快速凝固耐热铝合金的耐蚀性能

采用腐蚀失重法和极化曲线的测定,研究了FVS0611,FVS0812,2618,2618+Sc,2014,6061合金在人工模拟海水和海洋环境中的耐蚀性能。结果表明:2014,2618和2618+Sc合金腐蚀较严重,RS/PM生产的FVS0611,FVS0812和6061合金腐蚀速率较小,出现明显的钝化现象,并因为加工残余应力导致出现钝化-腐蚀-再钝化现象。     

Precipitation in rapidly solidified Al-Mn alloys

Precipitation at 450 °C was studied in melt-spun ribbons containing up to 15 wt pct Mn in solid solution in Al. The as-spun ribbons were microsegregation-free at compositions up to 5 wt pct Mn, but in more concentrated alloys a cellular microstructure was present. Upon annealing, four precipitate phases are observed, some of them being found preferentially on cell boundaries and others being found within the cells. Al₆Mn, G, and the Gℍ phase can coexist for long times at 450 °C, but the G phase appears to be slightly more stable. A less stable T phase was detected in Al-5 wt pct Mn foils following short annealing periods. The supersaturation of the Al matrix can persist for many hours in alloys containing up to 3 wt pct Mn, but is essentially gone after 1 hour in alloys with 5 wt pct Mn or more. On leave at the Center for Materials Research, The Johns Hopkins University, Baltimore, MD 21218. R.J.     

Beta-Eutectoid decomposition in rapidly solidified titanium-nickel alloys

The eutectoid reaction, β → α + Ti₂Ni, has been observed in as-produced rapidly solidified ribbons and flakes of hypoeutectoid and near-eutectoid beta titanium-nickel alloys prepared by chill block melt spinning (CBMS), pendant drop melt extraction (PDME), and electron beam melting/splat quenching (EBSQ) processes. Microstructural characterization of these materials was carried out by scanning electron microscopy, transmission electron microscopy, and X-ray energy dispersive spectroscopy. The occurrence of eutectoid decomposition in the rapidly solidified alloys was attributed to the breakaway of the ribbons or flakes (while still at an elevated temperature) from the quench wheel, resulting subsequently in a lower cooling rate. Fast quenching, as obtained in the hammer-and-anvil process, resulted in a martensitic structure free from products of eutectoid decomposition. The eutectoid morphology was nonlamellar in hypoeutectoid alloy ribbons, while a hitherto unreported lamellar eutectoid was observed in the near-eutectoid ribbons and flakes. The formation of this unusual lamellar eutectoid was rationalized in terms of the predominance of allotriomorphs of alpha phase and consequent availability of sufficiently mobile and maneuverable alpha/beta interphase boundaries in the fine-grained, rapidly solidified titanium-nickel alloys.     

Fatigue crack propagation in rapidly solidified aluminum alloys at 25 °C and 300 °C

The fatigue crack propagation performance of two rapidly solidified aluminum alloys was investigated in air at 25°C and 300°C. The results show that the crack propagation rates for continuous cycling tests of Al-8Fe-4Ce and Al-4.7-Fe-4.7Ni-0.2Cr alloys were similar at 25°C. Although the crack propagation rates of both alloys were increased at 300°C, the Al-Fe-Ce alloy exhibited the greater resistance to crack propagation. The inclusion of a tensile hold time in the fatigue loading cycle at 300°C produced an increase in the crack propagation rates for both alloys over the rates for continuous cycling. The fatigue crack propagation performance of the rapidly solidified alloys was not found to be superior when compared with the fatigue crack propagation performance of a wrought aluminum alloy tested under the same conditions. Transmission and scanning electron microscopy study of the tested specimens revealed that the crack propagation mode was primarily transgranular, with the metastable dispersoid particles providing impenetrable barriers to dislocation motion.     

Microstructural characterization of rapidly solidified Al-Ta alloys

Two Al-rich Al-Ta alloys containing by weight 3 and 6 pct Ta have been rapidly solidified from the melt using the ‘gun’ technique. The microstructures and the crystal structures of the phases in the as-solidified as well as those formed on subsequent decomposition of the supersaturated solid solution have been characterized. A supersaturated solid solution was obtained in both the alloys in the as-solidified condition indicating a solid solubility extension of Ta in Al to almost 6 wt pct. The supersaturated solid solutions formed in both the alloys have been found to be quite stable up to 673 K (for 1 hour). Annealing at higher temperatures resulted in the formation of rod-shaped precipitates inside the grains and massive precipitates along grain boundaries. The rod-shaped precipitates arranged in a regular pattern constitute a new metastable intermediate phase Al₇Ta having an ordered structure. The massive precipitates which form along grain boundaries constitute the equilibrium Al₃Ta phase with a tetragonal crystal structure. The transformation behavior and the morphology of the transformation products are detailed in this paper.     

Microstructures of rapidly solidified aluminum alloy submicron powders

The microstructures of electron transparent submicron aluminum alloy powders produced by an electrohydrodynamic process are described. The observations are coupled with thermodynamic, kinetic, and heat flow concepts to deduce the thermal history and solidification mode of the powders. The range of microstructures observed includes: homogeneous plane-front solidified single crystals; cellular crystals; and powders containing blocky segregates, multiple grains, and twins. In general, a decrease in the size of the submicron particles decreases the amount of segregation. This observation appears to be caused by an increase in undercooling prior to nucleation which leads to higher liquid/solid interface velocities. On the other hand, the same trend promotes multiple nucleation and increases the incidence of twin crystals. Liquid/solid interface breakdown from planar to cellular in the middle of the powders is correlated with solute “trapping” during recalescence and rejection of solute ahead of the solidification front after recalescence. The latter manifests itself as a solute depleted zone in the plane-front to cellular transition region. Finally, some of the microstructural observations lend credence to the recent extension of the morphological stability theory to rapid solidification. Formerly a Research Associate at the University of Illinois.     

Explosive consolidation of rapidly solidified aluminum alloy powders

Dynamic consolidation by explosive detonation was investigated for rapidly solidified Al-Cu-Li-Mg alloy powder. The densification, extent of interparticle bonding, microstructural modifications, and tensile properties were determined. The Al-alloy powders were prepared by vacuum atomization with cooling rates of 10³ to 10⁵ K per second. The powders were packed in steel cylinders, which were then evacuated to 1 to 7 Pa (1 to 5 × 10⁻² torr) for several hours and then sealed. Compaction experiments were conducted with the explosive surrounding the powder pack and detonated longitudinally. Variations in compaction pressure and detonation velocity were obtained by using different explosives and packing densities. The densification and interparticle bonding were correlated with explosive and powder parameters.     

Phase decomposition of rapidly solidified Fe-Mn-Al-C austenitic alloys

The phase decomposition of two (Fe_0.65Mn_0.35)_0.83Al_0.17-xC (x=3 and 4 at. pct orx=0.74 and 0.98 wt pct) austenitic alloys prepared by rapid solidification (RS) has been investigated on aging at 823 and 923 K by means of X-ray diffraction, transmission electron microscopy, and scanning electron microscopy. Under low bulk carbon supersaturation conditions (823 K aging of low carbon alloy and 923 K aging of high carbon alloy), zones formed preferentially at the cellular boundaries and in the bands in the {100} planes, giving rise to line broadening in the X-ray diffraction patterns. On the other hand, the initial aging under high, carbon supersaturation condition (823 K aging of high carbon alloy) resulted in the sideband formation, resulting from homogeneous structural modulation in the <100>_γ directions throughout the grain. The bulk carbon supersaturation dependence of initial decomposition modes indicates that carbon atom fluctuations are crucial in the initial state of phase decomposition, and that the observed {100} modulated structure corresponds to a structure consisting of alternate carbon-rich and carbon-poor zones. Together with the interstitial clustering process, an fcc-based substitutional ordering reaction concurrently took place. Later on these zones were replaced by a coherent metastable phase in the matrix, which was finally transformed into the cubic carbide (κ carbide) of (Fe, Mn)₃AlC_x chemical formula with the L'1₂ structure. However, at the end, a combined heterogeneous β-Mn and κ carbide precipitation seemed to finalize the decomposition process over the matrix κ carbide precipitation.     

Deformation of rapidly solidified dispersion strengthened titanium alloys

The influence of erbium on the behavior of titanium and titanium-aluminum based alloys has been investigated over a range of strain rates and temperatures (25 to 775°C). Data from hotextruded bulk specimens indicate that, although oxide dispersion strengthening can be large under certain conditions, the strengthening is minimal in fine-grain material subjected to low strain rate deformation at high temperatures. Both microstructural observations and an analysis of the flow data indicate profuse grain boundary sliding under these conditions. Grain coarsening anneals, which also coarsen the dispersoids and increase their population along grain boundaries, result in significantly improved high temperature flow strengths, especially in the high temperature/low strain rate regime. S. L. Kampe of Michigan Technological University, Houghton, MI     

Indentation characteristics of rapidly solidified Al-Cu-V and Al-Cu-Ti alloys

We report indentation characteristics of metallic glasses and nano-composites in Al-Cu-V as well as in Al-Cu-Ti systems. The melt spun ribbons of these alloys are employed to study the surface hardness characteristics of the specimen containing microstructural features at various length scales. The characterization of these materials has been done with the help of transmission electron microscope, scanning electron microscope and X-ray diffractometer. The surface hardness characteristics of melt spun ribbons with and without crystallization have been studied using a micro-hardness tester. We shall discuss the indentation behavior of ribbons in relation to their structures and microstructures.     

About the banded structure in rapidly solidified dendritic and eutectic alloys

A detailed analysis is presented for the formation of the so-called banded structure, consisting of a regular succession of dark and light bands, which has been found by several authors in rapidly solidified alloys. It is observed that the structures in the dark bands result from dendritic or eutectic growth depending on the alloy. Furthermore, it is concluded that the light bands are due to plane-front growth. The origin of these instabilities can be understood by considering the undercooling of the solid-liquid interface as a function of the growth rate for the two types of growth. Loss of local equilibrium and the influence of attachment kinetics at the solid-liquid interface are taken into account and quasi steady-state behaviour for growth inside each band is assumed. The proposed model is based upon a simple stability analysis leading to a periodic behaviour of the growth front when the isotherm velocity is in the range of negative slope of the interface undercooling-growth rate relationship. The predictions of the model are compared with detailed experimental results of dendritic AlFe, and dendritic/eutectic AlCu and AgCu alloys. Good agreement is found between the theoretical and experimental values of the transition velocities (from a fully dendritic or eutectic structure to a banded structure and from a banded structure to a precipitation-free structure) when reasonable values are chosen for the solute-trapping and attachment kinetic parameters. The calculated widths of the dark and light bands are also in agreement with experimental data when an upper limit is used for the thermal gradient.     

Solidification and microstructure analysis of rapidly solidified melt-spun Al-Fe alloys

Aluminum-iron alloys with Fe contents of up to 8 wt pct were rapidly quenched from the melt using the planar-flow melt-spinning technique. Microstructural changes that occur across the thickness of melt-spun ribbons were investigated as a function of initial melt composition and ribbon thickness. A transition exhibiting a dramatic change in the cell spacing from a microcellular to a coarse cellular region was observed for all the alloys examined. For a given alloy, the volume fraction of the microcellular region decreases significantly with increasing ribbon thickness. A theoretical explanation for the observed behavior is presented in which the heat flow and solidification characteristics are related to the undercooling achieved before the onset of nucleation.     

The structure of rapidly solidified Al- Fe- Cr alloys

Four aluminum alloys, designed for use at elevated temperatures, were studied. The alloys were supersaturated with iron and chromium, and one of them contained small amounts of Ti, V, and Zr. The starting materials were alloy powders made by the RSR (Rapid Solidification Rate) centrifugal atomization process. Extrusion bars were made from the four powders. The as-extruded microstructure and the microstructure of the alloys after annealing at 482 °C were investigated by optical and transmission electron microscopy and by X-ray diffraction. The microstructure consists of equiaxed grains of aluminum matrix and two types of precipitates, namely, Al₃(Fe ,Cr) and a metastable phase, Al6(Fe,Cr). The precipitates were different in their shape, size, distribution, and location within the grains.     

The microstructure of rapidly solidifiedβ-phase Cu-Zn-Al alloys

The structure of theβ phase in rapidly solidified Cu-Zn-Al alloys was studied by transmission electron microscopy. The structure is observed to contain many unusual features not found in normal solid-solutioned material. The microstructure is very fine-grained, homogeneous in composition, only partially ordered, and contains an abnormally high density of dislocations. No metastable phases are seen. The grain structure is refined about two orders of magnitude in grain size and is quite variable in size and shape in a given sample. The grain boundaries are highly structured, with drastic curvatures and large steps. Dislocations are seen to be emitted by grain boundaries, contributing to the higher than usual dislocation density. Subgrain structures and low-angle boundaries are present in some regions and isolated martensite plates are observed well above the measuredM _stemperature.     

Structures and tempering behavior of rapidly solidified high-carbon iron alloys

High-carbon iron alloys containing carbide formers of chromium and molybdenum were rapidly solidified by means of a single roller method. In the alloy containing a high level of both chromium and molybdenum (10Cr-5Mo) and a critical carbon content of about 4 pct, the metastable phases,ε phase and austenite, are retained after solidification. Theε phase could contain a large amount of carbon in solid solution so that during tempering at about 900 K, it decomposes to very fine ferrite and carbide, which bring about an enhanced hardness of 1300 DPN. Even after tempering at a high temperature around 1100 K, the hardness hardly deteriorates due to a remarkable dispersion of fine M₃C and M₇C₃ carbides. Thus, coaddition of chromium and molybdenum is effective in obtaining high hardness. Formerly Graduate Student, Kyushu Institute of Technology