Michal Landa

Newly developed Ti-Nb-Zr-Ta-Si-Fe biomedical beta titanium alloys with increased strength and enhanced biocompatibility.

Beta titanium alloys are promising materials for load-bearing orthopaedic implants due to their excellent corrosion resistance and biocompatibility, low elastic modulus and moderate strength. Metastable beta-Ti alloys can be hardened via precipitation of the alpha phase; however, this has an adverse effect on the elastic modulus. Small amounts of Fe (0-2 wt.%) and Si (0-1 wt.%) were added to Ti-35Nb-7Zr-6Ta (TNZT) biocompatible alloy to increase its strength in beta solution treated condition. Fe and Si additions were shown to cause a significant increase in tensile strength and also in the elastic modulus (from 65 GPa to 85 GPa). However, the elastic modulus of TNZT alloy with Fe and Si additions is still much lower than that of widely used Ti-6Al-4V alloy (115 GPa), and thus closer to that of the bone (10-30 GPa). Si decreases the elongation to failure, whereas Fe increases the uniform elongation thanks to increased work hardening. Primary human osteoblasts cultivated for 21 days on TNZT with 0.5Si+2Fe (wt.%) reached a significantly higher cell population density and significantly higher collagen I production than cells cultured on the standard Ti-6Al-4V alloy. In conclusion, the Ti-35Nb-7Zr-6Ta-2Fe-0.5Si alloy proves to be the best combination of elastic modulus, strength and also biological properties, which makes it a viable candidate for use in load-bearing implants.

Using finite element method for the determination of elastic moduli by resonant ultrasound spectroscopy

Resonant ultrasound spectroscopy is a recent experimental/numerical method for the determination of moduli of elastic materials. Generally, all 21 elastic components of the elastic tensor can be determined by the numerical procedure based on the knowledge of a mechanical spectrum of a specimen. This involves the solution of a demanding inverse problem. Presently, Levenberg–Marquardt’s (LM) algorithm with the Ritz method for the solution of eigenfrequencies is usually applied. The LM method is based on the modified Newton–Raphson procedure, where all the eigenfrequencies and eigenvectors must be known for the computation of relevant gradients. Finite element method offers an analogous but more general optimization. The tested specimen, for instance, can be made of a composite material consisting of several layers with different material properties; the form of the specimen can be of a more complex shape, etc. In the present work, the elastic moduli are optimized by the fixed point iteration method, which r...

Factors Controlling Superelastic Damping Capacity of SMAs

In this paper, questions linked to the practical use of superelastic damping exploiting stress-induced martensitic transformation for vibration damping are addressed. Four parameters, particularly vibration amplitude, prestrain, temperature of surroundings, and frequency, are identified as having the most pronounced influence on the superelastic damping. Their influence on superelastic damping of a commercially available superelastic NiTi wire was experimentally investigated using a self-developed dedicated vibrational equipment. Experimental results show how the vibration amplitude, frequency, prestrain, and temperature affect the capacity of a superelastic NiTi wire to dissipate energy of vibrations through the superelastic damping. A special attention is paid to the frequency dependence (i.e., rate dependence) of the superelastic damping. It is shown that this is nearly negligible in case the wire is in the thermal chamber controlling actively the environmental temperature. In case of wire exposed to free environmental temperature in actual damping applications, however, the superelastic damping capacity significantly decreases with increasing frequency. This was explained to be a combined effect of the heat effects affecting the mean wire temperature and material properties with the help of simulations using the heat equation coupled phenomenological SMA model.

Shape recovery mechanism observed in single crystals of Cu–Al–Ni shape memory alloy

Micromechanism of the shape recovery process is optically observed in single crystals of the Cu–Al–Ni shape memory alloy. Formation of X and λ-interfaces (interfacial microstructures with two intersecting habit planes) is documented, both in a thermal gradient and during a homogeneous heating. The observed growth mechanisms (i.e. mechanisms of nucleation and growth of the twinned structures) are described and analysed. Weakly non-classical boundaries between austenite and two crossing twinning systems are also documented.

Anomalous lattice softening of Ni2MnGa austenite due to magnetoelastic coupling

Elastic constants of the cubic Ni2MnGa austenite phase and corresponding mechanical damping were determined in the temperature range from 220 K to 400 K and magnetic field up to 2 T using ultrasound pulse-echo method and resonant ultrasound spectroscopy. The shear coefficient c′ increases from 3.6 GPa in the demagnetized state to 5.9 GPa at magnetic saturation, whereas the damping decreased nearly six times. The changes of other elastic constants, c11 and c44 with an applied field were less than 1%. In the ferromagnetic state, the c′ was proportional to the square of magnetization. Above the Curie point, the coefficient c′ and damping were field-independent. The anomalous shear softening is attributed to strong magnetoelastic coupling enhanced by low magnetic anisotropy.

Non-classical austenite-martensite interfaces observed in single crystals of Cu–Al–Ni

Interfaces between austenite and a crossing-twins microstructure consisting of four variants of 2H-martensite are optically observed in a single crystal of Cu–Al–Ni shape memory alloy. It is shown that these non-classical interfaces form during thermally induced transitions from compound twinned 2H-martensite into austenite, which is in agreement with theoretical predictions. Individual twinning systems and martensitic variants involved in the observed microstructure are identified. The corresponding volume fractions are estimated based on the compatibility conditions at the habit plane and the macroscopic geometry of the interface. Miscellaneous topics related to the observed microstructures (formation mechanism and planeness of the interface) are briefly discussed.

Linearized forward and inverse problems of the resonant ultrasound spectroscopy for the evaluation of thin surface layers

In this paper, linearized approximations of both the forward and the inverse problems of resonant ultrasound spectroscopy for the determination of mechanical properties of thin surface layers are presented. The linear relations between the frequency shifts induced by the deposition of the layer and the in-plane elastic coefficients of the layer are derived and inverted, the applicability range of the obtained linear model is discussed by a comparison with nonlinear models and finite element method (FEM), and an algorithm for the estimation of experimental errors in the inversely determined elastic coefficients is described. In the final part of the paper, the linearized inverse procedure is applied to evaluate elastic coefficients of a 310 nm thick diamond-like carbon layer deposited on a silicon substrate.

Contrast enhancement of ultrasonic imaging of internal stresses in materials

The ultrasonic methods, which detect applied or residual stress in materials, are based on nonlinear interaction of a small dynamic disturbance (acoustic waves) with the pre-deformed state of the solid. This weak phenomenon (acoustoelasticity) leads to a dependence of acoustic wave velocities on the initial stress, and a stress-induced anisotropy in the acoustical properties of the material. In anisotropic media, the transversal wave velocity depends on its polarization. The amplitude of the conical polarized shear wave, propagating through a plate specimen, is sensitive to pre-stress due to acoustoelastic birefringence. The resulting scan image is created by variations of the amplitude. The previous description is a basic principle of the approach used for stress mapping in Al-alloys by time-resolved acoustic microscopy. Disk specimens with central stress concentrators are loaded step by step. The acoustic scans are created during each loading step. Thermal stress detection is also shown on specimens with an Invar core. The original image processing procedure has been developed to improve edge detection of obtained stress maps. The acoustic images are compared with theoretically predicted isocline contours. The inherent material anisotropy and the structural inhomogeneities influence significantly the acoustoelastic measurements. Advantages and limitations of the nondestructive technique are summarized on the basis of presented experimental results.

Anisotropic elasticity of DyScO3 substrates

The full elastic tensor of orthorhombic dysprosium scandate (DyScO(3)) at room temperature was determined by resonant ultrasound spectroscopy (RUS). Measurements were performed on three 500 μm thick substrates with orientations (110), (100) and (001) in the Pbnm (a < b < c) setting. For this purpose, a modification of the RUS method was developed, enabling simultaneous processing of the resonant spectra of several platelet-shaped samples with different crystallographic orientations. The obtained results are compared with ab initio calculations and with elastic constants of other rare-earth scandates, and are used for discussion of the in-plane elasticity of the (110)-oriented substrate.

Crack growth in single crystals of α-iron (3 wt.% Si)

Experimental and theoretical investigations by the method of acoustic emission and atomistic simulations by a molecular dynamics technique show that brittle-ductile transition in α-iron is very sensitive to loading rate and that the character of acoustic emission is different when different processes operate at the crack tip. A self-similar concept for comparison of experimental and atomistic results is proposed for fracture tensile tests.