Al-4V Composites Research Articles

Transverse tensile behavior of SiC-fiber/ Ti–6Al–4V composites—1. experimental results

Under transverse tension (loads normal to the fiber axis), fiber/matrix debonding in fiber-reinforced titanium-matrix composites results in reductions in specimen stiffness and ultimate strength, and increased loading of the matrix. The interface structure and chemistry have a strong influence on the debond stress and therefore contribute significantly to the transverse composite behavior. In the present study a cruciform specimen geometry was employed over a range of specimen thicknesses to investigate the remote stress levels where debonding initiates in Ti–6Al–4V composites containing SCS-6 (C+Si coating), SCS-0 (no coating), and Trimarc 1 (carbon coating) SiC fibers. The different surface conditions of the fibers resulted in a range of remote interface debond stresses with SCS-0 being the strongest and Trimarc 1 being the weakest. Multiple-fiber, single-ply specimens of the three different fibers revealed that the remote debond stress did not vary for SCS-6 and Trimarc 1 compared to single-fiber tests, but were slightly lower for SCS-0. Part 2 of this study presents analytical and numerical modeling studies of the local stress state at the interface in relation to the applied stress, and discusses them with regard to these experimental results.

Evaluation of the strength of diffusion bonded joints in continuous fiber reinforced metal matrix composites

SiC continuous fiber reinforced Ti6Al4V composites with various fiber volume fractions were fabricated by the vacuum hot pressing process and were joined to a Ti6Al4V alloy plate by solid state diffusion bonding processes. The matrix alloy was perfectly joined to the Ti6Al4V plate within the bonding condition. The joint strength decreased with increasing fiber volume fraction. The joint strength was not directly proportional the matrix area fraction. A joint efficiency over 90% could be obtained when the fiber volume fraction was less than 30%. The SiC fibers were debonded from the matrix and cylindrical defects were formed at the end of the fiber under tensile stress. The defects produced a triaxial stress state at the matrix alloy and thus higher joint strengths than the values simply calculated from the area fraction of the matrix at the bonding interface. The composite was transient liquid phase bonded to itself using TiCuZr filler metal. The joint strength was degraded using thin interlayer less than 80 μm thick. An interlayer thickenss over 80 μm was found to be necessary to obtain a sound joint.

The interface debond stress in single and multiple SiC fiber/Ti-6Al-4V composites under transverse tension

The cruciform specimen, which effectively eliminates the influence of the fiber-free surface intersection, has recently been introduced to characterize the transverse tensile response of fiber-reinforced composites. In the present study, extensive transverse tensile testing of single and multiple-fiber specimens of SCS-6 SiC fiber/Ti-6Al-4V composites was performed. Poor consolidation and specimen damage prior to the test, when present, caused the measured interface strength to be low. The debond stress was relatively insensitive to specimen thickness between 200 and 550 μm. The average remote debond stress of well-consolidated, damage-free specimens was 322 ± 14 MPa, while a Weibull analysis indicated that the characteristic strength was 328 MPa and the Weibull modulus was 27. Stress analyses suggest that, at this level of remote stress, the interface supports a tensile stress of approximately 120 MPa, which is much greater strength than previously thought. Multiple-ply, multiple-fiber specimens were found to have nearly the same remote debond stress as the single-fiber specimens.

Determination of the interface failure mechanism during transverse loading of single fibre SiC/Ti-6Al-4V composites from torsion tests

During transverse loading of continuous fibre-reinforced composites, large tangential shear stresses develop at the interface at about 45° to the loading axis. These shear stresses may initiate interface failure and frictional sliding, increasing the load in the matrix and lowering the applied stress to cause normal fibre-matrix separation. In the present study, an attempt has been made to measure the tangential shear strength of the interface for single fibre SiC/Ti-6Al-4V composites with varying interfaces using torsion testing and finite element stress analysis. An energy based fracture mechanics approach was developed to predict the micro-mechanical response of the composite to torsional loading following interface crack initiation. The fracture mechanics analysis suggests that a non-linearity would appear in the strain versus angle of twist curve once debonding initiates. Based on the onset of non-linearity and finite element stress analysis, the tangential shear strength for SCS-6/Ti-6Al-4V interface (carbon-rich coated) was estimated at 50 MPa; that for the SCS-0/Ti-6Al-4V interface (uncoated) was greater than 80 MPa, and that for the AC1/Ti-6Al-4V interface (carbon coated) was about 40 MPa. Comparison of these results with experimental observations and finite element analysis of transverse tension tests indicates that failure in SCS-6/Ti-6Al-4V and AC1/Ti-6Al-4V composites is initiated by tangential shear failure of the interface. In SCS-0/Ti-6Al-4V composites, the results are not conclusive.

Materials characterization of silicon carbide reinforced titanium (Ti/SCS-6) metal matrix composites. II. Theoretical modelling of fatigue behaviour : Chiang, K.T., Loh, D.H., Liaw, P.K. and Diaz, E.S. Metall. Mater. Trans. A. (Dec. 1995) 26A (12), 3249–3256

Under transverse tension (loads normal to the fiber axis), fiber/matrix debonding in fiber-reinforced titanium-matrix composites results in reductions in specimen stiffness and ultimate strength, and increased loading of the matrix. The interface structure and chemistry have a strong influence on the debond stress and therefore contribute significantly to the transverse composite behavior. In the present study a cruciform specimen geometry was employed over a range of specimen thicknesses to investigate the remote stress levels where debonding initiates in Ti–6Al–4V composites containing SCS-6 (C+Si coating), SCS-0 (no coating), and Trimarc 1 (carbon coating) SiC fibers. The different surface conditions of the fibers resulted in a range of remote interface debond stresses with SCS-0 being the strongest and Trimarc 1 being the weakest. Multiple-fiber, single-ply specimens of the three different fibers revealed that the remote debond stress did not vary for SCS-6 and Trimarc 1 compared to single-fiber tests, but were slightly lower for SCS-0. Part 2 of this study presents analytical and numerical modeling studies of the local stress state at the interface in relation to the applied stress, and discusses them with regard to these experimental results.

Evaluation of the transverse response of fiber-reinforced composites using a cross-shaped sample geometry

The present study introduces the single-fiber, cross-shaped specimen for determination of the response of the fiber-matrix interface to transverse tension, in the absence of free-edge effects, which have been suggested to influence the behavior of uniform-gage samples. This test was found to successfully cause interface damage and matrix plasticity in the center section of the cross. A cursory stress analysis and experimental results were presented which indicate that the remotely-applied tensile stresses are uniform (along the x-axis) over a significant width in the center of the cross, and are not significantly different in magnitude from the normal stress in the uniform (vertical) section. This test was used to investigate SCS-6/Ti-6Al-4V and ACMF/Ti-6Al-4V composites. Earlier attempts to characterize the response of different fiber/matrix interfaces in uniform-gage samples by observing the interface at the free surface were unsuccessful; however, tensile tests using cross-shaped samples were able to distinguish between the two different interface responses. A simple method to calculate the interface tensile strength is presented and provides a preliminary value of 130 MPa for the SCS-6/Ti-6Al-4V interface, and 277 MPa for that of ACMF/Ti-6Al-4V. The value for SCS-6 is higher than previous estimates, where a strength of zero was reported to bestmore » fit the observations in straight-sided tests. Further experimental and theoretical studies with these systems are underway to determine the interface debond stress (and the scatter in the measurements) and the failure mechanism of the interfaces.« less

An effective joint of continuous SiC/Ti-6Al-4V composites by diffusion bonding

The continuous SiC/Ti-6Al-4V composite was fabricated through a hot pressing procedure. Solid state diffusion (SSD) and transient liquid phase (TLP) bonding processes were applied to join continuous SiC/Ti-6Al-4V composites to a Ti-6Al-4V plate and to themselves. The butt joint strength of the composite to the Ti-6Al-4V plate reached a maximum of approximately 850 MPa for V f = 30% composite. The maximum strength is 90% of the tensile strength of the Ti-6Al-4V alloy. When the composite was bonded directly to itself, a sound joint was not obtained. A joint strength equal to composite Ti-6Al-4V's joint strength was obtained using Ti-6Al-4V and Ti-Cu-Zr thin foils as filler metal. However, a fracture occurred not at the base metal but at the bonding interface. Scarf joint forms were also used to join a composite to a Ti-6Al-4V plate and to itself. When the scarf angle was less than 12°, the composite-composite joint strength reached a maximum value of 1380 MPa corresponding to 80% tensile strength of the base material. The composite-composite scarf joint was fractured at base material. The composite-Ti-6Al-4V scarf joint was also fractured at the Ti-6Al-4V plate when the scarf angle was less than 12°. It is possible to join the SiC/Ti-6Al-4V composite without any reinforcing parts, such as a doubler.

Enhanced fatigue crack growth resistance at elevated temperature in TiC/Ti-6Al-4v composite: Microcrack-lnduced crack closure

The fatigue crack growth behavior of TiC/Ti-alloy composite was examined at 450 °C and compared to the room-temperature behavior. Contrary to the temperature-dependent fatigue crack growth behavior of the monolithic alloy, fatigue crack growth resistance of the composite was improved at the elevated temperature. At 450 °C, the fatigue threshold of the composite was found to be 50 Pct higher than at room temperature. Such an improved fatigue crack growth resistance is shown to result from extensive microcracking of reinforcing particles, which promotes fatigue crack closure at the elevated temperature.

Interface contribution to the SiC-titanium and SiC-aluminium tensile strength prediction

Fragmentation tests of single SiC filaments embedded in an aluminium (1050 and 5083 alloys) or a titanium (Ti-6Al-4V) matrix have been analysed in an effort to obtain the interface contribution in terms that could be incorporated into a tensile fracture model for unidirectional composites. Depending on the matrix, two regimes of interfacial stress transfer can be distinguished within the whole range of tested temperatures. For the SCS2/5083 system, plastic deformation of the alloy limits the stress transfer, and the interface contribution thus finds its expression in the shear stress of the matrix. for the SCS6/Ti-6Al-4V system, friction is the leading process and the interface contribution strongly depends on the stress state around the fibre. Assuming a temperature dependent compressive radial stress up to 925‡C, an effective transfer shear stress may be easily calculated for unidirectional SCS6/Ti-6Al-4V composites.

Evaluation of the efficiency of TiB2 and TiC as protective coatings for SiC monofilament in titanium-based composites

The vigorous interfacial reactions in SiC/Ti-6Al-4V composites at elevated temperatures lead to the deterioration of the mechanical properties of the composites. TiB2 and TiC were selected as potential protective coatings for SiC fibres in titanium-based composites. These coatings were deposited on to fibres by the chemical vapour deposition technique. Comparisons and evaluations have been made of the effectiveness of these ceramics as protective coatings for SiC fibres by incorporating the coated fibres into a Ti-6Al-4V matrix using the diffusion bonding method. Emphasis has been placed on the chemical compatibility of the candidate coating with SiC and Ti-6Al-4V by examining the interfaces of the fibre/coating/matrix using microscopic methods and chemical analysis. A stoichiometric TiB2 coating was found to be stable with SiC and has proved an effective barrier to prevent the SiC fibre from reacting with the Ti-6Al-4V. The TiC coating showed no apparent reaction with a titanium-alloy matrix under the conditions studied, but was found to react with the SiC fibre substrate.

An interfacial study of a sigma fibre/Ti-6Al-4V composite using scanning Auger/EDX microscopy

The interfacial region of a Sigma fibre/Ti-6Al-4V composite has been studied by Auger/EDX microscopy. The SiC fibre has a duplex coating consisting of an inner layer of C and outer TiB x layer. Point analyses, line scans and images were acquired from the fibre/matrix interface region for a specimen taper sectioned at 10 o to the fibre axis and a sample fractured in situ along the fibre length. Fracture of the composite reveals the presence of a brittle B rich oxide layer probably of the form Ti x B y O at the boride coating/matrix interface. The fracture path occurs either through this oxide or the graphitic carbon layer. C is shown to have diffused through the entire boride coating

Diffusion bonding of SiC/Ti–6Al–4V composite to Ti–6Al–4V alloy and fracture behaviour of joint

AbstractContinuous SiC fibre reinforced Ti–6Al–4V composites were diffusion bonded to Ti–6Al–4V alloy. Bondability and the fracture mechanism of the joint were investigated. The joint strength increased with bonding time, and was a maximum at 850 MN m−2 for Vf=30% composite and 650 MN m−2 for Vf=45% composite. The bonding was completed sooner for Vf=30% composite than for Vf=45% composite. In Vf=30% composite, the maximum joint strength was about 85% of the tensile strength of Ti–6Al–4V. The void ratio at the interface between matrix and Ti–6Al–4V alloy decreased as bonding time increased. The joint strength was controlled by the bonding between the composite matrix and the Ti–6Al–4V alloy. The maximum joint strength was 100–150–MN–m−2 higher than the strength simply calculated from the area fraction of the bonded matrix/Ti–6Al–4V interface. Fibres were debonded from the matrix and the defects were produced around fibres, so the state of stress at the bond interface is triaxial owing to the defects and/or...

Microstructural characterization in diffusion‐bonded SiC/Ti–6Al–4V composites

SUMMARYThe microstructural evolution during the diffusion bonding consolidation of a Ti–6Al–4V/SiC fibre composite was investigated by optical, scanning and transmission electron microscopy. The effects of processing parameters, particularly temperature, on the microstructures of the matrix and the fibre and their bonding were considered. Processing at too high a temperature can result in growth of SiC crystals in the fibre in addition to rapid interfacial reaction, while interfacial bonding cannot be established if the temperature is too low. Various defects can be caused by inadequate fabrication practices. These include micro‐pores, matrix‐cracking, cracking, bending and impingement of fibres, and heterogeneous fibre distribution. Methods for avoiding these are discussed. A defect‐free and uniformly distributed fibre composite can only be achieved by optimizing the processing parameters (such as temperature, pressure, time and cooling rate) and adequately combining fibre spacing and matrix thickness with accurate fibre alignment.

ＳｉＣ／Ｔｉ‐６Ａｌ‐４Ｖ ＦＲＭの繊維／マトリックス界面反応による強度低下機構

The effect of the fiber-matrix chemical reaction which occurs during thermal exposure on the tensile strength of a SiC/Ti-6Al-4V composite was studied. The tensile strength of the composite decreased with an increase in the fiber-matrix reaction zone thickness. Especially, a drastic degradation occurred when the reaction zone thickness exceeded 1μm. The strength of the fibers extracted from the composite also decreased as the fiber-matrix reaction proceeded. This degradation in the fiber strength was caused by the surface roughness of the fiber due to the fiber-matrix reaction. The strength of the composite was approximately identical with the value estimated from the ROM (Rule of Mixture) using the strengths of the matrix and the extracted fibers.It was assumed from the observation of fracture surfaces of the composite and AE analysis during the tensile test that a fiber-matrix reaction zone debonding occurred before failure of the fiber. Therefore, it is concluded that the degradation in strength of the composite by thermal exposure can be attributed not to a notch effect of cracking in the reaction zone, but to the decrease in strength of the fiber itself.

Effect of the filament nature on fatigue crack growth in titanium based composites reinforced by boron, B(B4C) and SiC filaments

Crack propagation testing has been applied to synthetic metal matrix composites (MMC) in order to compare failure mechanisms in Ti-6Al-4V alloy reinforced by uncoated boron, B(B4C) and chemical vapour deposition (CVD) SiC filaments. The impeding effect of the fibres leads to low crack growth rates, compared to those reported for the unreinforced Ti-6Al-4V alloy and to higher toughness despite the presence of the reinforcing brittle phases. After long isothermal exposures at 850° C, the MMC crack growth resistance is reduced mainly due to fibre degradation, fibre-matrix debonding and an increase in matrix brittleness. However, for short-time isothermal exposures (up to about 10 h for B/Ti-6Al-4V, 30 h for B (B4C)/Ti-6Al-4V and 60 h for SiC/Ti-6Al-4V) the crack growth resistance is significantly increased. This improvement is related to the build up of an energy-dissipating mechanism by fibre microcracking in the vicinity of the crack tip. This damaging mechanism allowing matrix plastic deformation is already effective for boron and B(B4C) in the as-fabricated state, but occurs only after 10 h of thermal exposure at 850° C in the case of SiC/Ti-6Al-4V composites.

Effect of isothermal exposure on fatigue crack growth in boron-titanium composites

Fatigue crack growth testing has been applied to boron/Ti-6Al-4V composites in order to investigate simultaneously crack propagation mechanisms during the fatigue and overload portion of the experiments. It is concluded that linear elastic fracture mechanics (LEFM) for heterogeneous and anisotropic materials facilitates understanding of the rupture mechanisms and the assessment of failure work and toughness. The influence of isothermal exposure on crack propagation mechanisms has been pointed out. A short duration heat treatment at 850‡ C improves the composite toughness and reduces the fatigue crack growth rate although the fibrematrix (FM) interfacial bonding is increased. This effect has been related to a damage mechanism initiated in the interfacial reaction zone. In any case, the fatigue behaviour of the composite is controlled by the matrix and the capability of the fibres to function as crack arrestors. The impeding effect of the fibres is no longer effective when the thermal exposure duration is significant.

Fatigue behavior of SiC reinforced Ti(6AI-4V) at 650 °C

Axial, low cycle fatigue properties of 25 and 44 fiber vol pct SiC/Ti(6Al-4V) composites, measured at 650 °C, were compared with the fatigue properties of unreinforced Ti(6Al-4V) at the same temperature. A prior study of the fatigue behavior of this composite system at room temperature indicated that the SiC fiber reinforcement did not provide the anticipated improvement of fatigue resistance of this alloy. At 650 °C, the composite fatigue properties degraded somewhat from those at room temperature. However, these properties degraded more for the unreinforced matrix at 650 °C with the result that the composite fatigue strength was two to three times the fatigue strength of the matrix alloy. The reasons for this reversal are discussed in terms of crack initiation at broken fibers and residual matrix stresses.

Synthesis and material characterization of beryllium/Ti-6Al-4V composites

The synthesis of Be/Ti−6Al−4V alloy composites from mixed powders of the elemental constituents is described and bulk mechanical properties of the composites are compared to theoretical predictions. A synergistic strengthening effect is predicted based on interface constraints imposed on ductile beryllium fibers during plastic deformation. Composites fabricated with three separate beryllium compositions,i.e., 40, 50 and 60 vol. pct, had measured yield strengths between 240 MPa and 454 MPa and tensile moduli between 155 and 175 GPa. The ultimate fracture strengths for these compositions is between 825 MPa and 950 MPa and was found to be limited by the fracture strength of the Be/Ti-alloy interface.