Morphological studies on β−FeSiAI5\beta-FeSiAI_5 phase in AI-7Si-0.3Mg alloy with trace sdditions of Be, Mn, Cr, and Co

Although the detrimental effect of iron on the mechanical properties of A1-7Si-0.3Mg alloy has been well established, studies on neutralizing this effect have been very few and in- conclusive. The $\beta$-phase $(FeSiAl_5)$, seen in the interdendritic areas in the form of needles/plates, is responsible for this effect. In the present study, it has been observed that trace additions of Be, Cr, Mn, and Co, individually or combined, will tie up Fe to form new phases with altered morphologies such as Chinese scripts, polygons, stars, and irregular shapes. Cr-Fe and Mn-Fe phases are seen both in interdendritic regions and inside the a-A1 dendrites, while Co-Fe phase is mostly seen inside the a-A1 dendrites. However, Be-Fe phase is observed only inside the ~-A1 dendrites. Empirical formulas have been de- veloped from wavelength dispersive analysis on these relatively new Fe phases with trace additions. At higher Fe levels (0.8-1%) and with Be trace addition, individually or combined with Mn and Cr, fractographs reveal pyramids surrounded by cells, while other trace additions such as Mn, Cr, and Co reveal brittle cleavage fracture. All Be-added alloys, individually and combined with Mn and Cr additions, appear to neutralize the detrimental effect of Fe content on mechanical properties

Influence of Elastic-Constants on Mode-I Stress-Intensity Factors in 3-Dimensional Crack Problems

Plates with V-through edge notches subjected to pure bending and specimens with rectangular edge-through-notches subjected to combined bending and axial pull were investigated (under live-load and stress-frozen conditions) in a completely nondestructive manner using scattered-light photoelasticity. Stress-intensity factors (SIFs) were evaluated by analysing the singular stress distributions near crack-tips. Improved methods are suggested for the evaluation of SIFs. The thickness-wise variation of SIFs is also obtained in the investigation. The results obtained are compared with the available theoretical solutions

The formation of ?-FeSiAl5 and Be---Fe phases in Al---7Si---0.3Mg alloy containing Be

Thermal analysis and interrupted quench experiments have been carried out to study the formation of beta-FeSiAl5 and (Be-Fe)-BeSiFe2Al8 phases in Al-7Si-0.3Mg alloy with and without Be addition. In the base alloy with 0.6% Fe (without Be addition), a needle- and plate-shaped beta-phase is present in the interdendritic regions and is formed by a ternary eutectic reaction. In the Be- added alloy with 0.6% Fe, a Be-Fe phase of Chinese script and polygon shapes grows along with the primary alpha-Al dendrites, leading to superior mechanical properties. It is proposed that this Be-Fe phase is formed by a peritectic reaction. Be addition has also resulted in some grain refinement

Fatigue properties of sand cast, stircast and extruded Al-7Si-0.3 Mg alloy with trace additions of Be and Mn

Studies the fatigue behaviour of Al-7Si-0.3 Mg sand cast alloys at varying iron levels and with beryllium and manganese trace additions and when the alloy is stircast and extruded. From the stress (S)-number of cycles to failure (N)-(S-N) curves it is observed that the presence of higher amount of iron (0.76%) in sand cast alloys leads to a shorter fatigue life. Beryllium and manganese additions to a higher iron containing alloy (0.76%) show better fatigue properties than low (0.29%) and high (0.76%) iron containing sand cast alloys, thus countering the detrimental effect of iron. The better fatigue life of beryllium and manganese added high iron alloy is due to the presence of (Be,Mn)-Fe phases only inside α-Al dendrites. The fatigue life of stircast and extruded low iron (0.44%) alloy is superior to sand cast low (0.29%) iron alloys. Observation of fractured surfaces reveals that porosity/inclusions is the high stress concentrating point where the crack originates (stage-I) and then propagates (stage-II) depending on the presence of the second phases. In the case of low iron alloys (sand and extruded stircast) a crack propagates along eutectic silicon, while in the high iron alloys a crack propagates through the brittle β-phase. In beryllium and manganese added alloys, even though a crack nucleates on the (Be,Mn)-Fe phase, it will be arrested as it approaches α-Al dendrites and hence, the crack has to propagate along silicon particles. The fractured surface of the stircast and extruded alloys has revealed fine fatigue fracture with striation as compared to sand cast alloys. This aspect of crack nucleation and propagation is explained schematically