S. Murali et al., FATIGUE PROPERTIES OF SAND CAST, STIRCAST AND EXTRUDED AL-7SI-0.3MG ALLOY WITH TRACE ADDITIONS OF BE AND MN, Materials transactions, JIM, 38(1), 1997, pp. 28-36
Present study devotes to an understanding of the fatigue behaviour of
Al-7Si-0.3Mg sand cast alloys at varying iron levels and with berylliu
m and manganese trace additions and when the alloy is stircast and ext
ruded. From the stress(S)number of cycles to failure(N)-(S-N) curves i
t is observed that the presence of higher amount of iron (0.76%) in sa
nd cast alloys leads to shorter fatigue life. Beryllium and manganese
additions to 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. Better fatigu
e life of beryllium and manganese added high iron alloy is due to the
presence of (Be, Mn)-Fe phases only inside alpha-Al dendrites. Fatigue
life of stircast and extruded low iron (0.44%) alloy is superior to s
and cast low (0.29%) iron alloys. Observation of fractured surfaces re
veals that porosity/inclusions is the high stress concentrating point
where crack originates (stage-I) and then propagates (stage-II) depend
ing on the presence of the second phases. In the case of low iron allo
ys (sand and extruded stircast) crack propagates along eutectic silico
n, while in the high iron alloys crack propagates through brittle beta
-phase. In beryllium and manganese added alloys, even though crack nuc
leates on (Be, Mn)-Fe phase, it will be arrested as it approaches alph
a-Al dendrites and hence, crack has to propagate along silicon particl
es. 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 schemati
cally.