Shengcheng Mao

A Transforming Metal Nanocomposite with Large Elastic Strain, Low Modulus, and High Strength

S-T-R-E-T-C-H Me Most metals show elastic strain limits well below 1%, beyond which permanent plastic deformation occurs. Metal nanowires can be elastically stretched to much higher strains, on the order of 4 to 7%. However, when placed inside a metal matrix to form a composite, these nanowires can no longer be stretched to the same extent, even when the nanowires are well distributed and show good bonding with the matrix. Hao et al. (p. 1191; see the Perspective by Zhou) used a shape memory alloy as the matrix material to produce a much better (more elastic) composite. The use of a shape-memory metal alloy as a matrix better exploits the inherent elastic properties of niobium nanowires. [Also see Perspective by Zhou] Freestanding nanowires have ultrahigh elastic strain limits (4 to 7%) and yield strengths, but exploiting their intrinsic mechanical properties in bulk composites has proven to be difficult. We exploited the intrinsic mechanical properties of nanowires in a phase-transforming matrix based on the concept of elastic and transformation strain matching. By engineering the microstructure and residual stress to couple the true elasticity of Nb nanowires with the pseudoelasticity of a NiTi shape-memory alloy, we developed an in situ composite that possesses a large quasi-linear elastic strain of over 6%, a low Youngs modulus of ~28 gigapascals, and a high yield strength of ~1.65 gigapascals. Our elastic strain-matching approach allows the exceptional mechanical properties of nanowires to be exploited in bulk materials.

Cloning Nacre's 3D Interlocking Skeleton in Engineering Composites to Achieve Exceptional Mechanical Properties.

Ceramic/polymer composite equipped with 3D interlocking skeleton (3D IL) is developed through a simple freeze-casting method, exhibiting exceptionally light weight, high strength, toughness, and shock resistance. Long-range crack energy dissipation enabled by 3D interlocking structure is considered as the primary reinforcing mechanism for such superior properties. The smart composite design strategy should hold a place in developing future structural engineering materials.

The nano- and mesoscopic cooperative collective mechanisms of inhomogenous elastic-plastic transitions in polycrystalline TiNi shape memory alloys

The cooperative collective effects of strain-induced martensitic variant accommodation in nano- and micrograined polycrystalline TiNi shape memory alloys were discovered to form macroscopic strain-induced shearing bands—the Luders-like deformation bands. We use the in situ electron backscattered diffraction technique and the single surface trace analysis method to unambiguously elucidate the formation and propagation of shearing bands derived from the chain effect of intergrain selective martensite habit plane variants. The macroscopic characteristics of shearing bands can be well interpreted by mesoscale crystallographic features using phenomenological crystallographic theory and Schmid’s law with a strong correlation to texture. It was also discovered that the nonlinear elastic feature in the stress-strain curve prior to the stress-strain plateau correlates to lattice/grain rotations other than martensitic variant reorientation.

Crystallographic Study of Superelastic Deformation of Nitinol

Limitations of finite element analysis (FEA) in providing accurate localized stress and strain information in superelastic nitinol are well recognized. Understanding the parent texture and the crystallography of stress-induced martensitic transformation holds the key to bridge the gap between continuum mechanics and the microscopic stress-strain condition imposed by the phase transformation in understanding the deformation mechanism of this complex material. A scanning electron microscope equipped with an electron beam back scatter diffraction detector is a powerful tool that can extract microscopic crystallographic information from bulk specimens. The technique has been employed to study the crystallography of stress-induced martensitic transformation during tensile and bend deformations of superelastic nitinol. The results suggest that for tensile deformation, the transformation variants of stress-induced martensite (SIM) inside the Luders band follow maximum shear stress along the martensite shape change direction. The observation also confirms that the SIM transformation is incomplete, leaving a significant amount of retained B2 parent phase inside the Luders band. As tensile deformation proceeds, the Luders band propagates by nucleating new martensite plates instead of by thickening of the existing martensite variants. For bend deformation, SIM appears to transform much easier in the tension side than in the compression side, confirming previous studies on the asymmetrical tension-compression property in superelastic nitinol materials. Lastly, the local stress field at the tip of martensite plate has been computed by finite element (FEA) simulation based on the observed martensite morphology. The implication on local stress field and plasticity provides a rationalization in explaining why nitinol fatigue life appears to be insensitive to the mean strain effect.

Tunneling anisotropic magnetoresistance driven by magnetic phase transition

The independent control of two magnetic electrodes and spin-coherent transport in magnetic tunnel junctions are strictly required for tunneling magnetoresistance, while junctions with only one ferromagnetic electrode exhibit tunneling anisotropic magnetoresistance dependent on the anisotropic density of states with no room temperature performance so far. Here, we report an alternative approach to obtaining tunneling anisotropic magnetoresistance in α′-FeRh-based junctions driven by the magnetic phase transition of α′-FeRh and resultantly large variation of the density of states in the vicinity of MgO tunneling barrier, referred to as phase transition tunneling anisotropic magnetoresistance. The junctions with only one α′-FeRh magnetic electrode show a magnetoresistance ratio up to 20% at room temperature. Both the polarity and magnitude of the phase transition tunneling anisotropic magnetoresistance can be modulated by interfacial engineering at the α′-FeRh/MgO interface. Besides the fundamental significance, our finding might add a different dimension to magnetic random access memory and antiferromagnet spintronics.Tunneling anisotropic magnetoresistance is promising for next generation memory devices but limited by the low efficiency and functioning temperature. Here the authors achieved 20% tunneling anisotropic magnetoresistance at room temperature in magnetic tunnel junctions with one α′-FeRh magnetic electrode.

Revealing ultralarge and localized elastic lattice strains in Nb nanowires embedded in NiTi matrix

Freestanding nanowires have been found to exhibit ultra-large elastic strains (4 to 7%) and ultra-high strengths, but exploiting their intrinsic superior mechanical properties in bulk forms has proven to be difficult. A recent study has demonstrated that ultra-large elastic strains of ~6% can be achieved in Nb nanowires embedded in a NiTi matrix, on the principle of lattice strain matching. To verify this hypothesis, this study investigated the elastic deformation behavior of a Nb nanowire embedded in NiTi matrix by means of in situ transmission electron microscopic measurement during tensile deformation. The experimental work revealed that ultra-large local elastic lattice strains of up to 8% are induced in the Nb nanowire in regions adjacent to stress-induced martensite domains in the NiTi matrix, whilst other parts of the nanowires exhibit much reduced lattice strains when adjacent to the untransformed austenite in the NiTi matrix. These observations provide a direct evidence of the proposed mechanism of lattice strain matching, thus a novel approach to designing nanocomposites of superior mechanical properties.

Crystallographic mechanisms of fracture in a textured polycrystalline TiNi shape memory alloy

The crystallographic mechanisms of the crack initiation and propagation in a textured polycrystalline TiNi shape memory alloy were studied using scanning electron microscope and ex situ electron backscattered diffraction techniques. The results indicated that stress-induced martensitic transformation was triggered and formed a sandwich structure at the front of the crack tip prior to the crack initiation in the polycrystalline TiNi alloy. The crack propagated intragranularly and occasionally across the grain boundaries. The closed packed planes of the B2 parent phase such as {110} and {100} were revealed to favor the crack propagation. The stress-induced B2+B19′ sandwich structure determined the crack propagated by a microscopic sawtooth pattern and therefore a ductile feature in this textured polycrystalline TiNi sheet specimen.The crystallographic mechanisms of the crack initiation and propagation in a textured polycrystalline TiNi shape memory alloy were studied using scanning electron microscope and ex situ electron backscattered diffraction techniques. The results indicated that stress-induced martensitic transformation was triggered and formed a sandwich structure at the front of the crack tip prior to the crack initiation in the polycrystalline TiNi alloy. The crack propagated intragranularly and occasionally across the grain boundaries. The closed packed planes of the B2 parent phase such as {110} and {100} were revealed to favor the crack propagation. The stress-induced B2+B19′ sandwich structure determined the crack propagated by a microscopic sawtooth pattern and therefore a ductile feature in this textured polycrystalline TiNi sheet specimen.

Locality and rapidity of the ultra-large elastic deformation of Nb nanowires in a NiTi phase-transforming matrix

This study investigated the elastic deformation behaviour of Nb nanowires embedded in a NiTi matrix. The Nb nanowires exhibited an ultra-large elastic deformation, which is found to be dictated by the martensitic transformation of the NiTi matrix, thus exhibiting unique characteristics of locality and rapidity. These are in clear contrast to our conventional observation of elastic deformations of crystalline solids, which is a homogeneous lattice distortion with a strain rate controlled by the applied strain. The Nb nanowires are also found to exhibit elastic-plastic deformation accompanying the martensitic transformation of the NiTi matrix in the case when the transformation strain of the matrix over-matches the elastic strain limit of the nanowires, or exhibit only elastic deformation in the case of under-matching. Such insight provides an important opportunity for elastic strain engineering and composite design.

First-principles studies of the structural and electronic properties of the C14 Laves phase XCr2 (X = Ti, Zr, Nb, Hf and Ta)

The optimized structures, electronic properties and bonding characteristics of the hexagonal C14 Laves phase XCr2 (X = Ti, Zr, Nb, Hf and Ta) have been investigated using first-principles calculations. Our results reveal that the equilibrium formation enthalpies are not depends entirely on the atomic numbers. The total and the partial density of states and valence charge densities of Laves phases are also calculated and applied to reveal the nature of the bonding character in consideration of the different atomic numbers.

Elemental preference and atomic scale site recognition in a Co-Al-W-base superalloy

Using state-of-the-art atomic scale super energy dispersive X-ray spectroscopy and high angle annular dark field imaging this study reveals the elemental partitioning preference between the γ′ and γ phases in a Co-Al-W-Ti-Ta superalloy and the site preference of its alloying elements in the ordered L12 γ′ phase. A semi-quantitative analysis of atomic column compositions in the ordered L12 γ′ structure is provided. Co atoms were found to occupy the {1/2, 1/2, 0} face-center positions whereas Al, W, Ti and Ta atoms prefer to occupy the {0, 0, 0} cube corner positions in the L12 γ phase. These findings agree well with predictions from first principles simulations in the literature.