Adiabatic shear failure of high reinforcement content aluminum matrix composites

Dynamic failure behaviors of high reinforcement content TiB₂/Al composites were experimentally investigated using split Hopkinson pressure bar (SHPB). The TiB₂/Al composites showed high flow stresses and good plastic deformation ability at high strain rates. Adiabatic temperature rise decreased the flow stresses of TiB₂/Al composites, which was verified by the prediction of Johnson–Cook model. While the predictions by Cowper–Symonds model exhibited obvious strain hardening characteristic, the values of which were much higher than those of the Johnson–Cook model and the experimental. The composites were failed macroscopically in brittle fracture and some phase transformation bands were found on the shearing surfaces. The dynamic failure behavior of TiB₂/Al composites was predominated by the formation of adiabatic shear bands.     

Fabrication of TiC and TiB<Subscript>2</Subscript> locally reinforced steel matrix composites using a Fe–Ti–B<Subscript>4</Subscript>C–C system by an SHS-casting route

Steel matrix composites locally reinforced by in situ TiC and TiB₂ particulates were successfully fabricated using self-propagating high-temperature synthesis (SHS) in a Fe–Ti–B₄C–C system during casting. The locally reinforced steel matrix composites consist of three distinct regions: (i) a TiC and TiB₂ particulate-reinforced region, (ii) a transition region, and (iii) a steel matrix region. The TiC and TiB₂ particulates in the locally reinforced regions display a relatively uniform distribution, and their sizes decrease with the increase in Fe content from 10 wt.% to 40 wt.%. The wear resistance of the locally reinforced region of the steel matrix composites is much higher than that of the unreinforced steel matrix.     

Effect of tungsten addition on the microstructure and tensile properties of in situ TiB2/Fe composite produced by vacuum induction melting

In situ TiB₂ particulate reinforced Fe-based composite was produced by vacuum induction melting (VIM) technique. The effect of tungsten element on the microstructure and tensile properties of the composite was investigated. The results show that the tungsten can dissolve into the TiB₂ particulates and the segregation of TiB₂ is reduced. Meanwhile, with the addition of 3.0 wt.% tungsten, the composite is solid strengthened and an optimal tensile property can be obtained. The yield strength (YS), ultimate tensile strength (UTS) and elongation to rupture (Er) of the composite reach as high as 360 MPa, 690 MPa and 18.5%, respectively. The fracture morphologies also indicate that the addition of 3.0 wt.% tungsten results in the increase of plastic fracture.     

Cyclic deformation and lattice strain distribution of high Nb containing TiAl alloy

Low cycle fatigue of lamellar TiAl with 8.5 at.-%Nb was studied with a total strain amplitude of 0.28% at three temperatures: room temperature, 750°C and 900°C. At room temperature, the material exhibited cyclic hardening and the fracture mode was mainly interlamellar. At 750°C and 900°C, the material showed cyclic softening and the fracture mode was translamellar. The lattice strain in γ phase was almost tensile and larger tensile lattice strain in γ phase seems detrimental. Besides, the opposite direction of {201}_γ and {100}_α2 lead to crack propagation along α₂/γ interfaces. B2/β_o phase always suffered compressive lattice strain in the tests. The destruction of lamellar microstructure was the reason for colony refinement at 750°C and 900°C.     

Effects of microstructure on the strain-controlled fatigue failure behavior of an aluminium-alloy/ceramic-particle composite

A study has been made to understand the cyclic fatigue and cyclic fracture characteristics of a cast aluminium alloy metal matrix discontinuously reinforced with particulate silicon carbide. The Al/SiC_p composite was strained to failure over a range of strain amplitudes giving lives of less than 10⁴ cycles to failure. The specimens were cycled by using tension-compression loading under total strain control. In the as-cast condition, the aluminum-alloy/ceramic composite displayed combinations of cyclic hardening and softening to failure at higher cyclic-strain amplitudes, and progressive softening to failure at low cyclic-strain amplitudes. The spray-atomized and deposited composite exhibited softening to failure at the higher cyclic-strain amplitudes and combinations of softening and hardening behavior at the lower strain amplitudes. The observed hardening and softening behavior is a mechanical effect and attributed to concurrent and competing influences of interactions between cyclic deformation and composite microstructure during cyclic straining. The processed microstructure exhibited better cyclic ductility and cyclic-strain resistance than the as-cast composite microstructure. The cyclic fatigue behavior of the alloy is briefly interpreted in the light of composite microstructural effects, plastic strain amplitude and concomitant response stress.     

Superplastic behavior of Zn–Al eutectoid alloy with 2?% Cu

The effects of deformation temperature and strain rate on the superplastic behavior of the Zn–21Al–2Cu alloy (Zinalco alloy) were investigated by uniaxial tensile tests. Results were compared with those of the Zn–22Al eutectoid alloy without Cu. It was observed that additions of 2 % Cu leads to a decrease of the maximum strain attainable from 2600 % to 1000 %. The maximum strain in Zinalco alloy is obtained at lower strain rates. The presence of Cu increases the values of flow stress up to 600 % compared with those reported in the Zn-22Al alloy. Grain size sensitivity (p), true activation energy (Q _t ), and constant A of the constitutive equation were not affected by presence of Cu unlike the stress exponent (n) which increased from 2.5 to 3.9. The main effect of Cu was to decrease the plastic flow stability of the Zn–22Al alloy. The results indicate that presence of Cu in the Zinalco alloy causes a hardening effect at low strain rates leading to a decrease in the strain rate sensitivity which promotes the formation and growth of sharp necks. Microstructural characterization suggests that the large deformations at necking could possibly be due to the substantial elongation capability of the Zn-rich phase (η).     

Effect of in situ synthesized TiB2 on the reaction between B4C and Al in a vacuum infiltrated B4C–TiB2–Al composite

The in situ reaction equation of B₄C and TiO₂ was identified using thermodynamic calculations and XRD analysis. The optimum presintering process was determined according to investigating the effect of presintering temperature on the flexural strength and porosity of the porous B₄C–TiB₂ preform. The effect of in situ synthesized TiB₂ on the reaction products and initial reaction temperature of B₄C and Al was discussed based on DSC and XRD analysis. The results showed that the in situ synthesized TiB₂ could effectively improve the mechanical properties of the B₄C–TiB₂–Al composite, elevate the initial reaction temperature of B₄C and Al, change the reaction products, and moderate the reaction of B₄C and Al. The mechanism of reaction between B₄C and Al was discussed.     

Cyclic Behavior and Damage Analysis of Brass under Cyclic Torsional Loading

Fatigue behavior of brass was studied at a constant deformation rate of 5.6 × 10⁻³ s⁻¹ to understand the cyclic behavior and fatigue life under cyclic torsional deformation. Strains were in the range of 0.35 to 4.2%. In the as-drawn condition, it was found that the cyclic hardening/softening behavior strongly depends on the strain amplitude. For low strain amplitude, cyclic saturation occurred after an initial cyclic hardening stage, but for high strain amplitude, saturation could not be reached. Cyclic stress-strain (CSS) curve showed the presence of three distinct regions with a short quasi-plateau region in the intermediate amplitude range. Quantitative fatigue damage was assessed by microscopic observations of surface cracks.     

Fabrication of (TiB<Subscript>2</Subscript> − TiC)<Subscript>p</Subscript>/AZ91 magnesium matrix hybrid composite

Magnesium MMCs reinforced with TiB₂−TiC particulates were fabricated successfully via a master alloy route using a low cost Al-Ti-B₄C system as starting material system. Microstructural characterization of the (TiB₂−TiC)/AZ91 composite shows relatively uniform distribution of TiB₂ and TiC particulates in the matrix material. Moreover, the results show that the hardness and wear resistance of the composites are higher than those of the unreinforced AZ91 alloy.     

Room-temperature low-cycle fatigue behaviour of Ti-27Al-15Nb alloy

Low-cycle fatigue (LCF) behaviour of the alloy Ti-27Al-15Nb, in (α ₂+B2) heat-treated condition was studied in total axial strain control mode at different total strain amplitudes (Δɛ ₁/2) from ±0·65 to ±1·0% and room temperature. While there was little hardening of the material at the lowest strain amplitude (Δɛ _t/2: ±0·65%), pronounced hardening was observed at the higher strain amplitudes (Δɛ _t/2⩾0·83%). The cyclic stress, at high strain amplitudes, continuously increased from the beginning till fracture of the specimens. The LCF resistance of the material was found to be low and this was due to its poor ductility at room temperature. Dual slope was observed in the Coffin-Manson plot, with less slope of the upper segment than that of the lower one, as observed in several alloys. The fracture behaviour pointed to brittleness and faceted features were observed.     

The influence of Al2O3 particulate reinforcement on cyclic stress response and fracture behavior of 6061 aluminum alloy

The cyclic stress response characteristics and cyclic fracture behavior of aluminum alloy 6061 discontinuously reinforced with particulates of Al₂O₃ are presented and discussed. The 6061/Al₂O₃ composite specimens and the unreinforced 6061 aluminum alloy were cyclically deformed using tension-compression loading under constant total strain amplitude control. Both the composite and the unreinforced alloy exhibited softening to failure from the onset of cyclic deformation. The degree of softening was observed to increase at the elevated test temperature for both the composite and the unreinforced counterpart. The intrisic micromechanisms controlling the stress response characteristics during fully-reversed cyclic straining are highlighted and rationale for the observed behavior is discussed. The cyclic fracture behavior of the composite is discussed in terms of the competing influences of intrinsic microstructural effects, deformation characteristics arising from a combination of mechanical and microstructural contributions, cyclic stress response, and test temperature.     

Enhanced corrosion resistance of discontinuous anodic film on in situ TiB<Subscript>2p</Subscript>/A356 composite by cerium electrolysis treatment

The agglomerates of TiB₂ particulates and Si phases badly break the continuity of anodized film of in situ TiB_2p/A356 composite, which will restrict the improvement of corrosion resistance. In this study, cerium conversion coatings were successfully deposited on anodized TiB_2p/A356 composite by electrolysis treatment. Scanning electron microscope observations show that the conversion coatings effectively cover the whole surface of anodized composite. The conversion coatings on continuous porous anodic film are composed of many spherical nano-particulates; however, at the regions without anodic film the conversion coatings present a planar structure. The different morphologies are attributed to the different formation characteristics of cerium conversion coatings at different regions of anodized composite. X-ray photoelectron spectroscopy analysis indicates that the conversion coatings consist of CeO₂, Ce₂O₃, Ce(OH)₄, and Ce(OH)₃. The potentiodynamic polarization results testify that the integrated surface coatings of anodic film and cerium conversion coating provide a higher degree of protection for in situ TiB_2p/A356 composite in a chloride-containing environment.     

Microstructural Analysis of the Reinforced Al‐Cu5mgti/Tib2 5 wt % Alloy for Investment Casting Applications

The paper describes the influence of 5 wt % titanium diboride (TiB₂) particles on the microstructure of an Al‐Cu alloy produced by plaster casting process. The elaboration route leads to a composite material with 1% of in situ TiB₂ particles and 4% ex situ of TiB₂ particles. The comparison of the reinforced alloy with the corresponding non‐reinforced counterpart makes clear that the presence of TiB₂ particles has a large influence in the observed microstructure. The presence of TiB₂ particles decreases the grain sizes and the porosity level. It is also found that TiB₂ particles play an important role in the precipitation events of Al₂Cu precipitates that are formed during solidification at the TiB₂/aluminum matrix interfaces.     

Fatigue-life prediction of SiC particulate reinforced aluminum alloy 6061 matrix composite using AE stress delay concept

A study of the residual fatigue life prediction of 6061-T6 aluminum matrix composite reinforced with 15 vol % SiC particulates (SiC_p) by using the acoustic emission technique and the stress delay concept has been carried out. Fatigue damages corresponding to 40, 60 and 80% of total fatigue life were stimulated at a cyclic stress amplitude. The specimens with and without fatigue damage were subjected to tensile tests. The acoustic emission activities were monitored during tensile tests. It was found that a lower stress level was required to reach a specified number of cumulative AE events for specimens fatigued to higher percentage of the fatigue life. This stress level is called stress delay. Approximately a linear relation was found between stress delay and fatigue damage. Using the procedure defined in this study, the residual fatigue life can be predicted by testing the specimen in tension and monitoring the AE events. The number of the cumulative AE events increased exponentially with the increase of strain during tensile tests. This exponential increase occurred when the material was in the plastic regime and was attributed mainly to SiC particulate/matrix interface decohesion and linkage of voids. In high cycle fatigue, it was observed that the residual tensile strengths of the material did not change with prior cyclic loading damages since the high cycle fatigue life was dominated by the crack initiation phase.     

Effect of strain ratio on cyclic deformation and fatigue of extruded AZ61A magnesium alloy

Strain-controlled fatigue experiments were conducted on an extruded AZ61A magnesium alloy at three strain ratios (R_ɛ = −∞, −1, 0) using smooth tubular specimens. As the strain ratio decreased, stronger cyclic hardening, more asymmetric hysteresis loop, smaller stress amplitude, lower mean stress, and higher initial plastic strain amplitude were observed. These phenomena were associated with twinning in the compressive phase and detwinning in the tensile phase during cyclic deformation. At the same strain amplitude, fatigue life increased with decreasing strain ratio. The strain-fatigue life curve at each strain ratio exhibited a distinguishable kink. Such a kink point represents a demarcation point above which persistent twinning–detwinning occurs under cyclic loading. Two Smith, Watson, and Topper (SWT) fatigue criteria can predict the fatigue lives of the material at all strain ratios satisfactorily.     

Fatigue-creep interaction performance of Incoloy 825 nickel-based superalloy at 650 °C

The fatigue-creep interaction performance of Incoloy 825 nickel-based superalloy at 650 °C was investigated through introducing the tensile, compressive, and tensile-compressive strain hold time at the controlled total strain amplitude Δϵ_t = 0.3 %∼0.7 %. The results show that the Incoloy 825 nickel-based superalloy exhibits the cyclic hardening behavior, the cyclic hardening behavior followed by cyclic softening behavior and the cyclic hardening behavior followed by cyclic stability during the cyclic deformation with tensile strain hold time, while the alloy exhibits the cyclic hardening behavior and the cyclic hardening behavior followed by the cyclic stability during the cyclic deformation with compressive and tensile-compressive strain hold time. The relationship between both plastic and elastic strain amplitudes with reversals to failure for the alloy shows a single slope linear behavior, which can be described by the Coffin-Manson and Basquin equations, respectively. The deformation mechanism of the alloy under three loading condition of fatigue-creep interaction is mainly the planar slip. In addition, under three loading condition of fatigue-creep interaction, the cracks initiate and propagate in the transgranular mode.     

The relationship between TiB<Subscript>2</Subscript> volume fraction and fatigue crack growth behavior in the in situ TiB<Subscript>2</Subscript>/A356 composites

The relationship between TiB₂ volume fraction and fatigue crack growth behavior in the A356 alloy matrix composites reinforced with 3, 5.6, and 7.8 vol% in situ TiB₂ particles has been investigated. The mechanisms of crack propagation in the TiB₂/A356 composites were also discussed. The results show that the 3 vol% TiB₂/A356 composite has nearly the same crack growth behavior as the matrix alloy, while the 5.6 vol% TiB₂/A356 composite exhibits a little bit faster crack growth rate. The 7.8 vol% TiB₂/A356 composite presents the lowest resistance to crack growth, indicating that the crack growth is accelerated by increasing TiB₂ volume fraction. Fractographies reveal that an increase in TiB₂ volume fraction results in a change from the formation of striation and slip to the failure of voids nucleation, growth, and coalescence. Cracks tend to propagate within the matrix and avoid eutectic silicon and TiB₂ particles in the intermediate ΔK region, while prefer to propagate along interfaces of eutectic silicon and TiB₂ particles and link the fractured eutectic silicon particles in the near fractured ΔK region. Furthermore, the propensity for the separation of TiB₂ increases with the increase in TiB₂ volume fraction. The massive voids caused by fractured eutectic silicon and separated TiB₂ particles propagate and coalesce, and then accelerates the crack growth in TiB₂/A356 composites.     

In situ production of Al–TiB<Subscript>2</Subscript> nanocomposite by double-step mechanical alloying

An in situ Al–TiB₂ nanocomposite was synthesized by mechanical alloying (MA) of pure Ti, B and Al powder mixture in a planetary ball mill. A double-step process was used to prevent the formation of undesirable phases like Al₃Ti intermetallic compound. In the first step, a powder mixture was tailored to obtain nominal Al–90 wt% TiB₂ composition and the second step involved the addition of Al to the mixture in order to achieve Al–20 wt% TiB₂. The structural and thermal characteristics of powder particles were studied by X-ray diffractometry (XRD), scanning electron microscopy (SEM), differential scanning calorimetery (DSC), and transmission electron microscopy (TEM). The results showed that the MA process leads to the in situ formation of nanosized TiB₂ particles in an Al matrix with a uniform distribution. It was also found that the double stage addition of aluminum can prevent the formation of undesirable compounds even after annealing at high temperatures_.     

Understanding the influence of particle size on strain versus fatigue life,and fracture behavior of aluminum alloy composites produced by spray deposition

The strain versus fatigue life and fracture behavior of spray-formed Al–Si composites reinforced with SiC particles of two different sizes were studied under total strain amplitudes. Both composites exhibit short low-cycle fatigue (LCF) which follows a Coffin-Manson relationship, and display cyclic hardening at all strain amplitudes. The LCF endurance of the composite with large particles is higher than that of composite containing small particles in the high strain amplitudes, however, at low strains the difference in fatigue endurance between the two composites decreased. Moreover, the decrease in particle size results in a higher degree of hardening at low and middle strains, but reduces the magnitude of hardening at highest strain. Fractographic analysis reveals that particle/matrix debonding is the main mechanism of failure in composite with small particles, while fracture and debonding of SiC particle are predominant in the large particle reinforced composite.     

Effect of in situ synthesized TiB whisker on microstructure and mechanical properties of carbon–carbon composite and TiBw/Ti–6Al–4V composite joint

Carbon–carbon composite (C–C composite) and TiB whiskers reinforced Ti–6Al–4V composite (TiB_w/Ti–6Al–4V composite) were brazed by Cu–Ni + TiB₂ composite filler. TiB₂ powders have reacted with Ti which diffused from TiB_w/Ti–6Al–4V composite, leading to formation of TiB whiskers in the brazing layer. The effects of TiB₂ addition, brazing temperature, and holding time on microstructure and shear strength of the brazed joints were investigated. The results indicate that in situ synthesized TiB whiskers uniformly distributed in the joints, which not only provided reinforcing effects, but also lowered residual thermal stress of the joints. As for each brazing temperature or holding time, the joint shear strength brazed with Cu–Ni alloy was lower than that of the joints brazed with Cu–Ni + TiB₂ alloy powder. The maximum shear strengths of the joints brazed with Cu–Ni + TiB₂ alloy powder was 18.5 MPa with the brazing temperature of 1223 K for 10 min, which was 56% higher than that of the joints brazed with Cu–Ni alloy powder.