Particle Strengthening Research Articles

Tensile properties and strengthening mechanisms of Mo–Si alloy

Pure Mo and Mo–Si alloys with different silicon content were fabricated by powder-metallurgical and thermo-mechanical processing. Tensile properties of the pure Mo and Mo–Si alloys were measured at room temperature and the fracture surface was analyzed after test. The results indicate that Si can effectively reduced the grain size and improve the yield strength of Mo–Si alloys. With the decrease in grain size, the dominant fracture morphology is changed from intergranular to transgranular. The strengthening mechanisms were discussed and the yield strength was analyzed described with respect to grain size, solid solution hardening and Mo 3Si particle strengthening. Calculations show that the yield strength of Mo–Si alloys is governed by grain size.

Microstructure and mechanical properties of an Al–12.7Si–0.7Mg alloy processed by extrusion and heat treatment

Direct chill (DC) cast Al–12.7Si–0.7Mg alloy without chemical modification was hot extruded and subsequently heat-treated under various conditions. The microstructure and tensile properties of the extruded profile were investigated aiming at understanding the strengthening mechanisms. The results show that the refined microstructure characterized by fine Si particles uniformly distributed in the Al matrix of fine equiaxed grains was promoted by hot extrusion. The alloy after T6 treatment exhibits good ductility and much higher proof strength as well as tensile strength compared to 6063 alloy in general. The strengthening was attributed to be the combining effect of particle, grain boundary and precipitation strengthening.

Aging of Cu-3 at% Ti Alloys in Hydrogen Atmosphere: Influence of Hydrogen Pressure on Strength and Electrical Conductivity

The influence of hydrogen pressure during isothermal aging on the mechanical strength, electrical conductivity, and microstructure of Cu3 at% Ti alloys was investigated under various hydrogen pressures from 0 to 0.8 MPa. The variation of hardness with aging time was not significantly different among all specimens aged under the hydrogen pressures. This is because the hardness is improved primarily by the precipitation strengthening of Cu4Ti particles, which is less affected by hydrogen pressure. The electrical conductivity increased more significantly for specimens aged under higher hydrogen pressure, due to a rapid reduction in the concentration of Ti dissolved in the matrix, which is attributed to the accelerated formation of TiH2. The conductivity at peak-hardness was improved by a factor of approximately 1.4 in the specimens aged at both 773 and 723 K under the highest hydrogen pressure, compared to that for the specimen aged in vacuum. Therefore, aging under high hydrogen pressure assisted in the significant improvement of both strength and electrical conductivity.

Microstructure and Impression Creep Characteristics of Cast Mg-5Sn-xBi Magnesium Alloys

The microstructure and creep behavior of a cast Mg-5Sn alloy with 1, 2, and 3 wt pct Bi additions were studied by impression tests in the temperature range 423 K to 523 K (150 °C to 250 °C) under punching stresses in the range 125 to 475 MPa for dwell times up to 3600 seconds. The alloy containing 3 wt pct Bi showed the lowest creep rates and, thus, the highest creep resistance among all materials tested. This is attributed to the favorable formation of the more thermally stable Mg3Bi2 intermetallic compound, the reduction in the volume fraction of the less stable Mg2Sn phase, and the dissolution of Bi in the remaining Mg2Sn particles. These particles strengthen both the matrix and grain boundaries during creep deformation of the investigated system. The creep behavior of the Mg-5Sn alloy can be divided into the low- and high-stress regimes, with the respective average stress exponents of 5.5 and 10.5 and activation energies of 98.3 and 163.5 kJ mol−1. This is in contrast to the creep behavior of the Bi-containing alloys, which can be expressed by a single linear relationship over the whole stress and temperature ranges studied, yielding stress exponents in the range 7 to 8 and activation energies of 101.0 to 107.0 kJ mol−1. Based on the obtained stress exponents and activation energies, it is proposed that the dominant creep mechanism in Mg-5Sn is pipe-diffusion controlled dislocation viscous glide the low-stress regime and dislocation climb creep with back stress in the high-stress regime. For the Mg-5Sn-xBi alloys, however, the controlling creep mechanism is dislocation climb with an additional particle strengthening effect, which is characterized by the higher stress exponent of 7 to 8.

High-strength Mg–Al–Ca alloy with ultrafine grain size sensitive to strain rate

The microstructure, texture and mechanical properties of the 1 wt.% Ca-AZ31 alloy (1CaAZ31) processed by high-ratio differential speed rolling (HRDSR) were examined. After HRDSR, the grain size of the 1CaAZ31 decreased to ∼0.9 μm, and (Al, Mg) 2Ca particles were highly refined and uniformly dispersed into the matrix. The significant improvement in strength was attributed to the effective grain-size reduction. Contribution of the particle strengthening was relatively small. Strength of the ultrafine grained (UFG) 1CaAZ31 was sensitive to strain rate. As the result, the yield stress was as high as 412 MPa at 1 × 10 −1 s −1, while it was 292 MPa at 1 × 10 −5 s −1. The strain-rate-sensitivity of the UFG 1CaAZ31 was 0.031–0.041, which is about twice larger than that of its annealed counterpart having coarse grains (0.018). Uniform elongation of the UFG 1CaAZ31 was small compared to that of the coarse-grained 1CaAZ31 due to the reduced strain hardening ability, but its post-uniform elongation was large due to the increased strain-rate sensitivity.

Study of Nickel-Based Composite Coatings on H13 Steel Surface by Laser

NiCrBSi+Ti+C and NiCrBSi+TiC composite coatings were fabricated on H13 steel substrate by laser claddings to improve its wear resistance. The macroscopic qualities of the coatings were analyzed and the process parameters and the clad materials were optimized. The microstructure of the coatings was characterized by SEM. The microhardness of the coatings was examined using HXD-1000 microhardness tester. The wear properties were examined using M-200 wear test machine. The results show that the clad materials and the process parameters are the important factors to influence the quality of laser cladding. Ni+Ti+C organization is smaller, particle strengthening and grain refinement is more significant. The hardness of the clad coating of NiCrBSi+Ti+C coatings reaches HV900, which is improved by almost three times compared with the substrate H13 steel.

The influence of silicon content on microstructure and hardness of Mo–Si alloys

Mo–Si alloy sheets with different silicon content were fabricated by powder-metallurgical and thermo-mechanically processing. The effect of Si content and annealing temperature on microstructure and hardness of the Mo–Si alloys were studied by using optical microscopy (OM), transmission electron microscopy (TEM) and Vickers hardness tester, respectively. The results indicated that the presence of Si can effectively refine the grain sizes and improve the hardness of Mo–Si alloys. Si can also increase the recrystallization temperature of alloys and play a significant role in restraining the grain growth at high temperatures. Increasing the annealing temperature, the microstructure of Mo–Si alloy sheets is gradually coarsened and changed from fibrous to equiaxed structure, causing reduction in hardness. The hardening effect in the Mo–Si alloys came from the refined grain strengthening, solid solution strengthening, and second phase particle strengthening, which are closely dependent on the Si content and annealing temperature.

Synergic effects of grain refinement and precipitation strengthening

The article describes the combined effects of grain size and second phase particles on mechanical properties of CuCrZr alloy subjected to SPD processing and ageing in two sequences: (i) SPD processing followed by ageing and (ii) SPD processing of samples aged prior to deformation. It was revealed that each of these strengthening mechanisms acting alone gives a significant increase in mechanical strength (5 and 10 times in the case of ageing and SPD processing, respectively). However, it has been found in the present study that the strength of samples subjected to grain refinement and precipitation hardening is not a direct sum of the strengthening brought about by these two strengthening mechanisms acting alone. This finding is discussed in terms of the inter-dependence of grain size–particle strengthening in SPD nano-metals.

On the effects of particle strengthening and temperature on the VHCF behavior at high frequency

The fatigue properties in the high-temperature range at very high number of cycles of the precipitation-hardened nickel-base alloy Nimonic 80A were investigated. With the applied global strain amplitude being well below the elastic limit, cyclic life is dominated by heterogeneously distributed and localized plastic deformation in the microstructure. The material studied was fatigued in the peak-aged and overaged condition applying high frequency testing methods. Microstructural analysis by means of scanning and transmission electron microscopy was performed to determine the dominating damage mechanism. At room temperature planar single slip and at elevated temperatures partially wavy dislocation arrangements were found, with the wavy arrangement being accompanied by dislocation climbing processes as well as Orowan loops. At room temperature the overaged Nimonic 80A showed a slightly superior fatigue behavior compared to the peak-aged condition in the VHCF range, whereas at elevated temperatures the difference in fatigue behavior at high number of cycles seemed to be of less significance. The overaged condition showed a slight increase in fatigue life at 600 °C accompanied by a tendency towards a more pronounced homogeneous dislocation movement. Isothermal cyclic deformation at 800 °C led to a pronounced decrease of cyclic life with both heat treatments exhibiting a very similar fatigue behavior. Early failure was primarily ascribed to the formation of microcracks in the emerging oxide layers.

Effect of Ca additions on the microstructure, thermal stability and mechanical properties of a cast AM60 magnesium alloy

The effect of 0.5, 1.2, and 2.0 wt.% Ca additions on the microstructure, thermal stability, and mechanical properties of the cast Mg–6Al–0.3Mn alloy (AM60) was investigated. Addition of 0.5 wt.% Ca did not form any new phase but suppressed the discontinuous precipitation of the β-Mg 17Al 12 phase by being dissolved in both the second phase and magnesium matrix. In the materials containing higher amounts of Ca, however, the Al 2Ca phase with a lamellar morphology appeared in the microstructure and the volume fraction of β-Mg 17Al 12 diminished. It was found that Ca generally refines the microstructure and enhances the thermal stability of the alloy. This was documented by the retention of the initial fine dendritic microstructure, hardness, and ultimate shear strength (USS) of the Ca-containing materials after long-term annealing at 673 K. The observed improvements are believed to be caused by the solid solution strengthening of Ca in the Mg matrix, and the particle strengthening effects of the thermally stable Al 2Ca phase which are formed as an interconnected network. This behavior is in contrast to that of the base material which developed a coarse grain structure with decreased strength and hardness after the same thermal treatment.

Effect of oxygen precipitates on dislocation motion in Czochralski silicon

We have investigated the effect of oxygen precipitates on dislocation motion in Czochralski silicon by the indentation technique. It is found that the gliding distances of dislocations are much smaller in samples containing a high density of oxygen precipitates in the order of 10 9 cm −3 than in the control samples without remarkable oxygen precipitates. Transmission electron microscopy reveals that oxygen precipitates can indeed pin the dislocations generated at high temperatures. Such a pinning effect is proved to be dependent on the density and size of oxygen precipitates. The particle strengthening mechanism is tentatively adopted to explain the suppression of dislocation motion by the oxygen precipitates in CZ silicon.

Creep Behavior and Damage of Ni-Base Superalloys PM 1000 and PM 3030

Two oxide dispersion strengthening (ODS) nickel-base superalloys, a solely dispersion-strengthened alloy (PM 1000) and an additionally γ′-strengthened alloy (PM 3030) are investigated regarding creep resistance at temperatures between 600 °C and 1000 °C. The creep strength advantage of PM 3030 over PM 1000 decreases as the temperature increases due to the thermal instability of the γ′ phase. The particle strengthening contribution in both alloys increases linearly with load. However, solid solution softening leads to an apparent drop in particle strengthening in PM 1000. Deformation concentration in slip bands is more accentuated in PM 3030-R34 due to additional γ′ strengthening combined with strongly textured coarse and elongated grain structure. Finer, equiaxed grains reduce creep strength at higher temperatures due to grain boundary deformation processes and premature pore formation, but have only minor impact at low and intermediate temperatures.

Achieving high strength and high ductility in magnesium alloys using severe plastic deformation combined with low-temperature aging

A novel method of fabricating age-hardenable magnesium alloys with high strength and high ductility was developed using high-ratio differential speed rolling combined with low-temperature aging. The ultrafine-grained Mg–9Al–1Zn alloy processed by this technique exhibited a high yield stress of >400 MPa and tensile elongations of 12–14%. The high strength could be attributed to grain size and particle strengthening effects, while the high ductility could be attributed to weakening of the basal texture component.

Synthesis of ultra high strength Al–Mg–Si alloy sheets by differential speed rolling

The ultrafine-grained Al–Mg–Si alloy sheets, which were fabricated by severe plastic deformation (SPD) using a high-speed-ratio differential speed rolling (HRDSR) and subsequent low temperature aging, exhibited an ultra high strength (yield stress: 455 MPa, ultimate tensile strength: 489 MPa). The strengthening effect was impressive compared with the results obtained by using other SPD techniques. The achievement could be attributed to formation of very fine grains due to significantly increased dislocation density in solute supersaturated matrix, high Hall-Petch constant and particle strengthening gained by formation nano-scale precipitates during the low temperature aging after the HRDSR process.

The effect of Mo additions to high damping Ti–Ni–Nb shape memory alloys

The effect of Mo addition on the microstructure, phase transformation, mechanical behavior and damping capacity has been investigated. It is found that the Mo addition depresses the solubility of Nb in the TiNi matrix and results in abundant dispersive Nb-rich precipitation. The R-phase transformation is induced in Ti–Ni–9.0 at.%Nb alloys by the substitution of Mo for Ni. The yield strength and rupture strength increase with increasing Mo content due to the precipitation strengthening of Nb-rich particles and the solution strengthening of the Mo, while the elongation remains at a high level. With the increment of Mo content, the damping in the martensitic state and the damping during the phase transformation are enhanced. The increment of the martensite volume fraction and the presence of abundant Nb-rich particles are responsible for the high damping in the martensitic state. The improvement of the damping properties during the phase transformation is related to the volume fraction of the transformed martensite phase per unit time and the contribution of the R-phase transformation.

Microstructures and mechanical properties of Mg–Al–Zn–Ca alloys fabricated by high frequency electromagnetic casting method

The microstructures and mechanical properties of AZ31 and 1 wt%Ca-containing AZ31 billets fabricated using EMC (Electromagnetic Casting) and EMS (Electromagnetic Stirring) were examined. The results show a great potential of producing high-quality surface magnesium billets with fine-grained microstructure at a relatively high casting speed. Application of EMC + EMS for production of the 1 wt%Ca-AZ31 alloy billet with a diameter of 150 mm produced a reasonably homogeneous microstructure composed of fine grains with an average size of 45 μm. Attainment of the fine-grained and homogeneous microstructure by EMC + EMS was attributed to reduction of temperature gradient and fragmentation of dendrite structure under electromagnetic force. Strength of the EMC and EMC + EMS 1 wt%Ca-AZ31 billets was higher than that of the EMC AZ31 billet due to the grain size and particle strengthening effects.

Bulk Al Materials from Back Pressure Equal Channel Angular Consolidation of Mechanically Milled Particles

Mechanically milled pure aluminium powders were fabricated into bulk materials using back pressure equal channel angular consolidation (BP-ECAC) for four or eight passes at 373K. The bulk materials consolidated from 0 h and 4 h mechanically milled powders were characterised by Vickers hardness tests and density measurements. Thermal stability of the consolidated bulk materials was evaluated by isothermal heat treatments at 673K. The as-consolidated bulk material from the 0 h milled (i.e. unmilled) powder showed nearly full density. However, full density was not obtained with the 4 h milled powder even after eight passes. The HV values for the as-consolidated materials fabricated from the 0 h and 4 h milled powders after four passes and from the 4 h milled powder after eight passes were 57, 121 and 136, respectively. Softening was observed in the bulk material consolidated from the 0 h milled powder during the isothermal heat treatment. However, the hardness of the bulk materials consolidated from the 4 h milled powders after four and eight passes increased to maximum values of 137 and 141 after heat treatment for 28 h and 8 h at 673K, respectively. The maximum hardness was maintained for up to 100 h at 673K in both materials. The hardening and thermal stability in the bulk materials from the milled powders are attributable to dispersion strengthening of Al4C3 particles formed by solid-state reaction during the isothermal heat treatment.

Particle strengthening of the surface of copper electrode for electrical discharge machining

The manufacture of Electrical Discharge Machining (EDM) electrodes for the machining of mould cavities is often an intractable problem, which consumes much money and time. The combination of Rapid Prototyping (RP) with electroforming paves a way to fabricate copper EDM electrodes more rapidly and economically. However, the spark-resistance of the electroformed copper electrode is still not satisfying. To enhance the surface quality of copper electrodes, Cu-ZrB2 composite coating was electrodeposited and the EDM performance of this new material was studied. The hardness of the composite coatings increases with the increase in ZrB2 percentage, whereas the incorporation of ZrB2 particles has little effect on the electrical resistivity of composites. The effects of EDM parameters on the Material Removal Rate (MRR) and Tool Wear Ratio (TWR) of electrodes were investigated. The Cu-ZrB2 composite coating shows better EDM performance than that of the electroformed copper electrode.

Strengthening mechanisms in nanocrystalline alloys

The mechanisms for strengthening nanocrystalline metals by alloy additions are reviewed and a new model for nano-particle strengthening by Orowan bypassing in nano-grains is proposed. Recent experimental results for three different nanocrystalline alloy systems, Fe–Pb, Fe–Al 2O 3 and Al–Pb are presented and analyzed in terms of non-equilibrium solid solution strengthening, nano-composite strengthening and Orowan particle strengthening, respectively. Conflicting alloy hardening and softening effects observed in Al–Pb appear to be the result of interplay between Orowan particle hardening and a softening mechanism due to grain-boundary segregation. Preliminary MD simulations support the latter suggestion.

Preparation and Mechanical Properties of Particulate-Reinforced Powder Metallurgy Titanium Matrix Composites

This research focuses on the in-situ synthesis of titanium (Ti)-titanium carbide (TiC) composites by powder metallurgy (PM) technology. Newly designed Ti-1.5Fe-2.25Mo-1.2Nd-0.3Al (wt pct) alloy was chosen as the matrix, and the additive was Cr3C2. The microstructure, room/elevated temperature, tensile behavior, and sliding wear behavior of the matrix alloy and the composites are presented and discussed. Microstructural analysis of the composites reveals a uniform dispersion of the TiC reinforcements with near-equiaxed shape in the Ti matrix. Tensile property test results show that the tensile strength increases significantly at both room temperature (RT) and elevated temperature after adding Cr3C2 into matrix alloy. Results of sliding wear resistance examination indicate that the wear properties are improved through the synthesis of TiC reinforcements. High strength of the composites, with Cr3C2 added, is attributed to particle strengthening, grain refining, and solution strengthening of Cr element. Better wear resistance of the composites is attributed to strong interface between particles and Ti matrix and high hardness of the composites. Therefore, Cr3C2 is considered to be a promising additive for processing PM Ti matrix composites.