Acta Technica Corviniensis

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ACTA TECHNICA CORVINIENSIS – BULLETIN of ENGINEERINGcrystallization; is the possibility to used modifiercontaining Na as S, P, etc.[2]; Like disadvantagesATSi 7 Mg shows: reduced machining by cutting;with increasing concentration of Mg decreasesplasticity (mechanical R m increases and thecapability of machining); resistance to corrosionfrom HNO 3 ; reduced refractivity, which mayincrease with increasing concentration of Si, Mgor Cu.The hardening basic phase of ternary alloys isMg 2 Si noted as β’, the type chemicalintermetalic component (determined by normalvalence). Possess crystalline elementary cubenetwork and does not form solid solutions αwith its constituents, what is characteristic ioniccombinations in contrast with benthonic phasestype (ex: Mg 2 Al 3 ).Fig. 1 Structure of alloy ATSi 7 Mg (10000:1) aged a-135oC/15 h and b- a-135 o C/15 htemperatures and slow at ambienttemperature. Reset of atoms position hadplace with the distortion is strong crystallinenetwork and consequently there is as resultstrong growth mechanical resistance ofsiluminium type alloys at ambienttemperature and reducing refractivity oftemperature used in aging process (160-170oC and higher).2. There are a formations of small grains monoandtwo-dimensional phase metastabileβ’(Mg 2 Si) which possess crystalline hexagonalnetwork. It is believed that the training phaseMg 2 Si is the main cause of increasingresistance of alloys Al-Si-Mg, but with theapparition of stable phase Mg 2 Si therefractivity ternary alloys will be reduce .Thisclear happened at heated at 170-180 o Cretention time of 25h to return treatment.3. Stable phase Mg 2 Si is formed in alloys typesiluminium in process to maintain at 185-220 o C for several hours and at 300 o C for 30minutes to aging, with strong reduceresistance of alloy.The alloy aging process at 180-225 o C withreduced times of process can provide forATSi7Mg alloy high strength and low plasticity.Refractivity of these alloys can be strongincreased by two ways:It strengthens the solid solution α by acomplex process of alloying which assurethe separation of the granular stable phasescrystallized in form of branchesmake an bonding of Si free (elementary) inthe stabile component (Al 8 Si 6 Mg 3 Fe, Al 4 Si 2 Fe,Al 5 SiFe etc.) An example of thesecomponents is phase Mg 2 Si which is found inmost siluminiu alloys (ATSi 12 , ATSi 7 Mg, etc.)Fig.2 Structure of alloy ATSi 7 Mg(10000:1) aged a-165 o C in times:a15h b-25h, c-100hRegarding the influence of Si and Mg on theeffect of loss of mechanical resistance of alloysAl + Mg + Si at high temperatures [5], [6], [2],[8] ,it shows that the decomposition of α solidsolution of Mg and Si in Al folow the nextschedule:1. Appear in the crystal network of solidsolution the Guillet-P zone. There is achange to approaches atoms of Si and Mgand formation of metastabile β’ phase;(Mg 2 Si) - the process is very intense at highFig. 3 Structure of alloy ATSi 7 Mg (10000:1)aged a-175 o C/10 hMg 2 Si phase is formed through a series oftransformations that strongly distorts thecrystalline network of alloy and increases the422008/ACTA TECHNICA CORVINIENSIS/Tome I
ACTA TECHNICA CORVINIENSIS – BULLETIN of ENGINEERINGmechanical resistance at ambient temperature(due to hardening and aging processes).Fig. 4 Structure of alloy ATSi 7 Mg(10000:1)aged:a-185 o C /15h , b-200 o C /10h,c-300 o C /10h, d-300 o C /100hINTERPRETATIONSIn work [8], [9] it shows that the ternary alloysystem Al-Si-Mg, were first observed inaluminum matrix areas which characterizestages of ante-separation. There not wasestablished neither by X-ray observations thedifferences in areas structure for aging alloys.Despite lack evidence of structural changes,increased of mechanical properties of ternaryalloys is a fact. Only at a temperature of 150 o Cauthors [9] in 1958 established that theseparation distributed locally of Mg and Si takeplace.It is believed (with network vacancies which areformed during the hardening process) atoms ofalloying elements Ea begin to be collected inthe chains without any order, after which theatoms slowly sits in an order determined the bynetwork parameter (4.04 Å) and the chain getsthe same format with the elementary cell of thematrix. At high temperatures the atoms of Eaformed construction areas least different of thematrix. It shows [9] that this phase β’’ movegradually in the phase β’ rebuilding thecrystalline matrix is accompanied by a strongdistortion which is the main cause of increasingthe mechanical properties of alloys siluminium.But such a tensioned state of the crystallinenetwork helps to reduce refractory of alloys. Thisis observed as a strong fact, in changing thestructure of α solid solution alloy type ATSi 7 Mgat 165°C in the ageing process where thedecomposition of α solid solution takes placerelatively quickly.Figure 1shows the solid solution alloy ATSi 7 Mg inwhich ageing at 135 o C for 15 hours produceformation and deposition of fine dispersedelemental Si [5].In addition to this Si there aredeposits in the form round (white points)probably the Guillet areas or how to say „phase”β’’ .We say that such products of the decomposingsolid solution which is formed as chains in thealloys Al-Mg-Si is characteristic for the initialstage of aging. The structure of ATSi 7 Mg aged at150 o C for 15 h (Figure 1.b) confirms the words[16].At higher temperatures the return process andthe size of elementary high dispersed Siparticles and a separation (white point) increasedramatically. (Figure 2). Structure of α solidsolution after aging for 15 and 25 h (Figure 2 a, b)at 165 o C has many deposits of Si and formationof β’’ metastabile phase, and even the stablephase Mg 2 Si (fig. 2 c)From Figures 2 and 3 it is clear that withincreasing aging temperature there is thedecomposition of solid solution α is veryintense and is formed by deposition metastabilephases that increase substantially with agingtime. Very intense decomposition α solidsolution had place at 175 o C with themaintenance of 10 h.The figure 4 shows a large amount of fine andsmall and many particle high dispersed portionsof Si I (primary) that were formed after reset thenetwork. Particles Mg 2 Si (in the form of narrowwhite strips) are shown oriented.Very suggestive is the alloy ATSi 7 Mg old aged at185 o C for 15 h (Figure 1 a). But I clearly see thisin Figure 1 b α solid solution decomposition atthe aging temperature of 200 o C, virtually endsafter 10h. This produce reduced resistance ofalloy ATSi 7 Mg at temperature of 200 o C.2008/ACTA TECHNICA CORVINIENSIS/Tome I 43
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