3. Experimental Procedure 32
4.4. Results and discussion of material characterisation
4.4.6. Result and discussion: crack growth and crack growth threshold
A damage tolerance analysis is essential for any new material to ensure that parts are designed to support a slow and stable crack growth until the crack can be determined by any non-destructive inspection method. As the microstructural analysis of both materials displayed substantial differences from known microstructures of conventional aluminium alloys, a de-termination of the complete crack propagation behaviour of all three∆a/∆N -∆K regions (see Figure 2.11) was done for both materials Scalmalloy and SilmagAl. Specimens were manu-factured out of powder Batch I on either Platform P4 and Parameter Set 1 in HIP plus aged condition for Scalmalloy or on Platform P5 and Parameter Set 5 in HIP plus T6 condition for SilmagAl respectively. Fatigue crack growth and fatigue crack growth threshold measure-ments were executed for crack growth in the xy- and z- direction at load ratio R = 0.1 and R = 0.7. Figure 4.38 contains two∆a/∆N -∆K curves at R = 0.1 for Scalmalloy and two for SilmagAl with each crack growth direction xy and z. However, Figure 4.39 presents only three
∆a/∆N -∆K curves at R = 0.7 with Scalmalloy specimens tested with crack growth in xy- and z- direction and one SilmagAl specimen tested with crack growth in z- direction.
10-8
Fatigue Crack Propagation Rate - da/dN [mm/cycle]
Stress Intensity Factor Range - ∆K [MPa√m]
Scalmalloy Batch I - z Scalmalloy Batch I - xy SilmagAl Batch I - xy SilmagAl Batch I - z
Figure 4.38.: Scalmalloy and SilmagAl Batch I fatigue crack growth rate∆a/∆N -∆K includ-ing∆Kthvalues for load ratio R = 0.1 and crack growth direction z and xy
10-8
Fatigue Crack Propagation Rate - da/dN [mm/cycle]
Stress Intensity Factor Range - ∆K [MPa√m]
Scalmalloy Batch I - z Scalmalloy Batch I - xy Silmagal Batch I - z
Figure 4.39.: Scalmalloy and SilmagAl Batch I fatigue crack growth rate∆a/∆N -∆K includ-ing∆Kthvalues for load ratio R=0.7 and crack growth direction z and xy For both alloys, no difference is notable if the crack grows in xy- or z- direction at both load ratios. However, the near-threshold regime and the fatigue crack propagation under cyclic
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loading differ significantly between the two alloys. SilmagAl at a lower Rp0.2and Rmlevel, displays for load ratios R = 0.1 more favorable crack propagation values compared to Scal-malloy. The crack propagates in the Paris regime of the∆a/∆N -∆K SilmagAl curve (until
≈∆K = 18 MPa√
m) significantly less than in Scalmalloy. Threshold values for Scalmalloy specimens were measured already at a low level of∆Kth= 1.6 MPa√
min both crack growth directions xy and z. However, the crack propagation threshold values of SilmagAl test spec-imens are between 6 MPa√
m≤∆Kth≤7 MPa√
min xy- and z- direction. Crack closure effects are mainly eliminated at load ratio R = 0.7 which results in slightly different behaviour between the alloys. The crack in the SilmagAl sample grew only to∆K = 5.5 MPa√
mslower compared to cracks in Scalmalloy samples. At ∆K > 5.5 MPa√
mScalmalloy test samples changed into a slower crack propagation behaviour compared to SilmagAl. For both alloys, decreases the∆Kthwith the increasing load ratio, from∆Kth= 1.6 MPa√
mto < 1 MPa√ mfor Scalmalloy and from∆Kth= 7 MPa√
mto 2.3 MPa√
mfor SilmagAl (for crack growth in z-direction). Crack closure effects are hence more pronounced in SilmagAl than in Scalmalloy.
The near-threshold Regime I in fatigue crack curves is in general strongly dependent on mi-crostructural features around the crack tip. The influence of the very local microstructure around the crack tip is significantly reduced in upper Paris regime II but becomes dominant again in the final stage, Stage III, where the∆K reaches fracture toughnessKC and leads to unstable crack growth. Slipping planes in front of the crack tip are generally considered to be predominantly dependent on grain and precipitation size and distribution [84]. The very special microstructure of Scalmalloy with alternating ultra-fine grain and fine grain bands with homogeneously distributed full coherentAl3(Sc(1−x),Zrx)precipitations is hence assumed to be the main driver for its poor fatigue crack growth behaviour. In Scalmalloy, neither grain boundaries nor precipitations are able to prevent crack growth propagation under cyclic loads.
If the plastic zone in front of the crack tip is of the same size as the grains, the fracture can change from intra- to less favourable inter-granular, as shown for ultrafine grained Al-alloys in [133]. However, Pao [134] obtained very similar∆Kthvalues for fine grained Al7Mg material which was manufactured by powder metallurgy, but the noted fracture was still transgranular although the fracture paths are less tortuous compared to conventionally grained Al7Mg ma-terial. On the other side, it can be assumed that coarser grains and the size and distribution of precipitations in SilmagAl lead to a more favourable, lower crack propagation. Further investi-gations are necessary to investigate the mechanism that lead to the observed crack propagation in Scalmalloy and SilmagAl.
FINDINGS
• The crack growth behaviour confirm the extraordinary microstructure of alternating UFG and FG bands in AM Scalmalloy test samples, which are manufactured with Batch I on P4 with Parameter Set 1. The crack starts growing on surprisingly low threshold values for both load ratios R = 0.1 and R = 0.7.
• SilmagAl test coupons out of powder Batch I and manufactured on P5 with Parameter Set 5 result in higher threshold values and slower crack propagation behaviour in the lower Paris regime compared to Scalmalloy for both load ratios R = 0.1 and R = 0.7.
• Only in the upper Paris regime is the fatigue crack propagation in Scalmalloy test
sam-ples significantly slower compared to crack growth in SilmagAl specimens (or other conventional high strength Al- alloys).