Microstructural Evolution and Age Hardening in Aluminium Alloys ...

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102 S. P. Ringer and K. Honomaining problems in the physical metallurgyof age-hardenable aluminium alloys.This is also of current technological interest,given that variations in the heat treatment,strain conditions prior to aging, andalloy composition induce significant changesin microstructure, and there remains considerablepotential for the design of highstrengthaluminium alloys. Recent technologicalimpetus for exploring the alloy designand development has mainly comefrom the aerospace and automobile industries[1].The purpose of this review is to examinethe microstructural mechanisms underlyingthe aging processes in a selection ofwrought heat-treatable alloys. A feature ofthis review is the application of atom probefield-ion microscopy (APFIM) and transmissionelectron microscopy (TEM) in providingdirect observations of the evolutionof microstructure and an insight to the agehardeningprocesses.EXPERIMENTALMost of the studies reported here were performedon scientific alloys. Ingots wereconventionally cast into book molds, homogenizedat 525C for 72 h, and scalped(5mm from each face). Samples (5 10 10mm) were machined for hardness testing,while some material was cold rolled tosheet for TEM studies. Samples for APFIMxwere spark machined from the bulk to 0.3 0.3 10mm or obtained from materialdrawn to wire. Solution treatment was carriedout in either salt or Ar atmospheres,followed by cold-water quenching. Elevatedtemperature aging was performed insilicone oil baths, and hardness monitoredusing a standard Vickers hardness indentorunder a load of 5kg. Samples for APFIMand TEM were prepared using standardelectropolishing techniques [3, 4].Details describing the APFIM instrumentationare provided in references [5–7]. Animportant feature of the APFIM instrumentationused in these studies is the ability tocool specimens to 20K, which allows stablefield ion microscopy (FIM) of aluminiumalloys. Until recently, FIM and atomprobeanalyses of aluminium alloys wasuncommon, while other major industrialmetallic materials such as steels were extensivelystudied. This was mainly becauseof the low evaporation field of Al, whichmakes the observation of FIM images difficult.By lowering the specimen temperatureto 20K, the evaporation field of Albecomes as high as the ionization fields ofNe and He, allowing stable FIM observationsusing these imaging gases [3]. Thefirst FIM image of an Al alloy was obtainedby Boyes et al. [8], using a cold fingercooled down by liquid He. Later, using adedicated FIM, good-quality FIM images ofAl–Cu alloys were obtained by Abe et al.[9], Wada et al. [10], and Hono et al. [11]. Inan atom probe, however, a specimen-tiltingmechanism must be incorporated in theFIM, and achieving 20K is not trivial. Inall of the studies reviewed here, FIM imageswere obtained using either Ne gas at35K, or He gas at 25K. The one-dimensional(1D) atom probes employed eitherPoschenrieder or reflectron energy compensatinglenses. The 1DAP analyses weretypically performed with 0.5nm spatialresolution in the lateral direction andatomic layer resolution in the depth direction.A three-dimensional atom probe(3DAP), equipped with CAMECA’s tomographicatom probe [7] was also employed.Both 1D and 3DAP analyses were performedat a specimen temperature of 20–30K in ultrahigh vacuum (10 11 Torr) usinga pulse fraction of either 15 or 20% and apulse rate of 100 or 600Hz. Visualizationand analysis of the 3DAP data was carriedout using the Kindbrisk SDV 3DAP dataanalysis software running on the AdvancedVisualization System (AVS).The present work also contains resultsfrom conventional transmission electronmicroscopy (CTEM) and high-resolutiontransmission electron microscopy (HRTEM).A suite of microscopes were used includinga Philips CM12 (120kV), Philips CM20(200kV), JEOL 2000 EX (200kV), JEOL 4000
Microstructural Evolution in Aged Al Alloys 103EX (400kV), and a JEOL ARM operating at600kV.Al-Cu BASED ALLOYSThe most widely studied age-hardening alloysystem is Al–Cu, and several commercialalloys based on this system remain inuse in the 2xxx alloy series. Although thealloy is composed of only two elements, themicrostructural evolution is complex, andthe precipitation sequence varies dependingon the degree of supersaturation andthe aging temperature. Figure 1 shows theAl-rich corner of the equilibrium Al–Cuphase diagram, and includes the metastablesolvus boundaries for GP zones, and. Over many studies [9–29], it has beenproposed that the decomposition sequencein this system contains one or more of thefollowing processes:Supersaturated Solid Solution ( SSSS)→GP zones →θ'' →θ' →θThe complete precipitation sequence canonly occur when the alloy is aged at tem-peratures below the GP zone solvus (Fig.1). Various steps in this process may besuppressed by aging at temperatures closeto or above the intermediate solvus temperatures.Figure 2 shows a hardness–timeplot for Al–1.7Cu-aged at 130 and 190C,which represents temperatures below andabove the GP zone solvus temperature, respectively.The first stage of hardening at130C is attributed to the formation of GPzones. After reaching a critical diameter ofbetween 5 and 10nm, an incubation periodcommences, during which the zone sizeand the hardness remain constant [15–17].Further aging results in a second rise inhardness, attributed to precipitation. Theformation of is also followed by a shorterincubation period and the subsequent formationof the metastable phase. Prolongedaging results in the formation of theequilibrium phase. Each precipitationstage does not necessarily correspond tothe stages observed in the hardness curve,and more than two phases can coexist at agiven stage of the aging process. The mechanismof the transformation sequence fromone phase to another usually involves heterogeneousnucleation at the sites of earlierproducts, resulting in fine and uniform precipitatedispersions. However, under a suitabledegree of supersaturation, the productsin the above evolutionary sequencenucleate directly into the matrix [17]. ThisFIG. 1. Al-rich corner of the Al–Cu phase diagramshowing the metastable solvus boundaries for GPzones, and , together with the equilibrium solvusline for the phase. After Beton and Rollason [20] andMurray [21].FIG. 2. Hardness–time plot for Al–1.7Cu aged at 130and 190C, showing the effect of a 0.01 Sn addition.After Hardy [16].
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