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Poor chemical stability of graphene in titanium (Ti) is one critical problem to make use of the extraordinary properties of graphene for obtaining high-performance graphene/Ti composites. To overcome this problem, the selective laser melting (SLM) process with rapid heating and cooling characteristics was exploited to fabricate graphene nanosheet (GNS)-reinforced Ti–6Al–4V matrix composite in this study. Results showed that the SLMed composite registered super-high tensile strength (1526 MPa) and high Young's modulus (145 GPa), which were 73% and 26% lager than those of spark plasma sintered composites fabricated from the same powder mixture, respectively. It was found that the locally reacted GNSs and in-situ formed ultrafine TiC particles mainly contributed to the outstanding mechanical properties of SLMed GNSs/Ti composites. The underlying mechanism was thoroughly discussed based on microstructures and strengthening models. The results pave up the way towards the fabrication of super-strong Ti matrix composites. Image 1 • GNSs/Ti composites were fabricated by combination of facile mixing and SLM. • Nano-reinforcements and laser scanning strategy impacted on orientation of α-Ti laths. • Ti–TiC-GNSs interface in SLM GNSs/Ti composites were examined by HRTEM. • SLM GNSs/Ti composite possessed super high strength and high modulus. • Load transfer and CET were major strengthening mechanisms of SLM GNSs/Ti composites. [ABSTRACT FROM AUTHOR]

Dissatisfactory load transfer has been a critical issue in carbon nanotube (CNT)- and graphene-reinforced metal matrix composites (MMCs) mainly because of the intrinsically unpleasant carbon-metal interfaces. Here we show by introducing in-situ Al 2 O 3 nanoparticles at aluminum (Al)-CNTs interface, the load transfer efficiency can be noticeably enhanced in powder metallurgy CNTs/Al composites. From in-situ tensile tests, the nanoparticle-modified Al-CNTs interfaces result in CNT fracture, a sign of high load transfer efficiency; while clean interfaces without nanoparticles lead to CNT pulling-out. The nanoparticle-induced enhancement of interface strength can be explained by the increased sliding resistance of CNTs in MMCs at the wake of cracks under tensile loading. Our study provides a new strategy for designing strong carbon-metal interfaces to fabricate high-performance nanocarbon-reinforced MMCs. Image 1 • A type of strong Al-CNT interface was studied by TEM and in-situ tensile tests. • Clean Al-CNT interfaces led to CNT pulling-out. • Al-CNT interfaces with in-situ alumina nanoparticles resulted in CNT fracture. • Alumina nanoparticles increased interfacial strength without damaging CNT structure. [ABSTRACT FROM AUTHOR]

Oxygen and nitrogen have been both known to have a strong hardening effect on Ti and its alloys, while also imposing a serious embrittlement effect. In the present work, Ti samples with non-uniformly dispersed oxygen and nitrogen solid solutions were produced using high purity Ti via powder metallurgy (PM) methods. The experimental results suggested that, when the solutes are non-uniformly distributed, the high solute content region can serve as strengthening particles as that in metal matrix composites. In light of this, we propose a design of novel structure in high oxygen/nitrogen Ti materials to achieve both improved strengths and ductility. The highlight of the structure is a design of strain gradient at the matrix-particle interface, which can mitigate the strain compatibility that commonly reported in general MMCs as the crack initiator. Instead of using ceramics as reinforcements, here we propose to use the matrix phase itself that hardened by high oxygen/nitrogen solutes as reinforcing particles. [ABSTRACT FROM AUTHOR]

In this paper, we report that a selective laser melted (SLM) AlSi10Mg alloy has unique hierarchical microstructures. There are micro-sized Al grains and transgranular ultrafine cellular structures around primary Al, and the cell boundary is made up of alternate eutectic Si and eutectic Al phases. The SLM AlSi10Mg alloy has both high tensile strength and high strain-hardening capability, which are superior to Al-Si alloys fabricated by conventional powder metallurgy and cast methods. Post-tensile high-resolution microscopy analyses provide further evidence supporting Orowan looping as a strengthening mechanism of the SLM Al-Si alloy. [ABSTRACT FROM AUTHOR]

In the present work, we studied the effect of the aspect ratio of carbon nanotubes (CNTs) on strengthening aluminum metal matrix composites (Al MMCs). To this end, Al samples reinforced with CNTs of various aspect ratios were produced via three different powder metallurgy methods. Microstructural examination revealed that the CNTs were uniformly dispersed in the materials with a range of aspect ratios from 6.5 to 55. The tensile results showed that the CNTs exhibited a strong strengthening effect in the composites regardless of their aspect ratios. However, the post-loading examination and quantitative analysis indicated that there was a strengthening mechanism transition for CNTs, which was closely associated with the aspect ratio or length of CNTs. The origin of such transition was explored from the viewpoint of dislocation-CNTs interaction under loading. The findings may provide a new insight in understanding the strengthening behaviors of CNT-reinforced MMCs. [ABSTRACT FROM AUTHOR]

Dynamic recrystallization, grain refinement and strengthening mechanism were investigated in the ultrastrong and ductile α Ti-xSn alloy processed by spark plasma sintering (SPS) and hot extrusion. Compared with pure Ti, the yielding and ultimate tensile strength of Ti-xSn alloy were greatly increased by 140% (993.88 MPa) and 92% (1086.76 MPa). The alloyed solutes dragged nucleation and grains growth of the recrystallization process. The bimodal microstructure was occurred and the average grain size was refined with increasing the alloyed contents. The fraction of deformed strips was increased but the fraction of the recrystallized grains was decreased. Brittle Ti 3 Sn compound (∼100 μm) was formed in the high Sn content samples. The quantitative analysis of the relationship between the fraction of recrystallized grain, low angle grain boundary fraction, activation energy for grain growth and solid solute atoms indicated either a linear or parabolic relationship. Quantitative analysis of strengthening effects pointed out that solid-solution strengthening effect, strain hardening and boundary strengthening were the main contributors to the great strength enhancement. The toughening mechanism was discussed by the compatible uniform strain and postponed necking. Rapid cracks initiation and propagation after uniform strain led to gradual ductility sacrifice. The present work may provide a new understanding on the relationship among the mechanical properties, microstructure and processing of Ti alloy. [ABSTRACT FROM AUTHOR]

In the present work, we report a novel bimodal and multi-layered grain structure in pure titanium produced via powder metallurgy. It was found that the hot-extruded pure Ti consists of multiple grain layers, which exhibited substantially different mean grain sizes. The microstructural development during hot extrusion was then investigated for the pure Ti via an interrupted extrusion experiment. The influence of this unique structure on the mechanical properties of the material was also studied under uniaxial quasi-static tension. The experimental results showed that the samples with different arrangement of the grain layers exhibited very different mechanical behavior. Namely, with combining a small part of fine grain layers, the material showed significantly increased yield strength and slightly decreased uniform plastic strain. Yet, the elongation-to-failure was decreased markedly for the multilayered material. Postmortem examinations indicated that this may attribute to the absence of deformation twins in the fine grains that leads to formation of microvoids, which finally develop into large cracks. [ABSTRACT FROM AUTHOR]

Many studies have shown that carbon nanotubes (CNTs) have excellent strengthening effect as a reinforcement in pure aluminum (Al). However, it is unknown that whether such merit can be also achieved in strong Al alloys for achieving higher-strength Al matrix composites. In this study, CNTs were incorporated into pure Al and a high-strength Al alloy (AA5083) via the same powder metallurgy route. Results showed that during the dispersion stage, the high energy ball milling process caused different amounts of deformation to pure Al and AA5083, which led to a better dispersion state of CNTs in powders and composites of pure Al. Although the two composites had the same tensile strength coincidentally, the reinforcing effect of CNTs in pure Al composite was more than five folds larger than that in AA5083, suggesting a strong matrix effect on strengthening behavior of CNTs in Al matrix composites. The present finding may show significant guidance for the optimization of fabrication processes and development of high-performance CNTs/Al composites. [Display omitted] • Strengthening behavior of CNTs/pure Al and CNTs/AA5083 composites were compared. • The dispersion states of CNTs in the CNTs/pure Al and CNTs/AA5083 composites were compared especially. • The reinforcing efficiency of CNTs under poor dispersion state and fine dispersion state were discussed. [ABSTRACT FROM AUTHOR]

In this study, a pre-mixing process was applied to assist a high-energy ball milling (HEBM) process to fabricate high-strength aluminium (Al) matrix composites reinforced with carbon nanotubes (CNTs) by powder metallurgy. Effects of initial state on the dispersion quality of CNTs in composites and resultant mechanical properties were investigated using electron microscopy and tensile testing, respectively. Results showed that after pre-mixing, raw CNT agglomerations became flattened CNT clusters, and evolved into individually dispersed CNT fragments on flaky Al powder surface after HEBM for 12 h. However, CNT clusters remained in the same HEBM process without pre-mixing. Both individual CNTs and CNT clusters were observed inside cold-welded Al particles with a prolonged milling time of 24 h. By applying pre-mixing, the reinforcing effect in CNT/Al composites increased by more than 100%. The distinct evolution processes of CNT dispersion during HEBM with different initial states have been discussed. [ABSTRACT FROM PUBLISHER]

[Display omitted] • Developed dual-textured Ti extrudates via utilizing the β transus in CP-Ti. • Revealed the <0001>//ED and <1011>//ED texture on the firstly extruded portion and <1010>//ED texture on the rest extrudate. • Rationalized the formation mechanism of this novel dual-textured microstructure. • Established the correlation between elemental inhomogeneity and mechanical property deviation along the Ti extrudates. In contrast with titanium (Ti) alloys, the α+β region for commercially pure Ti is neglectable. Therefore, its β transus (T β) can be regarded as a watershed for β-Ti and α-Ti. Utilizing this characteristic, we fabricated dual-textured Ti extrudates through a powder metallurgy route via manipulating the hot extrusion temperature. The crystallographic texture along extrusion direction (ED) was analyzed. Different from the sole <1010>//ED texture throughout the entire extrudate in previous studies, our extrudates exhibited unique dual-textured microstructure. The firstly extruded portion manifested <0001>//ED and <1011>//ED, whereas the rest extrudate displayed <1010>//ED. This was attributed to the realtime extrusion temperature of each location in comparison with T β. On the other hand, the last portion of the extrudate contained much more interstitial nitrogen and oxygen, which led to extremely high strength via solid solution strengthening. This was resulted from a combination effect of diffusion-controlled element absorbing and material flow during extrusion. Our findings may open up a new direction in developing high-performance materials with variable microstructure and mechanical properties for medical applications like neoarthrosis. [ABSTRACT FROM AUTHOR]

• Nano SiCp was used to decorate the FLG-Ti interface in FLG reinforced Ti matrix composite. • The structural integrity of FLG in the composite was protected by SiCp decoration. • The in-situ chemical reaction between FLG and Ti was inhibited by SiCp coating. • The thickness of the in-situ TiC layer on the interface decreased due to the SiCp decoration. • The functionality of SiCp decoration was clarified via thermodynamic and kinetic calculations. [Display omitted] The high reaction tendency between graphene and titanium (Ti) matrix during fabrication of graphene/Ti composites aggravates the structural integrity of graphene and limits its outstanding strengthening potential. In order to solve this problem, in this study, few-layered graphene (FLG) decorated with SiC nanoparticles (SiCp@FLG) was synthesized to reinforce Ti-6Al-4V (Ti64) alloy via a powder metallurgy route. Microstructure and Vickers hardness of the developed SiCp@FLG/Ti64 composites were studied systematically. Results showed that the SiCp decoration changed the morphology of FLG/Ti interface from continuous TiC reaction layer to discontinuous TiC nanoparticles during hot-press sintering. This discontinuous interface not only kept the integrity of FLG by suppressing its reaction with Ti matrix but also led to direct bonding between FLG and Ti. This designing strategy of protecting the reinforcement by secondary particle decoration can be potentially applied to the development of high-performance metal matrix composites. [ABSTRACT FROM AUTHOR]

Graphite powder is the most widely used source of carbon in making powder metallurgy components. In this study, the graphite powder was replaced by the nanoscale carbon black powder. Compared to graphite powder, the carbon black powder provides more uniform distribution, higher apparent density, and faster flow rate for the Fe–0·8%C powder mixture. After sintering, the homogenisation of carbon, microstructure, and hardness are all similar between the two carbon powders. But a higher amount of spring back during ejection and thus lower green density was attained by using carbon black powders. Nonetheless, the compacts using carbon black powder had higher amounts of shrinkage, which compensated for the spring back effect during sintering, thus giving the same density as the graphite powder. [ABSTRACT FROM AUTHOR]

In the present work, a powder rolling process was utilized in the fabrication of fine grained pure Mg via powder metallurgy. Mg flakes were obtained after each rolling process, and broken into pieces for further rolling or sintering. Mg samples of experiencing 0, 5 and 10 rolling passes were obtained following spark plasma sintering (SPS) and hot extrusion. Microstructural results from electron backscatter diffraction (EBSD) revealed that, without experiencing powder rolling, the specimen contained a great number of residual tensile twins; in contrast, after powder rolling the specimen showed uniform and equiaxed grain structures. In addition, the average grain size was measured to be around 9.2, 2.9 and 2.1 µm for the samples subjected to 0, 5 and 10 rolling passes. The powder rolled specimens were found superior in mechanical properties. Post-loading microstructure examinations were also performed for the samples and a discussion regarding the relationship between their mechanical behavior and microstructures was provided. [ABSTRACT FROM AUTHOR]

Homogeneous graphene dispersion and suitable interface bonding are two key challenges to realize the high strengthening potential of graphene in metal matrix composites (MMCs). In this study, graphene nanosheets (GNSs) with deposited nickel (Ni) nanoparticles were synthesized to simultaneously overcome the two challenges. Ni-coated-GNS-reinforced aluminum (Al) MMCs were fabricated by graphene synthesis, Ni decoration and subsequent powder metallurgy. It was revealed that Ni-coated GNSs exhibited noticeably enhanced strengthening effect compared with graphene reinforcements in most previous studies. With the addition of 1.5 wt% Ni-coated GNSs, the composites exhibited 132% higher strength than that of unreinforced aluminum. The enhancement in Vickers hardness and Young's modulus further confirmed the remarkable strengthening effect. The high strength and high Young's modulus were examined by strengthening models, and a comparison between experimental and theoretical values was reached. The main strengthening mechanisms in the Ni-coated GNSs/Al composites were clarified as dislocation-related ones. [ABSTRACT FROM AUTHOR]

Strengthening-toughening mechanism of powder metallurgy Ti-Si alloy was investigated by interrupted in-situ tensile tests. The fabricated Ti-Si alloy was strong and ductile. Slip band was blocked mainly by solutes and grain boundaries. Narrow deformed strips, aligned low angle grain boundaries and twin boundaries also played a positive role in this blockage. Cross-slip in the strain hardening stage lowered the difference value of ultimate tensile and yielding strength. Two-stage toughening mechanism was studied (necking onset was demarcation point). In the homogeneous deformation period before necking, strain compatibility was improved by the intergranular rotation (rotation axis along [0001], [-1100] and [-1102]), high-angle intragranular rotation, intergranular sliding and minor twinning deformation. In the heterogeneous deformation period after necking, microcracks initiation/coalescence was dragged by uniform stress distribution. Cracks propagation was also suppressed by dense grain boundaries and front plastic grains. The present comprehensive analysis of strength and toughness provided an in-depth understanding about the relationship among the processing, mechanical properties and microstructure of Ti alloy. [ABSTRACT FROM AUTHOR]

Twinning mechanism of powder metallurgy Ti-Si alloy was investigated by interrupted in-situ tensile tests. Extension twins {10−12}<10-1-1> in the fine-grained Ti-Si alloy were found in the uniform deformation period, but no twinning in the coarse pure Ti. Three deformation twinning nucleation mechanisms were proposed: i) local stress concentration by neighboured slip incompatibility, ii) slip/twin oriented relationship in the parent grain and iii) slip/twin transfer by high Luster-Morris oriented relationship. The interior back-stress state, grains rotation and dislocations pile-up drove the occurrence of detwinning phenomenon. Silicon-facilitation twinning verified the hypothesis that the substitutional Si solutes affected the core structures and thus the mobility of screw dislocations. Enhanced driving force and decreased energy barrier of nucleation in the micro/atomic scale were further proposed in the twinning activation. It was expected to deepen the understanding of twinning/detwinning behaviors and supply direct evidences building immature twinning mechanism. In-depth understanding about the relationship among the processing, mechanical properties and microstructure of Ti alloy was facilitated in the present work. [ABSTRACT FROM AUTHOR]

• Heat treatment promotes the Ti/C interfacial reaction of the CFs/TC4 composite. • The thickness and structure of the TiC interface are examined. • The 900CFs/TC4 composite shows an excellent strength-plasticity match. • Suitable interface TiC thickness and grain size are beneficial for the mechanical performance. Weak interfacial bonding was a significant problem in titanium matrix composites (TMCs) reinforced with carbon fibers (CFs). In this study, heat treatment (HT) up to 1000 °C was applied on powder metallurgy Ti-6Al-4V matrix composites reinforced with discontinuous CFs to obtain different interfacial structures. Mechanical tests at both room-temperature and 500 °C showed that the best mechanical properties were attained at HT temperature of 900 °C. To understand the underlying mechanism, the phase structure, grain morphology and interfacial microstructure were examined. It was found that as HT temperature was increased, the interface thickness increases continuously and obvious grain growth happens at 1000 °C. Suitable interfacial reaction and fine grain size are identified as critical structural parameters to obtain high-performance CFs/TMCs. This study may provide new insight into the role of interfacial structure played in the strengthening effect of CFs/TMCs. [ABSTRACT FROM AUTHOR]