Fracture toughness and fatigue crack growth in rapidly quenched Nb-Cr-Ti In situ composites

In situ composites based on the Nb-Cr-Ti ternary system were processed by rapid solidification in order to reduce the size of the reinforcing intermetallic phase. Two-phase microstructures with small Cr₂Nb particles in a Nb(Cr, Ti) solid solution alloy matrix were produced for several compositions that previous work showed to produce high toughness composites in cast materials. The fracture and fatigue behaviors of these composites were characterized at ambient temperature. The results indicate that the fracture resistance increases with a decreasing volume of Cr₂Nb particles. Fracture toughnesses of the rapidly solidified materials with their smaller particle sizes were lower than for conventionally processed composites with larger particles of the intermetallic compound. The fatigue crack growth rate curves exhibit steep slopes and a low critical stress intensity factor at fracture. The lack of fracture and fatigue resistance is attributed to the contiguity of the intermetallic particles and the absence of plastic flow in the Nb solid solution matrix. The matrix alloy appears to be embrittled by (1) the rapid solidification processing that prevented plastic relaxation of residual stresses, (2) a high oxygen content, and (3) the constraint caused by the hard Cr₂Nb particles.     

Fatigue crack growth through alloyed niobium, Nb-Cr2Nb, and Nb-Nb5Si3 in situ composites

Fatigue cracks were grown through several niobium-based materials. For Nb-Cr-Ti composition materials, the single-phase alloy represented the matrix of two in situ composites with about 22 and 38 vol pct Cr₂Nb. Grain boundaries were coated with intermetallic in the lower-volume fraction material, while the 38 vol pct Cr₂Nb composite consisted of mainly spherical, dispersed intermetallic. The Nb-10Si composite was composed of about 28 vol pct primary Nb₅Si₃, with most of the matrix alloy in “fiberlike” shapes due to extrusion. Crack growth rates through the composites were generally faster than for unalloyed Nb, roughly in proportion to the volume fraction of intermetallic, although differences in microstructure make this comparison difficult. The presence of intermetallic greatly alters deformation of material near the crack tip. Particles of Cr₂Nb were broken during the crack growth process, leading to increased crack growth rates. These results suggest microstructural modifications that could be expected to enhance fatigue crack growth resistance.     

The fracture toughness of niobium-based,in situ composites

The fracture resistance of Nb-Cr-Ti alloys orin situ composites of three different compositions, Cr₂Nb, and a Nb-10Siin situ composite was studied at ambient temperature. The crack-tip deformation and fracture behaviors were characterized using near-tip measurement techniques and fractographic analyses. The relevant fracture and toughening mechanisms were identified and related to the microstructure. Despite fracture by a combination of cleavage and slip band decohesion, the Nb solid-solution alloy exhibited a resistance-curve behavior with a relatively high toughness and local ductility. The source of toughness was modeled and explained in terms of a cracking process that involved alternate slip band decohesion and cleavage. Thein situ composites, on the other hand, exhibited cleavage fracture but considerably lower toughness with little or no resistance-curve behaviors. The difference in the fracture behavior appears to arise from two factors: (1) the presence of a high constraint in the Nb solid-solution matrix in thein situ composites, and (2) the lack of plastic flow associated with cleavage of the constrained Nb solid-solution matrix.     

Strength and ductility of heavily drawn bundled Cu-Nb filamentary microcomposite wires with various Nb contents

The strength and ductility of heavily drawn bundled Cu-Nb filamentary microcomposite was examined as a function of Nb content. In order to predict the variation of the yield strength (YS) with Nb content, the interfilamentary spacing was calculated as a function of Nb content based on the assumption that Nb filaments are distributed regularly along the sides of a triangular unit cell in the transverse section. The yield stress can be described as the sum of the substructure strengthening component due to elongated grains, subgrains and/or cells, the phase boundary strengthening term associated with the Hall-Petch type interaction between dislocations and phase boundaries, and the precipitate strengthening component. The contributions from phase boundary strengthening, σ _PB (Cu-Nb), and precipitate strengthening, σ _ppt, increase with increasing Nb content. However, the contribution from substructure strengthening, σ _sub (Cu-Nb), decreases with increasing Nb content since more grain or subgrain boundaries are absorbed at Cu/Nb phase boundaries with increasing Nb content. The good agreement between the prediction and the experimental data suggests that the increase of the strength in Cu-Nb filamentary microcomposite with increasing Nb content results mostly from an increasing volume fraction of Nb filaments, which act as barriers to plastic flow.     

Effect of Nb Content on Mechanical Behavior and Structural Properties of W/(Zr55Cu30Al10Ni5)100−x Nb x Composite

In the present study, (Zr₅₅Cu₃₀Al₁₀Ni₅)_100−x Nb_(x=0,1,2,3) bulk metallic glass matrix/tungsten wire composites were fabricated by infiltration process. Structural studies were investigated by scanning electron microscopy and X-ray diffraction method. Also, mechanical behaviors of the materials were analyzed using quasi-static compressive tests. Results indicated that the best mechanical properties i.e., 2105 MPa compressive ultimate strength and 28 pct plastic strain before failure, were achieved in the composite sample with X = 2. It was also found that adding Nb to the matrix modified interface structure in W fiber/(Zr₅₅Cu₃₀Al₁₀Ni₅)₉₈Nb₂ since the stable diffusion band formation acts as a functionally graded layer. Finally, the observation of multiple shear bands formation in the matrix could confirm the excellent plastic deformation behavior of the composite.

     

Interfacial modification and impact properties of Nb/MoSi2 laminate composites by the addition of ZrO2, NbSi2, and SiC particles

In this study, the results of a thermodynamical estimation indicate that the impact properties of Nb/MoSi₂ laminate composites can be improved by suppressing the interfacial reaction. The effects of addition of SiC, NbSi₂, and ZrO₂ on the impact properties and interfacial reaction of Nb/MoSi₂ laminate composites are also examined. Laminate composites, which comprise alternate layers of matrix mixture and Nb foil, were fabricated by the hot press process. Addition of ZrO₂ particle is clearly demonstrated to increase both the impact value and the sintered density of Nb/MoSi₂ laminate composites. The thickness of the interfacial reaction layer in Nb/MoSi₂ laminate composites dramatically decreases with increasing volume fraction of ZrO₂ particle. The suppression of the interfacial reaction is caused by the formation of ZrSiO₄ in the MoSi₂-ZrO₂ matrix mixture. In addition, it appears that the reduction of the reaction layer promotes the plastic deformation of Nb foil and the interfacial delamination in laminate composites.     

Comparison of the strengths and microstructures of Cu-20% Ta and Cu-20% Nb in situ composites

Comparison of the strengths of wire drawn Cu-20% Ta and Cu-20% Nb in situ composites showed that Ta filaments were more effective obstacles than Nb filaments to plastic deformation. Strengthening in both Cu-Ta and Cu-Nb composites correlated with Ta and Nb filament spacing, respectively, according to a Hall-Petch relationship with the Hall-Petch slope being about 20% greater in the Cu-Ta composites. Analysis of strengthening in these composites showed that the degree of strengthening was most affected by increasing draw ratio, followed by decreasing initial dendrite size in the Cu-Ta and Cu-Nb arc melted castings and increasing elastic modulus of the composite.     

High-Performance dispersion-strengthened Cu-8 Cr-4 Nb alloy

A new high-temperature-strength, high-conductivity Cu-Cr-Nb alloy with a Cr:Nb ratio of 2:1 was developed to achieve improved performance and durability. The Cu-8 Cr-4 Nb alloy studied has demonstrated remarkable thermal and microstructural stability after long exposures at temperatures up to 0.98 T_m. This stability was mainly attributed to the slow coarsening kinetics of the Cr₂Nb precipitates present in the alloy. At all temperatures, the microstructure consists of a bimodal and sometimes trimodal distribution of strengthening Cr₂Nb precipitates, depending on precipitation condition, i.e., from liquid or solid solution, and cooling rates. These precipitates remain in the same size range, i.e., large precipitates of approximately 1 μm and small precipitates less than 300 nm, and effectively pin the grain boundaries, thus retaining a fine grain size of 2.7 μm after 100 hours at 1323 K. This grain-boundary pinning and sluggish coarsening of Cr₂Nb particles explain the retention of good mechanical properties after prolonged holding at very high temperatures, e.g., twothirds of the original yield strength after aging for 100 hours at 1273 K. The main sources of strengthening are the Hall-Petch and Orowan mechanisms due mostly to small particles. The coarsening kinetics of the large precipitates are most likely governed by grain-boundary diffusion and, to a lesser extent, volume diffusion mechanisms.     

Application of Finite Element Model and Artificial Neural Network in Characterization of Al Matrix Nanocomposites Using Various Training Algorithms

In this research, the effect of the volume fraction of the alumina nanoparticles on the mechanical properties of the Al-Si matrix composites was studied. The yield strength and tensile strength increase, but the elongation decreases with the increase in the volume fraction of the particles, indicating that increasing the volume fraction of the Al₂O₃ particles can improve the strength but degrade the plasticity of the composites. The mechanical properties modeling was carried out using an artificial neural network (ANN) and finite element model (FEM). The neural network was trained using different training algorithms, hidden layers, and neuron numbers in hidden layers in order to check the system accuracy of each training algorithm at the end of learning.     

Microstructural and fracture characterization of Nb-Cr-Ti mechanically alloyed materials

Three materials containing Nb, Cr, and Ti were fabricated by consolidating powders made by mechanical alloying. The Nb/Ti ratio was maintained at about 1.3 and Cr was increased to form the intermetallic Cr₂Nb. X-ray diffraction, metallography, and transmission electron microscopy were used to thoroughly characterize the microstructure and substructure of the materials. Fatigue and fracture toughness properties were also evaluated at ambient temperature. The alloyed powders contained only small amounts of intermetallic, but during the consolidation heat treatment, two of the materials precipitated large volume fractions of Cr₂Nb. In the third material, Cr₂Nb was precipitated by heat treatment, although this was not expected from the composition based on the Nb-Cr-Ti phase diagram. Maximum fracture toughness of the composutes was ≈ 11 MPa √m. The low fracture toughness was attributed to the high plastic constraint of matrix deformation by the Cr₂Nb and compositional change in the matrix.     

Influence of Nb and Cu Elements on the Intermetallic Particles Morphology in Ti/Al Multilayer Composite Processed Through Hot Pressing and Rolling

Ti–Al–Nb composites were produced by solid state diffusion bonding through hot pressing and rolling followed by annealing at 700 °C for 0.5, 1, 1.5 and 2 h. The morphologies of TiAl₃ intermetallics were investigated by Scanning Electron Microscopy combined with Energy-dispersive X-ray spectroscopy. Titanium tri-aluminide (TiAl₃) particles with blocky morphology were dispersed into Aluminum matrix. In the presence of niobium and copper, TiAl₃ particles were produced in different sizes and morphologies. The presence of Nb in the composite led to the formation of irregular angular morphology, while the copper resulted in cubic morphology of the intermetallic particles. The EDS results indicated that TiAl₃, (Ti, Nb)Al₃ and (Ti, Nb, Cu)Al₃ intermetallic compounds appeared near Ti zone, Nb Zone and in the presence of Cu, respectively.     

Transverse strength of metal matrix composites reinforced with strongly bonded continuous fibers in regular arrangements

The composite limit flow stress for transverse loading of metal matrix composites reinforced with a regular array of uniform continuous fibers is calculated using the finite element method. The effects of volume fraction and matrix work hardening are investigated for fibers of circular cross section distributed in both sqyare and hexagonal arrangements. The hexagonal arrangement is seen to behave isotropically with respect to the limit stress, whereas the square arrangement of fibers results in a composite which is much stronger when loaded in the direction of nearest neighbors and weak when loaded at 45° to this direction. The interference of fibers with flow planes is seen to play an important role in the strengthening mechanism. The influence of matrix hardening as a strengthening mechanism in these composites increases with volume fraction due to increasing fiber interaction. The results for a power law hardening matrix are also applicable to the steady state creep for these composites. The influence of volume fraction on failure parameters in these composites is addressed. Large increases in the maximum values of hydrostatic tension, equivalent plastic stain, and tensile stress normal to the fiber-matrix interface are seen to accompany large increases in composite strength.     

Toughening mechanisms in ductile niobium-reinforced niobium aluminide (Nb/Nb3Al) in situ composites

Anin situ study has been performed in the scanning electron microscope (SEM) on a niobium ductilephase-toughened niobium aluminide (Nb/Nb₃Al) intermetallic composite to examine the crack-growth resistance-curve (R-curve) behavior over very small initial crack extensions, in particular over the first ~500 μm of quasi-static crack growth, from a fatigue precrack. The rationale behind this work was to evaluate the role of toughening mechanisms, specifically from crack bridging, in the immediate vicinity of the crack tip and to define the size and nature of bridging zones. Although conventional test methods, where crack advance is monitored typically over dimensions of millimeters using compliance or similar techniques, do not show rising R-curve behavior in this material,in situ microscopic observations reveal that bridging zones resulting from both uncracked Nb₃Al ligaments and intact Nb particles do exist, but primarily within ~300 to 400 μm of the crack tip. Accordingly, rising R-curve behavior in the form of an increase in fracture resistance with crack growth is observed for crack extensions of this magnitude; there is very little increase in toughness for crack extensions beyond these dimensions. Ductile-phase toughening induced by the addition of Nb particles, which enhances the toughness of Nb₃Al from ~1 to 6 MPa√m, can thus be attributed to crack-tip shielding from nonplanar matrix and coplanar particle bridging effects over dimensions of a few hundred microns in the crack wake. formerly Research Student, Department of Materials Science and Mineral Engineering, University of California-Berkeley     

A study of void nucleation,growth, and coalescence in spheroidized 1518 steel

The ductile fracture of a spheroidized 1518 steel has been investigated using three types of tensile specimens — smooth tensile, notched tensile, and plane-strain tensile. It was found that void nucleation has two different modes (Type I and Type II) depending on local conditions, the most important of which are the size, shape, and distribution of the particles. By identifying the low-strain-range nucleation behavior (Type I), it was possible to determine the value of plastic strain, ε_N, after which void nucleation at average-sized carbide particles (Type II) begins; ε_N is 0.45 for the smooth tensile case, 0.30 for the notched, and 0.25 for the plane strain. The critical stress for Type II void nucleation, σ_c, is of the order of 1200 MPa. Void growth depends on the macroscopic stress-strain state: longitudinal growth is given by a linear function of applied plastic strain, ε_p, whereas lateral growth shows a linear dependence on the triaxial stress, σ_T. When the local value ofV _f reaches a critical volume fraction of voids (V _f ^cri = 5 ± 0.5 pct), void coalescence occurs in a catastrophic manner, leading to final separation within a highly localized zone. The stress concentration caused by the notched tensile specimen geometry and the localized mode of plastic flow caused by the constraint of the plane-strain state in a Clausing-type specimen were found to affect the substeps of void nucleation, growth, and coalescence. Formerly Graduate Research Assistant, Division of Engineering, Brown University. Formerly Professor, Division of Engineering, Brown University, Providence, RI.     

Microstructure and properties of In situ Al/TiB2 composite fabricated by in-melt reaction method

A novel in situ reaction process-in-melt reaction method was developed. TiB₂ particles form in situ through the reaction of TiO₂, H₃BO₃, and Na₃AlF₆ in an aluminum alloy melt. The results showed that the in situ TiB₂ particles formed were spherical in shape and had an average diameter of about 0.93 μm. Moreover, the distribution of TiB₂ particles in the matrix was uniform. The interface between the TiB₂ particles and the matrix showed good cohesion. The tensile strength and the yield strength of the composite increase with increasing TiB₂ content. When TiB₂ particle content in the matrix was 10 vol pct, the tensile strength, yield strength, and elongation of Al-4.5Cu/TiB₂ composite were 417 MPa, 317 MPa, and 3.3 pct, respectively.     

Strain distribution effects on the low-cycle fatigue behavior of Fe-C-Mo steels

The strain distributions obtained from monotonic finite element method (FEM) calculations have been employed to model the low-cycle fatigue (LCF) behavior of Fe-C-Mo dual-phase steels. The microstructures considered have a continuous ferrite matrix (with Mo₂C precipitates) surrounding martensite packets. Two microstructural parameters have been controlled: (1) the volume fraction of martensite and (2) the strength of the ferrite matrix. The FEM approximations show that highly strained regions dominate LCF lifetimes. The experimentally observed reductions in plastic strain life for increasing martensite volume fractions are described usingM _ε , the strain magnification factor, which is obtained from the FEM analyses. Strengthening the ferrite matrix or reducing the volume fraction of martensite reducesM _ε . The cyclic softening observed is qualitatively correlated with FEM predictions of increasing plastic strain in the martensite as the ferrite strength increases. The overall cyclic hardening-softening behavior results from the combination of ferritic hardening combined with martensitic softening. Formerly Graduate Student, Department of Materials Science, University of Virginia.     

Damage due to fracture of brittle reinforcements in a ductile matrix

A micromechanical model is developed for brittle particle reinforced metal matrix composites sustaining damage. A composite with uniformly distributed damage is modelled by a three-phase damage cell consisting of a cracked particle in a cylindrical matrix cell embedded in an undamaged composite cylinder. The fraction of broken particles to all particles is taken as the ratio of the broken-particle/matrix cell volume to the whole damage cell volume. Systematic analysis is carried out for aligned spherical and cylindrical particles in an elastic-perfectly plastic matrix subject to tensile loading normal to the plane of particle cracks. The influence of damage evolution paths on the composite stress-strain behavior is investigated. Results are given for the effects of damaged particle percentage, total particle volume fraction and particle shape on the overall composite limit flow behavior. Significant reduction in composite limit flow stress may occur if most of the particles are broken. For composite with spherical reinforcement, the reduction is found to be linearly dependent on the percentage of damaged particles.     

Design of multiscalar metallic multilayer composites for high strength,high toughness,and low CTE mismatch

We propose a new class of multilayer composites that consists of alternating tough and strong layers. Both the tough and the strong layers are metallic, effectively reducing the coefficient of thermal expansion (CTE) mismatch problem that often plagues metal-ceramic composites. The high-strength layers are themselves very fine-scale metallic multilayer composites. The high strengths result from Orowan strengthening of these very fine-scale layers. We present detailed analyses of the flow stress, toughness, and thermal stability of these multiscalar metallic multi-layer composites (M³C) as a guide for microstructural optimization. The dominant term in the flow stress is proportional to the volume fraction of the strong layers and scales inversely with thickness of the very fine-scale layers that make up the strong layer. The toughness is dominated by the plastic flow of the tough layers and is proportional to the volume fraction and flow stress of the tough layers, as modified by plastic constraint. The thermal stability of M³Cs is discussed in the context of solubility, length scales, and interdiffusivity of the two metals. Preliminary results suggest that M³Cs do exhibit an unusual combination of high toughness and strength.     

Ductile-phase toughening and Fatigue-Crack Growth in Nb-Reinforced Molybdenum Disilicide Intermetallic Composites

A study has been made of the role of ductile-phase toughening on the ambient temperature fracture toughness and fatigue-crack propagation behavior of a molybdenum disilicide intermetallicmatrix composite reinforced with 20 vol pct niobium spheres. Using disk-shaped compact DC(T) samples, only moderate improvements (∼24 pct) in fracture toughnessK _lcvalues were found for the composite compared to the unreinforced MoSi₂ matrix material. Moreover, (cyclic) fatigue- crack propagation was seen at stress intensities as low as 75 to 90 pct ofK _Ic, with growth rates displaying a high dependency (∼14) on the applied stress-intensity range. The lack of significant toughening due to the incorporation of ductile Nb particles is associated with an absence of crack/particle interactions. This is attributed to the formation of a weak reaction-layer interface and elastic mismatch stresses at the crack tip between the Nb and MoSi₂, both factors which favor interfacial debonding; moreover, the spherical morphology of Nb phase stabilizes cracking around the particle. Results suggest that increasing the aspect ratio of the distributed Nb rein- forcement phase with attendant interfacial debonding and eliminating possible Nb-phase em- brittlement due to interstitial impurity contamination are critical factors for the successful development of tougher Nb/MoSi₂ structural composites. Formerly with McDonnell Formerly with McDonnell