Effect of Misfit on Preload Maintenance of Retention Screws of Implant-Supported Prostheses

The aim of this study was to evaluate the effect of unilateral misfit at different levels on a crown-implant-retention screw system of implant-supported crowns. Hexagon castable UCLA abutments were cast in Co-Cr alloy to fabricate 48 metallic crowns divided into four groups (n = 12). Group A: crowns did not present misfit; Groups B, C and D: crowns were fabricated with unilateral misfit of 50, 100, and 200 μm, respectively. The crowns were attached by titanium retention screw with 30 N/cm to external hexagonal osseointegrated implants embedded in acrylic resin. After 2 min, the retention screw of each replica was submitted to detorque evaluation by an analogic torque gauge. Three retention screws were used to perform detorque evaluation at each replica and the procedure was repeated twice with each screw. Each group was submitted to 72 detorque measurements. Data were evaluated by ANOVA and Tukey test (P < 0.05). All groups exhibited significant decrease (P < 0.05) in preload and the lowest decrease occurred in Group A. Groups B, C, and D were statistically significant different from Group A (P < 0.05), but there was no statistically significant difference between Groups B and D (P > 0.05). Crowns with unilateral misfit presented higher preload decrease than crowns completely fitted to osseointegrated implants.     

Effect of TiC Particle Additions on Structure and Properties of Hypereutectic High Chromium Cast Iron

The present work studies the effects of titanium carbide (TiC) particle additions on the microstructure and mechanical properties of hypereutectic high chromium cast iron containing about 20%Cr and 4.0%C by the optical microscopy (OM), the transmission electron microscopy (TEM), the X-ray diffraction (XRD) analysis, the digital image analysis system, impact tester, and hardness tester. The carbides of high chromium cast iron are refined gradually and the shape of the primary M₇C₃-type carbides became more isotropic with the increase of TiC particles. When the addition of TiC particles exceeds 1.0 wt.%, the change of microstructure is not obvious. The hardness of high chromium cast iron has a slight increase and impact toughness has a obvious increase with the increase of TiC additions. However, when the addition of TiC particles exceeds 1.0 wt.%, the impact toughness has no obvious change.     

Microstructure and Mechanical Properties of a Low Alloyed MnB Cast Steel

The microstructure and mechanical properties of a low alloyed MnB cast steel designed for coupler castings of trucks were studied. The results show that the microstructure of the MnB cast steel after water quenching is lath martensite and a small amount of massive islands in the matrix of lath martensite. The average size of the martensite packets is about 10 μm in length. Carbides precipitated dispersively at the tempering temperature of 450 °C. The carbides are slender and fibrous, of which the microstructure was θ-phase (Fe, Mn)₃C characterized by TEM. The MnB cast steel has good hardenability and tempering stability. Excellent combination of strength, ductility and low-temperature toughness were obtained after water-quenching and 450 °C tempering: Rm = 960-1040 MPa, ReL = 880-900 MPa, A = 19-21%, Z = 56-58%. Especially, the impact energy of the Charpy V-Notch (CVN) specimens reached 70-88 J at −40 °C. The fracture mechanism is transcrystalline fracture both for ambient temperature uniaxial tensile test specimens and for CVN impact test specimens broken at −40 °C, where the whole surfaces were manifested as voids and dimples.     

Absence of the core–rim microstructure in TixTa1 − xCyN1 − y-based cermets developed from a pre-sintered carbonitride master alloy

(Ti,Ta)(C,N) solid solution-based cermets with cobalt as the binder phase were synthesised by a two-step milling process. The titanium–tantalum carbonitride solid solution (the ceramic phase) was obtained via a mechanically induced self-sustaining reaction (MSR) process from stoichiometric elemental Ti, Ta, and graphite powder blends in a nitrogen atmosphere. Elemental Co (the binder phase) was added to the ceramic phase, and the mixture was homogenised by mechanical milling (MM). The powdered cermet was then sintered in a tubular furnace at temperatures ranging from 1400 °C to 1600 °C in an inert atmosphere. The chemical composition and microstructure of the sintered cermets were characterised as ceramic particles grown via a coalescence process and embedded in a complex (Ti,Ta)–Co intermetallic matrix. The absence of the typical core–rim microstructure was confirmed.     

Protective coatings for induction casting of titanium

Commercially pure titanium is among the most biocompatible metals available for dental uses: it combines good mechanical properties with high corrosion resistance in simulated body fluids. However, titanium casting shows some problems mainly related to its high melting temperature and chemical reactivity with oxygen at T > 600 °C; as a consequence, molten titanium reacts with crucibles and mould components during casting. In this research, new Y₂O₃-based slurry was used to produce yttria coated mullitic crucibles and wax pattern coatings for induction melting of titanium; furthermore, a detailed comparison among 4 different investment materials (one silica-based, one ZrSiO₄-based and two MgO-based) was performed. The titanium surface after casting was investigated with optical and electron microscopy (SEM) and energy dispersion spectroscopy (EDS). A reaction zone was observed on Ti when cast in an uncoated crucible, while a clean, pure Ti surface was obtained by using yttria coated crucibles and yttria coated wax patterns before investing with silica-based and zircon-based investment.     

Preparation of fine-grained MoAlB with preferable mechanical properties and oxidation resistance

MoAlB has promising aeronautic and high-temperature applications due to its excellent mechanical property, favorable thermal conductivity and fantastic high temperature oxidation resistance etc. In this work, in order to obtain MoAlB with preferable properties, the fine-grained MoAlB ceramics were prepared by proprietary mechanical alloying and hot-pressing sintering technology. The microstructure observation showed that MoAlB with a small grain size of 1.26 μm could be obtained when the molar ratio of Mo, Al and B was 1:1.3:1 and the sintering temperature was 1300 °C. The testing results indicated that the as-prepared MoAlB ceramic showed better comprehensive mechanical properties compared with the reported results. The compressive strength, hardness, flexural strength and fracture toughness are 2420 MPa, 15.46 GPa, 382.87 MPa and 7.1 MPa·m^1/2, respectively. It also exhibited favorable high temperature oxidation resistance due to the rapid formation of alumina scales. These results can cast light for future development of ultrahigh-temperature ceramics.     

A fast method for determining favourable orientation relationships and interface planes: Application to titanium–titanium hydrides transformations

The transformation of titanium into several hydride phases has been studied by transmission electron microscopy at the surface of an acid-etched titanium substrate designed for biomedical applications. Four different orientation relationships have been found between face-centred cubic TiH_2−ε (ε ≪ 1), face-centred tetragonal TiH and hexagonal close-packed Ti. A fast method based on the alignment of diffraction spots perpendicular to interfaces (Δg condition) is proposed, to account for the experimental orientation relationships and interface planes. Different paths leading to the formation of the two above-mentioned hydride phases have been thus identified. A mechanism of misfit relaxation has been observed and described.     

Enhanced densification of metal powders by transformation-mismatch plasticity

The densification of titanium powders is investigated in uniaxial die pressing experiments carried out isothermally at 980°C (in the β-field of titanium) and during thermal cycling between 860 and 980°C (about the α/β phase transformation of titanium). Thermal cycling is found to enhance densification kinetics through the emergence of transformation-mismatch plasticity (the mechanism responsible for transformation superplasticity) as a densification mechanism. The isothermal hot-pressing data compare favorably with existing models of powder densification, and these models are successfully adapted to the case of transformation-mismatch plasticity during thermal cycling. Similar conclusions are reached for the densification of titanium powders containing 1, 5, or 10 vol.% ZrO₂ particles. However, the addition of ZrO₂ hinders densification by dissolving in the titanium matrix during the hot-pressing procedure.     

Accurate tracking controller design for high-speed drives

This paper presents a motion control strategy that utilizes a combination of vibration avoidance, sliding mode control, and torque ripple and friction compensation techniques to facilitate high bandwidth–high accuracy tracking in ball screw drives. Torsional vibrations are modeled with finite element analysis and experimentally identified. Excitation of the first and second mode is avoided by designing appropriate notch filters. Following the attenuation of structural vibrations, rigid body motion is controlled using adaptive sliding mode control, where a positioning bandwidth of 223 Hz is achieved. Nonlinear friction and motor/mechanical torque ripples are modeled and compensated in feedforward. With the proposed control strategy, an experimental tracking accuracy of 0.95 μm has been achieved while traversing the axis at a feedrate of 1000 mm/s with 1g acceleration.     

Surface protection of light metals by one-step laser cladding with oxide ceramics

Today, intricate problems of surface treatment can be solved through precision cladding using advanced laser technology. Metallic and carbide coatings have been produced with high-power lasers for years, and current investigations show that laser cladding is also a promising technique for the production of dense and precisely localized ceramic layers. In the present work, powders based on Al₂O₃ and ZrO₂ were used to clad aluminum and titanium light alloys. The compact layers are up to 1 mm thick and show a nonporous cast structure as well as a homogeneous network of vertical cracks. The high adhesive strength is due to several chemical and mechanical bonding mechanisms and can exceed that of plasmasprayed coatings. Compared to thermal spray techniques, the material deposition is strictly focused onto small functional areas of the workpiece. Thus, being a precision technique, laser cladding is not recommended for large-area coatings. Examples of applications are turbine components and filigree parts of pump casings.     

Evolution of multivariant microstructures with anisotropic misfit: A phase field study

We study, in two dimensions, the effect of misfit anisotropy on microstructural evolution during precipitation of an ordered β phase from a disordered α matrix; these phases have, respectively, 2- and 6-fold rotation symmetries. Thus, precipitation produces three orientational variants of β phase particles, and they have an anisotropic (and crystallographically equivalent) misfit strain with the matrix. The anisotropy in misfit is characterized using a parameter t = ?_yy/?_xx, where ?_xx and ?_yy are the principal components of the misfit strain tensor. Our phase field simulations show that the morphology of β phase particles is significantly influenced by t, the level of misfit anisotropy. Particles are circular in systems with dilatational misfit (t = 1), elongated along the direction of lower principal misfit when 0 < t < 1 and elongated along the invariant direction when ?1 ? t ? 0. In the special case of a pure shear misfit strain (t = ?1), the microstructure exhibits star, wedge and checkerboard patterns; these microstructural features are in agreement with those in Ti–Al–Nb alloys.     

CRSS of γ/γ′ phases from in situ neutron diffraction of a directionally solidified superalloy tension tested at 900 °C

Neutron diffraction was used to monitor elastic strains during in situ tension testing of a directionally solidified (DS) superalloy at 900 °C. Changes in misfit and thermal expansion coefficients of individual phases were obtained. In the γ phase, it is demonstrated that elastic strains saturate at 350 MPa, which is well below the yield strength of the alloy. This is interpreted as the onset of dislocation glide through less stressed vertical channels. The critical resolved shear stress (CRSS) of γ is found to be 143 ± 11 MPa, in agreement with a calculated CRSS that is dominated by Orowan bowing of dislocations through nanoscale-wide γ channels. This provides confirmation of Orowan bowing in plasticity/creep of the γ phase. Implications of CRSS and misfit in a “threshold stress” for creep and rafting are discussed. The CRSS of γ′ is found to be consistent with pairwise penetration of dislocations into γ′.     

Misfit strain–film thickness phase diagrams and related electromechanical properties of epitaxial ultra-thin lead zirconate titanate films

The phase stability of ultra-thin (0 0 1) oriented ferroelectric PbZr_1–xTi_xO₃ (PZT) epitaxial thin films as a function of the film composition, film thickness, and the misfit strain is analyzed using a non-linear Landau–Ginzburg–Devonshire thermodynamic model taking into account the electrical and mechanical boundary conditions. The theoretical formalism incorporates the role of the depolarization field as well as the possibility of the relaxation of in-plane strains via the formation of microstructural features such as misfit dislocations at the growth temperature and ferroelastic polydomain patterns below the paraelectric–ferroelectric phase transformation temperature. Film thickness–misfit strain phase diagrams are developed for PZT films with four different compositions (x = 1, 0.9, 0.8 and 0.7) as a function of the film thickness. The results show that the so-called rotational r-phase appears in a very narrow range of misfit strain and thickness of the film. Furthermore, the in-plane and out-of-plane dielectric permittivities ε₁₁ and ε₃₃, as well as the out-of-plane piezoelectric coefficients d₃₃ for the PZT thin films, are computed as a function of misfit strain, taking into account substrate-induced clamping. The model reveals that previously predicted ultrahigh piezoelectric coefficients due to misfit-strain-induced phase transitions are practically achievable only in an extremely narrow range of film thickness, composition and misfit strain parameter space. We also show that the dielectric and piezoelectric properties of epitaxial ferroelectric films can be tailored through strain engineering and microstructural optimization.     

Nonpolar ZnO film growth and mechanism for anisotropic in-plane strain relaxation

Using high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction, we investigated the strain relaxation mechanisms for nonpolar (1 1 ?2 0) a-plane ZnO epitaxy on (1 ?1 0 2) r-plane sapphire, where the in-plane misfit ranges from ?1.5% for the [0 0 0 1]ZnO6[1 ?1 0 ?1]sapphire to ?18.3% for the [?1 1 0 0]ZnO6[?1 ?1 2 0]sapphire direction. For the large misfit [?1 1 0 0]ZnO direction the misfit strains are fully relaxed at the growth temperature, and only thermal misfit and defect strains, which cannot be relaxed fully by slip dislocations, remain on cooling. For the small misfit direction, lattice misfit is not fully relaxed at the growth temperature. As a result, additive unrelaxed lattice and thermal misfit and defect strains contribute to the measured strain. Our X-ray diffraction measurements of lattice parameters show that the anisotropic in-plane biaxial strain leads to a distortion of the hexagonal symmetry of the ZnO basal plane. Based on the anisotropic strain relaxation observed along the orthogonal in-plane [?1 1 0 0] and [0 0 0 1]ZnO stress directions and our HRTEM investigations of the interface, we show that the plastic relaxation occurring in the small misfit direction [0 0 0 1]ZnO by dislocation nucleation is incomplete. These results are consistent with the domain-matching paradigm of a complete strain relaxation for large misfits and a difficulty in relaxing the film strain for small misfits.     

Kinetics of Failure Rate Accumulation and TiNi Shape Memory Effect Alloy Fracture Under Mechanical Cycling

Wire specimens, d = 0.1 mm, made of Ti-(50.6-50.8)at.%-Ni alloy were exposed to thermo-mechanical treatment (TMT), thus making the samples straight and providing them with high superelasticity (ε_se). It was established that the most effective method of TMT is annealing with deformation at 500 and 300 °C. The optimum mode of treatment was used in the research of mechanical fatigue of alloys with high superelastic properties. Two stages characterizing the alloy behavior under mechanical cycling were found out: failure accumulation and fracture. It was shown that the duration of the cycles is determined by the value of the preset deformation (σ_set) in relation to the deformation on the plateau-shaped area. The results of low-cycle fatigue of the alloys under investigation were processed by means of the method of least squares. The equations of prognosis of longetivity at the preset level of deformation are presented.     

Effect of Annealing Temperature on Microstructure and Mechanical Properties of Hot Swaged cp-Ti Produced by Investment Casting

The purpose of this study is to evaluate the influence of hot swaging (SW) and annealing treatment on microstructure and mechanical properties of commercially pure titanium (grade 4) produced by investment casting. During SW at 700 °C, the diameter of the cast titanium bars was reduced from 25 to 8.5 mm in 14 steps. After SW, material was annealed for 1 h at 500, 700, or 870 °C. The as-cast samples showed a typical microstructure consisting of a variety of α-morphologies, while the hot swaged samples exhibited a kinked lamellar microstructure. Annealing at 500 °C did not significantly change this microstructure, while annealing at both 700 and 870 °C led to recrystallization and formation of equiaxed microstructures. The cast bars exhibited a typical hard α-layer in near-surface regions with maximum depth and maximum hardness of 720 μm and 660 HV0.5, respectively. Due to SW, the tensile strength of the as-cast material drastically increased from 605 to 895 MPa. Annealing at 500 °C decreased this tensile strength slightly from 895 to 865 MPa while annealing at 700 °C led to a further pronounced drop in tensile strength from 865 to 710 MPa. No additional decrease in tensile strength was observed by increasing the annealing temperature from 700 to 870 °C. The tensile ductility of the as-cast and hot swaged samples was approximately the same in the range of 0.05 to 0.11, while the annealed samples showed values in the range of 0.25 to 0.53. In addition, the as-cast and hot swaged samples revealed a brittle cleavage fracture surfaces. However, the annealed samples showed a transgranular ductile fracture with formation of dimples.     

Glass–ceramic coatings on titanium alloys for high temperature oxidation protection: Oxidation kinetics and microstructure

A glass–ceramic coating is applied on Ti–6Al–4V alloy for oxidation protection at 800 °C. Its dynamic oxidation and microstructure evolution are investigated. The titanium alloy substrate is effectively protected by the glass–ceramic coating, of which the oxidation develops at constant rate. The linear relationship of oxidation is deduced dm/dt = Dρ(C₁ − C₂)/(bC′), and the diffusion coefficient of oxygen at 800 °C in glass is obtained. Oxygen diffusion through glass coating is the controlling step. After the initial firing, silicide interlayer forms between the glass coating and titanium alloy substrate, where the ratio of Ti/Si decreases after oxidation due to Si diffusion and Ti consumption.     

Microstructure,Mechanical Properties,and Unlubricated Sliding Wear Behavior of Air-Cooled MnSiCrB Cast Steels

Two medium carbon low-alloy MnSiCrB cast steels containing different Si contents (0.5 and 1.5 wt.%) were designed, and the effects of Si contents on the microstructure, mechanical properties, and unlubricated sliding wear behavior of the cast steels after air-cooling from 850 °C and subsequent tempering at 220 °C was studied. The results show that the microstructure of the cast steel containing 0.5 wt.% Si consists of granular bainite and lower bainite/martensite multi-phase. In the cast steel containing 1.5 wt.% Si, granular bainite was not observed. The microstructure consists of carbide-free bainite/martensite multi-phase. Excellent hardenability can be obtained at both low and high Si levels. The cast steel containing 0.5 wt.% Si exhibits excellent combination of strength, ductility, and impact toughness superior to the cast steel containing 1.5 wt.% Si. Also, the wear-resistance of the former steel is better than that of the latter in the unlubricated sliding wear condition. The air-cooled MnSiCrB cast steel containing low Si levels, with excellent mechanical properties and wear-resistance, is a potential high-performance and low-cost wear-resistant cast steel for unlubricated sliding wear condition.     

Low temperature synthesis of nanocrystalline titanium nitride from a single-source precursor of titanium and nitrogen

Nanocrystalline titanium nitride has been prepared via a convenient route from a single-source precursor of titanium and nitrogen (ammonium fluotitanate) in an autoclave at 650 °C. X-ray powder diffraction patterns indicate that the product is cubic titanium nitride, and the cell constant is a = 4.235 Å. Transmission electron microscopy image shows that it consists of particles with an average size of about 40 nm in diameter. The product was also studied by BET and TGA.     

High room-temperature plastic and work-hardening effect of the NiAl-matrix composites reinforced by particulates

The microstructures, interfaces, compression properties and work-hardening effect of the NiAl-matrix composites reinforced by 5–20 wt.% ceramic particulates (Nb₂C, NbC and NbB₂) fabricated by combustion synthesis and hot pressing (CSHP) have been investigated. The ultimate compression strength and yield strength increase with the increasing content of the ceramic particulates, while the fracture strain and work-hardening capacity (H_c) decrease. The NiAl-matrix composite with 5 wt.% ceramic particulates has a high true ultimate strength of 1497 MPa, a fracture strain of 18.3%, and work-hardening capacity H_c = 1.29. The good interface bonding between ceramic particulate and matrix, the high density dislocation in the NiAl matrix, the seriously distorted lattices and the intense interactions between dislocations and other crystal defects contribute to its prominent mechanical properties.