Microstructure and mechanical properties of carbon fiber reinforced multilayered (PyC–SiC)n matrix composites

Carbon fiber reinforced multilayered (PyC–SiC)_n matrix (C/(PyC–SiC)_n) composites were prepared by isothermal chemical vapor infiltration. The phase compositions, microstructures and mechanical properties of the composites were investigated. The results show that the multilayered matrix consists of alternate layers of PyC and β-SiC deposited on carbon fibers. The flexural strength and toughness of C/(PyC–SiC)_n composites with a density of 1.43 g/cm³ are 204.4 MPa and 3028 kJ/m³ respectively, which are 63.4% and 133.3% higher than those of carbon/carbon composites with a density of 1.75 g/cm³. The enhanced mechanical properties of C/(PyC–SiC)_n composites are attributed to the presence of multilayered (PyC–SiC)_n matrix. Cracks deflect and propagate at both fiber/matrix and PyC–SiC interfaces resulting in a step-like fracture mode, which is conducive to fracture energy dissipation. These results demonstrate that the C/(PyC–SiC)_n composite is a promising structural material with low density and high flexural strength and toughness.     

Influence of the microstructure evolution of ZrO2 fiber on the fracture toughness of ZrB2–SiC nanocomposite ceramics

ZrB₂–SiC nanocomposite ceramics toughened by ZrO₂ fiber were fabricated by spark plasma sintering (SPS) at 1700 °C. The content of ZrO₂ fiber incorporated into the ZrB₂–SiC nanocomposites ranged from 5 mass% to 20 mass%. The content, microstructure, and phase transformation of ZrO₂ fiber exhibited remarkable effects on the fracture toughness of the ZrO_2(f)/ZrB₂–SiC composites. Fracture toughness of the composites greatly improved to a maximum value of 6.56 MPa m^1/2 ± 0.3 MPa m^1/2 by the addition of 15 mass% of ZrO₂ fiber. The microstructure of the ZrO₂ fiber exhibited certain alterations after the SPS process, which enhanced crack deflection and crack bridging and affected fracture toughness. Some microcracks were induced by the phase transformation from t-ZrO₂ to m-ZrO₂, which was also an important reason behind the improvement in toughness.     

Microstructure and fracture-mechanical properties of carbon derived Si3N4 + SiC nanomaterials

The microstructure and basic mechanical properties, as hardness, fracture toughness, fracture strength and subcritical crack growth at room temperature were investigated and creep behavior at high temperatures was established. The presence of SiC particles refined the microstructure of Si₃N₄ grains in the Si₃N₄ + SiC nanocomposite. Higher hardness values resulted from introducing SiC nanoparticles into the material. A lower fracture toughness of the nanocomposite is associated with its finer microstructure; crack bridging mechanisms are not so effective as in the case of monolithic Si₃N₄. The strength value of the monolithic Si₃N₄ is higher than the characteristic strength of nanocomposites. Fractographic analysis of the fracture surface revealed that a failure started principally from an internal flaw in the form of cluster of free carbon, and on large SiC grains which degraded strength of the nanocomposite. The creep resistance of nanocomposite is significantly higher when compared to the creep resistance of the monolithic material. Nanocomposite exhibited no creep deformation, creep cracks have not been detected even at a test at 1400 °C and a long loading time, therefore the creep is probably controlled mainly by diffusion. The intergranular SiC nanoparticles hinder the Si₃N₄ grain growth, interlock the neighboring Si₃N₄ grains and change the volume fraction, geometry and chemical composition of the grain boundary phase.     

Microstructure evolution and mechanical properties of Nb–Si based alloy processed by electromagnetic cold crucible directional solidification

In this paper, the samples with the composition of Nb–22Ti–16Si–3Cr–3Al–2Hf (at.%) have been successfully processed by electromagnetic cold crucible directional solidification, and the microstructure evolution and mechanical properties of the samples were investigated as a function of withdrawal rate. The microstructures of all the samples were composed of primary Nbss, Nbss + α-(Nb, Ti)₅Si₃ eutectic and Nbss + Ti-rich (Nb, Ti)₅Si₃ eutectic. With the increase of withdrawal rate, the microstructure became finer, which could be characterized by the decreased mean diameter of cellular eutectic, mean diameter of eutectic Nbss particles and mean interphase spacing. Meanwhile, the room temperature fracture toughness decreased first, and then increased; the high temperature tensile strength increased gradually. The volume fraction, size and aspect ratio of Nbss and the number of Nbss/(Nb, Ti)₅Si₃ interfaces codetermined the fracture toughness. The more number of Nbss/(Nb, Ti)₅Si₃ interfaces was conducive to impeding the movement of dislocation, which caused the improvement of tensile strength.     

One-step preparation of dense TiC1 − xNx–Ni3Ti cermet by combustion synthesis

TiC_1 − xN_x-based cermets are attractive for high-speed cutting operations, and are usually produced by powder sintering at high temperatures and for a long time. In this paper, dense TiC_1 − xN_x–Ni₃Ti cermet is directly prepared in one step by combustion synthesis in 2 MPa N₂ atmosphere, from the reactant of (Ti + C + Ni) with a molar ratio of Ti:C:Ni = 1:0.7:0.4. The as-prepared TiC_1 − xN_x–Ni₃Ti cermet shows a homogeneous microstructure, in which equiaxed TiC_1 − xN_x grains of a few microns are bounded by Ni₃Ti grain-boundary phase, and a Vickers hardness of 12.1 ± 0.3 GPa. It is proposed that the deficiency of C in the starting reactant plays a key role in simultaneous densification during combustion synthesis. Compared with the conventional powder sintering approach, combustion synthesis may provide a one-step, ultrafast, and furnace-free way to prepare TiC_1 − xN_x-based cermets with reduced time and energy consumption.     

Mechanical properties of in-situ toughened Al2O3/Fe3Al

Mechanical properties of in-situ toughened Al₂O₃/Fe₃Al nano-/micro-composites were measured. Effects of Fe₃Al content, sintering temperature and holding time on properties and microstructure of the composites were investigated. The addition of Fe₃Al nano-particles decreased the aspect ratio and grain size of Al₂O₃, and changed the fracture mode of composites. The maximum bending strength and fracture toughness were 832 MPa and 7.96 MPa m^1/2, which were obtained in Al₂O₃/5 wt.% Fe₃Al sintered at 1530 °C and Al₂O₃/10 wt.% Fe₃Al sintered at 1600 °C, respectively. Compared to monolithic alumina, the strength increased by 132% and the toughness increased by 73%. The improvement in the mechanical properties of the composites was attributed to the change in fracture mode from intergranular fracture to transgranular fracture, the “in-situ reinforced effect” arising from the platelet grains of Al₂O₃ matrix, refined microstructure by dispersoids, as well as crack deflection and bridging of intergranular and intragranular Fe₃Al.     

The effect of fabrication processes on the mechanical and interfacial properties of SiCf/Cu–matrix composites

Three groups of SiC_f/Ti/Cu composites were prepared under conditions of 650 °C + 105 min (sample 1#), 750 °C + 85 min (sample 2#) and 840 °C + 50 min (sample 3#), respectively, by foil-fiber-foil method (FFF), and their room temperature tensile strengths were established. The aim is to model the reactive bonding states between Ti and SiC fiber and between Ti and Cu when Ti is used as interfacial adhesion promoters in SiC_f/Cu–matrix composites. The fracture surfaces, SiC_f/Ti interfaces and Ti/Cu interfaces were investigated by scanning electron microscopy (SEM), optical microscopy and energy dispersive spectroscopy (EDS). The tensile tests show that the tensile strengths of samples 1# and 2# are not obviously enhanced due to the weak bonding strength between SiC fiber and Ti, while those of sample 3# are achieved above 90% of ROM (the rule of mixtures) strength because of excellent bonding between SiC fiber and Ti. However, there are distinct Ti/Cu interfacial reaction zones after the three processes, which are approximately 5.4, 9.0 and 13.3 μm thick, respectively. The Ti/Cu interfacial reaction products are mainly distributed in four layers. In samples 1# and 2#, the products are predicted to be Cu₄Ti, Cu₃Ti₂, CuTi and CuTi₂ according to their chemical compositions determined by EDS, while in sample 3#, the products are Cu₄Ti, Cu₄Ti₃, CuTi and CuTi₂. Additionally, the relationships between the thickness of Ti interlayer and its reaction with C and Cu are also discussed, and an optimal thickness of Ti is introduced.     

Mechanical properties of a medical β-type titanium alloy with specific microstructural evolution through high-pressure torsion

The effect of high-pressure torsion (HPT) processing on the microstructure and mechanical biocompatibility includes Young's modulus, tensile strength, ductility, fatigue life, fretting fatigue, wear properties and other functionalities such as super elasticity and shape memory effect, etc. at levels suitable for structural biomaterials used in implants that replace hard tissue in the broad sense (Sumitomo et al., 2008 [4]). In particular, in this study, the mechanical biocompatibility implies a combination of great hardness and high strength with an adequate ductility while keeping low Young's modulus of a novel Ti–29Nb–13Ta–4.6Zr (TNTZ) for biomedical applications at rotation numbers (N) ranging from 1 to 60 under a pressure of 1.25 GPa at room temperature was systematically investigated in order to increase its mechanical strength with maintaining low Young's modulus and an adequate ductility.TNTZ subjected to HPT processing (TNTZ_HPT) at low N exhibits a heterogeneous microstructure in micro-scale and nano-scale consisting of a matrix and a non-etched band, which has nanosized equiaxed and elongated single β grains, along its cross section. The grains exhibit high dislocation densities, consequently non-equilibrium grain boundaries, and non-uniform subgrains distorted by severe deformation. At high N which is N > 20, TNTZ_HPT has a more homogeneous microstructure in nano-scale with increasing equivalent strain, ε_eq. Therefore, TNTZ_HPT at high N exhibits a more homogenous hardness distribution. The tensile strength and 0.2% proof stress of TNTZ_HPT increase significantly with N over the range of 0 ≤ N ≤ 5, and then become saturated at around 1100 MPa and 800 MPa at N ≥ 10. However, the ductility of TNTZ_HPT shows a reverse trend and a low-level elongation, at around 7%. And, Young's modulus of TNTZ_HPT decreases slightly to 60 GPa with increasing N and then becomes saturated at N ≥ 10. These obtained results confirm that the mechanical strength of TNTZ can be improved while maintaining a low Young's modulus in single β grain structures through severe plastic deformation.     

Production of TiC reinforced-aluminum composites with the addition of elemental carbon

TiC reinforced Al matrix composites were produced by the additions of elemental carbon to both Al + 4%Ti and Al + 5%Ti alloys. It is shown that the microstructure, phase composition as well as fracture behavior of the composites produced are controlled by the processing parameters, such as temperature, amount of excess carbon and duration. Composite microstructure subjected to 1300 °C for 15 min includes only TiC particles where the fracture occurs in a ductile manner whilst composites subjected to 1200 °C for 30 min contain Al₃Ti and TiC particles which show mixed mode of fracture behavior where Al₃Ti particles resulted in brittle fracture due to their coarser size.     

Effect of a weak fiber interface coating in ZrB2 reinforced with long SiC fibers

This study reports the microstructural analysis and mechanical properties of a ZrB₂ ceramic containing long BN-coated Hi-Nicalon SiC fibers. A composite was produced and thoroughly characterized by transmission electron microscopy to study the interfaces at the nanoscale level. Full densification was accomplished by hot pressing at 1450 °C. The fiber in the sintered material retained its pristine aspect, confirming that the coating was effective in preventing degradation due to interactions with the matrix. Pull-out was observed on fractured surfaces, but toughness values were about 4.5 MPa√m, which was comparable to those of ZrB₂ materials with SiC additions in the form of particles or short fibers. However, the composites exhibited a controlled fracture behavior, as confirmed by a notably higher work of fracture, 140 J/m², compared with 20–30 J/m² of unreinforced ZrB₂ or ZrB₂ containing chopped fibers.     

Effects of high temperature treatment on microstructure and mechanical properties of laser-clad NiCrBSi/WC coatings on titanium alloy substrate

Laser-clad composite coatings on the Ti6Al4V substrate were heat-treated at 700, 800, and 900 °C for 1 h. The effects of post-heat treatment on the microstructure, microhardness, and fracture toughness of the coatings were investigated by scanning electron microscopy, X-ray diffractometry, energy dispersive spectroscopy, and optical microscopy. The wear resistance of the coatings was evaluated under dry reciprocating sliding friction at room temperature. The coatings mainly comprised some coarse gray blocky (W,Ti)C particles accompanied by the fine white WC particles, a large number of black TiC cellular/dendrites, and the matrix composed of NiTi and Ni₃Ti; some unknown rich Ni- and Ti-rich particles with sizes ranging from 10 nm to 50 nm were precipitated and uniformly distributed in the Ni₃Ti phase to form a thin granular layer after heat treatment at 700 °C. The granular layer spread from the edge toward the center of the Ni₃Ti phase with increasing temperature. A large number of fine equiaxed Cr₂₃C₆ particles with 0.2–0.5 μm sizes were observed around the edges of the NiTi supersaturated solid solution when the temperature was further increased to 900 °C. The microhardness and fracture toughness of the coatings were improved with increased temperature due to the dispersion-strengthening effect of the precipitates. Dominant wear mechanisms for all the coatings included abrasive and delamination wear. The post-heat treatment not only reduced wear volume and friction coefficient, but also decreased cracking susceptibility during sliding friction. Comparatively speaking, the heat-treated coating at 900 °C presented the most excellent wear resistance.     

Effects of SiC interphase by chemical vapor deposition on the properties of C/ZrC composite prepared via precursor infiltration and pyrolysis route

Silicon carbide (SiC) interphase was introduced by chemical vapor deposition (CVD) process to prevent carbon fiber degradation and improve fiber–matrix interface bonding of C/ZrC composite prepared via precursor infiltration and pyrolysis (PIP) process. Moderate thickness of SiC interphase in fiber bundles could increase the density of the composite, but when the thickness of SiC interphase was over 0.5 μm, more close pores formed and the density of the composite decreased. The SiC interphase could protect carbon fiber effectively from carbo-thermal reduction, but could not enhance the mechanical properties of C/ZrC composite. The flexural strength and fracture toughness of C/ZrC composites with 0.05 μm thickness SiC layer were 252 MPa and 13.6 MPa m^1/2, and for those with 0.5 μm thickness SiC layer 240 MPa and 12.8 MPa m^1/2, both close to the value of the composite without SiC interphase (254 MPa and 14.5 MPa m^1/2), while those with 0.7 μm thickness SiC layer were only 191 MPa and 10.8 MPa m^1/2, respectively. Moderate content of SiC interphase could improve the ablation property of C/ZrC composites; however excessive content of SiC interphase would decrease the ablation property.     

Effects of sintering velocity on the microstructure and mechanical properties of hot-pressed ZrB2–SiC–ZrO2f ceramics

The effects of cooling and heating velocity on the microstructure and mechanical properties of the hot-pressed ZrB₂–20 vol.% SiC ceramics added with 15 vol.% Zirconia fiber were investigated in details. It was indicated that the appropriate cooling velocity could reduce the residual stress concentration, and the proper heating velocity could allowed the green compacts to get densified, which was favorable to enhance the flexural strength and fracture toughness to 1117 ± 98 MPa and 7.1 ± 0.5 MPa m^1/2, respectively. Due to the introduction of the Zirconia fiber, the observed toughening mechanisms were attributed to the fiber debonding, fiber pull-out and phase transformation toughening. The results presented here point out a promising way for improving the mechanical properties of ZrB₂-based ultra-high temperature ceramics.     

Preparation and mechanical properties of self-reinforced in situ Si3N4 composite with La2O3 and Y2O3 additives

Self-reinforced in situ Si₃N₄ composite material was prepared with high amount of La₂O₃ and Y₂O₃ additives by two-step hot pressing, and the optimum amount of additives was determined. The volume fraction of boundary glass phase was calculated based on the equilibrium of equivalent number in chemical reaction. For material with 15 mol% additives, flexural strength and fracture toughness at room temperature were 960 MPa and 12.3 MPa m^1/2, respectively. At temperature of 1350°C, flexural strength was maintained to 720 MPa and fracture toughness was significantly increased to 23.9 MPa m^1/2 because of the high refractory of oxynitride glass containing compositions of La and Y. Self-reinforced mechanism was mainly responsible for crack deflection along the elongated β-Si₃N₄ grains.     

Toughening effect of short carbon fibers in the ZrB2–ZrSi2 ceramic composites

The toughening effect of the short carbon fibers in the ZrB₂–ZrSi₂ ceramic composites were investigated, where the ZrB₂–ZrSi₂ ceramics without carbon fibers were used as the reference. The mechanical properties were evaluated by means of flexural and SENB tests, respectively. The microstructure was characterized by SEM equipped with EDS. The results found that the short carbon fibers were uniformly incorporated in the ZrB₂–ZrSi₂ matrix and the relative density was about 97.92%. The flexural strength of short carbon fiber-reinforced ZrB₂–ZrSi₂ composites is 437 MPa; the fracture toughness and the work of fracture are 6.89 MPa m^1/2 and 259 J/m², respectively, which increased significantly in comparing with composites without fibers. The microstructure analysis revealed that the improved fracture toughness could be attributed to the fiber bridging, the fiber–matrix interface debonding and the fiber pullout, which consumed more fracture energy during the fracture process.     

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.     

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.     

Microstructure and mechanical properties of BAS/SiC composites sintered by spark plasma sintering

High-density BAS/SiC composites were obtained from β-SiC starting powder by the spark plasma sintering technique. Various physical properties of the BAS/SiC composites were investigated in detail, such as densification, phase analysis, microstructures and mechanical properties. The results demonstrated that the relative density of the BAS/SiC composites reached over 99.4% at 1900 °C. The SiC grains were uniformly distributed in the continuous BAS matrix which is probably because of complete infiltration of the SiC particles in BAS liquid-phase formed during sintering. The pull-out of SiC particles, crack deflection and bridging were observed as the major toughening mechanism. The flexural strength and fracture toughness of the BAS/SiC composites sintered at 1900 °C were up to 560 MPa and 7.0 MPa·m^1/2, respectively.     

Preparation of YAG gel coated ZrB2–SiC composite prepared by gelcasting and pressureless sintering

In this paper, gelcasting and pressureless sintering of YAG gel coated ZrB₂–SiC (YZS) composite were conducted. YAG gel coated ZrB₂–SiC (YZS) suspension was firstly prepared through sol–gel route. Poly (acrylic acid) was used as dispersant. YZS suspension had the lowest viscosity when using 0.6 wt.% PAA as dispersant. Gelcasting was conducted based on AM–MBAM system. The gelcast YZS sample was then pressureless sintered to about 97% density. During sintering, YAG promoted the densification process from solid state sintering to liquid phase sintering. The average grain sizes of ZrB₂ and SiC in the YZS composite were 3.8 and 1.3 μm, respectively. The flexural strength, fracture toughness and microhardness were 375 ± 37 MPa, 4.13 ± 0.45 MPa m^1/2 and 14.1 ± 0.5 GPa, respectively.     

Analysis of tensile strength and microstructure of Ni–SiC composites prepared by electroforming

Ni–SiC metal matrix composites with two kinds of SiC content were prepared by electroforming in a nickel sulphamate bath. Tensile strength and microstructure of the composites before and after heat treatment were investigated. The maximum of tensile strength was obtained after heat treatment at 300 °C × 24 h. The values were 641 N/mm² and 701 N/mm² respectively. The complete reaction between nickel and SiC particles can produce shrinkage pores in the interface. The volume of shrinkage pores was equal to 8% of the volume of SiC particles in the composites. The interfacial reaction products were composed of Ni₃Si and a little amount of Ni₃₁Si₁₂ after heat treatment at 600 °C × 24 h. The fracture evolution went though microcracks initiation, growth and coalescence. Cracking of the matrix, debonding of Ni–SiC interfaces and cracking of particles were three types of cracking modes for Ni–SiC composites.