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PDF Titanium Alloys. Modelling of Microstructure, Properties and Applications

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The microstructure of the alloy was also investigated by transmission electron microscopy TEM using a Philips CM20 transmission electron microscope operated at kV. The Vickers microhardness value of a given alloy is the average of ten measurements. Five measurements were performed for each sample. From the stress-strain curves, the yield strength YS , the ultimate tensile strength UTS and the elongation at rupture EL can be determined. Two specimens were tested to rupture and the mean value of each property was calculated.

For this alloy, this reflection was unexpected. On figure 3 b , a few nanosized precipitates in black are detected. The selected area electron diffraction SAED pattern of the region corresponding to figure 3 b is indexed on figure 3 c. In addition to contain non-toxic and allergy free elements, Ti-Mo-Nb alloys have another advantage over Ti-6Al-4V alloy: a lower Young's modulus. Reducing stiffness mismatch between implant and bone is essential in order to avoid the stress shielding phenomenon.

Niobium and molybdenum have high solubility in titanium but also larger atomic radius. The calculated atomic radius is pm for Ti whereas it is pm for Nb and pm for Mo. At the contrary, the atomic radii of Al and V are smaller, respectively pm for Al and pm for V Liu et al. This presence reduces the binding force of the lattice by expanding unit cell volume. This can be attributed to its microstructure. Figure 4 shows the stress-strain curves at room temperature for the TiMo-8Nb alloys and indicates the mean values of yield strength YS , ultimate tensile strength UTS and elongation at rupture EL measured from these curves.

The explanation is that these properties strongly depend on the microstructure. Table 2 also contents the hardness values. As a general rule, an increase in hardness decreases the incidence of wear on implant material Ti-6Al-4V exhibits the highest hardness value.

On the other hand, the hardness of annealed TiMo-8Nb alloy is slightly higher than the hardness of aged TiMoNb. The two alloys have simultaneously different chemical compositions and microstructures. This does not allow a correlation between these chemical or structural differences and the measured hardness variation. These results are compatible with the studies of Xu et al.

The XRD patterns obtained by Xu et al. These authors did not perform TEM analysis. It could be possible that the presence of a small volume fraction of.


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For metallic materials, lower Young's modulus generally corresponds also to lower hardness. Therefore, it is necessary to find the best compromise between these mechanical properties. Hardness to Young's modulus ratio is often used as key indicator in order to evaluate the mechanical performance of metallic biomaterials for implant applications Higher is the ratio, more appropriate is the material.

TiMo-8Nb presents the highest hardness to Young's modulus ratio amongst the Ti alloys presented in Table 2. In addition, it also presents the highest hardness to Young's modulus ratio. Furthermore, this alloy is competitive compared to the previously studied TiMoNb and TiMoNb alloys, permitting to obtain lighter orthopedic implants at lower cost. One co-author is grateful to CNPq for his visiting researcher grant. Titanium alloys in total joint replacement-a materials science perspective.

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Microstructure

Ti based biomaterials, the ultimate choice for orthopaedic implants - A review. Progress in Materials Science. Metallic implant biomaterials. Materials Science and Engineering R. Titanium alloys for biomedical applications. Materials Science and Engineering: C. New developments of Ti-based alloys for biomedical applications. Mechanical properties of biomedical titanium alloys. Materials Science and Engineering: A. Biocompatibility of beta-stabilizing elements of titanium alloys. Journal of the Mechanical Behavior of Biomedical Materials. Journal of Alloys and Compounds.

Mechanical and electrochemical characterisation of new Ti-Mo-Nb-Zr alloys for biomedical applications. Comparing the diffractions patterns in Figs. In TEM, the magnetic field of the steel samples deflects the electron beam, in a manner that cannot be controlled, significantly deteriorating the 26 Maraging steels image quality. Not all sets of precipitate patterns are included in the schematic pattern b so spot 6 in a does not appear in b. Microstructure of maraging steels 27 After the long ageing, 50 hours, Ni3Ti precipitates grow, but they do not coarsen or dissolve into the matrix.

The average diameter and length are now about 17 and 40 nm, respectively, but their distribution remains almost the same as under peak hardness ageing condition Fig.

Maraging Steels: Modelling of Microstructure, Properties and Applications - PDF Free Download

The spherical precipitates are still present. Their average diameter is larger, about 13 nm Fig. For ageing up to 20 hours, there is no austenite reversion. The austenite peaks in X-ray diffraction are not statistically significant.

Reverted austenite is also clearly visible between martensite laths Figs. There is no precipitation within the austenite phase. Although the precipitation behaviour and hardening mechanisms in maraging steels have been widely studied, there are differing opinions about the nature of precipitation phases. For example, in the 18Ni series steels, there have been reports of many different types of precipitation phases including g-Ni3Mo, h-Ni3Ti Tewari et al.

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FeTi, Fe2Ti, w Tewari et al. Although research has concentrated less on cobalt-free grades, there are already, sometimes conflicting, reports on dispersion austenite, Ni3Ti, Fe7Mo6, and Fe2Mo, FeMo or FexTi multi-phase large particles. Many possible precipitation phases have similar structures and interplanar distances, which are further complicated by double diffraction and overlapping of diffraction from multiple phases. In addition, variations in alloy composition, ageing temperature and time also affect the type and morphology of precipitation phases.

Nanometre-sized ultra fine precipitates are difficult to analyse for many if not all available techniques. We have revealed the precipitation of highly dispersed needle or rod shape Ni3Ti at both low Section 2. Computer simulation of diffraction patterns of precipitates combined with composition analysis eliminated the possibility of Ni3Mo in a T cobaltfree maraging steel.

Atom probe field-ion microscopy was used to study the composition of the rod or needle shape precipitates in a T steel Fe— The results confirm that the precipitate is Ni3Ti rather than Ni3Mo, but with the replacement of a fraction of nickel and titanium in the precipitate by iron and aluminium, respectively. Of course, the composition differences between different maraging steels will affect the precipitation phases.

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Therefore, accurate identification and repeated confirmation are always necessary. TEM experiments have also revealed the extremely fine spherical precipitates in the cobalt-free maraging steel after low and medium temperature ageing. This type of precipitate, although yet unidentified in terms of crystal structure, also has strong resistance to coarsening.

Based on the lattice constants of possible precipitation phases of hexagonal, cubic and orthorhombic systems, the interplanar distances and indexing from diffraction patterns from this precipitate, no match with any type of precipitate mentioned above could be made. Therefore, it is likely that this spherical precipitate is a new, unknown type of phase or a complex of multiple intermetallic compounds.

Further investigation is necessary. Otherwise, a looping mechanism operates. The critical radius 15b in the current system would be about 3.


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After ageing for 12 hours, the Ni 3Ti precipitates have an average diameter of 10 nm and a length of 35 nm, much larger than this critical size. The type of orientation relationship between the precipitates and the matrix in maraging steels keeps the precipitates coherent with the matrix for significant ageing periods, limiting their growth and coarsening to larger, less s 0. This contributes to the ultra-high strength of the material. In maraging steels, a high dislocation density is produced during the phase transformation in the Fe—Ni matrix. The very low impurity inclusion levels Table 2.

Therefore, the matrix has good ductility and can resist a relatively large stress concentration. After ageing, during the dislocation looping through the nano-sized precipitates, dislocation pile-up and the associated stress concentration are not so severe, so the cracking between the precipitates and the matrix is minimised. Although the precipitate dispersion makes the dislocation movement difficult, when the dislocations start to move, their uniform movement in the matrix can be sustained over short distances.

Stress concentrations are more easily formed around impurity inclusions such as Ti C,N,S , leading to void formation, coalescence and enlarging. In addition, the use of molybdenum in this cobalt-free maraging steel minimises precipitation at prior austenite grain boundaries, thereby avoiding cracking along the grain boundaries. This also improves fracture toughness. The combination of these effects is the main reason why this cobalt-free maraging steel can sustain good ductility and toughness at strength levels above MPa.