pct Mn, and 0.18 wt pct Fe, on the other hand, the formation of the Al₁₂(FeMn)₃Si-based phase is suppressed, and two different Fe-bearing phases, based on Al-Fe-Cu-Mn (simple cubic with a=1.265 nm) and Al₇Cu₂Fe, respectively, form. Neither the Al-Fe-Cu-Mn-based phase nor the Al₇Cu₂Fe-based phase survive the hard anodizing treatment, and this results in a steady rise in the bath voltage with time and a relatively uniform growth of the anodic oxide film. Consideration of the size of the Fe-bearing particles reveals that the smaller the particle, the more uniform the growth of the anodic oxide film.     

The role of iron in the formation of porosity in Al-Si-Cu-based casting alloys: Part II. A phase-diagram approach

The mechanism by which iron causes casting defects in the AA309 (Al-5 pct Si-1.2 pct Cu-0.5 pct Mg) may be related to the solidification sequence of the alloy. Superimposing calculated segregation lines on the liquidus projection of the ternary Al-Si-Fe phase diagram suggests that porosity is minimized at a critical iron content when solidification proceeds directly from the primary field to the ternary Al-Si-βAl₅FeSi eutectic point. Solidification via the binary Al-βAl₅FeSi eutectic is detrimental to casting integrity. This hypothesis was tested by comparing the critical iron content observed in the standard AA309 alloy to that of a high-silicon (10 pct Si) variant of this alloy.     

Crystallization kinetics of amorphous magnesium-rich magnesium-copper and magnesium-nickel alloys

Amorphous magnesium-rich alloys Mg _y X_1-y (X=Ni or Cu and 0.82<y<0.89) have been produced by melt spinning. The crystallization kinetics of these alloys have been determined by in situ X-ray diffraction (XRD) and isothermal and isochronal differential scanning calorimetry (DSC) combined with ex situ XRD. Microstructure analysis has been performed by means of transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). Crystallization of the Mg-Cu alloys at high temperature takes place in two steps: primary crystallization of Mg, followed by simultaneous crystallization of the remaining amorphous phase to Mg and Mg₂Cu. Crystallization of the Mg-Cu alloys at low temperatures takes place in one step: eutectic crystallization of Mg and Mg₂Cu. Crystallization of the Mg-Ni alloys for a Mg content, y>0.85, takes place in two steps: primary crystallization of Mg and of a metastable phase (Mg_∼5.5Ni, with Mg content y=0.85), followed by the decomposition of Mg_∼5.5Ni. Crystallization of the Mg-Ni alloys for a Mg content y<0.85 predominantly takes place in one step: eutectic crystallization of Mg and Mg₂Ni. Within the experimental window applied (i.e., 356 K<T<520 K and 0.82<y<0.89), composition dependence of the crystallization sequence in the Mg-Cu alloys and temperature dependence of the crystallization sequence in the Mg-Ni alloys has not been observed.     

Phase chemistry and precipitation reactions in maraging steels: Part I. Introduction and study of Co-containing C-300 steel

This article introduces a series of studies of phase transformations in maraging steels. Atom-probe field-ion microscopy (APFIM) was the main research technique employed. Hardness measurements, transmission electron microscopy (TEM), and thermochemical calculations were also used. The composition and morphology of precipitates in the commercial-grade C-300 steel were compared for different aging times at 510 °C to investigate the aging sequence. Both Ni₃Ti and Fe₇Mo₆ were found to contribute to age hardening. The decomposition starts with the formation of small Mo-enriched Ni₃Ti particles at very short aging times. The Fe₇Mo₆ phase forms at a later stage of aging. The matrix concentrations of both Ti and Mo were measured and were found to be low after standard aging conditions. The observation of the Fe₇Mo₆ μ phase is supported by thermochemical calculations. Austenite reversion has been found at the aging temperature, and its composition approaches the predicted equilibrium composition after 8 hours of aging. Formerly Graduate Student with the Department of Materials, Oxford University.     

Coarsening kinetics of multicomponent MC-type carbides in high-strength low-alloy steels

Morphology and coarsening kinetics of MC-type carbide (MC-carbide) precipitating during the tempering process have been investigated in V- and Nb-bearing Cr-Mo martensitic steels. Detailed transmission electron microscopy (TEM) observations show that the addition of V and Nb stabilizes the B1-type MC-carbide instead of L’3-type M₂C-carbide. The morphology of the MC-carbide is characterized as disk-like with Baker and Nutting orientation relationships with the matrix. When the specimens are fully solution treated followed by quenching, the MC-carbide precipitates as a multicomponent system with continuous solid solution of VC, NbC, and MoC. The V-, Nb-, and Mo-partitioning control the lattice parameter of MC-carbide and consequently affect the coherency between MC-carbide and the matrix. The coherent MC-carbide grows into an incoherent one with the progress of tempering. The numerical analysis on TEM observations has shown that the coarsening kinetics of MC-carbide is equated to (time)^1/5 criteria, while the coarsening kinetics of the coexisting cementite is equated to (time)^1/3 criteria. It is thus suggested that the Ostwald ripening of MC-carbide is controlled by pipe diffusion of V, Nb, and Mo along dislocations. It has been confirmed that the coarsening rate of the multicomponent MC-carbide is affected by V, Nb, and Mo content. Applying the thermodynamic solution database, the rate equation for MC-carbide coarsening can be expressed as a function of V, Nb, and Mo content, and the activation energy for pipe diffusion can be estimated as ΔQ _v: ΔQ _Nb: ΔQ _Mo=1:3.9:0.6.     

Part I. The microstructural evolution in Ti-Al-Nb O+Bcc orthorhombic alloys

Phase transformations and the resulting microstructural evolution of near-Ti₂AlNb and Ti-12Al-38Nb O+bcc orthorhombic alloys were investigated. For the near-Ti₂AlNb alloys, the processing temperatures were below the bcc transus, while, for Ti-12Al-38Nb, the processing temperature was supertransus. Phase evolution studies showed that these alloys contain several constituent phases, namely, bcc, O, and α ₂; when present, the latter was in small quantities compared to the other phases. The transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray investigations of samples that were solutionized and water quenched were used to estimate the phase fields, and a pseudobinary diagram based on Ti=50 at. pct was modified. The aging-transformation behavior was studied in detail. For solutionizing temperatures between 875 °C and the bcc transus, the phase composition and volume fraction of the near-Ti₂AlNb alloys adjusted through relative size changes of the equiaxed B2, O, and α ₂ grains. The aging behavior followed three distinct transformation modes, dependent on the solutionizing and aging temperatures. Widmanstatten formation was observed when a new phase evolved from a parent phase. Thus, Widmanstatten O phase precipitated within the B2 phase for supertransus fully B2 microstructures, as well as for substransus α ₂+B2 microstructures. Similarly, Widmanstatten B2 phase can form from a fully O microstructure, a transformation that has not been observed before. In the case of equiaxed O+B2 solutionized and water-quenched microstructures, Widmanstatten O-phase formation occurred only below 875 °C. For the subtransus-solutionized and water-quenched microstructures, a second aging transformation mode, cellular precipitation, was dominant below 750 °C. This involved formation of coarse and lenticular O phase that grew into the prior B2 grains from the grain boundaries. A third transformation mode involved composition-invariant transformation, where the fully B2 supertransus-solutionized and water-quenched microstructure transformed to a fully O microstructure at 650 °C. This microstructure reprecipitated B2 phase out of the O phase with continued aging time. For Ti-12Al-38Nb, Widmanstatten O precipitation remained the only transformation mode. It is shown that subtransus processing offers flexibility in controlling microstructures through postprocessing heat treatments.