Orowan Strengthening Research Articles

Simultaneously improving strength and ductility of hybrid Al–Si matrix composite with polyphasic and multi-scale ceramic particles

Hybrid aluminum matrix composites have received widespread attention because of their superior performance to single-phase reinforced aluminum matrix composites. In situ VB2–AlN–Al4C3–VSi2/Al ribbons with polyphasic and multi-scale reinforcement particles were prepared by powder metallurgy method and controlled by melt-spinning and then verified the reinforcement efficacy in Al–12Si alloy. It was found that the addition of VB2–AlN–Al4C3–VSi2/Al ribbons not only refined the grain of α-Al, but also optimized the morphology of eutectic silicon for Al–12Si matrix composite. Thereafter, the mechanical properties of the VB2–AlN–Al4C3–VSi2/Al–12Si composite, including the tensile strength, ductility and hardness were increased by 36.2%, 82.2% and 33.4%, respectively, with the addition of VB2–AlN–Al4C3–VSi2/Al ribbons. The excellent mechanical properties can be attributed to the micron-sized VSi2 particles, nano-sized AlN, VB2, and Al4C3 as well as the well-bonded particle/matrix interface. In addition, multiple strengthening mechanisms have also made outstanding contributions to the improvement of the mechanical properties, including (1) Orowan strengthening; (2) Coefficient of thermal expansion (CTE) mismatch strengthening; (3) the Hall-Petch effect caused by grain refinement. The present experimental results provide an insight into the understanding of polyphasic and multi-scale reinforcement particles, and extend the effort for the development of Al–Si composite.

Microstructure evolution and texture tailoring of reduced graphene oxide reinforced Zn scaffold

Zinc (Zn) possesses desirable degradability and favorable biocompatibility, thus being recognized as a promising bone implant material. Nevertheless, the insufficient mechanical performance limits its further clinical application. In this study, reduced graphene oxide (RGO) was used as reinforcement in Zn scaffold fabricated via laser additive manufacturing. Results showed that the homogeneously dispersed RGO simultaneously enhanced the strength and ductility of Zn scaffold. On one hand, the enhanced strength was ascribed to (i) the grain refinement caused by the pinning effect of RGO, (ii) the efficient load shift due to the huge specific surface area of RGO and the favorable interface bonding between RGO and Zn matrix, and (iii) the Orowan strengthening by the homogeneously distributed RGO. On the other hand, the improved ductility was owing to the RGO-induced random orientation of grain with texture index reducing from 20.5 to 7.3, which activated more slip systems and provided more space to accommodate dislocation. Furthermore, the cell test confirmed that RGO promoted cell growth and differentiation. This study demonstrated the great potential of RGO in tailoring the mechanical performance and cell behavior of Zn scaffold for bone repair.

Precipitation strengthening of nano-scale TiC in a duplex low-density steel under near-rapid solidification

Precipitation strengthening of nano-scale TiC is a promising method to improve mechanical properties of Fe–16Mn–9Al–0.8C (wt.%) low-density steel. This work attempted to introduce nano-scale TiC precipitates by adding 1 wt.% Ti element. The experimental results show that these precipitates with the total fraction of about 2 vol.% were formed and no coarse precipitates were observed despite the high Ti addition. It was interesting that the polygonal and needle-shaped TiC precipitates were observed in γ-austenite and δ-ferrite, respectively. Ti addition also decreased the volume fraction of γ-austenite significantly. Correspondingly, the yield strength was increased, but the elongation was significantly decreased due to the significant decrease of γ-austenite. Comparing with the Ti-free steel, the formation of TiC precipitates was the main reason for the increase in yield strength of Ti-bearing steel, and TiC precipitates also led to a higher strain hardening index at the first deformation stage. TiC precipitates promoted the Orowan strengthening, resulting in a higher strain hardening capability than Ti-free steel reinforced by shearable κ-carbide.

Investigation on the elevated-temperature mechanical properties of TiB2/Al-4.5%Cu composites

Mechanical performance of TiB2/Al-4.5%Cu composites at elevated-temperature were evaluated by means of tensile behaviors and fracture mechanisms. Results indicated that the composite reinforced by nano-sized TiB2 particles exhibited excellent mechanical ability during deformation at elevated temperatures, the optimum addition level for dispersion, refinement and modification was 5wt.% TiB2/Al-4.5%Cu at 473K, i.e., yield strength is 358MPa and elongation is 4 %. The enhanced resistance of the composite was mainly attributed to Orowan strengthening and load transfer enhancement, additionally, interface debonding increasingly affects the damage and fracture of the composites.

Microstructures, mechanical properties, and grease-lubricated sliding wear behavior of Cu-15Ni-8Sn-0.8Nb alloy with high strength and toughness

Alloys used as bearings in aircraft landing gear are required to reduce friction and wear as well as improve the load-carrying capability due to the increased aircraft weights. Cu-15Ni-8Sn-0.8Nb alloy is well known for possessing good mechanical and wear properties that satisfy such requirements. In this study, the microstructure, mechanical properties, and grease-lubricated sliding wear behavior of Cu-15Ni-8Sn-0.8Nb alloy with 0.8 wt% Nb are investigated. The nanoscale NbNi3 and NbNi2Sn compounds can strengthen the alloy through the Orowan strengthening mechanism. A Stribeck-like curve is plotted to illustrate the relationship among friction coefficient, normal load, and sliding velocity and to analyze the grease-lubricated mechanism. The wear rate increases with normal load and decreases with sliding velocity, except at 2.58 m/s. A wear mechanism map has been developed to exhibit the dominant wear mechanisms under various friction conditions. When the normal load is 700 N and the sliding velocity is 2.58 m/s, a chemical reaction between the lubricating grease and friction pairs occurs, resulting in the failure of lubricating grease and an increase in wear.

Evolution of microstructure and mechanical properties influenced by nanosized precipitates during rolling deformation

In this research work, the 6111Al reinforced by rare earth element Er was subjected to severe plastic deformation with different rolling reduction ranging from 50%, 70%–90% respectively. Continuous deepening of the dynamic recrystallization occurred during the increment on rolling deformation and the average size of α-Al grains decreased to 7.9 μm eventually. The nanosized Al3Er particles were characterized by the combined function of inhibiting and promoting the DRX at the same time during the rolling process. And the competitive relationship between the driving force and the restraining force has been studied via systematic calculation here. A noticeable enhancement on 6111Al–Er alloy has been obtained with YS, UTS and elongation increasing to 359 MPa, 380 MPa and 27.4% respectively when the rolling deformation reached to 90%, which was attributed to the synergistic strengthening effect induced by the fine grain strengthening and the Orowan strengthening mechanisms. The evolution of microstructure was characterized by the equipments including Electron backscattered diffraction (EBSD), Scanning electron microscope (SEM) and High resolution transmission electron microscope (HRTEM).

Influence of heat treatment on microstructure evolution and mechanical properties of TiB2/Al 2024 composites fabricated by directed energy deposition

The influence of heat treatment on microstructure evolution and mechanical properties of TiB2/Al 2024 composites fabricated by laser-powder directed energy deposition (DED) were investigated. The results showed that the addition of TiB2 resulted in a significant grains refinement with random orientation. Compared with DED-Al 2024, a lot of network-like θ-Al2Cu phases was generated in the as-deposited TiB2/Al 2024 composite, leading to low plasticity. After 10h solution treatment, the θ phase disappeared, the thermally stable block T-Al20Cu2Mn3 phase was formed and the ultimate tensile strength (UTS) and elongation improved due to solution strengthening. During the aging process, Cu element was precipitated and semi-coherent θ’ phase was generated, improving the effect of Orowan strengthening. The composite material reached the maximum UTS of 453.65 MPa after 10h aging. The yield strength (YS) enhanced with the increase of aging time. After 15h aging, the YS and microhardness reached the maximum of 245.73 MPa and 133.50HV, respectively. This research provides a theoretical basis for the repair and manufacture of Al 2024 alloy and composite parts.

Effects of stress state, strain rate, and temperature on fracture behavior of in situ TiB2/2024 Al composite

To gain insights into the relationship between the fracture behavior and the unique microstructure of the in situ TiB2/2024 Al composite, systematic experiments were conducted over wide ranges of temperature (298–673 K), strain rate (0.001–4000/s), and stress triaxiality (−0.82–1.03). The results showed that the strengthening mechanism of the composite was related to Orowan strengthening and coefficient of thermal expansion (CTE) mismatch strengthening. Owing to the smaller particle size and sturdy interfacial bonding, the tensile strength of the in situ nano/submicron TiB2/2024 composite was higher than that of the ex situ composite and the in situ micron composite. Through a morphologic study by scanning electron microscopy (SEM), the microstructural deformation and crack propagation path were found to be largely dependent on the stress state, strain rate, and temperature. The melted matrix precipitated in the dynamic tensile fracture surface can hinder the crack propagation and is considered the determining factor that improves the ductility at room temperature. Furthermore, a 3D fracture locus in the space of the fracture strain, stress triaxiality, and the Lode angle parameter was formed. Finally, a new fracture criterion that considers the Lode angle parameter, the stress triaxiality cutoff value, and the coupling effect of strain rate and temperature was developed. The developed criterion exhibited a robust and accurate capability to describe the fracture behavior of the in situ TiB2/2024 Al composite.

Effect of Heat Treatment on Wear Behavior of A357/Cenosphere Composites

The present paper is based on the effect of heat treatment on the wear behavior of A357/cenosphere composites. The aluminum metal matrix composites were fabricated by the squeeze casting method. Both base alloy and composites are heat treated. The hardness of the matrix alloy and composites, both in as cast and heat-treated conditions, is measured by Vickers microhardness tester. The mechanical properties of the Al alloy/cenosphere composites got enhanced due to dispersion hardening, grain boundary strengthening and Orowan strengthening. The abrasive wear behavior was carried out on the composites using pin-on-disk wear tester with various operating parameters. The wear test results showed that the matrix alloy suffered higher wear rate than that of composites both in untreated and heat-treated conditions, depending on load.

Significantly improved particle strengthening of Al–Sc alloy by high Sc composition design and rapid solidification

Sc alloying is usually carried out by adding a small amount of Sc in typical aluminum alloys to improve the mechanical properties, however, the effect of high-Sc content on the microstructure and properties of aluminum alloys is rarely concerned. To further understand the role of Sc alloying, several Al–Sc binary alloys with high-Sc content are designed and the relationship between the Sc content and mechanical properties were investigated. It is found that the mechanical properties of Al–Sc binary alloy is determined by both the Sc content and the cooling rate. The addition amount of Sc shows relatively small effect on the mechanical properties at low cooling rate, but higher at high cooling rate, because cooling rate turns the size and distribution of Al3Sc particles. Through the high Sc content alloy design and melt spinning rapid cooling technology, the densely distributed Al3Sc precipitate of about 250 nm can be located inside the α-Al grains, resulting in the intracsystalline reinforcement. The combination of high tensile strength (344 MPa) and good plasticity (5.7%) were achieved at a grain size of ~500 nm in the hypereutectic Al–Sc binary alloys ribbons. The significant strengthening could be attributed to multiple strengthening mechanisms including (1) Orowan strengthening; (2) coefficient of thermal expansion (CTE) mismatch strengthening; (3) the Hall-Petch effect caused by grain refinement; (4) solid solution strengthening. When the cooling rate is increased, the effects of thermal mismatch strengthening and Orowan mechanism are more prominent, significantly improving the strength of the Al–Sc alloy. The present experimental results provide an insight into the understanding of microstructural evolution of nanoparticles, and extend the effort for the development of hypereutectic Al–Sc alloys as a member of high-strength structural materials.

Microstructural and mechanical evolution of 6111Al reinforced by in-situ ZrB2 particles during extrusion process

In this research work, the 6111Al matrix composites reinforced by different content of in-situ ZrB2 particles were conducted to hot extrusion process to investigate the characteristics of microstructural and mechanical evolution during deformation. The results showed that coarse particle clusters were crushed to pieces and the distribution of ZrB2 particles has been modified significantly with assistance of huge extruded force. Besides, the behavior of dynamic recrystallization (DRX) was intensified by nanosized particles causing severe dislocation pile-up around them, which resulted in tinier dynamically recrystallized (DRXed) grains generated in 6111Al/ZrB2 composites. The effect of enhancement induced by extrusion process on mechanical properties of the matrix alloy reached to the peak value when the content of ZrB2 particles was 3 wt%, where the YS, UTS and elongation of 6111Al-3wt% ZrB2 composites were 345 MPa, 396 MPa and 28.2% respectively, which was attributed to the uniform distribution of reinforcing particles, the elimination of casting defects and super-tiny DRXed grains in the microstructure. The particle stimulated nucleation (PSN) mechanism and strengthening mechanisms concerning Orowan strengthening, Dislocation strengthening, Grain refinement strengthening were systematically discussed in this paper.

Effect of micro-alloying element Ti on mechanical properties of Cu–Cr alloy

The mechanical properties and microstructure of Cu–Cr and Cu–Cr–Ti alloys after aging treatment were investigated via hardness measurements, electron backscattered diffraction analysis, and transmission electron microscopy. The results show that the hardness, yield strength, and softening temperature of the Cu–Cr–Ti alloy significantly improved. The interface of the Cu matrix and Cr-rich precipitates was semi-coherent as determined by calculations, and the strengthening mechanism of the Cr precipitates was via the Orowan strengthening relation. A high dislocation density was observed in the matrix, which was because of impediment by fine Cr precipitates in the Cu–Cr–Ti alloy. The effects of dislocation strengthening, solid-solution strengthening, and precipitate strengthening were shown to remarkably enhance the yield strength.

Effect of extrusion speed on mixed grain microstructure and tensile properties of a Mg-2.9Zn-1.1Ca-0.5Mn nanocomposite reinforced by a low mass fraction of TiCp

In this work, a novel TiC nanoparticle (0.5 wt %) reinforced Mg-2.9Zn-1.1Ca-0.5Mn nanocomposite, with a mixed grain microstructure exhibiting high strength and targeted for biocompatible/structural applications was successfully prepared by ultrasonic assisted semi-solid stirring and extrusion at ultra-slow speeds of 1, 0.1 and 0.01 mm/s. The experimental results revealed that the morphology of the eutectic Ca2Mg6Zn3 phase in the as-cast nanocomposite changed from plate-like to lamellar as a low mass fraction of TiCp was added. Both dynamically recrystallized grains and precipitates were gradually refined with decreasing extrusion speed. The finest recrystallized grains (∼0.46 μm), with a high volume fraction (∼4.3%) of fine precipitates, appeared after extrusion at 0.01 mm/s. The refined grain structure was not only due to dynamic recrystallization, but also the synergistic pinning effects from nano-TiCp as well as precipitated MgZn2 and α-Mn particles. The superior ultimate tensile strength (∼410.3 MPa), yield strength (∼384.5 MPa) and elongation to failure (∼4%) were obtained in the nanocomposite extruded at the slowest speed (0.01 mm/s) and had the potential to serve as a candidate material in orthopaedic applications. The improved strength was mainly related to grain refinement, thermal expansion effects and Orowan strengthening. Grain refinement, in particular, contributed to the largest strengthening increment. High tensile toughness of ∼66.6 KJ mm−3 was achieved for the nanocomposite extruded at a speed of 1 mm/s. Further, it exhibited a high strain hardening rate, θ, at stage IV. The fracture surface exhibited abundant dimples and consistent with high ductility. Remnant coarse Ca2Mg6Zn3 particles acted as crack initiators under high applied stress in tension, leading to structural failure.

Preparation and properties of Nano-HfO2 reinforced NiAl intermetallics fabricated with powder metallurgy

A new approach for in-situ synthesis of nano-HfO2 reinforced NiAl intermetallics was proposed for the preparation of a new NiAl-based composite material that is expected to function at 1200 °C. The phase composition, microstructure and mechanical properties of NiAl-HfO2 composites prepared by hot pressing sintering with Ni, Al, Hf and O2 as starting materials were studied. The composites with NiAl as the matrix and nearly spherical or capsule-shaped HfO2 with an average size of 80.19 nm as the dispersion strengthening phase were designed and prepared. The results indicated that the addition of HfO2 significantly increased the high-temperature strength and slightly improved the room-temperature plasticity of the NiAl matrix due to the design of the appropriate content, size and distribution of HfO2. The NiAl-9HfO2 (in wt.%) exhibited the most excellent mechanical properties with the compressive yield strength of 351.1 MPa at 1200 °C as well as the compressive yield strength of 1316.8 MPa and ultimate compression deformation of 25.8% at room temperature. The strengthening and toughening mechanisms of HfO2 on NiAl matrix were discussed in details, and the Orowan strengthening mechanism was emphasized.

Relationship and mechanism between double cold rolling-aging process, microstructure and properties of Cu–Ni–Si alloy prepared by two-phase zone continuous casting

Developing Cu–Ni–Si alloy with high strength and high electrical conductivity is a key issue in the electrical conductor industries. The C70250 copper alloy was prepared by two-phase zone continuous casting (TZCC) technology first, and then carried out by double cold rolling-aging process. The relationship between different double cold rolling-aging processes, microstructure, mechanical properties and electrical conductivity of the alloy was studied in detail, the mechanism was revealed. The results show that the C70250 copper alloy strip fabricated by TZCC technology can be subjected to large deformation double cold rolling-aging treatment. When the alloy is primary cold rolled by 97.5% reduction rate, primary aged at 400 °C for 30 min, second cold rolled by 60% reduction rate, second aged at 400 °C for 45 min, the tensile strength and electrical conductivity of the alloy can reach 879 MPa and 48.9% IACS respectively, owning excellent comprehensive properties. The Ni2Si phases inside C70250 copper alloy after primary aging effectively pin dislocations during the secondary cold rolling process, which forms high density dislocation entanglement, promotes the further precipitation of Ni2Si phase during the process of secondary aging, and finally produces a large number of fine, dispersed, multi-scale mixed and evenly distributed Ni2Si phases, the increase of alloy strength is mainly ascribed to dislocation and Orowan strengthening effect. Solute atoms inside the C70250 copper alloy get adequately precipitated after large deformation double cold rolling-aging treatment, the recrystallization is delayed, and the fibrous microstructure along the rolling direction is produced, the scattering effect of transverse grain boundaries and solute atoms on electrons is significantly reduced, which is helpful to improve the electrical conductivity of the C70250 copper alloy.

Effect of sintering temperature on microstructural and mechanical properties of SPS processed CoCrCuFeNi based ODS high entropy alloy

In the present investigation, CoCrCuFeNi based multicomponent nano-scale Y2O3–Ti dispersed oxide dispersion strengthened (ODS) alloys were prepared by mechanical alloying followed by spark plasma sintering (SPS) at three different temperatures (900 °C, 1000 °C and 1100 °C). The microstructures of the as processed SPS samples primarily consist of a combination of F1 and F2 based FCC phases in the matrix in addition to a minor amount of second (Co14·2Cr2·7Y2) phase. Further TEM analysis indicates bimodal grain size distribution with an increasing trend in grain size from 0.110 μm to 0.883 μm with an increase in sintering temperature. The best combination of ultimate compressive strength (1555 MPa) and hardness (468 HV) of the SPS processed bulk ODS-HEA has been obtained when sintered at 1000 °C. The contribution in yield strength has been estimated using three strengthening mechanisms and the Orowan strengthening model is the most dominating strengthening mechanism in the present ODS-HEA.

Laser powder bed fusion of Al–10 wt% Ce alloys: microstructure and tensile property

The increasing potential for additive manufacturing technology continuously drives the need for printable materials, including novel aluminum alloys. Ce is considered an economically feasible addition that is beneficial to corrosion resistance and high temperature performance of aluminum alloys. In this study, a binary Al–10Ce alloy was additively manufactured by laser powder bed fusion (LPBF) using gas-atomized powders for the first time. Initial investigation was carried out to determine the optimal LPBF parameters for the Al–10Ce alloy, which was found using the laser power of 350 W and scan speed of 1400 mm/s. Alloy samples with nearly full density and outstanding printability were obtained. Room temperature tensile tests of the as-built Al–10Ce alloy yielded 222 ± 2 MPa in yield strength, 319 ± 1 MPa in tensile strength and 10.8 ± 0.1% in elongation. This is superior to the cast counterpart reported in the literature. A uniform microstructure was observed throughout the alloy, and it primarily consisted of extremely fine-scale eutectic Al and Al11Ce3 intermetallic ribbons, arranged in a skeleton pattern. This fine microstructure would have originated from the rapid cooling inherent to the LPBF process, and strongly suggests the presence of the Orowan strengthening mechanism. This study demonstrated that the binary Al–10Ce alloy is a promising base composition with good printability that brings new possibilities for future ternary and higher-order alloy design for LPBF.

In-situ alloyed, oxide-dispersion-strengthened CoCrFeMnNi high entropy alloy fabricated via laser powder bed fusion

By using a blend of pre-alloyed CoCrFeNi powder and elemental Mn powder, quasi-equiatomic CoCrFeMnNi high entropy alloy (HEA) has been in-situ alloyed via laser powder bed fusion (LPBF). Besides being homogeneously dissolved into the HEA matrix, Mn also forms oxide particles with oxygen originating from both powder feedstock and printing atmosphere, resulting in an in-situ alloyed, oxide-dispersion-strengthened (ODS) HEA. The tensile strength of the ODS HEA is significantly improved to a yield strength of 620 MPa and a tensile strength of 730 MPa. The tensile ductility of the ODS HEA is lower than the ones prepared by using fully pre-alloyed powders, but an outstanding compression ductility maintains for the ODS HEA. The deforming behaviour of the alloy has been revealed, and the contribution of Orowan strengthening has been quantified along with other mechanisms. The study provides an approach to developing advanced HEAs with high strength and moderate ductility by LPBF.

Mechanical and Wear Properties of Aluminum-Based Nanocomposites Fabricated through Ultrasonic Assisted Stir Casting

Abstract The aluminum alloy AA6061 with 0.004, 0.008, 0.012, and 0.016 volume fractions of silicon carbide (SiCp) nanocomposites were fabricated using a semi-solid stirring assisted ultrasonic cavitation technique. Specimens are tested for morphology, density, mechanical, and wear properties. The density, strength, and microhardness of the nanocomposites increased with the increase of SiCp nano-reinforcement particles in the matrix. The yield strength of the SiCp-reinforced nanocomposites was evaluated by considering various strengthening mechanisms. It is found that the yield strength of the nanocomposites increased by 96 % for 0.004, 172 % for 0.008, 158 % for 0.012, and 206 % for 0.016 volume fractions of SiCp compared to the base alloy. The dislocation mismatch effect and Orowan strengthening effect are found to play a significant role in the SiCp-reinforced AA6061 nanocomposites. The dislocation mismatch effect increased with the reinforcement particles’ size reduction in the matrix. The Orowan strengthening effect increased up to 1.56 nm critical size of SiCp, and above that it decreased slightly. The experimental results are compared to predicted results obtained from various analytical models. The experimental results are observed to be in close agreement with the Mirza and Chen model. The wear loss of nanocomposites decreased with an increase in the nano-sized SiCp reinforcement quantity in the matrix. The amount of 0.004, 0.008, 0.012, and 0.016 volume fractions of nano-sized SiCp reinforcement additions to the AA6061 alloy matrix decreases the wear coefficient by 19.23 %, 38.46 %, 57.69 %, and 62.69 %, respectively. The worn surfaces were also analyzed using a scanning electron microscope and energy-dispersive X-rays.

Microstructure and tensile properties of magnesium nanocomposites fabricated using magnesium chips and carbon black

In this study, carbon black (0, 0.01, 0.03 and 0.08 wt%) and AZ31 (Mg–3Al–1Zn) magnesium chips were used to fabricate carbon black-reinforced magnesium matrix composites with extrusion or a combination of extrusion and high-ratio differential speed rolling. After hot pressing at 693 K and extrusion at 623 K with an extrusion ratio of 22, the magnesium chips coated with carbon black were soundly bonded into a bulk composite material. The grain sizes of the extruded materials were similar with a size of 48.2–51.5 µm despite the difference in the amount of carbon black. The yield strength and ultimate tensile strength increased from 177 to 191 MPa and from 240 to 265 MPa, respectively, as a result of the addition of 0.01% carbon black; however, a further increase in the strength was marginal with additional carbon black. The same trend was observed in the strain hardening behavior. The tensile elongation increased by to the addition of 0.01% carbon black (from 15.8% to 17.4%) due to the increased work hardening effect, but decreased with additional carbon black due to its agglomeration and poor dispersion at higher concentration. After high-ratio differential speed rolling (HRDSR) on the extruded materials and subsequent annealing, the AZ31 and AZ31 composites had a similar fine grain size of 16.3–17.9 µm. The annealed HRDSR composites showed the best mechanical properties at a higher content of carbon black (0.03%) compared to that (0.01%) for the extruded composites. This resulted from the enhanced dispersion effect of the carbon black due to the high shear flow induced during the HRDSR process. The extruded composites exhibited the three distinct hardening stages (stage II, stage III and stage IV), while the annealed HRDSR composites mainly displayed the stage III hardening. The addition of carbon black increased the strain hardening rate at all the strain hardening stages in both of the extruded and annealed HRDSR materials. At the initial hardening stage, the strain hardening rates of the extruded composites were higher than those of the annealed HRDSR composites, but this became reversed at the later stage of hardening. Possible explanations for this observation were discussed. The strength analysis suggests that dislocation-carbon black interaction by Orowan strengthening and dislocation generation due to a difference in thermal expansion between matrix and carbon black are the major strengthening mechanisms.