Thermal stability and phase transformations of martensitic Ti-Nb alloys.

Aiming at understanding the governing microstructural phenomena during heat treatments of Ni-free Ti-based shape memory materials for biomedical applications, a series of Ti-Nb alloys with Nb concentrations up to 29 wt% was produced by cold-crucible casting, followed by homogenization treatment and water quenching. Despite the large amount of literature available concerning the thermal stability and ageing behavior of Ti-Nb alloys, only few studies were performed dealing with the isochronal transformation behavior of initially martensitic Ti-Nb alloys. In this work, the formation of martensites (α' and α″) and their stability under different thermal processing conditions were investigated by a combination of x-ray diffraction, differential scanning calorimetry, dilatometry and electron microscopy. The effect of Nb additions on the structural competition in correlation with stable and metastable phase diagrams was also studied. Alloys with 24 wt% Nb or less undergo a [Formula: see text] transformation sequence on heating from room temperature to 1155 K. In alloys containing >24 wt% Nb α″ martensitically reverts back to ß0, which is highly unstable against chemical demixing by formation of isothermal ωiso. During slow cooling from the single phase ß domain α precipitates and only very limited amounts of α″ martensite form.

Formation of L10 Ordering in FeNi by Mechanical Alloying and Field-Assisted Heat Treatment: Synchrotron XRD Studies.

L10-ordered FeNi, tetrataenite, found naturally in meteorites is a predilection for next-generation rare-earth free permanent magnetic materials. However, the synthesis of this phase remains unattainable in an industrially relevant time frame due to the sluggish diffusion of Fe and Ni near the order-disorder temperature (593 K) of L10 FeNi. The present work describes the synthesis of ordered L10 FeNi from elemental Fe and Ni powders by mechanical alloying up to 12 h and subsequent heat treatment at 623 K for 1000 h without a magnetic field and for 4 h in the presence of 1.5 T magnetic field. Also, to address the ambiguity of L10 phase identification caused by the low difference in the X-ray scattering factor of Fe and Ni, synchrotron-based X-ray diffraction is employed, which reveals that 6 h milling is sufficient to induce L10 FeNi formation. Further milling for 12 h is done to achieve a chemically homogeneous powder. The phase fraction of L10-ordered FeNi is quantified to ∼9 wt % for 12 h milled FeNi, which increases to ∼15 wt % after heat treatment. Heat treatment of the milled powder in a magnetic field increases the long-range order parameter (S) from 0.18 to 0.30. Further, the study of magnetic properties reveals a decrease in magnetic saturation and a slight increase in coercivity with the increase in milling duration. At the same time, heat treatment in the magnetic field shows a considerable increase in coercivity.

Routes to control diffusive pathways and thermal expansion in Ti-alloys.

ß-stabilized Ti-alloys present several unexplored and intriguing surprises in relation to orthorhombic α″ phases. Among them are (i) the diffusion-controlled formation of transitional α″iso, α″lean and α″rich phases and ii) the highly anisotropic thermal expansion of martensitic α″. Using the prototypical Ti-Nb system, we demonstrate that the thermodynamic energy landscape reveals formation pathways for the diffusional forms of α″ and may lead to a stable ß-phase miscibility gap. In this way, we derive temperature-composition criteria for the occurrence of α″iso and resolve reaction sequences during thermal cycling. Moreover, we show that the thermal expansion anisotropy of martensitic α″ gives rise to directions of zero thermal strain depending on Nb content. Utilizing this knowledge, we propose processing routes to achieve null linear expansion in α″ containing Ti-alloys. These concepts are expected to be transferable to other Ti-alloys and offer new avenues for their tailoring and technological exploitation.

Giant thermal expansion and α-precipitation pathways in Ti-alloys.

Ti-alloys represent the principal structural materials in both aerospace development and metallic biomaterials. Key to optimizing their mechanical and functional behaviour is in-depth know-how of their phases and the complex interplay of diffusive vs. displacive phase transformations to permit the tailoring of intricate microstructures across a wide spectrum of configurations. Here, we report on structural changes and phase transformations of Ti-Nb alloys during heating by in situ synchrotron diffraction. These materials exhibit anisotropic thermal expansion yielding some of the largest linear expansion coefficients (+ 163.9×10-6 to -95.1×10-6 °C-1) ever reported. Moreover, we describe two pathways leading to the precipitation of the α-phase mediated by diffusion-based orthorhombic structures, α″lean and α″iso. Via coupling the lattice parameters to composition both phases evolve into α through rejection of Nb. These findings have the potential to promote new microstructural design approaches for Ti-Nb alloys and ß-stabilized Ti-alloys in general.

Factors influencing the elastic moduli, reversible strains and hysteresis loops in martensitic Ti-Nb alloys.

While the current research focus in the search for biocompatible low-modulus alloys is set on ß-type Ti-based materials, the potential of fully martensitic Ti-based alloys remains largely unexplored. In this work, the influence of composition and pre-straining on the elastic properties of martensitic binary Ti-Nb alloys was studied. Additionally, the phase formation was compared in the as-cast versus the quenched state. The elastic moduli and hardness of the studied martensitic alloys are at a minimum of 16wt.% Nb and peak between 23.5 and 28.5wt.% Nb. The uniaxial deformation behavior of the alloys used is characterized by the absence of distinct yield points. Monotonic and cyclic (hysteretic) loading-unloading experiments were used to study the influence of Nb-content and pre-straining on the elastic moduli. Such experiments were also utilized to assess the recoverable elastic and anelastic deformations as well as hysteretic energy losses. Particular attention has been paid to the separation of non-linear elastic from anelastic strains, which govern the stress and strain limits to which a material can be loaded without deforming it plastically. It is shown that slight pre-straining of martensitic Ti-Nb alloys can lead to considerable reductions in their elastic moduli as well as increases in their total reversible strains.

Elastic softening of ß-type Ti-Nb alloys by indium (In) additions.

Recent developments showed that ß-type Ti-Nb alloys are good candidates for hard tissue replacement and repair. However, their elastic moduli are still to be further reduced to match Young׳s modulus values of human bone, in order to avoid stress shielding. In the present study, the effect of indium (In) additions on the structural characteristics and elastic modulus of Ti-40 Nb was investigated by experimental and theoretical (ab initio) methods. Several ß-type (Ti-40 Nb)-xIn alloys (with x ≤ 5.2 wt%) were produced by cold-crucible casting and subsequent heat treatments (solid solutioning in the ß-field followed by water quenching). All studied alloys completely retain the ß-phase in the quenched condition. Room temperature mechanical tests revealed ultimate compressive strengths exceeding 770 MPa, large plastic strains (>20%) and a remarkable strain hardening. The addition of up to 5.2 wt% indium leads to a noticeable decrease of the elastic modulus from 69 GPa to 49 GPa, which is closer to that of cortical bone (<30 GPa). Young's modulus is closely related to the bcc lattice stability and bonding characteristics. The presence of In atoms softens the parent bcc crystal lattice, as reflected by a lower elastic modulus and reduced yield strength. Ab initio and XRD data agree that upon In substitution the bcc unit cell volume increases almost linearly. The bonding characteristics of In were studied in detail, focusing on the energies that appeared from the EDOSs significant for possible hybridizations. It came out that minor In additions introduce low energy states with s character that present antibonding features with the Ti first neighboring atoms as well as with the Ti-Nb second neighboring atoms thus weakening the chemical bonds and leading to elastic softening. These results could be of use in the design of low rigidity ß-type Ti-alloys with non-toxic additions, suitable for orthopedic applications.

Production of Porous ß-Type Ti-40Nb Alloy for Biomedical Applications: Comparison of Selective Laser Melting and Hot Pressing.

We used selective laser melting (SLM) and hot pressing of mechanically-alloyed ß-type Ti-40Nb powder to fabricate macroporous bulk specimens (solid cylinders). The total porosity, compressive strength, and compressive elastic modulus of the SLM-fabricated material were determined as 17% ± 1%, 968 ± 8 MPa, and 33 ± 2 GPa, respectively. The alloy's elastic modulus is comparable to that of healthy cancellous bone. The comparable results for the hot-pressed material were 3% ± 2%, 1400 ± 19 MPa, and 77 ± 3 GPa. This difference in mechanical properties results from different porosity and phase composition of the two alloys. Both SLM-fabricated and hot-pressed cylinders demonstrated good in vitro biocompatibility. The presented results suggest that the SLM-fabricated alloy may be preferable to the hot-pressed alloy for biomedical applications, such as the manufacture of load-bearing metallic components for total joint replacements.