Florian Pyczak

Microstructure and mechanical behavior of metal injection molded Ti–Nb binary alloys as biomedical material

The application of titanium (Ti) based biomedical materials which are widely used at present, such as commercially pure titanium (CP-Ti) and Ti-6Al-4V, are limited by the mismatch of Youngs modulus between the implant and the bones, the high costs of products, and the difficulty of producing complex shapes of materials by conventional methods. Niobium (Nb) is a non-toxic element with strong β stabilizing effect in Ti alloys, which makes Ti-Nb based alloys attractive for implant application. Metal injection molding (MIM) is a cost-efficient near-net shape process. Thus, it attracts growing interest for the processing of Ti and Ti alloys as biomaterial. In this investigation, metal injection molding was applied to the fabrication of a series of Ti-Nb binary alloys with niobium content ranging from 10wt% to 22wt%, and CP-Ti for comparison. Specimens were characterized by melt extraction, optical microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and transmission electron microscopy (TEM). Titanium carbide formation was observed in all the as-sintered Ti-Nb binary alloys but not in the as-sintered CP-Ti. Selected area electron diffraction (SAED) patterns revealed that the carbides are Ti2C. It was found that with increasing niobium content from 0% to 22%, the porosity increased from about 1.6% to 5.8%, and the carbide area fraction increased from 0% to about 1.8% in the as-sintered samples. The effects of niobium content, porosity and titanium carbides on mechanical properties have been discussed. The as-sintered Ti-Nb specimens exhibited an excellent combination of high tensile strength and low Youngs modulus, but relatively low ductility.

The Characterisation of a Powder Metallurgically Manufactured TNM™ Titanium Aluminide Alloy Using Complimentary Quantitative Methods

Abstract In order to be able to use intermetallic titanium aluminide in industrial applications, a quick and affordable method of quantitatively analysing their microstructures is required. In the presented work it was able to demonstrate on a powder metallurgical manufactured TNM™ alloy of nominal composition Ti-43.5Al-4Nb-1Mo-0.1B (at.%), that by electrolytic-polishing and colour etching a quick and cost effective quantitative microstructural analysis may be carried out via light-optical microscopic images. In doing so, the phase fractions and microstructural constituents of the various types of microstructures present are determined using complementary analysing techniques. Both light-optical and scanning electron microscopic images were captured from each of three different types of microstructures. These were then quantitatively evaluated using an image analysis program. The results were compared with those obtained from X-ray diffraction experiments. The possibilities and limits of the quantitative phase evaluation of light-optical microscopic images of colour etched microstructures are also explained and their relationship to the choice of parameters used for the colour etching and electro-polishing operations discussed.

Metal Injection Moulding of Titanium and Titanium-Aluminides

Metal injection moulding (MIM) attracts growing interest as an economic net-shape manufacturing technique for the processing of titanium and titanium alloys. Even for titanium-aluminides, intended for high-temperature applications, MIM is seen as a reasonable technique to overcome processing problems with conventional methods. In this paper, basic requirements in terms of raw materials, facilities and processing in order to produce high performance components are presented. Main focus is laid on the well-known Ti-6Al-4V alloy. It is shown that the tensile properties of specimens after MIM processing can exceed the requirements given by ASTM standards even without performing an additional HIP process. For an oxygen content ranging from 0.15 to 0.33 wt% plastic elongation yields excellent 14%. Fatigue measurements performed by means of 4-point-bending tests show that grain size is more important than residual porosity in order to achieve a high endurance limit. This is shown by addition of boron powder which refines the microstructure dramatically. The modified alloy Ti-6Al-4V-0.5B yields an endurance limit of 640 MPa compared to 450 MPa of MIM parts made from standard alloy powder. Sintered components from Ti-45Al-5Nb-0.2B-0.2C (at%) powder made by inert gas atomising (EIGA technique) and processed by MIM exhibit a residual porosity of only 0.2% and tensile properties comparable to cast material.

On the role of eutectics during recrystallization in a single crystal nickel-base superalloy - CMSX-4

Abstract The role of eutectics in recrystallization in a single crystal nickel-base superalloy – CMSX-4 – was studied by electron backscatter diffraction and electron probe micro-analysis techniques. It was shown that eutectics can initiate recrystallization by a particle stimulated nucleation mechanism which operates in interdendritic regions. During grain growth in the course of recrystallization, eutectics were observed to inhibit the motion of the advancing recrystallization grain boundary. Alternatively, a recrystallization grain boundary can migrate through eutectics by forming a large ′ phase particle behind the advancing front, either with the same crystal orientation as the eutectic the boundary advances into or, in some cases, with completely new orientations, i. e. neither the same as the eutectic it grows into nor the same as the recrystallized grains surrounding it.

Measurement of local elastic strains in the single-crystal nickel-based superalloy CMSX-6 by convergent-beam electron diffraction

Abstract High spatial resolution convergent beam electron diffraction (CBED) was used in this study as a powerful tool to investigate the local lattice parameters (and elastic strains) in the γ and γ′ phases of the single-crystal nickel-based superalloy CMSX-6 at room temperature, before and after high-temperature creep deformation in tension. With the aid of a newly implemented improved fast evaluation procedure, it was possible to determine the elastic strains in a large number of positions in the microstructure. The experimental results are in global agreement with earlier work but more specific with regard to details. Thus, it is confirmed that compressive elastic stresses exist in both horizontal and vertical γ channels in the initial state and that, after tensile creep at 980°C, in agreement with an earlier dislocation model, the elastic strains in the horizontal γ channels change from the compressive to a tensile state, acting perpendicular to the deformation axis, while the strain state in the vertical channels is only modified quantitatively. As a new result, it is noted that while the elastic strain in γ′ particles is initially homogeneous, an inhomogeneous distribution of the elastic strains in the γ′ phase is observed after high-temperature creep. The findings reported demonstrate the capability of the improved time-saving CBED evaluation procedure with respect to the reliable determination of local lattice parameters in fine-scale microstructures.

Influence of rhenium and ruthenium on the local mechanical properties of the γ and γ′ phases in nickel-base superalloys

The effect of rhenium and ruthenium on the hardness of the γ′ precipitates and the γ matrix in nickel-base superalloys was investigated using a nanoindenting atomic force microscope. The partitioning behaviour of the alloying elements and the lattice misfit between the γ and γ′ phase were determined in fully homogenised samples to explain the alloying effects. Rhenium strongly strengthens γ as it predominantly partitions to γ and has a strong solid solution-hardening effect. Ruthenium strengthens both γ and γ′ due to a more homogeneous partitioning behaviour. Ruthenium was found to cause less partitioning of rhenium to γ. This results in a stronger increase of the γ′ hardness. The change in the nanoindentation-derived hardness of both phases could be mainly attributed to the solid solution strengthening of Re and Ru.

Titanium carbide precipitation in Ti–22Nb alloy fabricated by metal injection moulding

Abstract Metal injection moulding was applied to fabricate Ti–22Nb alloy as a low modulus material for biomedical applications. Tensile test specimens were injection moulded, followed by debinding and sintering. Sintering was at 1500°C for 4 h under vacuum (10–3 Pa). Selected as-sintered Ti–22Nb samples were hot isostatically pressed at 915°C/100 MPa for 2 h. The nature of the titanium carbide precipitates in the as-sintered Ti–22Nb alloy was investigated. Selected area electron diffraction patterns revealed that the carbides are Ti2C with a fcc structure. The calculation of the phase diagram showed a significant decrease of carbon solubility in Ti–22Nb compared with that in Ti from 500 to 1500 °C, contributing to the carbide precipitation in Ti–22Nb. Due to the carbide precipitation, the as-hipped Ti–22Nb alloy exhibited higher tensile strength but lower elongation than conventionally processed Ti–22Nb.

Verification of a Commercial CALPHAD Database for Re and Ru Containing Nickel-Base Superalloys

The addition of rhenium and ruthenium to single crystal nickel-base superalloys improves the high-temperature properties of the alloys. In this work the applicability of the database TTNi7 (ThermoTech Ltd, UK) for developing 4th generation single crystal superalloys containing rhenium (Re) and ruthenium (Ru) was investigated. We systematically compared experimentally determined alloy properties to the predictions of ThermoCalc with the database TTNi7. The investigated properties were liquidus, solidus and ´ solvus temperature as well as incipient melting point and segregation. Calculations were based on thermodynamic principles with the assumption of either equilibrium or Scheil-Gulliver conditions, i.e. no diffusion in the solid and complete diffusion in the liquid. Furthermore the composition of the  and the  phase of a Re- and Ru-containing superalloy was measured and compared to calculations. Our results show that the database is capable of simulating general trends of 4th generation superalloys up to 6 weight percent (wt.-%) Re and 6 wt.-% Ru. The present work shows that Scheil-Gulliver calculations can only be used as a first approximation for nickel-base superalloys.

Diffusion Brazing of Single Crystalline Nickel Base Superalloys Using Boron Free Nickel Base Braze Alloys

Brazing is a well established repair technique for high temperature components in both industrial gas turbines and aero engines. Conventional nickel base braze alloys contain boron or silicon as melting point depressing elements. The major benefit of boron and silicon compared to other melting point depressants is its large effect on the melting point and its high diffusion coefficient in nickel base superalloys. However these elements promote precipitation of undesired brittle phases during the brazing process. To avoid these phases, transient liquid phase bonding in combination with boron and silicon free brazing alloys will be examined in this work. The influence of the brazing temperature on solidification and diffusion behaviour during transient liquid phase bonding for a single crystalline first generation and a second generation superalloy will be reported. Our experiments show that isothermal solidification without precipitation of brittle phases in the braze joint or the base material can be achieved. The brazed joint consists of fine γ/γ´ microstructure. EBSD measurements demonstrated that the single crystalline orientation of the base material was maintained throughout the joint. Electron probe micro analysis is used to characterize the diffusion behaviour. Solidification velocity will be compared with the theory of transient liquid phase bonding established by Tuah-Poku [1].

From titanium to magnesium: processing by advanced metal injection moulding

Abstract Metal injection moulding (MIM) is a good candidate for the economic mass production of complex shaped components. This is especially true for materials that are rather expensive and difficult to form, such as titanium alloys. However, the high affinity for interstitial elements such as oxygen and carbon presents a specific challenge with regard to powder purity, handling and sintering as well as the binder system and its removal. In this paper, three examples of the manufacture of high quality samples of advanced materials are shown in detail. These comprise an optimisation of the well known Ti–6Al–4V alloy with regard to MIM processing and fatigue resistance by adding 0·5 wt-% boron powder in order to effect a reduction in grain size. Second, the MIM processing of an intermetallic alloy Ti–45Al–5Nb–0·2B–0·2C (at-%) is intended for application in turbine engines and turbochargers. Third, the status of MIM of magnesium alloys is presented. In this case, the fabrication of biodegradable implants with adjustable porosity is the main motivation for the application of MIM.