*e-mail: spinelli@ufscar.br
Effects of Cellular Growth on Fatigue Life of
Directionally Solidified Hypoeutectic Al-Fe Alloys
Pryscilla Liberato Ribeiroa, Bismarck Luiz Silvab, Wanderson Santana da Silvaa, José E Spinellib*
aDepartment of Materials Engineering, Federal University of Rio Grande do Norte – UFRN,
CEP 59072-970, Natal, RN, Brazil
bDepartment of Materials Engineering, Federal University of São Carlos – UFSCar,
CEP 13565-905, São Carlos, SP, Brazil
Received: Dctober 31, 2013; Revised: January 13, 2014
Al-Fe hypoeutectic alloys are a family of casting alloys characterized by cell growth, low cost and appreciable formability. Ot is well known that fatigue strength is a requirement of prime importance considering the nature of load typically observed during operations involving the risers used in oil
extraction. The aim of this study is to examine the inluence of cell size and its intercellular phase
distribution on the fatigue life (Nf) of the directionally solidiied Al-0.5, 1.0 and 1.5wt% Fe alloys. A
water-cooled vertical upward unidirectional solidiication system was used to provide the castings.
Microscopy light and SEM microscopy were used. Ot was found that fatigue life decreases as cell spacing (λc) increases. Smaller cell spacing allows a homogeneous distribution of Al-Fe ibers to
happen within the intercellular regions, which tends to improve the mentioned fatigue property. Hall-Petch type correlations [Nf = Nf0+A(λc–1/2)-B(λ
c
–1); where A and B are constants] seems to be able to encompass the fatigue life variation along the Al-Fe alloys.
Keywords: solidification, Al-Fe alloy, fatigue life
1. Introduction
Al–Fe based alloys can be used in a variety of applications, e.g. packaging, architectural sheet and lithography sheet1. Al-Fe alloys always motivate commercial interest of the aluminum manufacturing industry once iron
is a dominant impurity in commercial grades at signiicant levels (0.2-1.0 wt %). It is also considered a major
contamination concerning purer grades of aluminium alloys.
Under equilibrium solidiication conditions, any iron in excess of its solid solubility limit of about 0.04 wt %Fe, forms
a eutectic of Al-rich α primary phase and an intermetallic Al3Fe. Figure 1 depicts the partial phase diagram of the Al-Fe system emphasizing both liquidus lines and eutectic
plateau. However, during non-equilibrium solidiication
typical of direct chill (DC) castings considerably different cooling rates occur from the ingot surface and the ingot center, causing the formation of metastable AlmFe and Al6Fe intermetallic phases in addition to the stable Al3Fe phase2.
The as-cast structures obtained from DC casting operations may be also characterized by high levels of inverse macrosegregation, which is a consequence of an enrichment of iron content near the surface of the Al-Fe castings3. Ot is well known that changes in the alloy chemistry and cooling
rate during solidiication can affect not only the cellular/
dendritic growth but also the distribution of eutectic mixture
and intermetallic particles. The effects of such modiication
can be especially important on mechanical strength and corrosion resistance4-10, which means that such properties are governed by either the scale of the microstructure or by
size and distribution of intermetallic particles. The dendrite
or cell ineness can be even more important in the prediction
of mechanical properties than grain size. Goulart et al.9,10
performed an investigation regarding to the solidiication
structure development of three hypoeutectic Al-Fe alloys,
which were directionally solidiied under unsteady-state heat low conditions. Considering the same cooling rate and
tip growth rate, the cellular spacing (λ1) was found to be independent on the alloy iron content. The experimental cell spacing variation with cooling rate (Ṫ) and tip growth rate (VL) was characterized by λ1=31(Ṫ)–0.55 and λ
1=17(VL) –1.1
experimental power laws, respectively.
Dne of the rare studies correlating mechanical properties with microstructural features of Al-Fe alloys can be found in Mondolfo11, who stated that formability of Al-Fe alloys depends mainly on size and distribution of intermetallics, since coarse Al3Fe particles tend to crack and produce notches that reduce formability and fatigue resistance. Dn
the other hand, inely dispersed Al6Fe does not have the same effect. The effects of the size and distribution of the Al6Fe ibers on fatigue properties of Al-Fe alloys deserve a deep investigation to be fully understood. On addition, Vinogrdov12 has stated that small grain size will have a positive effect on the yield and tensile strength while generally improving fatigue strength by retarding the crack initiation. The enhanced resistance to crack initiation is
considered as a result of high strength of ine grains that
other hand, according to Motyka et al.13 increase in strength under static load is not always accompanied by improved fatigue behaviour. This was proved by the results found for
the 442 aluminium alloy subjected to rapid solidiication and
plastic consolidations, which led to increase on the static mechanical properties but a reduction on its fatigue strength.
The inluence of casting-related porosity on the fatigue
life of cast aluminium components is always a concern especially regarding to design aspects on road and rail vehicle engineering. For example, if the degree of porosity
is increased from 0 to 8 according to ASTM E155, fatigue strength is reduced by about 17% for the age-hardened
Al-7Si-0.6Mg alloy14.
Some recent studies have pointed out the effect of microstructure, particularly of the scale of the dendritic array on the resulting tensile mechanical properties4-8. All the investigations have showed that if the dendrite arm spacing is decreased, the mechanical properties of the cast alloy may be improved. Goulart et al.15 have carried out a
study on the effects of iron content and cooling rate on the cell growth of hypoeutectic Al-Fe alloys and further on the development of tensile test parameters like ultimate tensile strength (σu), yield tensile strength (σy) and elongation (δ). Ot was shown that the microstructural arrangement strongly affects σu, σy and δ. Hall-Petch type relationships were proposed as follows corresponding to the ultimate tensile strength: σu=57.6+113.2(λ1
-1/2), σ
u=61.2+152.2(λ1 –1/2) and
σu=62.8+170.2(λ1–1/2) for Al-0.5wt%Fe, Al-1.0wt%Fe and
Al-1.5wt%Fe alloys, respectively.
The goal of present study is to develop experimental expressions, which correlate the fatigue life with the cell spacing for Al-Fe hypoeutectic alloys. Further, the work deals with both the analysis of microstructure features especially on the scale of the Al6Fe ibers within eutectic mixture and the aspects of the inal fracture surfaces.
2. Experimental Procedure
A solidiication system was designed in such a way
that the heat is directionally extracted only through a
water-cooled bottom made of low carbon steel (SAE 1020),
promoting vertical upward directional solidiication. More
details about such casting assembly can be found in previous articles8,9. A stainless steel split mold was used having an
internal diameter of 60 mm, height 157 mm and a 5 mm wall
thickness. The lateral inner mold surface was covered with a layer of insulating alumina to minimize radial heat losses. The bottom part of the mold was closed with a thin (3 mm) carbon steel sheet. The initial melt temperatures (Tp) were standardized at 10 °C above the liquidus temperature (TLiq).
Experiments were performed with Al-Fe hypoeutectic
alloys (0.5, 1.0 and 1.5 wt pct Fe). The chemical
compositions of metals that were used to prepare these alloys are presented in Table 1.
Each cylindrical ingot was subsequently sectioned (in a longitudinal way) along its vertical axis, ground and etched with an acid solution to reveal the macrostructure (Poulton’s
reagent: 5mL H2O; 5mL HF-48%; 30 mL HNO3; 60 mL
HCl). Transverse specimens were cut from the castings, as indicated in Figure 2a, and machined for fatigue testing following the dimensions shown in Figure 2b.
The fatigue tests were performed with 2000-2150 rpm frequency, under room temperature at using a WPM lexural
and rotation fatigue machine (UBM model, Germany). This machine permits a variation from 2000 rpm (for higher loads) to 6000 rpm (for minor loads). All experiments were performed under a constant load of 4 Kg, which corresponds
to lexural stress of 27.4MPa. The fatigue tests were run according to the standard DIN50113.
As the values of ultimate (UTS) and yield (YTS) tensile strength are well known for these alloys15, a reference value
concerning the lexural stress (27.4MPa) was adopted as 40% of the UTS value or 60% of the YTS obtained for the Al-0.5wt%Fe alloys, which are lower typically than the UTS and YTS values obtained for the Al-1.0 and 1.5wt%Fe
alloys. The value imposed for the fatigue tests conducted in the present study is quite reasonable since the literature16 indicates that fatigue strength limit is, in practice, around
50-60% of the UTS value for a given alloy. Figure 3 shows a
schematic representation of the mentioned equipment as well as a detail of the specimen assemblage on the machine axis. After performing the fatigue tests for each alloy, undamaged specimens from the broken fatigue samples
were extracted, polished and etched (a solution of 0.5%HF
in water) for metallography. An image processing systems
Olympus, GX51 (Olympus Co., Japan) was used to acquire
the images so that cell morphology and size could be proved as stated by Goulart et al.9,10 on previous studies. The broken samples were also used in order to examine the fracture surfaces concerning both general aspects and specific features. These surfaces were analyzed by use of a scanning electron microscopy (SEM, Philips, XL-30, Netherlands).
Figure 1. Partial phase diagram of the Al-Fe system.
Table 1. Chemical composition of metals.
Chemical Composition (wt pct)
Metal Fe Si Mn Cu Ni Al
Al 0.09 0.06 - 0.06 0.03 99.8
-3. Results and Discussion
Columnar macrostructures have prevailed along the directionally solidified castings of the experimentally examined Al-Fe alloys. Figure 4 depicts the macrostructures obtained for the three examined alloys. Columnar-to-equiaxed transitions can be observed for the Al-1.0 and
1.5wt% Fe alloy, which means that increase on Fe content
induces equiaxed grains to be formed, blocking the columnar development. On the present study only the columnar grain zones have been examined.
Figure 5 shows several aligned microstructures obtained
for each examined alloy and for distinct positions along the castings. These positions were the same adopted to perform fatigue tests. Ot can be clearly observed that an arrangement
of cells has prevailed in all examined samples solidiied
under unsteady-state conditions. The non-equilibrium Al6Fe rod-like intermetallic is usually connected to the
high cooling rates range during solidiication. According to
Goulart et al.9,10,15 the water-cooled solidiication setup used
in the present study is able to produce as-cast structures with prevalence of metastable Al6Fe particles. Oncrease of about 3 times in the cell spacing value can be seen if compared
the irst positions with the positions closer to the top of the
three castings. This behavior is translated from the variation
of the solidiication thermal parameters, as for example the tip cooling rate, which decreases as solidiication progresses.
On the range of the hypoeutectic compositions, the
solidiication microstructure is composed of a cellular
matrix rich in aluminum (α) surrounded by a eutectic mixture α + β, where α is rich in aluminum and β is enriched by the intermetallic Al6Fe, with a predominance of a rod-like morphology. Goulart et al.17 have found that the experimental development of interphase spacing and its distribution range as a function of growth rate for the Figure 2. (a) Removal of specimens for fatigue test; (b) Specimen
shape used for the fatigue testing (dimensions in mm).
Figure 4. Macrostructures obtained for the hypoeutectic Al-Fe alloys.
Al-1.5wt%Fe alloy follows the classical growth law for
eutectics: λc=1.6(v)–1/2, where v means eutectic growth rate. Ot can be inferred from this expression that if the cell spacing diminishes the interphase spacing is also decreased.
Ot can also be observed that increase Fe content resulted in a higher Al6Fe ibers density, with thicker intercellular walls. This Al6Fe seems to operate as a reinforcement of the ductile Al-rich matrix during loading.
Figure 6, where average spacings along with the standard variation are presented. The lines represent empirical power
law which it the experimental points. It can be observed that the cell spacing is not signiicantly inluenced by the alloy
solute content, which is evidenced by single experimental laws representing cellular spacing as a function of tip cooling rate.
Figure 7 shows the experimentally determined fatigue life for the three hypoeutectic Al-Fe alloys as a function of λc. Hall-Petch type relationships are able to encompass the fatigue life variation along the Al-Fe alloys once high
values of correlation coeficients, R2, may be inferred from the regressions. A single experimental relationship is able to encompass the experimental scatters obtained for both
Al-1.0 and 1.5wt%Fe alloys.
Figure 6. Cellular spacing as a function tip cooling rate for the
directionally solidiied Al-Fe alloys.
Figure 7. Fatigue life against cell spacing experimentally determined for (a) Al-0.5wt%Fe and (b) Al-1.0 and 1.5wt%Fe alloys. (c)
Comparison for all Al-Fe alloys corresponding to λc>15µm. R2 is the correlation coeficient. SEM images for two corresponding positions
Smaller cell spacing allows a homogeneous distribution
of Al-Fe ibers to happen within the intercellular regions,
which tends to increase the fatigue life. This can be especially observed in the case of alloys with higher solute
content such as 1.0 and 1.5wt%Fe (Figure 7b). In the case of the dilute Al-0.5wt%Fe alloy a less expressive variation was
observed concerning fatigue life. On general, the fatigue life increases with both increasing Fe content and decreasing λc.
The coarse Al6Fe ibers (associated with coarse λc) tend to crack reducing fatigue life. The fatigue life becomes quite constant and seems to be independent of solute content if considered λc values higher than 15 µm (Figure 7c).
Figure 8. Fracture SEM images obtained along the Al-1.5wt%Fe alloy casting: (a) P=7mm (b) P=37.8mm, (c) P=68.6mm. P is the
position from metal/mold surface.
Figure 9. Fracture SEM images obtained along the Al-1.5wt%Fe alloy casting: (a) P=7mm (b) P=37.8mm. P is the position from metal/
Dn the general point of view the fatigue fracture surface showed a complex aspect. The fatigue fracture surfaces are available in Figure 8. Figure 8a shows typical fatigue
grooves observed in the ine cells (white arrows). Further, it
is possible to identify in Figure 8b the radial marks initiated at some of the fatigue nuclei (indicated by the large grey arrow) together with some signs of cast porosity (indicated by large black arrows on the right side). At last, in Figure 8c it is possible note some nucleus surrounded by an intense deformation zone (indicated by the large white arrow on
the igure bottom).
The level of porosity was not controlled in the present investigation. Taylor et al.18 have stated that after 0.4wt%Fe further iron additions provoke increase on porosity level. The same was noted in the present results with higher porosity being formed for higher iron content even though porosity fraction has not been determined. Ot seems that even higher porosity level was not able to avoid fatigue life increase as iron content is increased. Porosity is obviously a drawback concern on engineering applications even though it may be tolerated even in safety parts13.
Some more details of fatigue surface are shown in Figure 9. Some signs of groove growth can be observed within the intercellular regions (indicated by small white arrows). On Figure 9b is possible to note the subsurface nucleus of fatigue process in a cell zone with some intercellular shrinkage.
4. Conclusions
The directional solidiication experiments with the
hypoeutectic Al-Fe alloys associated with the performance
of systematic lexural rotational fatigue tests permit the
following conclusions to be drawn:
• A complete cellular growth was characterized by
optical images along the three produced Al-Fe
alloys castings. The solidiication microstructure
is composed of a cellular matrix rich in aluminum (α) surrounded by a eutectic mixture α + β, where α is rich in aluminum and β is enriched by the intermetallic Al6Fe, with a predominance of a rod-like morphology. On all examined alloys, the cell spacing
varied roughly from 7µm to 20µm from the irst positions (iner) to the top of the casting (coarser); • Higher Al6Fe ibers density can be observed for Al-1.0
and 1.5wt%Fe alloys, which contributed to the higher
fatigue values associated with such alloys compared
with the Al-0.5wt%Fe alloy. The ineness of cells
can also increase the fatigue life values due to the
homogeneous distribution of Al-Fe ibers within the
intercellular regions.
Acknowledgements
The authors acknowledge the inancial support provided by FAPESP (The Scientiic Research Foundation of the
State of São Paulo, Brazil); CNPq (The Brazilian Research Council; grant 481292/2012-8); ANP (Petroleum National Agency).
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