Improvement of Superelastic Performance of Ti-Nb Binary Alloys for Biomedical Applications

Recently, β-Ti-based alloys consisting of non-cytotoxic elements and possessing a low elastic modulus attracted considerable attention for biomedical applications. In addition to low elastic modulus, these alloys must also present a high strength level required to endure stresses. However, it is not easy to find a thermomechanical route in order to achieve low elastic modulus and high strength simultaneously. In this study, we show that severe cold-rolling deformation followed by a short aging treatment on Ti-Nb binary alloys, in order to produce ultrafine grains and/or omega phase, is an effective way to improve both strength and superelasticity. High stress (900 MPa), low modulus (35 GPa), and high recoverable strain (2.5%) are obtained using this route. The obtained results on mechanical properties are explained in relation with microstructure evolution during thermomechanical processing.     

Pinning of dislocations and the origin of the stress anomaly in FeAl alloys

The anomalous stress rise found at intermediate temperatures in FeAl alloys may be caused by the presence of thermal vacancies produced as the temperature rises. Strong support for this hypothesis is provided by the demonstration that the same absolute values of stresses as well as stress increases are found both when testing at high temperatures and when testing at room temperature samples quenched from the same high temperatures. Examination of the superdislocations present after deformation shows strong pinning only at jogs produced by intersection with forest dislocations. Such sessile jogs on screw superdislocations lead to dipole and loop formation as the dislocations continue moving, hence producing the debris observed after deformation. In addition, edge superdislocations show a stepped morphology caused by line instabilities over a certain range of directions. There is no evidence of strong pinning and associated dislocation bowing by vacancy-aggregate type obstacles, and it is, therefore, deduced that the pinning obstacles responsible for the anomalous stress increase are probably relatively weak single vacancies, rather like solute atoms, and not stronger multi-vacancy defects.     

Effect of thermal stresses on the microstructure of the continuous cooling TiAl alloys

A series of continuous cooling tests were performed on TiAl alloys using a Gleeble3500 machine to investigate the effect of thermal stresses on the microstructure. The results show that macroscopic thermal stresses promote correlated nucleation of γ lamellae. The trend of the dominance of one twin-group γ variants in local regions is weakened, and the γ/γ interfaces tend to be true twin and pseudotwin boundaries rather than 120° rotational faults under macroscopic thermal stresses. Meanwhile, thermal-induced deformation generated under the effect of both microscopic and macroscopic thermal stresses results in numerous low angle grain boundaries (LAGBs) and dislocations. The LAGBs and dislocations distribute heterogeneously among lamellar colonies and phases. No mechanical twins are observed due to the low strain and low strain rate characteristics of the thermal-induced deformation. These findings could shed light on understanding and preventing the cracking of TiAl components during cooling process.     

Analysis of Variant Orientation Before and After Compression in Polycrystalline Ni50Mn29Ga21 MSMA

This paper deals with the microstructure and plastic deformation of Ni₅₀Mn₂₉Ga₂₁ ferromagnetic shape memory alloys. In contrast to conventional shape memory alloys, plastic deformation in the martensitic phase, which is due to twin boundary motion, may be caused not only by mechanical stress but also by an external magnetic field. The polycrystalline sample was prepared by directional solidification with a texture parallel to the heat flow. Afterwards, a heat treatment follows for chemical homogenization and stress relaxation in the austenitic state. The configuration of the twin boundaries was analyzed before and after compressing the samples. The microstructure after compression was related to the magnetic properties.     

Domain models for ferromagnetic shape-memory materials

A domain model for the twin variant and magnetic domain distribution in bulk systems of ferromagnetic shape-memory materials has been developed. The approach combines crystal elasticity, compatibility of a twinned microstructure with a tetragonal lattice structure, and micromagnetic domain theory. The model is applied to calculate phase diagrams under external magnetic fields and stresses for Ni–Mn–Ga as a magnetic system with easy-axis anisotropy and for Fe–Pd with easy-plane 4-fold anisotropies.     

Work-hardening model for polycrystalline metals under strain reversal at large strains

The mechanical behaviours under reversed strain of low carbon steels and aluminium alloys are reviewed and modelled with a simple approach based on the evolutionary laws of two dislocation densities related respectively to the forward and the backward straining. In essence, it is the competition between the annihilation of the dislocations that were created during the prestrain and the storage of newly created dislocations that lead to the observed stagnation of the hardening rate. Textural effects as well as back stresses are shown to extend or to reduce the stress–strain plateau but are not responsible for it.     

Atomistic characterization of pseudoelasticity and shape memory in NiTi nanopillars

Molecular dynamics simulations are performed to study the atomistic mechanisms governing the pseudoelasticity and shape memory in nickel–titanium (NiTi) nanostructures. For a 〈1 1 0〉 – oriented nanopillar subjected to compressive loading–unloading, we observe either a pseudoelastic or shape memory response, depending on the applied strain and temperature that control the reversibility of phase transformation and deformation twinning. We show that irreversible twinning arises owing to the dislocation pinning of twin boundaries, while hierarchically twinned microstructures facilitate the reversible twinning. The nanoscale size effects are manifested as the load serration, stress plateau and large hysteresis loop in stress–strain curves that result from the high stresses required to drive the nucleation-controlled phase transformation and deformation twinning in nanosized volumes. Our results underscore the importance of atomistically resolved modeling for understanding the phase and deformation reversibilities that dictate the pseudoelasticity and shape memory behavior in nanostructured shape memory alloys.     

Mechanical behavior of alloys with equiaxed microstructure in the semisolid state at high solid content

The mechanical response in compression of A2014 and Al–4 wt% Cu alloys with equiaxed microstructure in the semisolid state at high volume fraction of solid (>0.6) is studied. When the amount of liquid is low, deformation in compression is highly non-uniform due to strain localization. In the range of conditions examined no phase segregation was detected. The stress–strain curve exhibits a peak at low strain, around 5–10%, and then decreases to a plateau value. Secondary phase particles in A2014 that remain solid in the semisolid range of the alloy are responsible for the lower peak and higher plateau stress compared to Al–4 wt% Cu alloy. The effect of the grain size for the alloys tested was small. Strain rate jump experiments show that the instantaneous strain rate sensitivity in the semisolid state is similar to that of the solid phase. As the volume fraction of solid decreases, the plateau stress becomes rate insensitive to the strain rate jump. The results allow the relative importance to be assessed of various mechanisms responsible for resistance to flow of semisolid alloys with equiaxed microstructure in compression.     

Hot deformation behavior of Cu–Fe–P alloys during compression at elevated temperatures

Hot compression tests of the Cu–Fe–P alloys (KFC alloy and C194 alloy) were performed on Gleeble 1500 system at strain rates ranged between 0.01 and 10 s^?1 and temperatures between 650 and 850 °C. The results show that the flow stress and deformed microstructure strongly depend on the Fe contents. For the KFC alloy, the true stress–true strain curves are characterized by multiple peaks or a single peak flow, followed by a steady state flow stress, mainly related to dynamic recrystallization. For C194 alloy, the true stress–true strain curves exhibit a peak stress in the initial stage of deformation, after which the flow stress decreases monotonically, dynamic particles coarsening seemed to be responsible for flow softening. The values of deformation activation energy for the KFC alloy and the C194 alloy are 289 and 316 kJ/mol, respectively. Both are higher than that of the polycrystalline copper. The increase of deformation activation energy of the Cu–Fe–P alloys is due to the existence of the Fe-rich particles, which act as obstacles for the dislocation movement, and lead to a increase of the flow stress.     

Effect of chromium on the mechanical properties of powder-processed Fe–0.45 wt.% P alloys

The present investigation shows the role of chromium in Fe–P binary and Fe–P–Cr ternary alloys. The compositions are characterized in terms of microstructure, porosity content, hardness and tensile properties. The alloys were made using a hot powder forging technique. In this process mild steel encapsulated powders were hot forged into slabs. Then the slabs were hot rolled and annealed to relieve the residual stresses. Densifications as high as 98.9% of theoretical density have been realized. Microstructures of these alloys consist of single-phase ferrite only. Both Fe–0.45P and Fe–0.45P–3Cr alloys showed very high strength. As forged and hot rolled Fe–0.45P alloy showed low elongation. It was observed that, the addition of Cr to Fe–P based alloys caused an increase in strength associated with the reduction in ductility. Alloys developed in the present investigation were capable of hot working to very thin gage of sheets and wires.     

The controlling creep processes in TiAl alloys at low and high stresses

The creep response of a nearly-lamellar Ti–47Al–4(W, Nb, B) alloy is studied at 760 °C in a wide stress range 100–500 MPa. The alloy exhibits excellent creep resistance with a minimum creep rate of 1.2×10⁻¹⁰/s at 100 MPa and the time to 0.5% creep strain of 1132 h at 140 MPa. The controlling creep process is probed by analysis of the post-creep dislocation structure and by observation of incubation period during stress reduction test. The results indicate that creep is controlled by dislocation climb at low stresses (Class II type) and by jog-dragged dislocation glide at high stresses (Class I type). The transition from Class II to Class I type creep occurs at about 180 MPa. The excellent creep resistance of the studied alloy compared to other W containing TiAl alloys is attributed to its highly stable lamellar microstructure consisting eventually of coarse gamma laths.     

Effects of cold working degrees on grain boundary characters and strain concentration at grain boundaries in Alloy 600

Strain concentration at grain boundaries and grain boundary microstructure in cold worked Alloy 600 were characterized. Excluding the annealing twin boundaries, the base and 20% cold worked alloys exhibited higher random grain boundary fractions than the 8% and 40% cold worked alloys. An increased low angle boundaries and decreased annealing twins were observed with deformation. The 20% cold worked alloy displayed a maximum strain concentration at grain boundaries. The stress corrosion cracking growth in cold worked Alloy 600 in high temperature water showed a strong correlation with the strain concentration at grain boundaries.     

Analysis of the solidification and deformation behaviors of twin roll cast Mg-6Al-X alloys

In this study, the solidification and deformation behaviors in twin roll cast (TRC) Mg-6Al-X alloys have been investigated. The TRC simulation results showed that the AX60 alloy tended to have lower segregation while the AZ60 had the highest segregation due to the different solidification behavior and thermal properties. Compared to the as-cast microstructure, the segregation area was well matched with the melt to roll nip distance predicted in simulation. Mg alloys with Ca or Sr elements showed weaker textures when compared to A6 alloys rolled at 350 °C. In addition, there was a significant change in (0002) pole figures from strong basal textures to random textures when the rolling temperature increased from 350 °C to 450 °C. This may be attributed to the non-basal slip system activity at high temperatures. The results of visco-plastic self-consistent simulation revealed that critical resolved shear stress of the tension twin increased with increasing rolling temperature. This led to tension twin suppression in compression, which were associated with enhancing the yield isotropy of Mg alloys. Furthermore, the relative activities of basal <a> slip in AX60 alloy were higher than the other Mg alloys. This means they were responsible for enhancing the formability and yield isotropy of Mg alloys.     

Hot hardness and creep of Fe3Al-based alloys

Hot hardness and creep studies were carried out on Fe₃Al and Fe₃Al containing Cr or Ti. Indentation and impression creep testing methods were employed to characterize the creep behaviour. Compared to the binary alloy, Fe₃Al–Cr exhibits a lower hardness indicating solid-solution softening effect of Cr. On the other hand, solid-solution hardening effect of Ti is significant in the temperature range 300–900 K. Results from indentation creep indicates that a power-law creep behaviour (n between 6 and 8) is observed in the binary and Cr containing alloys at temperatures greater than 753 K. At lower temperatures in the above two alloys and in the Ti-containing alloy even at higher temperatures, there is a power-law break down. On the other hand at low stress levels covered in the impression creep studies, power-law creep is observed in all the alloys in the stress and temperature range of investigation. Under these conditions, all the alloys exhibit a stress exponent value of around 3 for the steady state creep rate. The activation energy for creep is estimated to be in the range 325 and 375 kJ mol. Among the alloys studied, Fe₃Al–Ti exhibits the best creep resistance. The results indicate that in the B2 region, viscous glide controls the creep rate at low stresses while climb of dislocations may be rate controlling at higher stresses.     

Solute strengthening from first principles and application to aluminum alloys

Alloys containing substitutional solutes exhibit strengthening due to favorable solute fluctuations within the alloy that hinder dislocation motion. Here, a quantitative, parameter-free model to predict the flow stress as a function of temperature and strain rate of such alloys is presented. The model builds on analytic concepts developed by Labusch but introduces key innovations rectifying shortcomings of previous models. To accurately describe the solute/dislocation interaction energies in and around the dislocation core, density functional theory and a flexible-boundary-condition method are used. The model then predicts the zero temperature flow stress, the energy barrier for dislocation motion, and thus the finite temperature flow stresses. The model is used to predict the flow stresses of various Al alloys. Excellent results are obtained for Al–Mg and Al–Mn. Al–Fe with ppm levels of Fe is not predicted well but, using experimental results for Fe, results for the quasi-binary Al–Cr–(Fe) and Al–Cu–(Fe) alloys agree well with experiments. The model is also consistent with the “stress equivalency” postulate of Basinski. This parameter-free model using first-principles input thus provides a basis for achieving the long-sought goal of computational design of alloys, within the context of solute-strengthening mechanisms.     

Diffusion creep of intermetallic TiAl alloys

The creep behaviour of γ-TiAl with L1₀ structure without second phases, γ-TiAl with precipitated particles of α₂-Ti₃Al with D0₁₉ structure, and γ-TiAl with the H-phase Ti₂AlC has been studied at low stresses in the temperature range 900–1200°C. The obtained data allow the construction of creep deformation mechanism maps for the studied alloys which may be used for an extrapolation of the observed creep behaviour. At higher stresses dislocation creep occurs in all alloys, which is well described by the Dorn equation with stress exponents in the range 3–5. Extended Coble creep with threshold stress was observed only for the studied two-phase alloys. A strong temperature dependence of the threshold stress for Coble creep was found for the TiAl alloy with carbide particles.     

Effect of external stress on γ nucleation and evolution in TiAl alloys

We have shown in a previous paper that twin boundary area fraction in lamellar γ-TiAl based alloys depends on collective or correlated nucleation induced by long-range elastic interactions among nucleating precipitates or between pre-existing and nucleating precipitates. In this study we investigate the effects of applied stresses on γ nucleation and evolution by elastic interaction energy calculations and phase field simulations. It is found that the elastic interaction energy reaches minimum when two variants are twin related because of their self-accommodation of the coherency strain. When external stresses are present, such autocatalysis during nucleation could be either enhanced or suppressed. Compression perpendicular or tension parallel to the basal plane of the α phase is found to enhance the internal elastic interactions between nucleating precipitates, promote twin variant selection and reduce the overall nucleation rate, while isostatic pressing, compression parallel, tension perpendicular to the basal plane does the opposite. Shear along the interface always reduce the twin boundary area fraction, regardless of the shear direction. These findings could shed light on optimizing processing routes to increase twin boundary area fraction in lamellar microstructures of γ-TiAl based alloys.     

Texture,strain and alloying in sputtered granular magnetic films

An extensive characterization of the microstructure of a large variety of CoFe–Ag(Cu) and CoFeCu granular alloys prepared by r.f. sputtering is presented. The resulting microstructure is described in terms of the ferromagnetic concentration, from the low concentration limit (x∼0.10 by volume) to values near the volume percolation threshold (x_th=0.50–0.55). The influence of the annealing procedure on the sample texture, particle size, crystal structure and strains, substrate–film stress and CoFe dilution in the matrix, among others, is also studied. This work is relevant to the complex magnetic and magnetotransport properties of granular alloys and their subtle and crucial dependence on the microstructure.     

Potentialities of Raman Imaging for the Analysis of Oxide Scales Formed on Zircaloy-4 and M5® in Air at High Temperature

Micro-Raman imaging was used to investigate oxide scales formed on Zircaloy-4 and M5^® alloys in air, in the 800–1,000 °C temperature range. To create the 2D spectral images, the data were processed by different ways. The results clearly show that a microscopic picture of the scales in terms of microstructure and internal stresses can be developed from Raman spectral maps at the micron scale. Data on the microstructure, crystallography, and composition, are presented. They confirm that the crystallographic phases observed for the Zircaloy-4 and M5^® alloys are different, since, for Zircaloy-4, we clearly observed additional Raman signatures which most probably track the presence of nitrogen in the layers well before the occurrence of the kinetic transition. In particular, they show the presence of cubic zirconia in the layers, and strongly suggest the presence of zirconium nitride and oxynitride. Results also suggest the presence of strong stress gradients in the oxide scales.