Materials Science and Engineering A (2007)

Transcription

1 Materials Science and Engineering A (2007) Effects of individual and combined additions of Be, Mn, Ca and Sr on the solidification behaviour, structure and mechanical properties of Al 7Si 0.3Mg 0.8Fe alloy S.S. Sreeja Kumari, R.M. Pillai, T.P.D. Rajan, B.C. Pai Regional Research Laboratory (CSIR), Thiruvananthapuram , Kerala, India Received 17 May 2006; received in revised form 20 January 2007; accepted 22 January 2007 Abstract The effects of individual and combined additions of Be, Mn, Ca and Sr on the solidification, structure and mechanical properties of Al 7Si 0.3Mg 0.8Fe alloy has been investigated. Thermal analysis study shows that all additions except Be show a peak corresponding to -Fe intermetallic phase formation. Ca and Sr additions decrease the eutectic temperature (by 6.6 and 8.7 K, respectively) compared to the untreated alloy and lead to modification of eutectic Si from platelet to fibrous form. The peak corresponding to Be Fe phase has been identified by DTA. Mn (0.4%) and Be (0.2%) additions to Al 7Si 0.3Mg 0.8Fe alloy change the morphology of platelet -phase to script form leading to significant improvement in tensile and both tensile and impact properties, respectively. Addition of Be, Ca and Sr to Mn added Al 7Si 0.3Mg 0.8Fe alloy improves the impact strength significantly due to the modification of the eutectic Si from acicular to fibrous form and refinement of platelet -phase in addition to its morphological change from platelet to Chinese script form. Combined additions of Be + Mn, Ca + Mn and Sr + Mn result in a reduction in the impact strength compared to individual additions of Be, Ca and Sr. The best combinations of both tensile and impact properties have been obtained with the combined additions of Be + Mn, Ca + Mn and Sr + Mn to Al 7Si 0.3Mg 0.8Fe alloy. A trace amount of Be (0.005%) addition to Ca and Sr leads to superior tensile properties compared to individual additions of Ca and Sr Elsevier B.V. All rights reserved. Keywords: Al 7Si 0.3Mg alloy; Fe intermetallics; Beryllium; Manganese; Calcium; Solidification 1. Introduction Cast Al Si alloys find widespread applications especially in the automotive and aircraft industries due to their excellent combination of both casting characteristics and mechanical properties. Over the years, these alloys have been specially developed to meet the increasing demands of today s industry, which has resulted in the production of smaller and light-weight components to comply with property, environmental and other specifications [1]. Recently, appeal for recycling of resources is becoming more and more intensive with increasing public awareness on conservation of materials and energy and protection of environment. However, the increasing use of recycled aluminium casting alloys warrants strict process control to remove the ill effects of impurity elements [2]. Corresponding author. Tel.: ; fax: address: rmpillai@rediffmail.com (R.M. Pillai). Iron is the most common and harmful impurity in cast aluminium alloys forming platelet form of iron intermetallic phase ( ), which is detrimental to the mechanical properties and fracture toughness. Aluminium foundry alloys always contain a certain quantity of iron. In addition, it is sometimes unintentionally added to the molten metal by various means, particularly through the dissolution of the iron tools often used during melting and casting. The raw materials such as aluminium and silicon used for the preparation of alloys also contain a certain unavoidable amount of iron [3,4]. In order to achieve the best mechanical properties, either the Fe content has to be maintained as low as possible or the size, shape and distribution of intermetallic compounds should be modified. Among the various methods available to neutralize the ill-effects of iron, neutralization by adding trace elements [5] is commonly practiced. Be has been found to be the most effective element in improving the mechanical properties of Al Si Mg alloys [6 9]. However, Be is carcinogenic. Manganese is the most commonly used and the least expensive element for Fe neutralization in /$ see front matter 2007 Elsevier B.V. All rights reserved. doi: /j.msea

2 562 S.S. Sreeja Kumari et al. / Materials Science and Engineering A (2007) Al Si alloys. However, higher amounts of Mn required for iron neutralization lead to sludge formation affecting machinability [6,10 12]. Further, it has been reported that Mn does not significantly reduce the deleterious effect of Fe on fracture toughness [10]. The authors have carried out a detailed study on the Fe neutralizing effect of Ca in Al 7Si 0.3Mg xfe alloy [13]. It has been found that Ca modifies the eutectic Si from acicular to fibrous form and refines the platelet -phases leading to improvement in elongation and impact strength of Fe containing Al 7Si 0.3Mg alloy significantly. There are considerable evidences [14 16] for the Fe neutralizing effect of Sr in Al Si Cu alloys. Hence, a combination of Mn and/or Be, Ca, Sr can improve both the tensile and impact properties of Fe containing Al 7Si 0.3Mg alloy. On the other hand, it has been reported that [13,17] higher amounts of Ca and Sr lead to porosity in the castings. Addition of a small amount of Be in Al Si alloys is known to have several benefits. It preferentially oxidizes forming BeO on the surface of the casting, reducing Mg loss and making Mg available for forming the required volume of Mg 2 Si for strengthening [18]. Hence, addition of a trace amount of Be in Ca and/or Sr containing Al 7Si 0.3Mg 0.8Fe alloy can reduce oxidation of the melt, and hence the porosity leading to improved mechanical properties. In view of this, the present study aims to investigate the effects of Be, Mn, Ca and Sr (individually and in combination) on the solidification behaviour, structure and mechanical properties of Al 7Si 0.3Mg 0.8Fe alloy. were immersed in the melt at 1003 ± 5 K using a coated and preheated mild steel plunger and stirred for 2 3 min to enable complete dissolution and homogenization. The melt was held at this temperature for about 20 min (holding time). All percentages in the text, figures and table are in weight percent only. The melt was thoroughly stirred, skimmed and poured (pouring temperature ranged between 993 and 1003 K) into a preheated rectangular permanent mould (preheat temperature 573 K) and cylindrical permanent and sand moulds (preheat temperatures 473 and 573 K, respectively). The rectangular casting (19 mm diameter 200 mm length) was used for making tensile and impact specimens. Cylindrical castings (27.5 mm diameter 55 mm length) were utilized for metallography (Leitz Metalloplan Optical microscope) and Scanning Electron Microscopy/EDX (Jeol Scanning Electron Microscope). The solidification behaviour of the alloys was studied using Melt- Lab Aluminium Thermal Analyser and Differential Thermal Analysis (DTA) using a SETSYS apparatus from SETARAM. All the tensile and impact specimens were solutionized at 808 K for 12 h, quenched in hot water (353 K) and aged at 2. Materials and methods Commercial grade Al 7Si 0.3Mg [LM25 (BS 1490)/356] alloy was used as the base alloy. The chemical composition of the alloy is given in Table 1. For each experiment, about 3 kg of the alloy was melted in a clay graphite crucible of 5 kg capacity using an electric resistance furnace. The melt was subjected to fluxing and degassing using commercially available fluxes and degassing tablets, respectively. The main reason for the choice of a high Fe content alloy (about thrice the usual limit) was to promote the formation of large intermetallics in the microstructure. Amount of Be and Mn to be added was chosen as per the optimum level reported for the alloy in the literature [6]. It has been found [19] that the optimum amount of Ca for Fe neutralization lies in the range of %. Since the properties obtained with Ca modification are at par with those obtained with the well known modifier viz., Sr (0.02%), the amount of Ca and Sr for the comparison and interaction study has been taken as 0.04%. For all the experiments, the processing temperature of the melts ranged between 993 and 1013 K. Required amounts of Be, Mn, Ca, Sr and Fe were added in the form of Al 5% Be, Al 10% Mn, Al 10% Ca and Al 10% Sr master alloys and ALTAB Fe compact (75% Fe and 25% Al), respectively. The alloying additions Table 1 Chemical composition (wt.%) of the Al 7Si 0.3Mg (LM25/356) alloy Si Fe Cu Mn Mg Ni Zn Sn Pb Ti Balance Al Fig. 1. Typical microstructures of permanent mould cast Al 7Si 0.3Mg alloy with (a) 0.8% Fe, circled area showing the branching of -platelet and (b) higher magnification of the branched platelet, arrow showing nucleation of eutectic Si by -platelets.

3 S.S. Sreeja Kumari et al. / Materials Science and Engineering A (2007) Fig. 2. (a d) SEM micrograph of permanent mould cast Al 7Si 0.3Mg 0.8Fe alloy showing branched -platelet. 438 K for 9 h prior to tensile and impact testing. Tensile and impact testing were carried out in an Instron Universal Testing Machine at a crosshead speed of 2 mm/min and a Pendulum type Charpy Impact Testing Machine (PSW-423), respectively. For each composition, four tensile specimens made from the permanent mould casting were tested and the average value was computed. 3. Results 3.1. Microstructure The authors have observed [13] that increasing Fe content in Al 7Si 0.3Mg alloy results in the precipitation of long platelet -phase, whose amount and size increase with increasing Fe content. Fig. 1(a and b) shows the typical as cast microstructures of permanent mould cast Al 7Si 0.3Mg 0.8Fe alloy. It has been seen that platelet -phases are present in the interdendritic regions as well as precipitated along with the eutectic Si. Smaller platelet -phases are branching out from the parent needle spanning across the matrix (Fig. 1(a) circled area). This can be clearly seen in the SEM micrograph (Fig. 2(a d)) of the deep etched sample. The network of platelet -phases extending over large distances in the matrix may affect the feedability of the alloy during casting. Fig. 1(b) is a typical micrograph showing eutectic Si nucleating at several locations along the length of a large platelet -phase Effect of Be, Mn, Ca and Sr Fig. 3 shows the typical as cast microstructures of permanent mould cast Al 7Si 0.3Mg 0.8Fe alloy with Be (0.2%) addition. It is seen that Be addition replaces the platelet phases by globular and Chinese script Fe-intermetallic phases. It is to be noted that majority of the Be Fe phases exist in the interdendritic regions while very few inside the -Al. It is also observed that Be added sample cast in permanent mould shows refined Chinese script phases. From the XRD pattern (Fig. 4), the secondary phases identified in the Al 7Si 0.3Mg 0.8Fe and Al 7Si 0.3Mg 0.8Fe 0.2Be alloys are (Al 9 FeSi 3 and Al 5 FeSi) and Al 2 FeBe 3, respectively. Fig. 5 shows the typical as cast microstructure of permanent mould cast Al 7Si 0.3Mg 0.8Fe alloy with 0.4% Mn addition. It is seen that addition of 0.4% Mn to Al 7Si 0.3Mg 0.8Fe alloy results in both platelet -phases and Chinese script Fe-intermetallics with the dominance of the latter. The EDS analysis of the Al Mn Fe Si compound indicates the presence of Al, Si, Fe and Mn and their distributions are shown in Fig. 6. Fig. 7(a and b) shows the typical as cast microstructures of permanent mould cast Al 7Si 0.3Mg 0.8Fe alloy with calcium and strontium additions, respectively. It is seen that both Ca and Sr modifies the eutectic Si to fine fibrous form and reduces the size of the platelet -phases. It has also been observed that the platelet -phases are precipitated along with the eutectic Si.

4 564 S.S. Sreeja Kumari et al. / Materials Science and Engineering A (2007) Fig. 4. X-ray diffraction patterns of (a) Al 7Si 0.3Mg 0.8Fe and (b) Al 7Si 0.3Mg 0.8Fe 0.2Be alloys. to the replacement of platelet -phases to Chinese script Mn Fe phases (Fig. 9) Solidification behaviour Thermal analysis Figs show the cooling curves and their first derivatives and the microstructures (at the centre of the thermal analysis sample, where the thermocouple tip was located) and Table 2 gives the alloys, thermal arrests and phases formed in Al 7Si 0.3Mg 0.8Fe alloy without and with Be (0.2%), Mn (0.4%), Ca (0.04%) and Sr (0.04%) additions, respectively. The first derivative is a measure of the instantaneous cooling rate along the cooling curve and is used here to signal the presence of minor slope changes on the curves. For each alloy sample, two cooling curves were taken to check repeatability. Fig. 10 shows first thermal arrest point at K, where the formation and growth of Al nuclei occur. After this arrest, the temperature of the solidifying alloy continues to decrease. Primary Al dendrites grow and the liquid is progressively enriched in Si and Fe. Second thermal arrest occurs at 858 K. This is caused Fig. 3. (a c) Typical as cast microstructures of permanent mould cast Al 7Si 0.3Mg 0.8Fe 0.2Be alloy Interaction of Be, Mn, Ca and Sr It has been observed that addition of a trace amount of Be (0.005%) to Ca (0.04%) and/or Sr (0.04%) added Al 7Si 0.3Mg 0.8Fe alloy has no significant effect on the microstructure. However, both Mn Fe and Be Fe Chinese script intermetallic compounds and acicular eutectic Si have been observed (Fig. 8) in the combined addition of Be (0.15%) and Mn (0.15%) in Al 7Si 0.3Mg 0.8Fe alloy. The combined addition of Mn (0.3%) and Ca or Sr (0.04%) to Al 7Si 0.3Mg 0.8Fe alloy leads not only to the modification of eutectic Si from acicular to fibrous form, and refinement of platelet -phases but also Fig. 5. SEM micrograph of Al 7Si 0.3Mg 0.8Fe alloy with 0.4% Mn addition.

5 S.S. Sreeja Kumari et al. / Materials Science and Engineering A (2007) Fig. 8. SEM micrograph of Al 7Si 0.3Mg 0.8Fe alloy with Be (0.15%) + Mn (0.15%) addition. -phases with Ca and Sr additions. Further, Ca and Sr additions decrease the eutectic temperature compared to the untreated alloy and the difference between the eutectic temperature of the untreated and Ca and Sr added sample is 6.6 and 8.7 K, respectively. This leads to the modification of eutectic Si from platelet to fine fibrous form by Ca and Sr additions as shown in the microstructures. Fig. 6. EDS elemental X-ray mapping and elemental distribution of Chinese script Mn Fe phase. by the latent heat fusion of the platelet -phase. Next arrest occurs at K corresponding to the eutectic Si formation and growth. The final arrest point occurs at K when the Mg 2 Si phase is formed. On the other hand, the peak corresponding to platelet -phase is completely absent with Be (0.2%) addition. This shows that the platelet -phase has been changed to -(Be Fe) phase as shown in the microstructure. However, the peak corresponding to platelet -phase formation is seen in the case of Mn, Ca and Sr additions. This is also evident from the microstructures (Figs ) showing both platelet and Chinese script phases with Mn addition and refined platelet Differential thermal analysis Differential Thermal Analysis (DTA) has been carried out in Al 7Si 0.3Mg 0.8Fe and Al 7Si 0.3Mg 0.8Fe 0.2Be alloys at a heating and cooling rate of 2 K/min and the corresponding DTA curves are given in Figs. 15 and 16. The peak corresponding to platelet -phases could not be detected since it merges with the eutectic Si reaction as shown in the cooling curve (Fig. 15). It is seen from the Fig. 16 that the temperature of Be Fe phase dissolution (heating curve) and the precipitation (cooling curve) are higher than the peak corresponding to platelet -phase formation (858 K from thermal analysis). As a result, the amount of free iron available to precipitate in the form of platelet -phase is diminished. This is supported from the microstructural observation of the replacement of platelet -phases by Chinese script phases. Fig. 7. Typical microstructures of permanent mould cast Al 7Si 0.3Mg 0.8Fe alloy treated with (a) 0.04% Ca and (b) 0.04% Sr.

6 566 S.S. Sreeja Kumari et al. / Materials Science and Engineering A (2007) Fig. 9. Typical microstructure of permanent mould cast Al 7Si 0.3Mg 0.8Fe alloy with (a) Mn (0.3%) + Ca (0.04%) and (b) Mn (0.3%) + Sr (0.04%) additions. Fig. 10. (a) Cooling curve and its first derivative and (b) microstructures of Al 7Si 0.3Mg 0.8Fe alloy thermal analysis sample Mechanical properties Tensile properties The ultimate tensile strength and % elongation of Al 7Si 0.3Mg 0.8Fe alloy treated with Be, Mn, Ca and Sr individually or in combination and cast in permanent mould are shown in Fig. 17(a and b), respectively. It has been observed that all additions except Mn, Ca and Sr increase the UTS compared to the untreated alloy. The highest improvement in UTS has been observed in the combined addition of Be (0.15%) and Mn (0.15%) to Al 7Si 0.3Mg 0.8Fe alloy. On the other hand, Mn (0.4%), Ca (0.04%) and Sr (0.04%) additions have reduced the UTS by 8.6, 9.2 and 5.3%, respectively compared to that of untreated Al 7Si 0.3Mg 0.8Fe alloy. It has also been observed that the addition of Be, Mn, Ca and Sr both individually and in combination increases the % elongation of the alloy (Fig. 17(b)) compared to the untreated alloy. This is due to the morphological change of platelet -phases to Chinese script form with Be and Mn additions and the refinement of platelet -phases with Ca and Sr additions, respectively. Addition of Mn (0.3%) to Al 7Si 0.3Mg 0.8Fe 0.04% Ca and Al 7Si 0.3Mg 0.8Fe 0.04% Sr alloys leads to 75 and 167% improvement in elongation, respectively, compared to individual addition of Ca and Sr. Be (0.2%) addition has shown significant improvement in elongation (125%). It is interesting to note that trace amount of Be (0.005%) with Ca or Sr (0.04%) has improved the UTS (2 and 5.5%, respectively) and elongation (100 and 167%, respectively) compared to the untreated and Ca and Sr alone added alloys Impact strength The effect of Be, Mn, Ca and Sr addition individually and in combination on the impact strength of T6 heat-treated Al 7Si 0.3Mg 0.8Fe alloy is shown in Fig. 18. It has been found that all additions except Mn improve the impact strength compared to the untreated alloy. However, addition of Be, Ca and Sr to Mn added Al 7Si 0.3Mg 0.8Fe alloy improves the impact strength significantly. On the other hand, combined additions of Be + Mn, Ca + Mn and Sr + Mn lead to a reduction in the impact strength compared to individual additions of Be, Ca and Sr. Fig. 19(a f) shows the SEM fractographs of the impact tested Al 7Si 0.3Mg 0.8Fe alloy without and with Mn, Ca, Sr, Ca + Mn and Sr + Mn additions, respectively. It is seen that Al 7Si 0.3Mg 0.8Fe alloys without and with Mn show cleavage fracture typical of brittle failure, whereas Ca and Sr added alloys show more dimples typical of ductile failure. On the other

7 S.S. Sreeja Kumari et al. / Materials Science and Engineering A (2007) Fig. 11. (a) Cooling curve and its first derivative and (b) microstructures of Al 7Si 0.3Mg 0.8Fe 0.2Be alloy thermal analysis sample. hand, combined additions of Mn with Ca and/or Sr show both dimples and cleavage fractures. 4. Discussion The large size of platelet -phases observed in the untreated alloys indicates that they must have formed at a higher temperature, i.e., as a pre-eutectic reaction (providing a longer time for its growth). On the other hand, the much smaller size of the platelet -phases observed indicates that they are very likely the products of a co-eutectic reaction. It has been reported [20 22] that unlike the Si eutectic temperature, which is only marginally affected by variations in cooling rate, the onset of platelet phase onset temperature decreases with decreasing iron content, increasing cooling rate and increasing melt superheat until it eventually merges with the Si eutectic temperature. Though the platelet -phase continues to crystallize until the end of the silicon eutectic reaction, the length of the primary platelet phases greatly depends on the time interval between the platelet -phase onset temperature and the silicon eutectic temperature. With decreasing platelet -phase onset temperature, the platelet -phase growth time and therefore the length and volume fraction of this phase decrease until the platelet -phase onset temperature merges with the silicon eutectic temperature. The branched structure of the platelet -phase observed in the present work has been further supported from the work of Ashtari et al. [23] and Samuel et al. [14]. Platelet -phases are often found associated with shrinkage pores [24,25], and this observation is understood to be the direct result of their effect in increasing restrictions to feeding through the small interdendritic channels in the later stages of feeding. It has been reported [26] that the platelets were observed to form an interlocking network and grow around pre-existing features in the microstructure. These relatively large and long platelet -phases intersect and reduce the size of the interdendritic feeding paths, limiting feeding and thus increasing the driving force for pores to form. This has been supported from the findings of the work of Samuel et al. [24] on 319 alloy, which shows that as the Fe content is increased from 0.37 to 1.4, the percentage porosity also increases. It has been observed that the eutectic Si nucleates on the surface of the platelet -phases (Fig. 1(b)). This is supported

8 568 S.S. Sreeja Kumari et al. / Materials Science and Engineering A (2007) Fig. 12. (a) Cooling curve and its first derivative and (b) microstructures of Al 7Si 0.3Mg 0.8Fe 0.4Mn alloy thermal analysis sample. by the work of Taylor et al. [27]. It has been reported that the Si particles are often observed to grow from multiple locations along a single platelet -phase. For eutectic Si to nucleate and grow on the platelet -phases, the latter must form and present prior to the former. This is the case for the platelets that form as either a primary platelet -phase or as binary platelet -phase (a component of the Al -Al 5 FeSi binary eutectic). Such platelets are well developed by the time the ternary eutectic Si begins to grow. This has been confirmed by the appearance of a peak at 858 K in Fig. 10 corresponding to platelet -phase formation. Be (0.2%) has changed the morphology of platelet -phase to Chinese script and fine globules. Both Mn and Be additions lead to the precipitation of either coarse or fine Chinese script form of intermetallic phase with Fe at slow (thermal analysis sample, cooling rate 1.5 K/min) and fast cooling rates (permanent mould), respectively. It has also been observed in the present work that the Chinese script phases with Be addition forms only inside the -Al at slow cooling rate. Murali et al. [7] have reported that in the case of Be (0.27%) added sand cast Al 7Si 0.3Mg alloy, the Be Fe phase is formed only inside the -Al, which is probably the result of a peritectic reaction. Wang and Xiong [28] studying the effect of Be in Al 7Si 0.4Mg 0.2Ti (sand cast) alloy have reported that the Be Fe particles may act as the nuclei for -Al during solidification and hence most of the Be Fe phases are located inside the -Al. Contrary to this observation at high cooling rates, very few Be Fe phases are formed Fig. 13. (a) Cooling curve and its first derivative and (b) microstructure of Al 7Si 0.3Mg 0.8Fe 0.04Ca alloy thermal analysis sample (arrows show -Fe intermetallic phase). inside -Al with majority appearing in the interdendritic areas. This has been further confirmed by DTA analysis (Fig. 16). It has been observed that the Be Fe phase precipitate after the -Al peak at temperatures much higher than the formation temperature of the platelet -phase. At high cooling rates, the growth kinetics favour the development of aluminium dendrites and the Be Fe phases are constrained to interdendritic areas. Addition of Mn changes the morphology of platelet -phase to Chinese script form. This is because of the replacement of Mn by Fe, which can substitute each other, being transitional elements. In order to obtain the crystallization of the iron compound in the Chinese script form and avoid the needle like and polyhedral crystal morphology, a certain critical ratio of iron: manganese is required and this ratio depends on the cooling rate. Bakerud et al. [29] have clearly explained the solidification sequence with Mn addition in Al 7Si 0.3Mg alloy. According to Mondolfo [30], (Fe,Mn)Al 6 is the first phase to form in the Al Fe Mn Si system, which encompasses many commercial alloys. In many alloys, (Fe,Mn)Al 6 reacts peritectically with the liquid Al to form (FeMn) 3 Si 2 Al 15 [31 34]. The refinement of -platelets with Ca and/or Sr addition is due to the fragmentation of platelet -phases by Ca and/or Sr. The reason for this fragmentation of the platelets could be attributed to the rejection of Si at the platelet -phase/al matrix

9 S.S. Sreeja Kumari et al. / Materials Science and Engineering A (2007) Fig. 14. (a) Cooling curve and its first derivative and (b) microstructure of Al 7Si 0.3Mg 0.8Fe 0.04Sr alloy thermal analysis sample (arrows show -Fe intermetallic phase). Table 2 Alloys, thermal arrests and the phases formed in Al 7Si 0.3Mg 0.8Fe alloy without and with Be, Mn, Ca and Sr additions Alloy Thermal arrests (Temperature (K)) Phases formed Al 7Si 0.3Mg 0.8Fe Primary -Al Iron intermetallics Eutectic Si Mg 2 Si phase Al 7Si 0.3Mg 0.8Fe 0.2Be Primary -Al Eutectic Si Mg 2 Si phase Al 7Si 0.3Mg 0.8Fe 0.4Mn Primary -Al Iron intermetallics Eutectic Si Mg 2 Si phase Al 7Si 0.3Mg 0.8Fe 0.04Ca Primary -Al Iron intermetallics Eutectic Si Mg 2 Si phase Al 7Si 0.3Mg 0.8Fe 0.04Sr Primary -Al Iron intermetallics Eutectic Si Mg 2 Si phase Fig. 15. DTA curves of Al 7Si 0.3Mg 0.8Fe alloy: (a) heating and (b) cooling at 2 K/min. interface. The higher diffusion coefficient [35] of Si in Al (10 11 to 10 8 cm 2 s 1 ) compared to Fe (10 13 to cm 2 s 1 )in the temperature range 673 to 873 K would facilitate the destabilization of the platelet -phase in the presence of Ca and/or Sr. This is further supported by the work of Samuel et al. [14] and Pennors et al. [15]. In the combined addition of Be (0.15%) and Mn (0.15%) to Al 7Si 0.3Mg 0.8Fe alloy, both Mn Fe and Be Fe Chinese script intermetallic compounds have been observed. As a result, the amount of free iron available to precipitate in the form of platelet -phase is diminished. Addition of Ca (0.04%) and/or Sr (0.04%) to Mn (0.3%) added Al 7Si 0.3Mg 0.8Fe alloy results in a microstructure consisting of Al dendrites, modified eutectic Si, Mn Fe Chinese script phases and refined platelet -phases. Mn (0.3%) addition changes the morphology of platelet -phase to Chinese script form. Ca (0.04%) and/or Sr (0.04%) play the dual role of modifying the eutectic Si from platelet to fibrous Si and refining the large platelet -phases to smaller sizes. The solidification process starts by the formation of Al dendrites and the interdendritic liquid becomes enriched in Fe, Mn and Si.

10 570 S.S. Sreeja Kumari et al. / Materials Science and Engineering A (2007) Fig. 16. DTA curves of Al 7Si 0.3Mg 0.8Fe alloy with 0.2%Be addition: (a) heating and (b) cooling at 2 K/min. Fig. 17. Effect of Be, Mn, Ca and Sr individually and in combination on the (a) UTS and (b) % elongation of Al 7Si 0.3Mg 0.8Fe alloy. The second reaction involves the formation of Mn Fe Chinese script phase until the eutectic composition is reached. Later, Al, Si and platelet -phases will crystallize together within the ternary eutectic reaction. The microstructural constituents such as size and shape of eutectic silicon, size of platelet Fe-intermetallic phases and morphology of the Fe-intermetallics affect the mechanical properties of the alloys studied. The mechanical properties (tensile and impact) of the Al 7Si 0.3Mg 0.8Fe alloy are low compared to the other alloys studied. It has been reported that cracks nucleate on Si particles [36] and platelet -phases [37]. The microstructural observation shows that the platelet -phases are continuous throughout the interdendritic regions. Thus crack propagation can also take place easily resulting in reduced mechanical properties. Fig. 19(a) clearly shows the cleavage fracture (typical of brittle failure) of impact tested Al 7Si 0.3Mg 0.8Fe alloy. Be (0.2%) addition has significantly improved the tensile and impact properties due to the morphological change of platelet -phase to globular and Chinese script form. The microstructures of Mn and Be added alloys in the T6 treated condition show the presence of Chinese script phases. Further, it has been Fig. 18. Effect of Be, Mn, Ca and Sr individually and in combination on the impact strength of Al 7Si 0.3Mg 0.8Fe alloy.

11 S.S. Sreeja Kumari et al. / Materials Science and Engineering A (2007) Fig. 19. SEM fractographs of impact tested simples: (a) Al 7Si 0.3Mg 0.8Fe, (b) Al 7Si 0.3Mg 0.8Fe 0.3Mn, (c) Al 7Si 0.3Mg 0.8Fe 0.04Ca, (d) Al 7Si 0.3Mg 0.8Fe 0.04Sr, (e) Al 7Si 0.3Mg 0.8Fe 0.3Mn 0.04Ca and (f) Al 7Si 0.3Mg 0.8Fe 0.3Mn 0.04Sr alloy. reported [38] that -AlFeMnSi particles have quite perfect crystals and hence the dissolution process is hindered by the lack of defects. According to Wikle [18], addition of 0.25% Be to 356 aluminium alloy (T6-temper-sand mold casting) increases the UTS and % elongation by 21 and 71%, respectively. Murali et al. [6] have observed that among the neutralising elements Be, Cr, Mn and Co, Be is very effective in converting the platelet -phase into polygonal or Chinese script particles in 356 alloy containing 0.1 1% Fe. The improvement in elongation and impact strength with Sr or Ca additions is due to the fragmentation of the platelet phase and modification of the eutectic Si. This observation is in agreement with the work of Samuel et al. [24] on Sr addition to Al Si Cu (319) alloy. The fragmentation of platelet -phase is clearly explained elsewhere [19]. It has been found that Mn addition improves the % elongation of the alloy and not impact strength. This may be due to the presence of both acicular eutectic silicon and hard Mn Fe intermetallic phases in the Mn added alloy. Richard [39] has reported that the impact strength is more sensitive to very small variations in the microstructure than elongation. Similar observations have been made by Tsukuda [40], who has indicated that impact values are more sensitive to the as cast microstructure of Al Si alloys than the tensile properties, and the influence of porosity on impact property is not as significant as on tensile properties. Fig. 19(b) clearly shows the brittle cleavage fracture of impact tested Al 7Si 0.3Mg 0.8Fe 0.4Mn alloy. However, addition of Ca and Sr to Mn added Al 7Si 0.3Mg 0.8Fe alloy improves the impact strength significantly. This is due to the modification of the eutectic Si from acicular to fibrous form and refinement of

12 572 S.S. Sreeja Kumari et al. / Materials Science and Engineering A (2007) platelet -phases. Fig. 19(e and f) shows dimples (typical of ductile failure) as well as cleavage fracture (typical of brittle failure). Similarly, combined addition of Be (0.15%) and Mn (0.15%) also improves the impact strength which is due to the formation of both Be Fe and Mn Fe phases. Be Fe Chinese script phases are known [6] to improve the impact strength. On the other hand, combined additions of Be + Mn, Ca + Mn and Sr + Mn lead to a reduction in the impact properties compared to individual additions of Be, Ca and Sr. This may be attributed to the presence of hard brittle Mn Fe intermetallic phases in the alloys, which needs further probing. Further, the best combinations of both tensile and impact properties have been obtained in the combined additions of Be + Mn, Ca + Mn and Sr + Mn to Al 7Si 0.3Mg 0.8Fe alloy. Similarly, combined addition of a trace amount of Be (0.005%) to Ca (0.04%) or Sr (0.04%) improves the tensile properties significantly. This is due the fact that Be in trace amount in Al Si alloys preferentially oxidizes forming BeO on the surface of the casting, cleans the melt surface and reduces gas pick up (and hence porosity) and Mg loss [Pilling Bedworth ratio (PB) of Be is 1.68 which is much higher than Mg (0.78)] making Mg available for forming the required volume of Mg 2 Si for strengthening [18,41 43]. 5. Conclusions Thermal analysis study shows that all additions except Be show a peak corresponding to -Fe intermetallic phase formation. Ca and Sr additions decrease the eutectic temperature (by 6.6 and 8.7 K, respectively) compared to the untreated alloy and lead to modification of eutectic Si from platelet to fibrous form. Mn (0.4%) addition to Al 7Si 0.3Mg 0.8Fe alloy changes the morphology of platelet iron phase to script form leading to significant improvement in tensile properties. However, there is no significant improvement in impact strength. Be (0.2%) changes the platelet morphology of iron phase to Chinese script form, which are seen only inside the -Al in slow cooling (sand cast), and both in the interdendritic as well as inside the -Al in fast cooling (permanent mould casting) condition. This morphological change is responsible for the significant improvement in both the tensile and impact properties. Addition of Be, Ca and Sr to Mn added Al 7Si 0.3Mg 0.8Fe alloy improves the impact strength significantly. This is due to the modification of the eutectic Si from acicular to fibrous form and refinement of the platelet -phases. 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