Failure mechanisms in metal/metal hybrid nanocrystalline microtruss materials
Introduction
Nanocrystalline electrodeposition can be used to reinforce microtruss cellular metals, creating new types of metal/metal nanocrystalline hybrids [1], [2], [3]. There are three distinct levels of structural hierarchy controlling the strength of these materials. First, there is the length scale of the crystalline building blocks making up the nanocrystalline reinforcement. This level of structure controls the yield strength, ductility and strain-hardening ability of the electrodeposited sleeves (e.g. see reviews in Refs. [4], [5], [6]). Secondly, there is the strut or ligament connectivity within the cellular architecture of the starting pre-form. This level of structure determines whether externally applied loads are resolved axially or transversely to the constituent struts or ligaments (e.g. see reviews in Refs. [7], [8]). When the connectivity is such that Maxwell’s stability criterion is not satisfied (e.g. [7]), failure occurs by bending-dominated mechanisms and the specific strength increase is much lower than what can be obtained if the conformal network of nanocrystalline material satisfies the stability criterion and deformation is stretch-dominated [9]. Finally, there is a level of structure intermediate between the first two, which describes the internal shape factor of the nanocrystalline tubes. Since the electrodeposited material is optimally positioned away from the neutral bending axis of the starting pre-form microtruss struts, even very small amounts of nanocrystalline material can have a large effect on the overall hybrid performance because of its large second moment of area [1], [2], [3].
For each of the hybrid systems studied to date, adding the nanocrystalline sleeve significantly enhanced the mechanical properties; in each case the metal/metal hybrids failed during uniaxial compression by a combination of inelastic buckling collapse and nanocrystalline sleeve fracture mechanisms [1], [2], [3]. In the case of aluminum microtrusses reinforced with nanocrystalline Ni–Fe [1] or nanocrystalline Ni [2] (n-NiFe/Al and n-Ni/Al hybrids, respectively) sleeve fracture occurred before the peak inelastic buckling strength of the microtruss cellular architecture was reached. On the other hand, there was no evidence of crack formation until well after the peak stress in plain carbon steel microtrusses reinforced with nanocrystalline Ni (n-Ni/steel hybrid) [3]. The extent to which each mechanism controls the overall strength is not yet clear, nor is there a good model for predicting the strength of these new hybrid materials. The present study begins to address these issues by employing experimental and finite-element methods to understand the stress evolution and failure mechanisms over the complex cellular architecture of nanocrystalline microtruss cellular materials.
Section snippets
Experimental
The n-Ni/Al microtrusses were fabricated from aluminum alloy AA3003 (thickness t0 = 0.76 mm) precursor sheets having the same starting perforation sheet geometry as the n-Ni/steel microtrusses studied previously [3]. The 25.81 mm2 square perforations were punched on a two-dimensional square lattice (unit cell size of 6.35 mm), creating an open area fraction of ϕ = 0.64. The pyramidal cores (strut geometry shown in Fig. 1a) were fabricated by deforming alternating nodes above and below the starting
Experimental
The mechanical properties of the n-Ni/Al hybrid microtrusses were evaluated in uniaxial compression. Fig. 2 presents typical stress–strain curves for samples in the as-fabricated (uncoated) condition and after electrodeposition. The uncoated samples collapsed by inelastic buckling of the microtruss struts at a peak strength of σ = 2.01 ± 0.03 MPa. This failure mode is typical of aluminum alloy microtruss materials having struts with intermediate slenderness ratios [19]. The peak strength increased
Inelastic buckling failure of composite struts
The evolution in resolved axial stress along a given strut forms the basis for understanding inelastic buckling in the composite nanocrystalline struts and for developing upper-bound predictive strength models. For the uncoated aluminum microtruss, there is uniform axial stress over the strut cross-section during the initial period of elastic loading. The axial stress eventually exceeds the proportional limit of the parent metal, but remains uniformly distributed until the point of stress
Conclusions
A complex set of failure mechanisms is involved in the structural collapse of the interconnected network of nanocrystalline tubes reinforcing conventional microtruss cores during uniaxial compression. For the n-Ni/Al hybrids of the present study, failure included inelastic strut buckling, plastic sleeve wrinkling, delamination at the metal/metal interface and nanocrystalline sleeve fracture. There was a transition from inelastic buckling dominated failure for the thinnest n-Ni sleeves to
Acknowledgment
Financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC) is gratefully acknowledged.
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Mechanical anisotropy in electrodeposited nanocrystalline metal/metal composite foams
2012, Materials Science and Engineering: AArchitectural design in stretch-formed microtruss composites
2012, Composites Part A: Applied Science and ManufacturingCitation Excerpt :As a result, at a given relative density, the stiffness and strength of microtruss materials can be up to an order of magnitude higher than those of conventional metal foams which fail in bending [21,22]. The open and low-relative density internal architecture of microtruss cellular metals, combined with the non-line-of-sight characteristics of electrodeposition also allows them to be reinforced by thin sleeves of nanocrystalline metal [23–25]. Nanocrystalline materials exhibit large strength increases by virtue of grain size reduction to the nm-scale [26].
Structural ceramic coatings in composite microtruss cellular materials
2011, Acta MaterialiaCitation Excerpt :In this paper, an electrochemical anodizing process is used to oxidize the surface of 3003 aluminum alloy microtruss structures possessing a relative density of ∼0.07. The Al2O3 provides nearly the same strength increase per unit coating thickness to the starting microtruss as was seen in the case of nanocrystalline Ni [9]. However, unlike nanocrystalline electrodeposition, the strength increase in the present case is accomplished at virtually no additional weight penalty.