The precipitation of silicon in rapidly solidified AlSi alloys was studied. For alloys with 2.4 and 11.0 wt % Si (2.3 and 10.3 at % Si, respectively) the lattice parameters of the AIrich and of the Si-rich phases were measured after ageing at 397,425 and 448 K. For alloys with 2.6 and 13.0wt %Si crystallite sizes and lattice strains were determined by analysis of the X-ray diffraction line broadening. After ageing the lattice parameters of the AI-rich and the Si-rich phases were influenced by the difference in thermal expansion between both phases. After correction for this effect the amount of silicon dissolved in the AI-rich phase was estimated as a function of ageing time. Quenched-in (excess) vacancies influenced the precipitation kinetics. Activation energies for precipitation appeared to depend on the extent of transformation. Further, quenched-in vacancies caused anomalous maxima in the lattice parameter curves. The behaviour of the lattice microstrains on ageing was explained as a result of the disappearance of stresses due to quenching and the introduction and subsequent dissipation of stresses due to precipitation. After completed precipitation stresses due to the difference in thermal expansion between both phases still exist at room temperature.
The precipitation of copper and silicon from the Al-rich matrix in an AI-1.3 at. pct Cu-19.1 at. pct Si alloy was investigated by differential scanning calorimetry (DSC). Both as-extruded (AE) and extruded and solution treated and quenched (solid-quenched, SQ) specimens were studied. The DSC curves of the SQ specimens showed two exothermic effects, A and B. Effect A corresponded to the simultaneous precipitation of silicon and copper, whereas effect B was caused by the transition from the state with the intermediate copper-containing phase, 0', to the state with the equilibrium copper-containing phase, 0. The heat contents of effect A and B could quantitatively be described in terms of solid solubilities before and after precipitation and the heats of precipitation of the phases involved. From this description, it was derived that for heating rates-<20 K/min, copper precipitated as the 0' phase, while for heating rates->40 K/min, copper precipitated mainly as the 0 phase. In SQ specimens, Guinier-Preston (GP)zone formation occurred during aging at room temperature with a rate approximately 104 times slower than in the corresponding binary A1-Cu alloy. For the AE specimens, it was found that during extrusion, precipitation of copper and silicon proceeded to a large extent. However, from DSC experiments and from hardness measurements as a function of aging time at 453 K, it was deduced that copper precipitation had not finished during extrusion. The hardness increase as observed during aging directly after extrusion was interpreted to be due to formation of the semicoherent intermediate 0' phase.
DnHerenlnal ,,¢annnng eah~rnmelry and X ray dnHuaclnun were u,'~ed I1= 'dudv Ihe o~olnng arid healing hale dL'perlderlce .~1 precnpnlal.m nn an AI-I (~h al % ('u alhw Al-let h, miogernznng, oH)Innl, al a tale ~I 2~ K ram-~ (SC22)ns ,,uffncnenl I~ relann all e~pper nr= s~dnd seduln, m (.iP z,~ne lormal.m dunng su [v~equenl heal Irealmenl ns hnndered, lhns n,, ascnhed h~ an nnsulhcnenl numher ~I (excessl vaeancne.~ Aller a waler quench (W()) a lar/.;e number ~I (_iP .',,~nes are h~rmed durnng ,,uhsequenl ,,l~rag, e al r~m lemperalure h~r I h The heal c~mlenl ~I lhe (_iP z,~me dnss~duln~n eHecl can quanlnlal,vely he do, crnl-wd m lerms ~I lhe heal ~I pre~np~laln~n ~I (.iP [ z~me'~ and lhe s~dnd ,.duhnlnlnes as dernved In~m lhe (_iF' I z,~nc .,,dvu,,, The heal omlenl ~I lhe o)mhnned O' //9 phaw preelpllall~m eHe¢l appeared h~ he pr~p~rl.mal I~ lhe number ~I c~pper al~m', preenp~laled, yneldnn~, an averaE, e value h~r lhe heal ~I copper preenpnlal.~n ~I "lh kl m~l ~ c~pper 'The acl~val,)rl energy h~r O' pha,,e h)rmalu~n n~. I. Inlrodu¢lionRecenlly nl has been shuwn lhal lhe heal ~I' precipn lal~n m a suhd quenched AI-Cu allay uennl'~rced wilh snluc'~m parlncles can quanlnlalnvelv he descrnlwd nn terms ~1 the heats ol preopnlaln~n ~1 holh alluynng elemenls and ol s,~lnd solubnlnlies ,~1' Ihe cemslnlulnng hnnaryalhwsll] Further, ulappearedlhal e,~pper precnpnlaliem was dependenl on Ihe healnng rate; al I¢~w healing rales (2() K mnn-i ,~r less)~.'¢~pper preeipilaled mannly as Ihe melaslable O' phase, whereas at hJgh healing rules (.-I()K mnn-n or =nore) ~.'~pper precipitated mannly as the equilibrium 0-phase AIIhough lhe preenpilaln,~n Ir¢~m a supersaluraled AI rneh phase in binary AI-Cu alloys during m~n ~s~-Ihermal agenng has often been sltndned ]2-4], Ihe relaln, m between precnpnlal,_m eHeels and Ihe healing rales applied has, h~ ~ur knowledge, nell been nnvesH galed unlnl n~w The Iull preenpnlal~on sequem.'e nn quenched AI-Cu alh~yn us usually given as Iolh~ws' supersaluraled s~lnd s~lknln¢~n ---," (_;P I ---,. (._;1:' II ---0'--. o (l)where GP 1/11 ,,.,lands I'or Gumner-Preshm 'z,~mes, O' us a Iransnln~m phase havnng a slruclure whnch us a lelrag~mal distorlnon ot the CaF, slruclure and whnch has a e~mposnlnon AI,C'u and 0 ns Ihe equnlnbrnum phase havnng a body cenlred lelragtmal slruclure wnlh the same chemncal eompusitnon as #' (LJnlnl recenl pknhliculn~ms, dnseussnon ahuul the nalknre ~I GP I and GP II zones has persisted I I. GP II zones are omsndered h~ be a slighlly in~dnhed (mullnlayered)h~rm ul (predomnnantlv mLm~dayered)(.;F' I zones, ipn ti~ be a dnfferenl phase (then Ihe nndicaln~m /-/"-phase nnslead ul' GP II zunes us usually prelerred). In Ihns paper Ihe nndncaln~m GP II us used In any ease, GP II us subsequent h~ (.iP I) According I~ Nakamtnra cl ,11 Ihe sLy-called X phase w~uld ~wcur helween (jP II and O' T'hns phase shl~uld be responnnhle h~r the peak hardness durnng agenng i~l quenched AI-Cu alh~ys The X phase has, however, nol been ndenlnlned by X ray t~r Iransmnss,~)n elecl run mncrosc...
Quenched and aged specimens of the A1-1.3 at. pct Cu-19.1 at. pct Si alloy were studied by X-ray diffraction. Since this alloy contains a high volume percentage (-20 vol pct) of secondphase Si particles, it is regarded as a model for a metal matrix composite (MMC). During isothermal aging of the solid-quenched Al-l.3 at. pct Cu-19.1 at. pct Si alloy, the Cu and Si atoms precipitate. This causes the Al-rich phase lattice parameter to increase from a value lower to a value higher than the lattice parameter of pure unstrained aluminum. Due to thermal misfit after quenching from heat-treatment temperatures, all lattice parameters are influenced by residual stresses. A model describing the elastic/plastic accommodation of a misfitting spherical inclusion in an infinite matrix is adapted for the case of misfitting inclusions in a finite matrix. This model describes the measured lattice parameter shifts of the Si phase reasonably well. Comparison of the model for elastic accommodation and the model for elastic/plastic accommodation with measured stresses shows significant discrepancies for the low-temperature range (AT < 200 K). These discrepancies may be related to the volume effect of defects (dislocations, vacancies) created in the plastic zone.
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