Thermal Mismatch Strengthening Research Articles

The Preparation and Mechanical Properties of a Pure Titanium-Based Matrix Composite Reinforced with Graphene Nanoplatelets

A lightweight titanium matrix composite material was fabricated by vacuum sintering using semi-powder metallurgy. The graphene nanoplatelets (GNPs) were used as a reinforcement for the titanium matrix composites. Fabricating the composite materials used three steps: dispersion, formation, and sintering. In particular, GNPs were dispersed by ionic liquid through a centrifugal testing machine instead of ball milling in the process. The better pressure for composite forming was 600 MPa. At the same time, the better sintering temperature and holding time were 1200 °C and 3 h. The influences of the GNP addition on the density, microstructure, and microhardness of the Ti/GNP composites were investigated. For the mechanical properties of the composites, we focused on the tensile strength with different GNP contents. The Ti 0.075 wt% and Ti 0.15 wt% GNP composites exhibited yield strengths of 850 and 948 MPa, which demonstrated 66% and 85% increase compared to those of extruded titanium materials with no GNP additive (512 MPa yield strength). The main strengthening mechanisms of Ti/GNP composites are grain refinement strengthening, thermal mismatch strengthening, and dispersion strengthening.

Microstructure and Wear Properties of Cu–La2O3 Composites Prepared by Spark Plasma Sintering

The Cu–La2O3 composite was prepared by internal oxidation method. The friction and wear behavior was examined under different applied load and sliding velocity according to ASTM G99 standard. The results showed the size of La2O3 particles was about 150 nm, which were homogeneous distributed in the Cu matrix. The mean size of Cu grains in the Cu–1.32 wt% La2O3 composite decreased by 60% compared with that of pure Cu. The ultimate tensile strength of the Cu–1.32 wt% La2O3 composite was 328 MPa. The dominant strengthening mechanisms were the grain size strengthening and thermal mismatch strengthening. The wear rate of Cu–1.32 wt% La2O3 composites was lower than that of pure copper and it increased with the increase of applied load and sliding velocity. The mild to severe wear transition appeared at different load and velocity. The wear mechanisms were abrasive wear and oxidative wear in mild wear. However, it changed to plastic deformation and delamination wear in severe wear.

Nanoparticulate dispersion, microstructure refinement and strengthening mechanisms in Ni-coated SiCp/Al-Cu nanocomposites

Ni-coated SiCp/Al–Cu nanocomposites with different addition levels (0, 1, 2 and 3 wt% Ni-coated SiCp nanoparticulates) were successfully fabricated by semisolid stir casting combined with hot extrusion. The microstructures of the as-cast and hot-extruded nanocomposites showed that the grain size could be substantially refined. The dispersion mechanism of the SiCp nanoparticulates was discussed. Because the Al3Ni that was formed on the SiCp surface improved the wettability, more Ni-coated SiCp nanoparticulates were captured as the solid/liquid interface advanced, causing the Ni-coated SiCp to enter the α-Al interior and reduce agglomeration in the grain boundaries. The strengthening mechanism of the SiCp nanoparticulates at room temperature and high temperatures was studied. At 298 K, the strengthening was attributed to the hindrance of dislocation movement by the uniformly distributed SiCp nanoparticulates, grain size refinement and thermal mismatch strengthening. At 453 K and 493 K, the strengthening effect was ascribed to the impeded dislocation climb by the SiCp nanoparticulates distributed in the grain interior and the pinning effect of nanoparticulates on the grain boundaries.

Synergetic effect of GNPs and MgOs on the mechanical properties of Mg–Sr–Ca alloy

Novel Mg composite was fabricated to explore the synergetic effect of GNPs and MgOs on the microstructure evaluation, deformation behavior and mechanical properties of Mg–Sr–Ca alloy. The addition of hybrid reinforcement into the Mg–Sr–Ca alloy led to considerable grain refinement (~53%). The GNPs were homogeneously dispersed in the matrix alloy where they were randomly distributed revealing the pulling out and pulling off performance. The MgOs were accumulated on the surface of GNPs as a result of the synergetic effect of 1 + 1<2 of hybrid reinforcement by Orowan and thermal mismatch strengthening mechanisms. The yield strength of the hybrid composite at both tensile and compressive tests was improved 19.5% and 38.9% respectively. Significantly synergetic effect of hybrid reinforcement was presented by improvement of failure strain at both tensile and compressive tests (~86.5% and ~20.4%, respectively) due to grain refinement and reduction of eutectic phase (~85%). Conversion of detwinning into the dominant slip mechanism in tensile loading and reduction of twin boundaries motion in compressive loading due to addition of hybrid reinforcement resulted hardening phenomena. Also, the Mg–Sr–Ca/GNP + MgO composite illustrated a low corrosion rate in the SBF solution. Comparative plot indicated that novel composites can be considered for the wide range of applications.

Microstructure and mechanical properties of the ultra-fine grained ZK60 reinforced with low content of nano-diamond by powder metallurgy

ZK60 Mg alloys reinforced by Nano-diamond (ND) particles (0.05 wt%) were successfully fabricated by powder metallurgy followed with hot extrusion. ND particles and the precipitated MgZn2 phase were uniformly distributed in the matrix. Compared with the ZK60 Mg alloy, the tensile and compressive yield strengths of the composite exhibited an considerable improvements of ∼390 MPa and ∼430 MPa, an increase rate of ∼15% and ∼44%, respectively. The composite with 0.05 wt% ND exhibited an ultra-high strengthening efficiency, with tension yield strengthening and compressive yield strengthening efficiencies at 302 and 887, respectively. Furthermore, the composite showed an improved tension-compression yield symmetry. Based on theoretical analysis, the high strength of the composites is mainly attributed to thermal mismatch strengthening, grain refinement, Orowan strengthening and texture strengthening.

Fabrication, microstructure refinement and strengthening mechanisms of nanosized SiCP/Al composites assisted ultrasonic vibration

The performances of particulate–reinforced aluminum matrix composites are strongly dependent on alloying elements, precipitates and added particulates. To reveal the sole influence mechanisms of high volume fraction of nanosized particulates on the solidification behavior, microstructure and mechanical properties of aluminum alloys, nanosized SiCP (60 nm) was incorporated into commercial pure Al at different volume fractions (i.e., 0, 1, 3, 5, 7 and 9 vol%) by stir–casting assisted ultrasonic vibration. The results reveal that a fairly uniform dispersion of nanosized SiCP throughout the matrix was achieved at a volume fraction as high as 7 vol%. Average α–Al dendritic sizes were significantly refined from 270 µm for the matrix to 90 µm in the solidified microstructure of nanocomposites. Thermal analysis during solidification indicates that the presence of nanosized SiCP increased the nucleation temperature of α–Al, whilst recalescence during solidification process disappeared. Additionally, the yield and ultimate tensile strength of the nanosized SiCP/Al composites at both ambient temperature and 453 K were remarkably improved, whilst remaining suitable fracture strain. Theoretical analysis suggests that the significant strength increments induced by nanosized SiCP at ambient temperature could be attributed to thermal mismatch strengthening, Orowan strengthening and grain refinement strengthening, while the pinning effect of nanosized SiCP could predominantly account for the strengthening effect at 453 K.

Strengthening effects of encapsulating graphene in SiC particle-reinforced Al-matrix composites

This work aims to reveal the strengthening effects of encapsulating graphene in the tensile strength of SiC particulate Al-based composites. Modified theoretical models are discussed regarding the thermal mismatch, Orowan and fine-grain strengthening mechanisms. It is found that the thermal mismatch strengthening is the dominant mechanism among them, followed by the Orowan strengthening and the fine-grain strengthening to be the last. While comparing the effects of two different types of graphene, we find that sheet graphene makes more strengthening contributions to both thermal mismatch and Orowan strengthening mechanisms compared with encapsulating graphene. Finite element simulations are employed to investigate the variation patterns of these effects with varies particle sizes and volume fractions. A better enhancing effect of encapsulating graphene can be achieved with a larger volume fraction and a smaller particle size.

Atomic-level study on mechanical properties and strengthening mechanisms of Al/SiC nano-composites

Molecular dynamics (MD) models for the study on the mechanical properties of β-SiC particles-reinforced aluminum matrix nano-composites (Al/SiC nano-composites) are established. The nano-composites in the model are fabricated by a powder metallurgy (P/M) process, followed by a hot isostatic pressing and then annealing to room temperature. The fabricated nano-composites have dense and even distributions of reinforced particles. Then representative volume elements (RVEs) of the fabricated nano-composites are built by adding periodic boundary conditions (PBCs). In this way, RVEs with different volume fractions and particle sizes of SiC are produced and put into the simulation of tension tests. The elasticity and strength in single axial tension obtained from MD analysis are in good agreement with those calculated according to the rule of mixture. It is found that the dispersion of SiC particles into the Al matrix leads to a significant enhancement in the strength of nano-composites compared to pure Al, which is also dramatically affected by both the volume fraction and particle size. Additionally, the Al/SiC nano-composites with finer SiC particles get greater enhancement in the mechanical behavior than those with coarser ones. MD analysis clearly shows the contributions of load-transfer effect, thermal mismatch strengthening and Orowan strengthening to the strengthening of Al/SiC nano-composites.

Strengthening mechanisms in ultrasonically processed aluminium matrix composite with in-situ Al3Ti by salt addition

Abstract Al3Ti reinforced Aluminium composites with different weight percent of Al3Ti particles were developed by in-situ reaction of aluminium alloy with potassium hexafluorotitanate (K2TiF6). Ultrasonication of the aluminium melt during salt reaction was carried out to refine the cast microstructure and achieve better dispersion of in-situ formed Al3Ti particles. The in-situ composites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The Al3Ti particles generated in the melt promoted heterogeneous nucleation, which was responsible for grain refinement of the cast microstructure. The well dispersed Al3Ti significantly improved the mechanical properties including ductility, yield strength (YS), ultimate tensile strength (UTS) and hardness. The dominant strengthening mechanism in the composite was the thermal mismatch strengthening followed by Hall-Petch strengthening.

Effect of Al2O3 Nanoparticles as Reinforcement on the Tensile Behavior of Al-12Si Composites

Al2O3 nanoparticle-reinforced Al-12Si matrix composites were successfully fabricated by hot pressing and subsequent hot extrusion. The influence of weight fraction of Al2O3 particles on the microstructure, mechanical properties, and the corresponding strengthening mechanisms were investigated in detail. The Al2O3 particles are uniformly distributed in the matrix, when 2 and 5 wt. % of Al2O3 particles were added to the Al-12Si matrix. Significant agglomeration can be found in composites with 10 wt. % addition of Al2O3 nanoparticles. The maximum hardness, the yield strength, and tensile strength were obtained for the composite with 5 wt. % Al2O3 addition, which showed an increase of about ~11%, 23%, and 26%, respectively, compared with the Al-12Si matrix. Meanwhile, the elongation increased to about ~30%. The contribution of different mechanisms including Orowan strengthening, thermal mismatch strengthening, and load transfer strengthening were analyzed. It was shown that the thermal mismatch strengthening has a more significant contribution to strengthening these composites than the Orowan and load transfer strengthening mechanisms.

A quantitative understanding on the mechanical behaviors of carbon nanotube reinforced nano/ultrafine-grained composites

The mechanical responses of carbon nanotube (CNT) reinforced nano/ultrafine-grained metal matrix composites (MMCs) with the reinforcement located at grain boundaries and atomically disordered regions near the reinforcement have not been discussed yet. Here a theoretical model based on dislocation-controlling constitutive relations of metal matrix and strain gradient effect is adopted to quantitatively understand the contributions of strengthening mechanisms to the mechanical response of these composites. CNT-reinforced nano/ultrafine-grained MMCs will be treated as a two-phase composite with CNT-free regions surrounded by CNT-containing domains, whose mechanical behaviors are different from that of the matrix-only regions. The predictions of this model are in excellent agreement with available experimental results. During the process of deformation, geometrically necessary dislocations induced by strain gradient and thermal mismatch are limited within CNT-containing domains, and the thicknesses of these regions are obtained to be linearly proportional to matrix grain size by fitting with the experimental results. When the size of the reinforcement is large, the effect of thermal mismatch strengthening mechanism on the strength of MMCs is slight and can be ignored compared with the roles of Hall–Petch strengthening or load transfer effect.