Effects of molybdenum content on the structure and mechanical properties of as-cast Ti-10Zr-based alloys for biomedical applications

The effects of molybdenum on the structure and mechanical properties of a Ti-10Zr-based system were studied with an emphasis on improving the strength/modulus ratio. Commercially pure titanium (c.p. Ti) was used as a control. As-cast Ti-10Zr and a series of Ti-10Zr-xMo (x = 1, 3, 5, 7.5, 10, 12.5, 15, 17.5 and 20 wt.%) alloys prepared using a commercial arc-melting vacuum pressure casting system were investigated. X-ray diffraction (XRD) for phase analysis was conducted with a diffractometer. Three-point bending tests were performed to evaluate the mechanical properties of all specimens. The experimental results indicated that these alloys had different structures and mechanical properties when various amounts of Mo were added. The as-cast Ti-10Zr has a hexagonal α′ phase, and when 1 wt.% Mo was introduced into the Ti-10Zr alloy, the structure remained essentially unchanged. However, with 3 or 5 wt.%, the martensitic α″ structure was found. When increased to 7.5 wt.% or greater, retention of the metastable β phase began. The ω phase was observed only in the Ti-10Zr-7.5Mo alloy. Among all Ti-10Zr-xMo alloys, the α″-phase Ti-10Zr-5Mo alloy had the lowest elastic modulus. It is noteworthy that all the Ti-10Zr and Ti-10Zr-xMo alloys had good ductility. In addition, the Ti-10Zr-5Mo and Ti-10Zr-12.5Mo alloys exhibited higher bending strength/modulus ratios at 20.1 and 20.4, respectively. Furthermore, the elastically recoverable angles of these two alloys (26.4° and 24.6°, respectively) were much greater than those of c.p. Ti (2.7°). Given the importance of these properties for implant materials, the low modulus, excellent elastic recovery capability and high strength/modulus ratio of α″ phase Ti-10Zr-5Mo and β phase Ti-10Zr-12.5Mo alloys appear to make them promising candidates.     

Study of the hot deformation behaviour in Ti-5Al-5Mo-5V-3Cr-1Zr

Structural applications of near beta titanium alloys are gradually increasing in the aerospace industry because of their high specific mechanical properties and good corrosion resistance. Furthermore, a wide range of microstructures can be obtained by thermomechanical processes. This work determines by the use of EBSD technique the mechanism of restoration active in the near beta titanium alloy Ti-5Al-5Mo-5V-3Cr-1Zr for deformations in both α + β and β field near to the β transus temperature (T_β = 803 °C). Hot compression tests are carried out up to 0.7 true strain by means of a Gleeble^® 1500 machine at strain rates of 0.01, 0.1 and 1 s⁻¹. Dynamic recovery of β phase and rotation of the α grains take place predominantly in the α + β field. Further deformation produces continuous dynamic recrystallization of the β phase influenced by the strain rate. Dynamic recovery is observed during deformation above the T_β, where the misorientation is increasing towards the grain boundaries forming new small grains with a substructure at high strain rates and larger deformation. The stress exponent and the apparent activation energy for the sinh constitutive equations are determined and the microstructural features are correlated with the Zener-Hollomon parameter.     

Effects of modification on microstructure and properties of ultrahigh carbon (1.9 wt.% C) steel

The effects of a modifier that contains Rare Earths (RE), low melting point alloy (Al-Bi-Sb) and Ca-Si alloy on an ultrahigh carbon steel containing 1.9 wt.% C were studied. Microstructure characterization was carried out with optical microscopy (OM) and scanning electronic microscopy (SEM) combined with energy-dispersive spectrometry (EDS). Upon modification, the continuous eutectic carbide network structure was broken up and changed to a partly isolated and finer blocky structure in the as-cast alloy. Differential scanning calorimetry (DSC) revealed that the eutectoid temperature increased and the eutectic temperature decreased for the modified alloy. Modification also improved the impact toughness of the tempered steel, with a significant increase from 6.5 to 12.6 J cm⁻², despite the hardness remaining around 66 HRC. Furthermore, in pure sliding under loads of 20, 60 and 100 N for 600 s against a zirconia ball, the modified alloy shows slightly higher friction coefficient at all loads than the non-modified one. In addition, the friction coefficient for the steel specimen decreased with load from 20 N to 100 N attributing to a reduction in metallic wear and the formation of a thicker oxide film on the surfaces.     

The variation of tensile characteristics of Al-2 wt.%Ag and Al-2 wt.%Ag-1 wt.%Sn alloys during transformation

Effect of tin addition and heat treatment on the work hardening characteristics of Al-2 wt.%Ag alloy during phase transformation has been studied in the deformation temperature range from 503 K to 583 K. The fracture strain ?_f, and dislocation slip distance L increased with increasing deformation temperature. On the other hand the coefficient of work hardening χ = δσ²/δ?, the fracture time t_f, yield stress, σ_y fracture stress, σ_f decreased with increasing deformation temperature and exhibited an abrupt increase at about 553 K. The Sn-free samples were generally harder than the ternary alloy. The activation energy of the fracture mechanism in both alloys was around 26.8 ± 3 kJ/mol and 34.6 ± 3 kJ/mol before and after transformation temperature (553 K), respectively. The quenched samples are harder than those of slowly cooled samples. From X-ray analyses it is clear that when lattice strain ? and dislocation density ρ increases, the crystallite size η decreases.     

Relationship between microstructure,hardness and corrosion resistance in 20 wt.%Cr, 27 wt.%Cr and 36 wt.%Cr high chromium cast irons

The microstructures, hardness and corrosion behavior of high chromium cast irons with 20, 27 and 36 wt.%Cr have been compared. The matrix in as-cast 20 wt.%Cr, 27 wt.%Cr and 36 wt.%Cr high chromium cast irons is pearlite, austenite and ferrite, respectively. The eutectic carbide in all cases is M₇C₃ with stoichiometry as (Cr_3.37, Fe_3.63)C₃, (Cr_4.75, Fe_2.25)C₃ and (Cr_5.55, Fe_1.45)C₃, respectively. After destabilization at 1000 °C for 4 h followed by forced air cooling, the microstructure of heat-treatable 20 wt.%Cr and 27 wt.%Cr high chromium cast irons consisted of precipitated secondary carbides within a martensite matrix, with the eutectic carbides remaining unchanged. The type of the secondary carbide is M₇C₃ in 20 wt.%Cr iron, whereas both M₂₃C₆ and M₇C₃ secondary carbides are present in the 27 wt.%Cr high chromium cast iron. The size and volume fraction of the secondary carbides in 20 wt.%Cr high chromium cast iron were higher than for 27 wt.%Cr high chromium cast iron. The hardness of heat-treated 20 wt.%Cr high chromium cast iron was higher than that of heat-treated 27 wt.%Cr high chromium cast iron. Anodic polarisation tests showed that a passive film can form faster in the 27 wt.%Cr high chromium cast iron than in the 20 wt.%Cr high chromium cast iron, and the ferritic matrix in 36 wt.%Cr high chromium cast iron was the most corrosion resistant in that it exhibited a wider passive range and lower current density than the pearlitic or austenitic/martensitic matrices in 20 wt.%Cr and 27 wt.%Cr high chromium cast irons. For both the 20 wt.%Cr and the 27 wt.%Cr high chromium cast irons, destabilization heat treatment gave a slight improvement in corrosion resistance.     

Comparative determination of the α/β phase fraction in α+β-titanium alloys using X-ray diffraction and electron microscopy

A comparison is made between the measured α/β phase fractions in Ti-6246 using X-ray diffraction (XRD) and electron microscopy. Image analysis of SEM and TEM images was compared to the phase fraction estimate obtained using electron backscattered diffraction, lab and high-energy synchrotron XRD. There was a good agreement between the electron microscopic and diffraction techniques, provided that the microstructural parameters of grain size and texture are estimated correctly when using quantitative Rietveld refinement.     

Ultrafine 316 L stainless steel particles with frozen-in magnetic structures characterized by means of electron backscattered diffraction

Electron Backscatter Diffraction (EBSD) studies clearly revealed a different crystallographic structure of the smallest particle size fraction of gas-atomized AISI 316 L stainless steel powder (< 4 μm) compared with larger sized fractions of the same powder (< 45 μm). Despite similar chemical compositions, the predominating structure of the smallest particle size fraction was ferritic (i.e., has ferromagnetic properties) whereas the larger sized particle fractions and massive 316 L revealed an expected austenitic and non-magnetic structure. From these findings, it follows that direct magnetic separation can be applied to separate very fine sized particles. These structural differences explain previously observed dissimilarities from corrosion and metal release perspectives.     

Microstructural stability and high-temperature mechanical properties of AZ91 and AZ91 + 2RE magnesium alloys

The effects of 2 wt.% rare earth element addition on the microstructure evolution, thermal stability and shear strength of AZ91 alloy were investigated in the as-cast and annealed conditions. The as-cast structure of AZ91 consists of α-Mg matrix and the β-Mg₁₇Al₁₂ intermetallic phase. Due to the low thermal stability of this phase, the strength of AZ91 significantly decreased as the temperature increased. The addition of rare earth elements refined the microstructure and improved both thermal stability and high-temperature mechanical properties of AZ91. This was documented by the retention of the initial fine microstructure and ultimate shear strength (USS) of the rare earth elements-containing material after long-term annealing at 420 °C. The improved stability and strength are attributed to the reduction in the volume fraction of β-Mg₁₇Al₁₂ and retention of the thermally stable Al₁₁RE₃ intermetallic particles which can hinder grain growth during the annealing process. This behavior is in contrast to that of the base material which developed a coarse grain structure with decreased strength caused by the dissolution of β-Mg₁₇Al₁₂ after exposure to high temperature.     

Investigation on the residual stress and microstructure of (TiB + TiC)/Ti-6Al-4V composite after shot peening

Titanium matrix composites reinforced with TiB and TiC are fabricated by in situ technology. The distribution of residual stresses and microstructure are investigated after shot peening. The domain sizes and the microstrain of surface deformation layers are calculated from the full width at half maximum of Ti (2 1 3) peak. The results reveal that both compressive residual stresses and microhardness increase with the improvement of shot peening intensity in the surface deformation layers. And the domain sizes are refined and the microstrain become severely in surface layers after shot peening. In the progress of investigation, it is found that these variations are influenced by both the volume percentages of reinforcements and shot peening intensities. Comparing to the reinforcements, the effects of SP intensities were distinct and dominate.     

Influence of the supersaturated silicon solid solution concentration on the effectiveness of severe plastic deformation processing in Al-7 wt.% Si casting alloy

A comparative study of room temperature severe plastic deformation (SPD) of a hypoeutectic Al-7 wt.% Si casting alloy by high pressure torsion (HPT) and equal channel angular pressing (ECAP) has been performed. Microstructural parameters and microhardness were evaluated in the present work. Three different initial Si solid solution contents have been considered: as cast (C sample, 1.6 wt.% Si), annealed and quenched (Q sample, 1.2 wt.% Si) and annealed and furnace cooled (S sample, 0.7 wt.% Si). The samples processed by ECAP have smaller average Si particle sizes (0.9-1.7 μm), than those for samples processed by HPT (2.4-4.4 μm). The initial supersaturated Si solid solution is the major factor affecting the microstructure and the mechanical properties of the material. Fine deformation-induced Si precipitates from the supersaturated solid solution were responsible of the large grain refinement obtained by both SPD processing methods, which was considerably higher than that reported for pure aluminium. Q samples, processed by both SPD methods, containing an intermediate concentration of Si in solid solution, show the highest hardness due to the finest and most homogeneous microstructure. The finest and homogeneous grain size was ∼0.2 μm for the HPTed and ∼0.4 μm for the ECAPed Q samples.     

Influence of the filler metal on the properties of Ti-6Al-4V electron beam weldments. Part I: Welding procedures and microstructural characterization

The use of different procedures for electron beam welding of 17 mm thick Ti-6Al-4V plate and the difficulties found in this process are analysed. When this alloy was welded autogeneously the presence of significant amounts of α martensite was observed, recommending looking for another solution. In the early trials a V joint design was used but distortions and defects were detected in the welds when multi-pass procedures were considered. Consequently, for the remaining weldments K or I joint configurations were selected. Initially, Ti-6Al-4V wire was preferred in order to match mechanical properties with base material but no significant improvement was found leading to consideration of using a less alloyed filler metal. Different commercially pure titanium filler metals have been employed to optimise the performance of the fusion zone of electron beam weldments. In a second paper [1] the influence of the welding procedure on the mechanical properties of the various joints will be discussed.     

The effects of post annealing on the mechanical properties, microstructure and texture evolutions of pure copper deformed by twist extrusion process

X-ray diffraction peak broadening analysis showed that performing a proper heat treatment between the twist extrusion passes of commercially pure copper decreased the coherent domain size and increased the microstrain. Moreover, SEM micrographs illustrated that annealed material contained new formed grains that could not grow due to lack of sufficient time. Under such circumstances, the ultimate strength was elevated about 45 MPa. The deformed material showed texture of simple shear deformation, changing by applying the post annealing.     

The effect of laser energy input on the microstructure,physical and mechanical properties of Ti-6Al-4V alloys by selective laser melting

Selective laser melting is an advanced manufacturing process which can control the microstructure evolution and mechanical properties of as-manufactured products via various processing parameters. In this study, the porosity/relative density, surface quality, microstructure and mechanical properties were investigated on the selective laser melted Ti-6Al-4V alloy specimens fabricated with a wide range of laser energy inputs. It was found that the microstructure of selected laser melted Ti-6Al-4V alloys is typical of acicular martensites α′. Quantitative analysis reveals that the relative density, martensitic lath size and microhardness increase with the laser energy input. The surface quality is also substantially affected by the energy input.     

The effect of Si additions on the sintering and sintered microstructure and mechanical properties of Ti-3Ni alloy

Thermodynamic predictions suggest that silicon has the potential to be a potent sintering aid for Ti-Ni alloys because small additions of Si lower the solidus of Ti-Ni alloys appreciably (>200 °C by 1 wt.% Si). A systematic study has been made of the effect of Si on the sintering of a Ti-3Ni alloy at 1300 °C. The sintered density increased from 91.8% theoretical density (TD) to 99.2%TD with increasing Si from 0% to 2%. Microstructural examination reveals that coarse particles and/or continuous networks of Ti₅Si₃ form along grain boundaries when the addition of Si exceeds 1%. The grain boundary Ti₅Si₃ phase leads to predominantly intergranular fracture and therefore a sharp decrease in ductility concomitant with increased tensile strengths. The optimum addition of Si is proposed to be ≤1%. Dilatometry experiments reveal different shrinkage behaviours with respect to different Si contents. Interrupted differential scanning calorimetry (DSC) experiments and corresponding X-ray diffraction (XRD) analyses clarify the sequence of phase formation during heating. The results provide a useful basis for powder metallurgy (PM) Ti alloy design with Si.     

Phase transformations of Ni-15 wt.% B powders during mechanical alloying and annealing

The formation of Ni-B binary intermetallic compounds was investigated by mechanical alloying (MA) of the Ni-15 wt.% B (≈ Ni-48 at.% B) powder mixture and subsequent heat treatment. It was found that an interstitial Ni(B) solid solution was formed at the early stage of milling, followed by the formation of Ni₃B intermetallic compound after 25 h of milling. On further milling, the Ni₃B transformed to Ni₂B and o-Ni₄B₃ (orthorhombic). Phase transformation during heating of Ni(B) solid solution phase up to 800 °C could be represented by Ni(B) → Ni₃B → Ni₂B. Other intermetallics can be formed by heat treatment of Ni(B) solid solution at temperatures above 800 °C.     

Effect of bismuth and silver on the corrosion behavior of Sn-9Zn alloy in NaCl 3 wt.% solution

The effect of silver (Ag) and bismuth (Bi) on the corrosion resistance of Sn-9Zn alloy in NaCl 3 wt.% solution was investigated using electrochemical techniques. The results showed that the addition of Bi and Ag lead to the increase of corrosion rate and the corrosion potential E_corr is shifted towards less noble values. After immersion, X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive of spectroscopy (EDS) analysis of the corroded alloy surface revealed the nature of corrosion products. EDS and XRD analyses confirmed the oxide of zinc (ZnO and Zn₅(OH)₈Cl₂H₂O) as the major corrosion product formed on the outer surface of in the tested three solder alloys.     

Heterophase structures and their quantitative characteristics in (1 − x)Pb(Fe1/2Nb1/2)O3 − xPbTiO3 near the morphotropic phase boundary

Features of phase coexistence in solid solutions of (1 − x)Pb(Fe_1/2Nb_1/2)O₃ − xPbTiO₃ with the perovskite-type structure are studied at 0.05 ≤ x ≤ 0.08. The role of elastic matching of the tetragonal P4mm and monoclinic Cm phases of the ferroelectric nature is considered near the morphotropic phase boundary. Model concepts on the stress relief in heterophase structures are developed and applied to interpret the phase content in (1 − x)Pb(Fe_1/2Nb_1/2)O₃ − xPbTiO₃. Good agreement between the calculated and experimental dependences of the volume fraction of the tetragonal phase on x is observed. It is shown that the studied phase coexistence under conditions for the complete stress relief can take place at elastic matching of the single-domain monoclinic phase and the tetragonal phase split into two types of 90° domains.     

Microstructure and texture studies on twin-roll cast AZ31 (Mg-3 wt.%Al-1 wt.%Zn) alloy and the effect of thermomechanical processing

The microstructure and texture of the twin-roll cast (TRC) AZ31 (Mg-3 wt.%Al-1 wt.%Zn) sheet, with a thickness of 6 mm, have been investigated. The TRC AZ31 exhibits a dendritic microstructure with columnar and equiaxed grains. These contain Al-Mn and Mg-Al-Zn second-phase particles that are approximately 1 μm in size. This heterogeneous structure is attributed to the effect of the cooling rate, which varies from 325 °C/s on the surface to ∼150 °C/s in the mid-thickness of the sheet. No surface segregation, but a certain degree of macrosegregation is observed in the mid-thickness which persists after annealing and rolling. Recrystallization at 420 °C leads to a bimodal grain-size distribution, while a fine-grain structure is obtained after rolling and annealing. The TRC AZ31 sheet exhibits basal textures in the (i) as-received, (ii) rolled and (iii) rolled-annealed conditions. However, post-annealing of the TRC AZ31 at 420 °C produces a relatively random texture that has not been previously observed in the conventional AZ31 sheet. The texture randomization is attributed to the particle-stimulated nucleation of new grains in the TRC structure. The preliminary evaluation of mechanical properties indicates that such annealing treatment slightly increases the ultimate tensile strength (UTS), but significantly improves elongation.     

Pyrochlore type semiconducting ceramic oxides in Ca-Ce-Ti-M-O system (M = Nb or Ta)—Structure, microstructure and electrical properties

A new series of pyrochlore type ceramic semiconducting oxides in Ca-Ce-Ti-M-O (M = Nb or Ta) system has been synthesized by the conventional ceramic route. The electrical conductivity measurements show that these oxides exhibit semiconducting behavior and the conductivity increases with the Ce content in the compound. Activation energy of the current carriers is in the range of 0.5-1.6 eV. The electrical conductivity in these oxides is due to the presence of Ce³⁺, which remains in the reduced state without being oxidized to Ce⁴⁺ by structural stabilization. The photoluminescence and X-ray photoelectron spectroscopy analysis corroborate the presence of Ce in the 3+ state. Impedance spectral analysis is carried out to evaluate the transport properties and indicates that the conduction in these compounds is mainly due to electronic contribution. The X-ray powder diffraction and Raman spectroscopy analysis establishes that these oxides belong to a cubic pyrochlore type structure.