Effect of martensitic transformation in Ti–15 at %V β-phase particles on lamellar boundary decohesion in γ-TiAl Part II Finite element analysis of crack-bridging phenomenon

The commercial finite element package ABAQUS has been used to analyse the crack bridging process by Ti-15 at%V -phase particles dispersed in -TiAl matrix in the presence of particle–matrix decohesion. Both the particle–matrix decohesion potential and the -phase materials constitutive relations are found to have a major effect on the ductility, fracture toughness and failure mode of the – two-phase material. The interface potential is found to primarily affect the distribution of the normal interface strength ahead of the advancing interfacial crack and the mode (gradual versus sudden) of decohesion. The -phase materials constitutive relations are found to influence the location of nucleation of the interfacial cracks and, in turn, the mode of decohesion. A metastable -phase that can plastically deform at low stress levels by undergoing a stress-assisted martensitic transformation, but experience a high rate of strain hardening is found to give rise to the largest levels of ductility and fracture toughness is the – two-phase material. © 1998 Kluwer Academic Publishers     

Effect of thermal cycling on martensitic transformation temperatures in Ti–Ni–Cu shape memory alloys

Abstract

Changes in martensitic transformation temperatures during thermal cycling in Ti–Ni–Cu shape memory alloys have been investigated by means of electrical resistivity measurements, thermal cycling tests under constant load and transmission electron microscopy. During thermal cycling without applied stress, the B2→B19′ transformation temperature M _s decreased, while the B2→B19 transformation start temperature M _s′ kept almost constant. During thermal cycling with applied stress, in solution treated Ti–45Ni–5Cu alloy, changes in M _s depended on the amount of applied stress. That is, M _s decreased when the applied stress was 39.2 MPa, while its value kept almost constant when a stress of 117.2 MPa was applied. It was also found that M _s′ increased during thermal cycling in the solution treated Ti–35Ni–15Cu and Ti–30Ni–20Cu alloys, irrespective of the amount of applied stress. All changes in M _s and M _s′ during thermal cycling with applied stress in Ti–Ni–Cu alloys were explained well by a combination of the thermal cycling effect and the structural refinement effect.     

Coherency strains influence on martensitic transformation of γ-Fe particles in compressed Cu-Fe alloy single crystals

Single crystals of a Cu-Fe alloy, which contained spherical -Fe particles, were compressed up to shear strain of 0.17 in liquid nitrogen bath. In the process of straining the structural (optical and TEM) observations in as-deformed and post-deformation annealed samples were provided. The substructure of deformed samples was characterized by slightly developed cell structure and lack of distinct layer-like arrangements of dislocations. Three kinds of particles were found: coherent and semi-coherent f.c.c. -Fe and martensiticly transformed b.c.c. -Fe. The critical diameter for coherency loss was found to be 58 nm at the initial stage of deformation decreasing with strain to 50 nm. These values coincided with the theoretical estimations. It was suggested that relaxation of coherency strains around -Fe particles by the generation of interface dislocations might occur prior to the martensitic transformation. This assumption might explain the particle size dependency of martensitic transformation.     

Effect of grain boundary on martensite transformation behaviour in Fe–32 at.%Ni bicrystals

Two types of Fe–32 at.%Ni bicrystals containing a 90°{211} tilt or a 90°<211= twist grain boundary were prepared to investigate the effect of grain boundary character on the martensitic transformation behaviour. The martensite-start temperature (Ms) of bicrystals with the tilt boundary was significantly higher than that of single crystals, while Ms of bicrystals with the twist boundary showed no significant difference from that of single crystals. Near the tilt boundary, the coarse lenticular martensites were symmetrically formed in the neighbouring grains. In contrast, the tiny martensites were homogeneously distributed in bicrystals with the twist boundary, similar to those in single crystals. In the vicinity of the tilt and the twist boundaries, some variants with the habit plane almost parallel to the boundaries were preferentially selected among 24 variants; moreover, the equivalent variants in neighbouring grains were adjoined at the tilt boundary. As a result, the compatibility of shape strains across the boundary was maintained in the case of the tilt boundary, resulting in increasing the Ms. Such characteristic nucleation of martensites can be regarded as an example of self-accommodation across the boundary, which is called cooperative nucleation (C–N). From a crystallographic viewpoint, C–N can occur only at the symmetric tilt boundary. Effects of pre-deformation and applied stress on the heterogeneous nucleation at the boundary were also examined; C–N was always confirmed to occur at the tilt boundary, and the advantage of the boundary for nucleation of martensites did not change even under pre-deformation or applied stress. Furthermore, the martensite-start stress (σ_M) and the morphology of martensites in the stress-assisted transformation were strongly influenced by C–N.     

Effect of grain boundary on martensite transformation behaviour in Fe–32 at.%Ni bicrystals

Two types of Fe–32 at.%Ni bicrystals containing a 90°〈211〉 tilt or a 90°{211} twist grain boundary were prepared to investigate the effect of grain boundary character on the martensitic transformation behaviour. The martensite-start temperature (Ms) of bicrystals with the tilt boundary was significantly higher than that of single crystals, while Ms of bicrystals with the twist boundary showed no significant difference from that of single crystals. Near the tilt boundary, the coarse lenticular martensites were symmetrically formed in the neighbouring grains. In contrast, the tiny martensites were homogeneously distributed in bicrystals with the twist boundary, similar to those in single crystals. In the vicinity of the tilt and the twist boundaries, some variants with the habit plane almost parallel to the boundaries were preferentially selected among 24 variants; moreover, the equivalent variants in neighbouring grains were adjoined at the tilt boundary. As a result, the compatibility of shape strains across the boundary was maintained in the case of the tilt boundary, resulting in increasing the Ms. Such characteristic nucleation of martensites can be regarded as an example of self-accommodation across the boundary, which is called cooperative nucleation (C–N). From a crystallographic viewpoint, C–N can occur only at the symmetric tilt boundary. Effects of pre-deformation and applied stress on the heterogeneous nucleation at the boundary were also examined; C–N was always confirmed to occur at the tilt boundary, and the advantage of the boundary for nucleation of martensites did not change even under pre-deformation or applied stress. Furthermore, the martensite-start stress (σ_M) and the morphology of martensites in the stress-assisted transformation were strongly influenced by C–N.     

The Effect of Low-Temperature Heat Treatment on the Superelasticity in Ti–50.6 at % Ni Single Crystals

Technical Physics Letters - As a result of low-temperature aging at 573 K for 1.5 h of Ti–50.6 at% Ni [001]-oriented single crystals quenched from 1253 K, a high-strength state is obtained...     

Effect of Grinding of β-Cyclodextrin and Glibenclamide on Tablet Properties. Part I. in vitro

Abstract

Physical properties including dissolution characteristics of glibenclamide (GB) tablets were studied. Directly compressed and wet-granulated GB tablets gave only 35% and 40% drug dissolved, respectively. Physical mixing, kneading, and grinding of β-cyclodextrin (CD) with GB were investigated. It was found that the grinding method could markedly enhance the release of drug from the tablets. The physical properties of these tablets were unchanged after they had been stored at 40°C and 75% RH for at least 3 months. The GBKD mixture at a ratio of 1 to 4, ground for 24 or 48 hr, exhibited superior dissolution and chemical stability. Differential scanning calorimetry indicated that an inclusion complex was produced. Decreasing grinding time or CD concentration could result in incomplete formation of the inclusion complex. It was concluded that pretreatment of the drug with CD by the grinding method could significantly improve the dissolution and stability of GB tablets.     

Effect of Mo on microstructural characteristics and coarsening kinetics of γ' precipitates in Co–Al–W–Ta–Ti alloys

The solvus temperature,volume fraction,coarsening behavior of Y' precipitates and the partitioning behavior of alloying elements as well as lattice misfit of Y/Y' phases influence the creep behavior of Ni-and Co-base superalloys.However,few investigations about the microstructural characteristics and the coarsening behavior of Y' precipitates were reported in multicomponent novel Co-base superalloys during thermal exposure.Two alloys containing different contents of molybdenum and tungsten have been investigated to explore the effect of molybdenum on Y' solvus temperature,Y + Y' microstructure and Y' coarsening in Co–Al–W–Ta–Ti-base alloys.The results showed that the Y' solvus temperature decreases with the addition of Mo.Mo addition reduces the Y' volume fractions after aging above 1000?C,but results in negligible influence on the Y' volume fractions aging at 900?C.Meanwhile,Y' coarsening is controlled by diffusion in experimental alloys after aging at 900?C and 1000?C,and the kinetics of Y'growth in experimental alloys are consistent with the predictions of LSW theory.     

Effect of SiO2 nanoparticles on the phase transformation of TiO2 in micron-sized porous TiO2–SiO2 mixed particles

Spherical and nanoporous TiO₂ and TiO₂–SiO₂ mixed micro-particles with four different compositions (20/80, 50/50, 80/20, 90/10 in weight ratio of TiO₂/SiO₂) were prepared by spray drying method from colloidal mixtures of amorphous silica and anatase titania nanoparticles. The as-prepared particles were heat-treated at 900 °C for 0.5–5 h. The TiO₂ and TiO₂–SiO₂ particles were spherical in shape and the average particle diameter was about 1 μm. The anatase mass fraction and the specific surface area of TiO₂–SiO₂ (50 wt.% SiO₂) mixed particles were kept to 61.5% and 30.6%, respectively, of their initial values after 5 h heat-treatment whereas these values of TiO₂ particles were rapidly decreased to 13.0% and 1.2% of their initial values, respectively, within 30 min after heat-treatment. And the anatase mass fraction and specific surface area increased as SiO₂ content in the TiO₂–SiO₂ mixed particles increased.     

Effect of process parameters on stir zone microstructure in Ti–6Al–4V friction stir welds

The effects of the main process variables on the stir zone microstructure in friction stir welds were investigated for Ti–6Al–4V. Welds were produced by employing varying welding speeds under a constant rotation speed or different rotation speeds at a constant welding speed. The stir zone microstructure was examined by optical microscopy and transmission electron microscopy. It was found that the stir zone microstructure was determined by the parameters controlling temperature and deformation history during the friction stir welding. A bimodal microstructure characterized by primary α and transformed β with lamellar α + β or a full lamellar microstructure composed of basket-weave α + β lamellae could be developed in the stir zone. The microstructural evolution mechanism in the stir zone was discussed.     

Effect of room-temperature aging on shape memory characteristics of Ti–10Nb–10Zr–11Ta (at.%) alloy

In this study, the effect of room-temperature (RT) aging on the shape memory characteristics of the Ti–10Nb–10Zr–11Ta (at.%) alloy was investigated by tensile tests. Ingots were prepared using the arc melting method and then cold-rolled at a reduction of up to 95%. After cold-rolling, the plates were solution-treated at 1173 K for 1.8 ks, followed by aging at RT and temperatures up to ∼573 K for various periods of times. Superior superelasticity was observed at RT in the solution-treated specimen. The critical stress for inducing martensitic transformation (σ_SIM), tensile strength, and critical stress for slip (σ_S) of specimens aged at RT increased with increasing aging time up to 60 days, showing no noticeable changes with further increases in the aging time. On the other hand, in the specimens aged at 373 K, 423 K, and 473 K for 3.6 ks, the values of σ_SIM and the tensile strength increased with increasing aging temperature, while the specimen aged at 573 K exhibited mature fractures. There were little change in σ_SIM and σ_S of the specimen that was solution-treated followed by aging at 373 K for 3.6 ks during RT aging. This result indicated that aging at 373 K resulted in good resistance against the effect of RT aging.     

In vitro evaluation of human osteoblast adhesion to a thermally oxidized γ-TiAl intermetallic alloy of composition Ti–48Al–2Cr–2Nb (at.%)

Ti–48Al–2Cr–2Nb (at.%) (γ-TiAl), a gamma titanium aluminide alloy originally designed for aerospace applications, appears to have excellent potential as implant material. Thermal treatment of γ-TiAl renders this alloy extremely corrosion resistant in vitro, which could improve its biocompatibility. In this study, the surface oxides produced by thermal oxidation (at 500°C, and at 800°C for 1 h in air) on γ-TiAl were characterized by X-ray photoelectron spectroscopy (XPS). hFOB 1.19 cell adhesion on thermally oxidized γ-TiAl was examined in vitro by a hexosaminidase assay, scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM) after 1, 7 and 14 days. Ti–6Al–4V surfaces were used for comparison. Hexosaminidase assay data and CLSM analysis of focal contacts and cytoskeleton organization showed no differences in cell attachment on autoclaved and both heat-treated γ-TiAl surfaces at the different time points. SEM images showed well organized multi-layers of differentiated cells adhered on thermally oxidized γ-TiAl surfaces at day 14. Unexpectedly, thermally oxidized Ti–6Al–4V surfaces oxidized at 800°C exhibited cytotoxic effects on hFOB 1.19 cells. Our results indicate that thermal oxidation of γ-TiAl seems to be a promising method to generate highly corrosion resistant and biocompatible surfaces for implant applications.     

Effect of hot working on flow behavior of Ti–6Al–4V alloy in single phase and two phase regions

Hot deformation behavior of the alloy Ti–6Al–4V was investigated via conducting hot compression tests at temperatures of 800–1150 °C and at strain rates, ranging from 0.001 s⁻¹ to 1 s⁻¹, at an interval of an order of magnitude. The apparent differences of flow stress curves obtained in dual phase α + β and single phase β regions were analyzed in term of different dependence of flow stress to temperature and strain rate and different microstructural evolutions. The values of strain rate sensitivity and apparent activation energy were obtained respectively as 0.20 and 530 kJ/mol for two phase microstructure. However, for single phase β microstructure they were approximated as 0.19 and 376 kJ/mol, respectively. It was found that in two phase region the values of strains corresponding to peak point, ε_p, and the highest rate of flow softening, ε^*, are almost independent to Zenner–Hollomon parameter. In single phase region, ε_p and ε^* exhibited a direct relationship to Z parameter and the corresponding empirical equations were proposed.     

Effect of phase transformation and intermetallic compounds on the microstructure and tensile strength properties of diffusion-bonded joints between Ti–6Al–4V and AISI 304L

The occurrence of a phase transformation and the effect of intermetallic compounds on the microstructure and tensile strength properties of diffusion-bonded (DB) joints between Ti–6Al–4V and AISI 304L were studied in the temperature range of 875–950 °C with an interval of 25 °C, a bonding time of 60 min and pressures of 4 MPa and 8 MPa. A maximum tensile strength of 242.6 MPa, was observed for diffusion-bonded joints that were processed at a temperature of 900 °C, bonded for 60 min at a pressure of 4 MPa and annealed for 2 h at 750 °C. Optical microscopy and scanning electron microscopy (SEM) were used to examine the grain growth and the fine details of the interface structure. Energy dispersive X-ray analysis (EDAX) and X-ray diffraction analysis (XRD) revealed the existence of intermetallic compounds and corroborated the phase transformation.     

Effect of heat treatment on microstructure and microhardness evolution in a Ti–6Al–4V alloy processed by high-pressure torsion

A Ti–6Al–4V alloy was heat-treated to give two types of microstructures with different volume fraction of equiaxed α phase and lamellar (α + β) microstructure. Disks were cut from the heat-treated rods and processed by quasi-constrained high-pressure torsion (HPT) at room temperature with an applied pressure of 6.0 GPa and torsional straining from 1/4 to 20 turns. The results show that there is a gradual evolution of homogeneity in microhardness and grain size with increasing numbers of revolutions in HPT such that the microhardness values attain a maximum constant value across the disk after processing by HPT for 10 turns and the measured equilibrium grain sizes after 20 turns are ~130 nm in Ti64-1 and ~70 in Ti64-2. The results show also that a larger fraction of lamellar (α + β) in the microstructure of Ti–6Al–4V leads to a higher hardenability after processing by HPT.     

Effect of preform microstructure on the hot working mechanisms in ELI grade Ti–6Al–4V: transformed β v. equiaxed (α + β)

Abstract

Processing of Ti–6Al–4V includes hot working above and below the β transus and the various stages of manufacture involve preforms with either transformed β (Widmanstätten colony type) or equiaxed (α + β ) microstructure. For achieving defect free products during manufacture, it is important to understand the response of these two preform microstructures to the imposed hot working conditions. In this paper, the influence of the preform microstructure on the hot working mechanisms of extra low interstitial (ELI) grade Ti–6Al–4V has been studied with the help of hot compression experiments conducted in the temperature range 750–1100°C and strain rate range 0·001–100 s^-1. The data have been analysed using the standard kinetic approach as well as the more recent approach of processing maps. In the α–β range, the stress–strain behaviour of transformed β preform is marked by a higher flow stress and a continuous flow softening while the equiaxed (α + β ) preform exhibits steady state flow at lower strain rates. By deforming in the α–β range, the transformed β microstructure is converted into an equiaxed one by a process of globularisation. On the other hand, the equiaxed (α + β ) preform deforms superplastically with an associated minor change in its microstructure. In the β range, the transformed β deforms by a process of large grained superplasticity involving the sliding of prior colony (Widmanstätten) boundaries with an associated diffusion accommodated flow. However, dynamic recrystallisation of β occurs in the equiaxed preform. Deformation near the transus for both the preforms is associated with local minima in the tensile ductility indicating the possibility of void nucleation. At strain rates higher than about 0·1 s^-1, both the preforms exhibit flow instabilities manifested in the form of flow localisation due to adiabatic shear band formation which is severe in the case of transformed β preform.     

Electrochemical depositions of calcium phosphate film on commercial pure titanium and Ti–6Al–4V in two types of electrolyte at room temperature

Electrochemical depositions of calcium phosphate film on commercially pure titanium (cp-Ti) and Ti–6Al–4V in two types of electrolytes, mono-calcium phosphate monohydrate (MCPM) based aqueous solution and supersaturated calcium phosphate solution (SCPS), were carried out by the cathodic polarization. The calcium phosphate coating layer was successfully deposited on cp-Ti and Ti–6Al–4V under 3 mA/cm² current density for 45 min. The major phases that appeared in the film were DCPD (brushite) co-existed with octa-calcium phosphate (OCP). After soaking in revised simulated-body-fluid (r-SBF), the amorphous bone-like apatite became a single phase within a day for the specimen obtained from MCPM electrolyte. The specimens obtained from SCPS electrolyte took a week to complete the apatite precipitation in r-SBF. From the scratch test, the highest critical load of 87.2 N was obtained from the as-deposited rod-like DCPD on cp-Ti substrate. The critical load was decreasing almost 50% for all of the specimens after apatite precipitation in r-SBF. The decrease of critical load might result from loosening of the film structure during immersion of the specimens in r-SBF. The thickness of the films was 44–55 μm in average, which satisfied the qualification for using as an implant material.     

Effect of duplex aging on microstructure and mechanical behavior of beta titanium alloy Ti–15V–3Cr–3Al–3Sn under unidirectional and cyclic loading conditions

The microstructure and mechanical behavior of beta titanium alloy Ti–15V–3Cr–3Al–3Sn (Ti15-3) under unidirectional and high cycle fatigue (HCF) loading conditions were studied after single aging (SA) and duplex aging (DA). After SA, well-developed grain boundary α and micro precipitate free zones (micro PFZs) were present in the microstructure. Whereas after DA, grain boundary α was poorly developed and micro PFZs were absent. DA resulted in smaller size, higher density and volume fraction of alpha particles. DA led to higher work hardening rate, better strength-ductility combination and higher HCF life. Improved mechanical behavior after DA is explained based on microstructural observations.     

Effect of in situ synthesized TiB whisker on microstructure and mechanical properties of carbon–carbon composite and TiBw/Ti–6Al–4V composite joint

Carbon–carbon composite (C–C composite) and TiB whiskers reinforced Ti–6Al–4V composite (TiB_w/Ti–6Al–4V composite) were brazed by Cu–Ni + TiB₂ composite filler. TiB₂ powders have reacted with Ti which diffused from TiB_w/Ti–6Al–4V composite, leading to formation of TiB whiskers in the brazing layer. The effects of TiB₂ addition, brazing temperature, and holding time on microstructure and shear strength of the brazed joints were investigated. The results indicate that in situ synthesized TiB whiskers uniformly distributed in the joints, which not only provided reinforcing effects, but also lowered residual thermal stress of the joints. As for each brazing temperature or holding time, the joint shear strength brazed with Cu–Ni alloy was lower than that of the joints brazed with Cu–Ni + TiB₂ alloy powder. The maximum shear strengths of the joints brazed with Cu–Ni + TiB₂ alloy powder was 18.5 MPa with the brazing temperature of 1223 K for 10 min, which was 56% higher than that of the joints brazed with Cu–Ni alloy powder.     

Effect of hot extrusion on wear properties of Al–15 wt.% Mg2Si in situ metal matrix composites

Al–15 wt.% Mg₂Si composites were prepared by in situ casting and characterized in wear tests. Previous to the extrusion of specimens at 470 °C – varying extrusion ratio (7.4, 14.1 and 25), the as-cast composites were homogenized at 500 °C for 5 h, followed by slow furnace cooling. The microstructure, hardness and sliding wear behavior were characterized for both, the as-cast and hot extruded composites. Results show that increasing the extrusion ratio causes a significant improvement in hardness and wear resistance. This is ascribed to the observed decrease in average size and better distribution of Mg₂Si particles, in tandem with a remarkable decrease in porosity percentages, which goes from 5.63 in the as-cast condition, to 0.47 at the extrusion ratio of 25. It was found that abrasion is the dominant wear mechanism in all extruded composites, whilst a combination of adhesion and delamination appears to be the governing mechanism for as-cast composites.