Microscale damage evolution and stress redistribution in Ti–SiC fiber composites

Local damage evolution in a composite is the primary micromechanical process determining its fracture toughness, strength, and lifetime. In this study, high energy X-ray microdiffraction was used to measure the lattice strains of both phases in a Ti–SiC fiber composite laminate. The data provided in situ load transfer information under applied tensile stress at the scale of the microstructure. To better understand damage evolution, predictions of a modified shear lag model were compared to the strain data. This comparison (1) demonstrated the importance of accounting for the matrix axial and shear stiffness, (2) optimized the stiffness ratio for load transfer, and (3) improved the interpretation of the ideal planar geometry commonly used in micromechanical composite models. In addition, the results proved the matrix within and around the damage zone sustained substantial axial load and locally yielded. It was also shown that an area detector is essential in such a diffraction study as it provides multi-axial strain data and helps eliminate the “graininess” problem.     

Thermal properties of pressureless melt infiltrated AlN–Si–Al composites

Thermal properties of AlN-Si-Al composites produced by pressureless melt infiltration of Al/Al alloys into porous α-Si₃N₄ preforms were investigated in a temperature range of 50-300 °C. SEM and TEM investigations revealed that the grain size of AlN particles was less than 1 μm. In spite of sub-micron grain size, composites showed relatively high thermal conductivity (TC), 55-107 W/(m.K). The thermal expansion coefficient (CTE) of the composite produced with commercial Al source, which has the highest TC of 107 W/(m.K), was 6.5×10^?6 K^?1. Despite the high CTE of Al (23.6×10^?6 K^?1), composites revealed significantly low CTE through the formation of Si and AlN phases during the infiltration process.     

Mechanical spectroscopy of thermal stress relaxation at metal–ceramic interfaces in Aluminium-based composites

The micromechanisms of thermal stress relaxation in aluminum-based metal-matrix composites (MMCs) have been investigated by mechanical loss and dynamic shear modulus measurements during thermal cycling between 100 and 500 K. A transient mechanical loss maximum, which is absent in the monolithic material, appears during cooling. This damping maximum is strongly dependent on the measurement parameters: oscillation frequency, oscillation amplitude and cooling rate. In addition, it increases with the volumetric reinforcement content and decreases if the matrix strength is improved. The shear modulus evolution during thermal cycling shows that no detectable interfacial debonding occurs. Compared with alloyed MMCs, Al4N-based MMCs show the highest damping maximum simultaneously with a plateau in the elastic shear modulus. The mechanical loss maximum is attributed to dislocation generation and motion near the interfaces, resulting from the differential thermal contraction of matrix and reinforcement. A new model is proposed which describes this specific mechanical behavior of MMCs in terms of the development of microplastic zones in the matrix near the metal–ceramic interfaces.     

Infiltration kinetics of pressureless infiltration in SiCp／Al composites

The pressureless infiltration kinetics was investigated by plotting the infiltration distance as function of the infiltration time. The effects of key process parameters such as time, temperature, Mg content on the pressureless infiltration of silicon carbide particle compacts were studied and quantified. The preform with high volume fraction SiC was obtained by mixing SiC particles with bimodal size distribution, whose diameters are 5 and 50 btm, respectively. The results show that an incubation period exists before infiltration, the influence of temperature on the incubation time exceeds that of Mg content, infiltration rate increases with the increasing temperature and Mg content, infiltration rate decreases as Mg consumes. A model of macroscopical infiltration and microscopical infiltration of liquid alloy in porous SiC preform was proposed.     

Fracture resistance curve behavior of multilayered alumina–zirconia composites produced by centrifugation

The single-edge-V-notched-beam (SEVNB) testing method was used to measure the crack growth resistance (R-curve) behavior of multilayered alumina–zirconia composites. Crack initiation and extension from the V-notch tip were observed via in situ optical microscopy. The resulting R-curves were compared with an R-curve measured from a monolithic composite having a similar composition and a homogeneous microstructure, where the influence of layer–layer interfaces, gradient microstructures, and the direction of crack propagation on the resulting R-curves were observed. Additionally, the stress intensity factor for crack initiation from the V-notch tip was ∼0.2 MPa·m^1/2 higher than the stress intensity factor to further extend this crack.     

Phase transformations and phase equilibria in a Ti–37% Al–20% Mn alloy

The solid-state phase transformations and phase equilibria in a Ti–37 at% Al–20 at% Mn alloy have been investigated. The alloy was prepared by plasma-arc melting and the microstructures of high-temperature-annealed and water-quenched or furnace-cooled samples were studied. The results show that at 1235°C β-phase and (Mn,Al)₂Ti (Laves phase) are present whereas below 1000°C the phases (γ+α/α₂+(Mn,Al)₂Ti) are formed. A comparison between calculated equilibrium phase compositions and values measured by EDXA shows reasonable agreement for the β, γ and α/α₂ phases over a range of temperatures but agreement for the (Mn,Al)₂Ti phase is less good, particularly at the higher temperatures. The transformation kinetics in this system appear to be sluggish and true equilibrium does not appear to have been achieved in all samples annealed below 1145°C. DTA analysis was also undertaken and the heating thermogram obtained is interpreted with the aid of the calculated phase equilibria.     

Effect of rare earth additives on alumina fiber development in Ti-Al intermetallic matrix composites

Al2O3（1）/TiAl composites were synthesized by an exothermic reaction method using Ti, Al and TiO2 powders doped with Nb205 and La2O3. The effect of Nb205 and La2O3 additives on the growth and morphology of the fibers, the phases and microstructure of the composites were investigated by means of XRD and SEM. The result indicates that the in situ alumina fiber can be developed in Ti-Al matrix with the Ti/Al mole ratio of 1：2 1：7, and the addition of rare earth powders can improve the dispersion of the fibers in the matrix and increase the length-to-diameter ratio of the fibers.     

Wetting,spreading and joining in the alumina–zirconia–Inconel 738 system

A study is presented on the wetting and spreading behaviour of Ag–Cu–Ti on zirconia and alumina–zirconia ceramics and on the Inconel 738. The results are discussed in terms of thermodynamic and kinetics of spreading, and on the basis of the morphological analysis, including microhardness tests, of the interfacial layers, both in the wetting couples and in the different joints.     

Determination of the habit plane characteristics in the β–α′ phase transformation induced by stress in Ti–5Al–2Sn–4Zr–4Mo–2Cr–1Fe

The characteristics of the habit planes of the α″ orthorhombic martensite plates, induced by stress within a commercial sheet of Ti–5Al–2Sn–4Zr–4Mo–2Cr–1Fe have been investigated. First, the orientations of several grains in BCC phase which presented at least one α″ orthorhombic martensite plate, were determined by EBSD. The Miller indexes of habit planes within a parent grain were deduced from the intersection lines of the martensite plates on two perpendicular sides of the sample and from the orientation of the parent grain. These data are compared to the characteristics of the habit planes, calculated from the phenomenological theory of the martensitic transformation proposed by Wechsler, Liebermann and Read. The different results and their dispersions are analysed in this contribution.     

Interfacial microstructure of 20% SO2/Al matrix composites reinforced with in situ formation ceramic phases

Al matrix composites reinforced with in situ ceramic phases by adding 20% SiO2 were fabricated by powder metallurgy process.The interfacial microstructure of the composites was studied by means of SEM and HREM.It was shown that the ceramic phases mainly composed of spinel MgAl2O4 are formed in situ in the original SiO2 particle and the size of small MgAl2O4 crystallites is about dozens of nanometers,which can adjacent to Al and Si.MgO could not found in original SiO2 particle but a little in matrix and may exist with Si,Mg2Si and Al.Si is mostly distributed in matrix and forms some segregation zones.The size of Mg2Si is about 50-100nm and can usually be seen in the matrix.     

Anomalies of the plastic yield stress in the intermetallic compound Fe–30 at.% Al

The plastic properties of Fe–30 at.% Al were investigated in compression, cyclic tension/compression and shear tests between room temperature and 870 K. At elevated temperatures in all tests a positive temperature dependence of the yield stress was observed. At room temperature an asymmetry of the flow stress with respect to the deformation direction (tension or compression) was found, which disappeared for temperatures T⩾570 K. The results are discussed on the basis of the present state of knowledge about the decoupling of D0₃-superdislocations at elevated temperatures and the deformation behaviour of b.c.c. metals at low temperatures.     

Peculiarities of ball-milling induced crystalline–amorphous transformation in Cu–Zr–Al–Ni–Ti alloys

An amorphization process in (Cu₄₉Zr_45−xAl_6+x)_100−y−zNi_yTi_z (x = 1, y, z = 0; 5; 10) induced by ball-milling is reported in the present work. The aim was investigation of the effect of Ni and Ti addition to Cu₄₉Zr₄₅Al₆ and Cu₄₉Zr₄₄Al₇ based alloys as well as type of initial phases on the amorphization processes. Also the milling time sufficient for obtaining fully amorphous state was determined. The entire milling process lasted 25 h. Drastic structural changes were observed in each alloy after first 5 h of milling. In most cases, after 15 h of milling the powders had fully amorphous structure according to XRD except for those ones, where TEM revealed a few nanosized crystalline particles in the amorphous matrix. In (Cu₄₉Zr₄₅Al₆)₈₀Ni₁₀Ti₁₀ alloy the amorphization process took place after 12 h of milling and the amorphous state was stable up to 25 h of milling. In the case of (Cu₄₉Zr₄₄Al₇)₈₀Ni₁₀Ti₁₀ alloy the powders have fully amorphous structure between 12 h and 15 h of milling.     

Study on creep-fatigue damage evaluation for advanced 9%–12% chromium steels under stress controlled cycling

Creep-fatigue interaction is one of the main damage mechanisms in high temperature plants and their components. Assessment of creep-fatigue properties is of practical importance for design and operation of high temperature components. However, the standard evaluation techniques, i.e. time fraction rule and ductility exhaustion one have limitations in accounting for the effects of control mode on the cyclic deformations. It was found that conventional linear cumulative damage rule failed in accurately evaluating the creep-fatigue life under stress controlled condition. The calculated creep damages by time fraction rule were excessively high, which led to overly conservative prediction of failure lives. In the present study, it was suggested that such over estimation of creep damage was mainly caused by anelastic strain upon stress loading. For precise assessment under conditions of stress control, a modified creep damage model accounting for the effect of anelastic creep was proposed. The assessments of creep fatigue data under stress controlled condition were performed with the new approach developed in this paper for a rotor material and a boiler material used in ultra supercritical power plants. It was shown that a more moderate amount of creep damage was obtained by the new model, which gave better predictions of failure life.     

Thermally induced surface roughness in austenitic–ferritic duplex stainless steels

Austenitic–ferritic duplex steels hot forged to rods demonstrate a complex deformational behaviour even in the absence of mechanical loads: purely thermal cycling in the temperature interval from 20 °C to 900 °C can cause either accumulation of inelastic strains (thermal ratchetting) or plastic shakedown, depending on microstructure morphology and thermomechanical properties of the constituting phases. The structural macroscopic response of entire specimens to cyclic thermal loading is investigated by dilatometry experiments. The influence of traction-free external surfaces on the microscopic deformation of ferrite and austenite is examined by measuring the surface roughness evolution by three-dimensional profile scans. Besides an increase of the roughness parameters due to thermal cycling, the measured surface profiles also reveal a strong anisotropy linked to microstructural orientation. Spatial surface roughness parameters based on the autocorrelation function allow a quantitative correlation between roughness and microstructure. Measurements are compared with micromechanical finite-element predictions.     

Deformation induced phase rearrangement in near eutectic tin–lead alloy

Deformation induced microstructural changes in the near eutectic tin (Sn)–lead (Pb) alloy are investigated. This study is motivated by the thermomechanical reliability of solder materials used in microelectronic packages, with the primary objective of gaining fundamental understandings of pure mechanical effects without the influence of diffusion related phenomena. Bulk specimens, having an initial microstructure of equiaxed Pb-rich particles within the Sn-rich matrix, are subject to relatively fast deformations of tension, compression and bending. Microhardness indentation is employed to characterize the sliding behavior along grain boundaries and interphase boundaries. It is observed that phase rearrangement induced by boundary sliding commences when the overall strain is still relatively small, giving rise to the gradual development of aligned regions free of the Pb-rich phase. These Pb-free zones, or equivalently Sn grain clusters, are aligned in the direction of macroscopic normal stresses. A micromechanical model, supported by further experimental evidences, is proposed to explain the observed microstructural changes. This study also illustrates the possibility of mechanically induced phase coarsening in Sn–Pb solder alloys.     

Tensile creep of directionally solidified NiAl–9Mo in situ composites

Well-aligned Mo fiber-reinforced NiAl in situ composites were produced by specially controlled directional solidification. The creep behavior parallel to the growth direction was studied in static tensile tests at temperatures between 900 °C and 1200 °C. A steady-state creep rate of 10^?6 s^?1 was measured at 1100 °C under an initial applied tensile stress of 150 MPa. Compared to binary NiAl and previously investigated NiAl–Mo eutectics with irregularly oriented Mo fibers, this value demonstrates a remarkably improved creep resistance in NiAl–Mo with well-aligned unidirectional Mo fibers. A high-resolution transmission electron microscope investigation of the NiAl/Mo interface revealed a clean semi-coherent boundary between NiAl and Mo, which enabled an effective load transfer from the NiAl matrix to the Mo fibers, and thus leads to the remarkably increased creep strength. The stress exponent, n, was found to be between 3.5 and 5, dependent on temperature. The activation energy for creep, Q_c, was measured to be 291 ± 19 kJ mol^–1, which is close to the value for self-diffusion in binary NiAl. Transmission electron microscopy observations substantiated that creep occurred by dislocation climb in the NiAl matrix. The Mo fiber was found to behave in a quasi-rigid manner during creep. A creep model for fiber-reinforced metal matrix composites was applied for an in-depth understanding of the mechanical behavior of the individual components and their contribution to the creep strength of the composite.     

Reactive sintering of TiAl–Ti5Si3 in situ composites

TiAl with between 0 and 20 vol%Ti₅Si₃ was produced by reactive sintering (700 °C for 15 min in vacuum) of cold pressed compacts of elemental Ti, Al and Si powder. The results show that adding Si to Ti and Al reduces the swelling associated with reactive sintering of TiAl, as composites containing more than 5 vol%Ti₅Si₃ densified during reactive sintering. However, composites containing more than 10 vol%Ti₅Si₃ did not retain their shape and the TiAl+20 vol%Ti₅Si₃ composite completely melted during the sintering process. A thermodynamic analysis indicated that the simultaneous formation of TiAl and Ti₅Si₃ increases the adiabatic flame temperature during the reaction between the powders. In fact, the analysis predicted that the maximum temperature of the reaction associated with the formation TiAl+20 vol%Ti₅Si₃ should exceed the melting point of TiAl, and this was observed experimentally. Differential thermal analysis (DTA) revealed that an Al–Si eutectic reaction occurred in mixtures of Ti, Al and Si powders prior to the formation of the TiAl and Ti₅Si₃ phases. There was no such pre-reaction formation of a eutectic liquid in Ti and Al powder mixtures. The formation of the pre-reaction liquid and the increase in adiabatic flame temperature resulted in the melting that occurred and the enhanced densification (minimization of swelling) during reactive sintering of the in situ composites.