Microstructure and mechanical properties of TiC/Ti–6Al–4V composites processed by in situ casting route

Abstract

TiC/Ti–6Al–4V composites containing various volume fractions of TiC were produced by induction skull melting and common casting utilising in situ reaction between titanium and carbon powder. The microstructure and room tensile properties of as cast and heat treated TiC/Ti–6Al–4V composites were investigated. Bar-like or small globular eutectic TiC were found in 5 vol.-%TiC/Ti–6Al–4V composite, whereas the equiaxed or dendritic primary TiC particles were found to be the main reinforcements in 10 and 15 vol.-%TiC/Ti–6Al–4V composites. The as cast TiC/Ti–6Al–4V composites have shown higher strength but lower ductility than those of monolithic Ti–6Al–4V alloy. The shape and fracture of TiC particles can strongly influence the fracture and failure of the composites, and so the ultimate tensile strengths and elongations of as cast composites reduce with the increase in volume fraction of TiC. TiC particles appear to be spheroidised, and titanium precipitation can be found within large TiC particles after heat treatment at 1050°C for 8 h, which can promote the resistance to fracture of composites. Therefore, the elongations of the composites increase significantly, and the ultimate tensile strengths also have marginal increase especially for the 10 and 15 vol.-%TiC/Ti–6Al–4V composites after heat treatment.     

Damage evolution of angle-ply SCS-6/Ti composites under static and fatigue loading

The effect of fibre orientation and laminate stacking sequence on the tensile and fatigue behaviour of SCS-6/Ti 15-3 composites were investigated. The laminates used in this study were: (90)₆, (0/ ± 45)_s, (0/90)_s, and (90/ +-45)_s. The initiation and progression of microstructural damage at various stress levels was thoroughly characterized. It was found that fatigue life at high applied stresses were controlled by fibre fracture; progressive damage involving fibre fracture, interfacial debonding and matrix cracking became dominant at low applied stresses. Observation of the damage mechanisms in the angle-ply laminates under cyclic loading suggests that increasing the fibre-matrix bonding strength may improve the load carrying capability and fatigue life of laminates containing off-axis plies.     

Microstructure and mechanical properties of in situ formed TiC-reinforced Ti–6Al–4V matrix composites

Carbon nanotubes were blended into a Ti–6Al–4V matrix to synthesize titanium carbide (TiC) in situ, via spark plasma sintering. The microstructure and mechanical properties of both the monolithic Ti–6Al–4V alloys and the TiC/Ti–6Al–4V composites were studied to evaluate the strengthening effects of TiC on the Ti–6Al–4V matrix. The morphologies obtained by scanning electronic microscopy and optical microscopy indicated that the grain size of both the Ti–6Al–4V alloy and the TiC/Ti–6Al–4V composite decreased with increasing planetary ball-milling (PBM) speed, leading to an increase in the hardness of the investigated materials. The compressive yield strength of the monolithic Ti–6Al–4V alloys and the TiC/Ti–6Al–4V composites initially increased and then decreased with increasing PBM speed. The strengthening and fracture mechanisms were studied.     

Microstructure of interfacial reaction zone in SCS-6 SiC fibre reinforced super α2 composites

Abstract

The microstructure of the interfacial reaction zone in SCS-6 SiC/super α2 composites heat treated at 700°C for 3000 h was investigated by means of analytical transmission electron microscopy. The very fine grained reaction layer adjacent to the carbon coating of the SiC fibre was found to consist of two sub layers, determined to be (Ti, V)C and (Ti, V,Nb)₅Si₃. The second layer is (Ti,Nb)C with large equiaxed grainsfollowed by the third layer consisting of the (Ti,Nb)₃(Al,Si)C phase. This layer is separated from the matrix by a fourth layer with the phase composition (Ti,Nb)₅(Si,Al)₃. At some interface positions, the two layers of(Ti,Nb)C and (Ti,Nb)₃(Al,Si)C are separated by an additional layer of the (Ti,Nb)₃(Si,Al) phase. The thickening of the interfacial reaction zone at 700°C is mainly due to the layers of (Ti, Nb)₃(Al,Si)C and (Ti,Nb)₃(Si,Al). The growth of these two layers is probably responsible for the degradation of the mechanical properties of the composites.     

Effect of carbon fibres on the static and fatigue mechanical properties of fibre metal laminates

It is crucial to understand the characteristic fatigue crack initiation and its growth mechanisms, as well as the relationship between the mechanical properties and the fatigue damage evolution in fibre metal laminates (FMLs). Two types of FML were studied in this work: a polyacrylonitrile‐based carbon fibre epoxy matrix composite sandwiched by Ti‐6Al‐4V (Ti‐alloy) sheets (IMS60‐Ti) and a pitch‐based carbon fibre epoxy matrix composite sandwiched by Ti‐alloy sheets (K13D‐Ti). The static and fatigue mechanical properties of IMS60‐Ti and K13D‐Ti were investigated. The increased failure strain of the FML was greater than that of carbon fibre‐reinforced polymer (CFRP) matrix composites. The fatigue life of IMS60‐Ti was much longer than that of K13D‐Ti. The fatigue damage process in IMS60‐Ti was related to the fatigue creep behaviour of the Ti‐alloy face sheet and mode II cracking at the CFRP/Ti‐alloy interface, and the damage in K13D‐Ti was related to the K13D CFRP laminate.     

Characterisation of diffusion bonds formed between Ti–6Al–4V and titanium aluminide Super Alpha-2

Abstract

Isostatic diffusion bonds were produced between Ti–6Al–4V and Super Alpha-2 sheet materials at temperatures of 920–940°C and pressures of 6–10 MPa. The diffusion bonds were evaluated using optical microscopy, lap shear testing, and scanning electron microscopy of the lap shear fracture faces. The results of X-ray microanalysis carried out on the bond interface region showed that substantial diffusion of niobium and molybdenum had occurred from the Super Alpha-2 to the Ti–6Al–4V. This caused a local suppression of the β transus in the Ti–6Al–4V alloy and on subsequent cooling the β phase decomposed to give a microstructure of fine acicular α phase and retained β phase in the bond region.

MST/3421     

Determination of residual stresses in SiC monofilament reinforced metal-matrix composites using Raman spectroscopy

Raman spectroscopy has been used to follow the deformation of chemical vapour deposition type SCS-6 and Sigma 1140+ SiC monofilaments and to determine residual stresses in these SiC monofilaments reinforced metal-matrix composites. Raman bands at 1330 and 1600 cm⁻¹ due to carbon have been observed on the monofilament surface and it has been shown that both bands shift linearly to lower wavenumbers during tensile deformation. The residual stresses in SiC monofilament reinforced composites arising from thermal expansion mismatch have also been determined by measuring the shifts of carbon bands from the same monofilaments embedded in a Ti–6Al–4V matrix. The axial residual stresses in the carbon coating are found to be around −850 MPa for the SCS-6 composite and −540 MPa for the Sigma 1140+ composite.     

Transient liquid phase diffusion bonding of continuous SiC fibre reinforced Ti–6AI–4V composite to Ti–6AI–4V alloy

Abstract

A continuous SiC fibre reinforced Ti–6Al–4V composite was diffusion bonded in transient liquid phase to Ti–6Al–4V alloy plate using Ti–Cu–Zr amorphous filler metal. Joint strength increased with bonding time up to 1·8 ks and reached the maximum value of 850 MN m^?2 which corresponded to 90% of the tensile strength of Ti–6Al–4V. The extent of deformation of Ti–6Al–4V in the vicinity of the bonding interface was small compared with that of solid diffusion bonding because of the low bonding pressure. The bonding layer had an acicular microstructure which was composed of Ti₂Cu and α titanium with dissolved zirconium. Brittle products such as (Ti, Zr )₅ Si₃ or (Ti, Zr )₅ Si₄ were formed at the interface between the SiC fibres and the filler metal. These products existed only at the end of fibres, in very small amounts, therefore joint strength was not significantly affected by the products.

MST/1989     

(Al2O3)f/Al复合材料在强界面结合下的疲劳损伤模式

在拉-拉载荷下测定了(Al₂O₃)_f/Al复合材料的疲劳寿命(S-N)曲线。通过夭折试验以及SEM疲劳断口和纵截面组织结构分析,研究了复合材料的疲劳损伤模式。研究结果表明,(Al₂O₃)_f/Al复合材料的疲劳极限为750MPa,远高于SCS-6碳化硅纤维增强钛基复合材料。该复合材料兼有钛基和树脂基纤维复合材料疲劳损伤的特点,高应力下由单个裂纹的起源和生长导致复合材料的失效;低应力下,疲劳损伤模式包括纤维劈裂、众多基体裂纹和单个基体裂纹的横向扩展。其中纤维劈裂是主控机制。其更高的疲劳极限可归因于低应力下纤维的纵向劈裂。     

Strain-controlled fatigue properties of dissimilar welded joints between Ti–6Al–4V and Ti17 alloys

The aim of this study was to evaluate the microstructure, hardness and cyclic deformation behavior of electron beam welded dissimilar joints of Ti–6Al–4V and Ti17 (Ti–5Al–4Mo–4Cr–2Sn–2Zr) titanium alloys. The welding resulted in a significant microstructural change across the joint, with hexagonal close-packed (hcp) martensite α′ and orthorhombic martensite α″ in the fusion zone (FZ), α′ in the heat-affected zone (HAZ) of Ti–6Al–4V side, and coarse β in the HAZ of Ti17 side. A characteristic asymmetrical hardness profile across the dissimilar joint was observed with the highest hardness in the FZ and a lower hardness on the Ti–6Al–4V side than on the Ti17 side, where a soft zone was observed. The dissimilar joint exhibited a lower Young′s modulus and higher cyclic strain hardening exponent than both Ti–6Al–4V and Ti17 base metals (BMs), and had the monotonic and cyclic yield strengths lying in-between those of two BMs with higher values for Ti17 alloy. Both BMs and joint showed essentially symmetrical hysteresis loops and equivalent fatigue life, and exhibited cyclic stabilization at lower strain amplitudes up to 0.6%, while cyclic softening occurred after initial cyclic stabilization at higher strain amplitudes. The initial cyclic stabilization was shortened with increasing strain amplitude. In the Ti–6Al–4V BM fatigued at a high strain amplitude of 1.2%, a short initial cyclic hardening emerged, corresponding to the presence of twinning and its resistance to the dislocation movement. Fatigue failure of the dissimilar joint occurred in the HAZ of Ti17 side where the soft zone was present, with crack initiation from the specimen surface or near-surface defect and crack propagation characterized by typical fatigue striations.     

Fatigue properties and fracture analysis of a SiC fiber-reinforced titanium matrix composite

Tension–tension fatigue properties of SiC fiber reinforced Ti–6Al–4V matrix composite (SiC_f/Ti–6Al–4V) at room temperature were investigated. Fatigue tests were conducted under a load-controlled mode with a stress ratio 0.1 and a frequency 10 Hz under a maximum applied stress ranging from 600 to 1200 MPa. The relationship between the applied stress and fatigue life was determined and fracture surfaces were examined to study the fatigue damage and fracture failure mechanisms using SEM. The results show that, the fatigue life of the SiC_f/Ti–6Al–4V composite decreases substantially in proportion to the increase in maximum applied stress. Moreover, in the medium and high life range, the relationship between the maximum applied stress and cycles to failure in the semi-logarithmic system could be fitted as a linear equation: S_max/μ = 1.381 − 0.152 × lgN_f. Fractographic analysis revealed that fatigue fracture surfaces consist of a fatigued region and a fast fracture region. The fraction of the fatigued region with respect to the total fracture surface decreases with the increase of the applied maximum stresses.     

Identification of failure mechanisms in retrieved fractured dental implants

Dental implants treatment complications include mechanical failures. These complications were considered minor until now but several clinical trials showed that mechanical complications are common in implantology and in implant rehabilitation. The aim of the study was to perform a detailed systematic failure analysis on Ti–6Al–4V and CP-Ti retrieved dental implants.A total number of 10 CP-Ti and 8 Ti–6Al–4V retrieved fractured dental implants and implant parts were collected and there metal composition was identified using SEM–EDX (energy dispersive X-ray spectroscopy).The identification of the implants failure mechanisms was done by comparing the fracture surfaces of retrieved fractured dental implants to fracture surfaces of implants fractured in lab conditions in room air, and also in an environment mimicking the intraoral environment, which includes artificial saliva and fluoride (exemplar testing). The analysis was done by using Scanning Electron Microscopy (SEM).The overall fracture mechanisms that were identified on the retrieved Ti–6Al–4V and of CP-Ti dental implants were identical to those found on fatigue fracture surfaces of the specimens’ fractured in lab conditions. No evidence was found for corrosion products on the metal surface, which might suggest the operation of a corrosion processes participating in the crack formation.This study clearly shows that fatigue is the main failure mechanism for Ti–6Al–4V and CP-Ti retrieved dental implants. The fractographic analysis showed that implants and their parts might be broken at relatively low cyclic load levels, of the kind that matches the load levels generated during mastication.     

Experimental characterisation of fibre failure and its influence on crack growth resistance in fibre reinforced titanium metal matrix composites

Abstract

The present paper addresses the effects of fibre failure on the fatigue crack growth resistance of a Ti-6AI-4V (wt-%) alloy matrix unidirectionally reinforced with continuous Sigma (SM1240) SiC fibres. Fibre fracture was monitored in situ using a PAC Locan acoustic emission (AE) analyser, and the exact spatial locations of the individual fibre failure events were identified using novel experimental techniques. A fibre probe technique has been illustrated to be a viable method with which to identify whether a fibre is broken or remains intact within a testpiece. Examination of exact spatial locations of fibres is possible, and evidence suggests that individual fibre failure is of ten followed by another fibre failure within the same row of a single mat lay up. Experimental observations and AE data reveal that crack arrest occurs if relatively few fibres fail in the crack wake as they are breached by matrix fatigue crack growth, and that fibre failure occurs only in the crack wake and behind the growing fatigue crack tip.     

Acoustic emission from stress corrosion cracks in aligned GRP

Acoustic emission (AE) produced by the propagation of stress corrosion cracks in an aligned glass fibre/polyester resin composite material has been recorded. Tests have been carried out over a range of crack growth rates and the variation of AE with crack velocity/applied stress intensity has been examined. The main source of AE is fibre fracture and there is a one-to-one relationship between the number of fibre fractures and the number of high-amplitude AE signals. This enables crack growth to be monitored directly from acoustic emission. The amplitude of AE signals produced by fibre failure appears to be proportional to the fracture stress of the fibres, although further analysis requires a greater understanding of the generation, transmission and detection of AE signals. This work demonstrates that stress corrosion cracking is an ideal source for the study of AE produced by fibre fracture without complications caused by interface effects, such as fibre debonding or pullout.On leave from the Technical University of Wroclaw, Wroclaw, Poland.     

Wavelet transform of acoustic emission signals in failure of model composites

The fundamental characteristics of acoustic emission (AE) signals, such as the attenuation, and frequency dependency of AE signals, were investigated and the fracture process of the single fiber composite (s.f.c.) was examined. As a result, the frequencies of AE signals were almost unchanged, while the amplitudes attenuated greatly with the increment of the propagation length. This proved that the frequency analysis is an effective way in processing AE signals of composite materials. In the fracture process of the s.f.c., the number of AE events was in a good agreement with the number of fiber breakages, and the sources of AE signals were the failure modes at fiber breakages. Using the proposed time-frequency method of wavelet transform (WT) to process AE signals, the microfailure modes at a fiber breakage and the microfracture mechanism, such as the sequence of each failure mode and their interaction, were made clearer. These indicated that both processing methods of AE signals, fast-Fourier transform and WT, were powerful for identifying the microfailure modes and for elucidating the microfracture mechanisms in composite materials.     

Tensile and fatigue evaluation of Ti–15Al–33Nb (at.%) and Ti–21Al–29Nb (at.%) alloys for biomedical applications

In this work the fatigue and tensile behavior of Ti–15Al–33Nb (at.%) and Ti–21Al–29Nb (at.%) was evaluated and compared to that for other titanium-based biomedical implant alloys, in particular Ti–6Al–4V (wt.%). The mechanical properties of interest were fatigue strength, tensile strength, elastic modulus, and elongation-to-failure. Fatigue stress versus life curves were obtained for tests performed at room temperature in air as well as in Ringer's solution at R = 0.1 for maximum stresses between 35% and 90% of the ultimate tensile strength. The results indicated that the fatigue strength and lives and elastic modulus of these alloys is comparable to that for Ti–6Al–4V (wt.%). Considering the data scatter and deformation behavior, the Ringer's solution did not significantly affect the fatigue behavior. Heat treatment reduced the tensile strength and this corresponded to a reduction in the fatigue strength. The tensile strength of the as-processed Ti-Al-Nb alloys was slightly lower than that for Ti–6Al–4V (wt.%), and the Ti–15Al–33Nb (at.%) exhibited lower strengths and higher elongations than Ti–21Al–29Nb. Based on the current results, it is proposed that titanium–aluminum–niobium alloys will be of considerable future interest for biomedical applications.     

Microstructure and mechanical properties of in situ synthesised (TiC+TiB)/Ti–6Al–4V composites prepared by powder metallurgy

Abstract

Titanium matrix composites (TMCs) reinforced with hybrid reinforcements were synthesised by blending Ti–6Al–4V, Ti, B₄C and C powders followed by reactive hot pressing. The phases were identified by X-ray diffraction, and the microstructures were examined by optical microscopy and scanning electron microscopy (SEM). Mechanical properties were tested at room temperature (RT), 400, 450 and 500°C respectively. The results show that Ti–6Al–4V produced by hot pressing has higher strength and better plasticity than by casting; there are four kinds of reinforcements in TMCs, and the TMCs’ strength increases significantly with the addition of reinforcements both at RT and elevated temperature; the TMCs with 5 vol.-% of reinforcements have higher strength than that with 10 vol.-% at high temperature. The fracture surfaces were examined by SEM. It shows that the bond between the reinforcements and matrix is not so well that reinforcements’ debonding occurs even at RT.     

In situ joining of titanium to SiC/Al composites by low pressure infiltration

A method of in situ joining of titanium to SiC/Al composites by low pressure infiltration was proposed. The effect of infiltration temperature on microstructure and bending strength of in situ joining composites was investigated and the best infiltration temperature was confirmed to be 710 °C. The interfacial region of SiC/Al/Ti composites was consisted of Ti substrate, Al–Ti interfacial layer, Al layer and SiC/Al composite. The bending strength of SiC/Al composites kept nearly constant as the infiltration temperature changed while that of SiC/Al/Ti composites was influenced significantly by the infiltration temperature. The fracture occurred at the Al–Ti and Al–SiC/Al interfaces alternately as infiltrated at 670 °C. But as the infiltration temperature was increased to 710 °C, the fracture occurred only at the Al–SiC/Al interface which shows a great interfacial bonding at the Al–Ti interface. The formation of Al–Ti brittle intermetallics and the effect of crystallization and grain coarsening are two possible reasons which lead to the decrease of bending strength when the infiltration temperatures were increased from 710 °C to 730 °C.     

长径比对TiB/Ti6Al4V网状复合材料力学行为的影响

目的 针对高性能钛基复合材料开发过程中所面临的强韧性倒置问题,对网状构型钛基复合材料拉伸行为进行仿真,以揭示增强体长径比对材料强度与韧性的影响机理。方法 针对TiB/Ti6Al4V网状构型复合材料体系,构建增强相长径比不同的复合材料有限元模型,分别进行拉伸行为仿真,并对其应力-应变曲线、应力集中系数、应力云图和应变云图等进行预测与分析。结果 随着增强相TiB长径比的增大,复合材料的断裂伸长率单调递增,弹性模量与抗拉强度则呈先下降后上升的趋势。结论 增强相长径比是影响复合材料力学性能的重要参数。增强相的长径比和局部体积分数的共同作用导致复合材料模量和强度随长径比的增大先降低后升高。此外,随着TiB长径比的增大,断口更加曲折,主裂纹多次偏转扩展方向,并沿着TiBw/Ti6Al4V-Ti6Al4V“限域”界面扩展,进而消耗了大量的体系能量,这对材料韧化有积极影响。     

Fatigue and wear evaluation of Ti-Al-Nb alloys for biomedical applications

In this work the fatigue and wear behavior of Ti–15Al–33Nb(at.%) and Ti–21Al–29Nb(at.%) was evaluated and compared to that for other titanium-based biomedical implant alloys, in particular Ti–6Al–4V(wt.%). Fatigue stress versus life curves were obtained for tests performed at room temperature in air at a stress ratio of R = 0.1 for maximum stresses between 75%–90% of the ultimate tensile strength. The results indicated that the fatigue strength and lives of the as-processed alloys are comparable to that for Ti–6Al–4V(wt.%). Heat treatment significantly increased the orthorhombic-phase volume fractions in the alloys and resulted in reduced fatigue strength. The wear resistance for the alloys was significantly greater than that for Ti–6Al–4V(wt.%). Based on the current results, it is proposed that titanium–aluminum–niobium alloys will be of considerable future interest for biomedical applications.