Microstructure of rapidly solidified laser molten AI-4.5 Wt Pct cu surfaces

The microstructure of rapidly solidified laser molten or electron beam molten Al-4.5 wt pct Cu alloyed surfaces has been investigated. A variety of electron microscopy techniques was employed. The epitaxially resolidifying melt undergoes three different solidification modes: about 3 μm to 5 μm of material near the fusion line resolidify in a plane front mode. The next bulk resolidifies in a cellular dendritic mode, which then turns dendritic. The major impact of resolidification is refinement of the surface microstructure, which by itself shows various characteristics for the different resolidification modes. Particularly, the validity of the relationship of secondary interdendritic arm spacing, as inversely dependent on the one-third power of the average local cooling rate, was verified for cooling rates up to 10⁶ K per second.     

快速凝固喷射沉积制备Al-40Si组织分析

针对应用广泛的低密度、低膨胀、高热导、高比强的高硅铝合金,采用快速凝固喷射沉积技术制备了Al-40Si高硅铝合金锭坯,通过光学显微镜及扫描电镜对其组织进行了分析.结果表明：合金组织特征为初生Si相均匀弥散分布于α-Al基体中,未出现共晶Si组织;随着合金中Si含量的增加,初生硅相的数量增加,平均尺寸增大;本试验所制备的Al-40Si合金中初晶硅粒子大小为5～30μm,并随合金锭坯部位的不同,初生Si大小不同,中心部位最小,底部次之,边部最大.     

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.     

Structure and initial precipitation in a rapidly solidified nickel superalloy

A nickel base superalloy (Nimonic 80A) has been rapidly solidified at cooling rates of between 10⁵ to 10⁶ K.S^-1 by pendant drop melt extraction and by chill block melt spinning in an evacuated chamber backfilled with helium or argon. The internal structure is described in terms of process variables pertaining to rotating chill block quenching techniques. Both transmission electron microscopy and atom-probe field-ion microscopy have been employed to give structural and constitutional data on quenched and aged specimens. The as-quenched structure is homogeneous apart from fluctuations in titanium concentration which upon aging undergoes a spinodal phase decomposition to form disordered Ni₃(Ti,Al,Cr) precipitates in the matrix, which after prolonged aging produces ordered γ (Ni₃(Ti,Al)). inin6 particles form readily on grain boundaries and also appear in conjunction with ordered γ, via a discontinuous reaction, after short aging times.     

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.     

Phase distributions in rapidly solidified stainless steels

Phase distributions and the internal magnetic fields have been determined in rapidly solidified stainless steels (Fe-nCr-8Ni-0.05C, Fe-nCr-5Ni, and Fe-nCr withn in the range of 10 to 24) by transmission and conversion electron Mössbauer spectroscopy (TMS and CEMS). Based on these results, a modification of the phase boundaries in the Schaeffler diagram is suggested to account, in particular, for rapidly solidified stainless steels. The suggested modification is primarily an expansion of the austenite field toward higher Cr and lower Ni equivalent contents. Combining CEMS and TMS makes it possible to determine the phase distributions both in the near surface region (outmost 300 nm) and in the bulk of the ribbons. For the low-Cr alloys, the content of the bcc phase (martensite) in the surface region is higher than in the sample as a whole. In the high-Cr alloys, the content of the bcc phase (ferrite) is lower in the surface than in the bulk. This disparity is ascribed to the different mechanisms of formation of martensite (diffusionless) and ferrite (nucleation and growth) in relation to the higher cooling rates of the surface layers. The determinations of the internal magnetic field are in good agreement with earlier investigations on conventionally processed Fe-Cr steels, where it was found that the internal magnetic field decreases with increasing Cr content.     

Microstructural characterization of a rapidly solidified Al-Mg-Mn powder alloy

A detailed investigation concerning the microstructural characteristics of a rapidly solidified Al-Mg-Mn powder alloy has been carried out. A range of microstructures was observed in the atomized powder: dendritic, cellular, and featureless morphologies. Variations in structure have been related to cooling rate, nucleation undercooling, and recalescence during solidification. A metastable Al-Mn phase was created during atomization and was present in two distinct morphologies. Decomposition behavior during thermal and thermomechanical processing has been studied. An interesting feature was the retention of the metastable phase through the initial consolidation stages.     

Microstructural investigation of a rapidly solidified 12Cr-Mo-V steel

Rapidly solidified martensitic stainless steel (11.59Cr-0.98Mo-0.28V (in wt pct) ribbons have been produced by the melt-spinning process. The microstructure of the ribbons showed three distinct zones: a columnar, a cellular, and a cellular-dendritic zone. The height of the columnar grain zone is independent of the process parameters such as the wheel material or the wheel velocity. Due to a high level of undercooling and a high growth velocity of the solid/liquid interface, the rapid solidification process is found to suppress the formation of δ-ferrite and enhance the formation of austenite. The austenite is transformed into martensite upon cooling. In comparison with conventional solidification, a reduction in the initial austenite grain size has been found to result in a very fine lath martensite (M) structure. Investigations of the texture within the ribbons along the growth direction show a weak fiber texture. Transmission electron microscopy (TEM) has revealed a [111]_M1 ‖ [001]_M2 and (011)_M1 ‖ (110)_M2 orientation relationship between two neighboring martensite laths. The observed orientation relationship is a result of a superposition of both the Kurdjumov-Sachs (K-S) and Nishiyama-Wasserman (N-W) orientation relations.     

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。     

Two-dimensional grain growth in rapidly solidified succinonitrile films

The kinetics and topological mechanisms of normal grain growth have been examined throughin situ dynamic studies on rapidly solidified succinonitrile (SCN). Thein situ studies allowed for continuous monitoring of the evolution of individual grains during growth. We have assessed the Mullins—Von Neumann topological grain growth law and the Burke—Turnbull parabolic rate law and have determined rate constants that describe grain growth. This work demonstrates that both laws are both obeyed globally and consistently. Thesein situ studies permit one to follow the unit operations associated with grain growth kinetics. This article demonstrates the usefulness of succinonitrile as a model analog system for studying grain growth. This article is based on a presentation made in the symposium “Fine Grains And Their Growth in Rapidly Solidified Materials,” TMS Materials Week ’93, Pittsburgh, PA, October 18–21, 1993.     

Microstructural characterization of rapidly solidified Al-Li-Co powders

A study of the combined effect of alloying elements and melt superheat has been carried out on the as-solidified structure of rapidly solidified Al-Li-Co powders. Three alloys,viz., Al-3 pct Li, Al-3 pct Li-0.4 pct Co, and Al-3 pct Li-0.8 pct Co were chosen, and the liquid melt in each alloy atomized from the temperatures 1173 and 1073 K, using the centrifugal atomization technique. The microstructural characterization was done using light, scanning, and transmission electron microscopy. Four types of microstructures,viz., dendritic, cellular, equiaxed-type, and featureless structures, were observed by light microscopy. The cooling rate, as determined from the same, lay in the range 10⁴ to 10⁶ Ks⁻¹, but was seen to go beyond 10⁷ Ks⁻¹ when estimated from TEM micrographs. On the micro-level, the Al-Li powders were found to exhibit dendritic structures with differing morphologies, whereas low-angle cell walls with perturbed interfaces were the main structural features observed in the Al-Li-Co alloys. Increasing both cobalt content and powder particle diameter favored transition from dendritic into cellular structure. The featureless zone was comprised mainly of elongated columnar grains (0.2 μm width and 1.5 μm length). A mechanism describing the cellular structure formation has been proposed. Aging of the melt-quenched powders at 473 K for times up to 100 hours results in the dissolution of the cellular structure. A mechanism for the same has been postulated. The difference in the superheats chosen in the present work is found not sufficient to cause drastic microstructural changes. Central Metallurgical Research and Development Institute, National Research Centre, Dokki, Cairo, Egypt     

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.     

Identification of precipitate phases in a mechanically alloyed rapidly solidified Al-Fe-Ce alloy

The stable and metastable precipitate phases which form during mechanical alloying of a rapidly solidified (RS) Al-8.4Fe-3.4Ce alloy have been unambiguously identified using X-ray diffraction, transmission electron microscopy (TEM), and energy dispersive spectroscopy techniques. The metastable Al₁₀Fe₂Ce and stable Al₁₃Fe₃Ce and Al₁₃Fe₄ intermetallic phases, with crystal structures and lattice parameters as reported in the literature, have been identified. It is shown that the metastable Al₁₀Fe₂Ce intermetallic phase particles have elongated shapes and their sizes range between 100 and 200 ran and are free of any localized faults, whereas the equilibrium Al₁₃Fe₃Ce and Al₁₃Fe₄ intermetallic phases are equiaxed in shape and have particle sizes ranging from 200 to 500 nm. It is suggested that the presence of the metastable Al₁₀Fe₂Ce in this material is due to its incomplete transformation to the equilibrium Al₁₃Fe₃Ce phase.     

Phase transformation-induced grain refinement in rapidly solidified ultra-high-carbon steels

Superplastic forming offers a promising approach for reducing the cost of high-performance metal components with complex shapes. Severe thermomechanical deformation is one method for producing the fine grain structure needed to permit superplastic forming economically. Our approach to generating fine-grained microstructures is by cyclic heat treatment of rapidly solidified material. First, a metastable structure is produced by rapid quenching of the liquid metal. Then, solid-state phase transformations at modest temperatures are employed to refine this structure. In the ultra-high-carbon steels (UHCS) studied, the brittle as-cast structure of martensite and austenite was transformed, after cyclic heat treatment, to a ductile mixture of 1-μm ferrite and 0.25-μm carbide. Varying the heat-treat temperatures by 100 °C within the transformation range had little effect on the scale of the microstructure. Higher C resulted in coarser carbide spheroids, addition of Al refined the microstructure, and the finest mean carbide size was obtained with an intermediate level (5 pct) of Cr. Refinement of the martensite plates retained austenite via cyclic tempering and austenitization was found to be the key step in the overall mechanism for phase transformation-induced grain refinement in rapidly solidified UHCS.