The effect of molybdenum on the microstructure and creep behavior of Ti–24Al–17Nb–<Emphasis Type="Italic">x</Emphasis>Mo alloys and Ti–24Al–17Nb–<Emphasis Type="Italic">x</Emphasis>Mo SiC-fiber composites

The effect of molybdenum (Mo) on the microstructure and creep behavior of nominally Ti–24Al–17Nb (at.%) alloys and their continuously reinforced SiC-fiber composites (fiber volume fraction = 0.35) was investigated. Constant-load, tensile-creep experiments were performed in the stress range of 10–275 MPa at 650 °C in air. A Ti–24Al–17Nb–2.3Mo (at.%) alloy exhibited significantly greater creep resistance than a Ti–24Al–17Nb–0.66Mo (at.%) alloy, and correspondingly a 90°-oriented Ultra SCS-6/Ti–24Al–17Nb–2.3Mo metal matrix composite (MMC) exhibited significantly greater creep resistance than an Ultra SCS-6/Ti–24Al–17Nb–0.66Mo MMC. Thus, the addition of 2.3 at.% Mo significantly improved the creep resistance of both the alloy and the MMC. An Ultra SCS-6 Ti–25Al–17Nb–1.1Mo (at.%) MMC exhibited creep resistance similar to that of the Ultra SCS-6/Ti–25Al–17Nb–2.3Mo (at.%). Using a modified Crossman model, the MMC secondary creep rates were predicted from the monolithic matrix alloys’ secondary creep rates. For identical creep temperatures and applied stresses, the 90°-oriented MMCs exhibited greater creep rates than their monolithic matrix alloy counterparts. This was explained to be a result of the low interfacial bond strength between the matrix and the fiber, measured using a cruciform test methodology, and was in agreement with the modified Crossman model. Scanning electron microscopy observations indicated that debonding occurred within the carbon layers of the fiber-matrix interface.

---

Microstructure,tensile properties,and creep behavior of high-pressure die-cast Mg–4Al–4RE–0.4Mn (RE = La,Ce) alloys

High-pressure die-cast (HPDC) Mg–4Al–4RE–0.4Mn (RE = La, Ce) magnesium alloys were prepared and their microstructures, tensile properties, and creep behavior have been investigated in detail. The results show that two binary Al–Ce phases, Al₁₁Ce₃ and Al₂Ce, are formed mainly along grain boundaries in Mg–4Al–4Ce–0.4Mn alloy, while the phase composition of Mg–4Al–4La–0.4Mn alloy contains only α-Mg and Al₁₁La₃. The Al₁₁La₃ phase comprises large coverage of the grain boundary region and complicated morphologies. Compared with Al₁₁Ce₃ phase, the higher volume fraction and better thermal stability of Al₁₁La₃ have resulted in better-fortified grain boundaries of the Mg–4Al–4La–0.4Mn alloy. Thus higher tensile strength and creep resistance could be obtained in Mg–4Al–4La–0.4Mn alloy in comparison with that of Mg–4Al–4Ce–0.4Mn. Results of the theoretical calculation that the stability of Al₁₁La₃ is the highest among four Al–RE intermetallic compounds supports the experimental results further.     

Microstructure features of failure and mechanical properties of ultra-fine grained Ti–6AL–4V ELI alloy at 300–77 K

Experimental investigation of mechanical characteristics and failure regularities of the ultra-fine grained Ti–6Al–4V ELI alloy, produced by equal channel angular pressing (ECAP), have been carried out under the uniaxial tension at 300 and 77 K. These characteristics have been compared for the different structural states of the Ti–6Al–4V ELI alloy, distinguished by the average grain size and by the morphology of α and β phases. It has been established that the combination of the heat treatment, ECAP, and the extrusion of the Ti–6Al–4V ELI alloy leads to a considerable increase of the alloy’s strength in comparison with the initial state (54% at 300 K and 78% at 77 K) with preserving the reasonable values of the ductility (3–4% elongation to the neck beginning and 5–10% to the failure). For all investigated structural states of the Ti–6Al–4V ELI alloy only ductile failure was observed at 300 and 77 K. The fracture surface consists of regions failed under normal and shear stresses. In shear failure regions of fracture surface only elongated dimples were present.     

Electrochemical and mechanical behavior of laser processed Ti–6Al–4V surface in Ringer’s physiological solution

Laser surface modification of Ti–6Al–4V with an existing calcium phosphate coating has been conducted to enhance the surface properties. The electrochemical and mechanical behaviors of calcium phosphate deposited on a Ti–6Al–4V surface and remelted using a Nd:YAG laser at varying laser power densities (25–50 W/mm²) have been studied and the results are presented. The electrochemical properties of the modified surfaces in Ringer’s physiological solution were evaluated by employing both potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) methods. The potentiodynamic polarizations showed an increase in the passive current density of Ti–6Al–4V after laser modification at power densities up to 35 W/mm², after which it exhibited a decrease. A reduction in the passive current density (by more than an order) was observed with an increase in the laser power density from 25 to 50 W/mm². EIS studies at the open circuit potential (OCP) and in the passive region at 1.19 V showed that the polarization resistance increased from 8.274 × 10³ to 4.38 × 10⁵ Ω cm² with increasing laser power densities. However, the magnitudes remain lower than that of the untreated Ti–6Al–4V at OCP. The average hardness and modulus of the laser treated Ti–6Al–4V, evaluated by the nanoindentation method, were determined to be 5.4–6.5 GPa (with scatter <±0.976 GPa) and 124–155 GPa (with scatter <±13 GPa) respectively. The corresponding hardness and modulus of untreated Ti–6Al–4V were ~4.1 (±0.62) and ~148 (±7) GPa respectively. Laser processing at power densities >35 W/mm² enhanced the surface properties (as passive current density is reduced) so that the materials may be suitable for the biomedical applications.     

Indentation creep of the wrought AZ31 magnesium alloy

Creep behavior of a wrought Mg–3Al–1Zn (AZ31) alloy was investigated by long-term Vickers indentation testing under constant loads of 5 and 10 N and at temperatures in the range 423–523 K. Based on the steady-state power-law creep relationship, the stress exponents were determined. The creep behavior can be divided into two stress regimes with different stress exponents and activation energy values. The low-stress regime activation energy of 96.2 kJ mol⁻¹, which can be interpreted as that for the activation energy for Al diffusion in Mg, and stress exponents of about 3.0–3.4 suggest that the operative creep mechanism is dislocation viscous glide governed by the diffusion of aluminum atoms in magnesium. This behavior is in contrast to the high-stress regime, in which the average values of n = 6 and Q = 132.4 kJ mol⁻¹ imply that dislocation climb-controlled creep is the dominant deformation mechanism. Stress exponents and activation energies obtained by different analysis methods of the indentation tests are in good agreement with each other and with those of the conventional tensile creep tests on AZ31 magnesium alloy reported in the literature. The localized indentation creep tests are, thus, considered capable of acquiring reliable information on the creep behavior of wrought magnesium alloys.     

Production of Ti–13Nb–13Zr alloy for surgical implants by powder metallurgy

The Ti–13Nb–13Zr near-β alloy was developed aiming the replacement of the traditional Ti–6Al–4V alloy in surgical implants owing to its larger biocompatibility. Samples of this alloy were obtained using the blended elemental technique from hydrided powders. The isochronal sintering of the compacts for 2 h was carried out in the range 900–1,400 °C with a heating rate of 20 °C min⁻¹. In this work, the behavior of the elementary powders during sintering and the corresponding microstructural evolution were investigated. The alloy was characterized by means of scanning electron microscopy (SEM) in the backscattered mode, X-ray diffraction, and density measurements. The results indicate that the homogenization of the alloy is diffusion-controlled. With increasing temperature, homogenization of the alloy takes place and a fine plate-like α + β structure is found throughout the microstructure in temperatures above 1,300 °C. The process variables were defined aiming to minimize interstitial pick-up (C, O, and N) and avoiding intensive grain growth.     

Effect of chromium addition on microstructure, tensile properties and creep resistance of as-cast Ti-48Al alloy

The effect of Cr on the microstructure, tensile properties and creep resistance of as-cast Ti–48Al–xCr (x = 0, 2, 4 at.%) alloys were studied. The dependence of the tensile properties and creep resistance of as-cast TiAl on the solid solution strengthening and formation of β phase due to addition of Cr was investigated.     

Influence of zirconium on grain refining efficiency of Al–Ti–C master alloys

The influence of Zirconium on the grain refinement performance of Al–Ti–C master alloys and the effect mechanism has been studied in this paper. The experimental results show that Zr not only results in poisoning the Al–Ti–B master alloy, but also poisons the Al–Ti–C master alloys. The poisoning effect is more obvious at higher melting temperature. When 0.12%Zr is added into the melt, the grain refinement performance of Al–5Ti–0.4C refiner with 0.2% addition level absolutely disappears at 800 °C. The experimental results also show that it is difficult to refine the commercial purity Al containing 0.15%Zr by Al–5Ti–0.4C master alloy. Further experiments show that the Zr element can interact with both TiAl₃ and TiC phases. If both of them are present, Zr preferentially reacts with TiAl₃ phase.     

Tensile properties and fracture behaviour of an ultrafine grained Ti–47Al–2Cr (at.%) alloy at room and elevated temperatures

An ultrafine grained (UFG) Ti–47Al–2Cr (at.%) alloy has been synthesized using a combination of high energy mechanical milling and hot isostatic pressing (HIP) of a Ti/Al/Cr composite powder compact. The material produced has been tensile tested at room temperature, 700 and 800 °C, respectively, and the microstructure of the as-HIPed material and the microstructure and fracture surfaces of the tensile tested specimens have been examined using X-ray diffractometry, optical microscopy, scanning electron microscopy and transmission electron microscopy. The alloy shows no ductility during tensile testing at room temperature and 700 °C, respectively, but very high ductility (elongation to fracture 70–100%) when tensile tested 800 °C, indicating that its brittle to ductile transition temperature (BDTT) falls within the temperature range of 700–800 °C. The retaining of ultrafine fine equiaxed grain morphology after the large amount of plastic deformation of the specimens tensile tested at 800 °C and the clear morphology of individual grains in the fractured surface indicate that grain boundary sliding is the predominant deformation mechanism of plastic deformation of the UFG TiAl based alloy at 800 °C. Cavitation occurs at locations fairly uniformly distributed throughout the gauge length sections of the specimens tensile tested at 800 °C, again supporting the postulation that grain boundary sliding is the dominant mechanism of the plastic deformation of the UFG TiAl alloys at temperatures above their BDTT. The high ductility of the UFG alloy at 800 °C and its fairly low BDTT indicates that the material a highly favourable precursor for secondary thermomechanical processing.     

Effect of CaCl2 hydrothermal treatment on the bone bond strength and osteoconductivity of Ti–0.5Pt and Ti–6Al–4V–0.5Pt alloy implants

To achieve osteoconductivity, Ti–0.5Pt and Ti–6Al–4V–0.5Pt alloys were hydrothermally treated at 200°C in 10 mmol/l CaCl₂ aqueous solution for 24 h (HT-treatment). We conducted histological investigations of the HT-treated materials by using Wistar strain rats (SD rats) to evaluate the usefulness of the treatment. To measure the bone bond strength, the specimens were implanted in the tibia of SD rats, and a pull-out test was conducted. From the early postoperative stages, direct bone contact was obtained for the HT-treated implants. Within 1–4 weeks of implantation, the bone contact ratios and bone bond strengths of the HT-treated implants were higher than those of the non-treated implants. The Ti–0.5Pt and Ti–6Al–4V–0.5Pt alloys with HT-treatment showed the potential to develop a new implant with a high bone bond strength and rapid osteoconduction.     

Influence of additions of Zr, Ti–B, Sr, and Si as well as of mold temperature on the hot-tearing susceptibility of an experimental Al–2% Cu–1%?Si alloy

This study was undertaken to investigate the effects of chemical composition and mold temperature (MT) on the hot-tearing susceptibility (HTS) of an experimental Al–2% Cu–1% Si alloy using a constrained rod casting mold. The HTS results were then compared with 206 (Al–5 wt% Cu) alloys containing the same additions. In general, the Al–2% Cu–1% Si based alloys exhibited higher resistance to hot-tearing than did the 206-based alloys. It was found that an elevated MT is beneficial in reducing the HTS of the Al–2% Cu–1% Si and 206 alloys in that the HTS value decreased from over 21 to less than 5, as the MT was increased from 250 to 450 °C. Increasing the Si content reduced the HTS of the Al–2% Cu–1% Si alloy considerably; this reduction may be attributed to an increase in the volume fraction of eutectic in the structure. The addition of Sr caused deterioration in the hot-tearing resistance of the base alloy due to the formation of Sr-oxides and an extension of the freezing range of the alloy. The refinement of the grain structure obtained with the Zr–Ti–B addition decreased the severity of hot-tearing as a result of an increase in the number of intergranular liquid films per unit volume and a delay in reaching the coherency point. It was also observed that α-Fe intermetallic particles may impede the propagation of hot-tearing cracks. The Al–2% Cu–1% Si alloy with 1 wt% Si addition was judged to be the best composition in view of its low HTS.     

Microstructural evolution after creep in aluminum alloy 2618

The microstructural evolution of Al–2.24 Cu–1.42 Mg–0.9 Fe–0.9 Ni (AA2618) alloy after 195 °C/18 h aging, as well as after 180 and 240 °C/100 h creep, has been studied by transmission electron microscopy and high resolution electron microscopy (HREM). The Guinier–Preston–Bagaryatsky (GPB) zones/co-clusters, S″, S, and Al₉FeNi phases co-exist in the alloys after the 195 °C/18 h aging. After creep, precipitates become coarser and the transformation of GPB zones/co-clusters and S″ to S phase take place. A large number of GPB zones/co-clusters as those in aging state exist after 180 °C/100 h creep which possibly dynamically precipitates during the creep process. After the 240 °C/100 h creep, most of the precipitates are S variants with a few GPB zones and S″ phase. More dislocations appear upon which precipitate colonies form after creep. HREM images show that most of the early precipitates less than about 5 nm cannot exhibit perfect lattice image for the existence of stress. However, certain GPB/co-clusters possessing coherent relationship with the matrix can also be observed. HREM demonstrates that certain S particles viewed along [100]_S and [013]_S have classic orientation relationship with the matrix, and that those upon the dislocations depart from the standard orientation.     

In vitro fibroblast response to ultra fine grained titanium produced by a severe plastic deformation process

The in vitro response of the mouse fibroblast cell line 3T3 on the surface of ultrafine grained titanium [produced by a severe plastic deformation (SPD) process] has been studied in this work. SPD Ti showed much higher strength than the coarse grained Ti and equivalent to that of Ti–6Al–4V alloy. Better cell proliferation was observed on SPD Ti compared to conventional Ti and Ti–6Al–4V alloy. This could be attributed to the increased surface free energy by reduction in the grain size and possibly the presence of a large number of nano size grooves at the triple point junctions in SPD Ti sample. There was no significant difference in the results of cytotoxicity tests of fine and coarse grained materials.     

First measurement of the heat effect of the grain boundary wetting phase transition

The melting of coarse- and fine-grained Al–Zn–Mg alloys was studied by the differential scanning calorimetry. The asymmetric shape of the melting curve indicates the heat effect of the grain boundaries (GBs) wetting. The difference between melting heat of coarse- and fine-grained samples permitted to estimate the GB wetting heat effect per GB unit area. It is ~0.15 J/m² for the Al–5 wt% Zn–2 wt% Mg alloy and ~1 J/m² for the Al–10 wt% Zn–4 wt% Mg alloy.     

Enhanced fatigue properties of ultrafine-grained Ti rods processed by ECAP-Conform

This study is focused on enhancement of the fatigue properties of Ultrafine Grain titanium (UFG Ti) processed by ECAP-Conform and subsequent drawing. By examining specific combinations of ECAP-Conform and drawing, we have shown that the size and shape of grains, dislocation substructure formation, and grain boundary state are among the main microstructural parameters that determine the strength and ductility in UFG Ti. The increase in fatigue resistance of UFG Ti to 610 MPa was attributed to the high values of strength and ductility (UTS = 1290 MPa with elongation = 13 %) This value is comparable with the fatigue resistance of the titanium alloy Ti–6Al–4V.     

Decyl bis phosphonate–protein surface modification of Ti–6Al–4V via a layer-by-layer technique

Titanium and its alloys have been applied in orthopedics due to their biocompatibility, mechanical, and physical properties. Here we use decyl bis phosphonate (DBP) and collagen I to modify Ti–6Al–4V through layer-by-layer technique in order to improve its bioactivity. The abilities of bovine serum albumin (BSA) adsorption and biomimetic mineralization of different sample surfaces were studied. X-ray photoelectron spectroscopy (XPS) and water contact angle data showed that DBP and collagen I were assembled on substrates successfully. The absorbance of BSA solution acquired from ultraviolet spectrophotometer (UV) indicated that samples of Ti–6Al–4V/DBP/Collagen and Ti–6Al–4V/DBP/Collagen/DBP adsorbed BSA most, followed by Ti–6Al–4V/DBP and Ti–6Al–4V. Scanning electron microscope (SEM) photos and X-ray diffraction (XRD) data showed that sample of Ti–6Al–4V/DBP/Collagen had better bioactivity in inducing HA formation than other samples tested in this investigation.     

Bone tissue response to titanium implant surfaces modified with carboxylate and sulfonate groups

The present study assessed in vivo new bone formation around titanium alloy implants chemically grafted with macromolecules bearing ionic sulfonate and/or carboxylate groups. Unmodified and grafted Ti–6Al–4V exhibiting either 100% carboxylate, or 100% sulfonate, or both carboxylate and sulfonate groups in the percent of 50/50 and 80/20 were bilaterally implanted into rabbit femoral condyle. Neither toxicity nor inflammation were observed for all implants tested. After 4 weeks, peri-implant new bone formation varied as a function of the chemical composition of the titanium surfaces. The percent bone-implant contact (BIC) was the lowest (13.4 ± 6.3%) for the implants modified with grafted carboxylate only. The value of BIC on the implants with 20% sulfonate (24.6 ± 5.2%) was significantly (P < 0.05) lower than that observed on 100% sulfonate (38.2 ± 13.2%) surfaces. After both 4 and 12 weeks post-implantation, the BIC value for implants with more than 50% sulfonate was similar to that obtained with the unmodified Ti–6Al–4V. The grafted titanium alloy exhibiting either 100% sulfonate or carboxylate and sulfonate (50% each) groups promoted bone formation. Such materials are of clinical interest because, they do not promote bacteria adhesion but, they support new bone formation, a condition which can lead to osseointegration of bone implants while preventing peri-implant infections.     

Interface characteristics in diffusion bonding of a γ-TiAl alloy to Ti–6Al–4V

In the present study, diffusion bonding of a γ-TiAl alloy to a Ti–6Al–4V alloy at the different temperatures ranging from 1073 to 1173 K under an applied stress of 100 MPa for 2 h was investigated. The observation of the microstructure revealed that sound joints between the γ-TiAl Alloy and the Ti-alloy without any pores or cracks could be achieved through diffusion bonding at temperatures over 1073 K under the applied stress of 100 MPa for 2 h. The bond was composed of two zones, and its width increases with the increase of the bonding temperature. The EDS chemical composition profiles indicated that there is a diffusion flux of Al-atoms from γ-TiAl alloy towards the Ti-alloy and of Ti-atoms in the opposite direction. The microhardness of the diffusion bond was in the range of 310–450 HV, and increased monotonously from the side near the γ-TiAl alloy to the side near the Ti-alloy. In this bonding process, the diffusion flux of Ti atoms in interface is mainly controlled by grain boundary diffusion.     

The elevated-temperature fatigue behavior of boron-modified Ti–6Al–4V(wt.%) castings

This work investigated the effect of nominal boron additions of 0.1 and 1.0 wt.% on the elevated-temperature (455 °C) fatigue deformation behavior of Ti–6Al–4V(wt.%) castings for maximum applied stresses between 250 and 450 MPa (R = 0.1 and 5 Hz). Boron additions resulted in a dramatic refinement of the as-cast grain size, and larger boron additions resulted in larger titanium-boride (TiB) phase volume percents. The boron-containing alloys exhibited longer average fatigue lives than those for Ti–6Al–4V, which was suggested to be related to the reduced as-cast grain size and the addition of strong and stiff TiB phase. The Ti–6Al–4V–0.1B alloy exhibited the longest average fatigue lives. The TiB phase cracked during the fatigue experiments and this resulted in a decreasing Young's modulus with increased cycle number. Each alloy exhibited α-phase cracking and environmentally assisted surface edge cracking.     

Influence of oxygenation time on crack growth in titanium alloy under cyclic bending

We present the results of fatigue crack-growth tests for Ti–6Al–4V titanium alloy. Plane specimens 4 mm in thickness with a unilateral notch were tested. The analysis of crack growth was carried out for specimens without and after oxygenation process for 2 and 4 h. The fatigue crack growth rate was determined. It follows from the test results that the fatigue crack growth in the specimens without thermochemical treatment differs from the fatigue crack growth in the specimens after oxygenation. It has been also found that the applied surface treatment increases the fatigue life of the considered alloy.