Search

Ultra‐high molecular weight polyethylene monofilament is prepared by melt spinning. Differential scanning calorimetry, X‐ray diffraction, scanning electron microscopy, and tensile testing are used to characterize the crystallinity, crystal form, surface morphology, and tensile strength of the monofilament at various draw temperatures and ratios, as well as the interaction between them. The lamellar crystal structure in the fiber is stable at lower draw temperatures, and an excessive draw ratio will aggravate the stress concentration of the intergranular molecular chain in the drawing process, which can easily lead to microfiber fracture. Increasing the draw temperature encourages not only molecular chain disentanglement in the amorphous region of the fiber, but it can also weaken the crystal structure constraint on molecular chain activity, promote the transformation of lamellar structure into extended chain structure, and improve the loadable draw ratio. The elevated draw ratio drives the orientation of the molecular chain, the crystal structure is motivated to draw along the axis, and the crystal grains are refined, resulting in a more compact internal structure of the fiber. Excessive temperature causes drastic changes in the internal structure of the fiber, which is detrimental to improving draw ratio and fiber strength. [ABSTRACT FROM AUTHOR]

In this work, the thermal stability of nanocrystalline (NC) AZ31/TiB2 magnesium matrix composites was investigated, while the microstructure evolution and mechanical properties were analyzed. The results indicated the AZ31/TiB2 still maintained NC structure after annealing at 350 °C for 180 min. Even at the high annealing temperature of 400 °C and 450 °C for 180 min, their average grain size just reached about 124 nm and 155 nm, indicating excellent thermal stability. The TiB2 particles with sub-micron size uniformly distributed in Mg matrix had no change in size and no reaction with matrix during the annealing treatment. Due to the strong Zener pinning effect of TiB2 particles, the grain growth of Mg matrix at high temperature was effectively inhibited. Meanwhile, the solution and precipitation behavior of Al atoms were completed in a short time, due to the existence of many grain boundaries and structural defects. By calculation, the grain growth kinetics was described by the kinetics equation D 8 - D 0 8 = k t and the activation energy Eg for grain growth was 131.6 kJ/mol, which was much higher than that of pure Mg (92 kJ/mol). Due to their excellent thermal stability, the decrease in both compressive yield strength and ultimate compressive strength was no more than 12.2% after annealing treatment. Even annealing at 450 °C for 180 min, the CYS and UCS of the samples were still above 283 MPa and 295 MPa, respectively. [ABSTRACT FROM AUTHOR]

In present work, a novel Co-free (CrFeNi 2) 100- x Y x (x = 0, 3, 6, 9) medium entropy alloys (MEAs) were successfully prepared by vacuum arc-melting method. The results revealed that the addition of rare-earth element Y had a great effect on the phase composition of CrFeNi 2 MEA, which was transformed from the single FCC phase structure to the duplex structure of the FCC and HCP (YNi 5 type) phases. Meanwhile, the phase evolution of the current alloy system was evaluated using the previous criteria (ΔH mix - δ , Ω - δ , Δ χ , VEC and Λ). In addition, the volume fraction of the HCP phase increased significantly with the increasing Y content, which caused the obvious grain refinement. On the other hand, the nanoindentation test results revealed that the nanohardness of the HCP phase (∼10.7 GPa) was more than three times of the FCC phase (∼3.4 GPa). The dislocation nucleation types of the FCC phase (3.68 GPa) and the HCP phase (6.46 GPa) were discussed by calculating their maximum initial plastic shear stress. With the increasing Y content, both the hardness and strength of the alloy were significantly improved, but the plasticity deteriorated. The second phase strengthening played a dominant role in the strength improvement of (CrFeNi 2) 100- x Y x MEAs, while both the solid solution strengthening and grain boundary strengthening contributed slightly to the strength increment. • The expensive Co elements were replaced with atoms by the lower-cost Ni element to reduce costs. • Phase precipitation behavior and mechanical properties of (CrFeNi 2) 100- x Y x (x = 0,3,6,9) MEAs were investigated for the first time. • The differences in mechanical properties and deformation behavior of FCC and HCP phases were explored. • The yield strength and hardness of (CrFeNi 2) 100- x Y x (x = 9) MEAs were increased to 1265 MPa and 485HV, respectively. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Ultrafine-grained Ti/AZ31 magnesium matrix composites were prepared by powder metallurgy. • AZ31 alloys distributed uniformly by submicron/nanometer sized titanium were obtained. • The strengthening mechanisms for Ti/AZ31 composites at room temperature were analyzed quantitatively. • Outstanding strength at elevated temperature of ultrafine-grained Ti/AZ31 composites was confirmed experimentally. Abstract The microstructure of ultrafine grain for magnesium alloys can result in drastic enhancement in their room temperature strength, but the issue of low strength at elevated temperature becomes more serious as well due to grain boundary slide. Here ultrafine-grained Ti/AZ31 magnesium matrix composites with high strength at both room and elevated temperature were prepared by vacuum hot pressing and subsequent hot extrusion. The microstructure of the composite samples before and after consolidation processing was characterized, and the mechanical properties of the as-consolidated bulk samples were measured at room and elevated temperatures. The results indicate that after extrusion ultrafine-grained magnesium alloys were obtained and Ti particulates with particulate size of ∼310 nm disperse in Mg matrix. The magnesium grain of AZ31-15at.%Ti grows from 66 nm to 800 nm. Meanwhile, the relative densities of Ti/AZ31 composites are higher than 99%. The yield strength (YS) of extruded AZ31-15at.%Ti composite at room temperature is 341 MPa, being 2.4 times higher than original AZ31 alloy. Theoretical estimation shows that remarkably enhanced room-temperature mechanical strength attributes to grain boundary strengthening with the contribution ratio of 74%. In addition, the peak stress of extruded AZ31-15at.%Ti composite at 573 K is 82 MPa and ultrafine Ti dispersions are responsible for the enhanced strength. [ABSTRACT FROM AUTHOR]

In this work, nanocrystalline AZ31 magnesium alloy powders, reinforced by submicron TiB2 particles, were prepared via mechanical milling. It was found that TiB2 particles stimulated the fracture and welding of AZ31/TiB2 powders, leading to the acceleration of the milling process. Meanwhile, the TiB2 particles were refined to submicron-scale size during the milling process, and their distribution was uniform in the Mg matrix. In addition, the matrix was significantly refined during the milling process, which was also accelerated by the TiB2 particles. The formation of grain boundary segregation layers also led to the weakened TiB2 peaks in the XRD patterns during the mechanical milling. The grain sizes of AZ31–2.5 wt % TiB2, AZ31–5 wt % TiB2 and AZ31–10 wt % TiB2 powders were refined to 53 nm, 37 nm and 23 nm, respectively, after milling for 110 h. Under the combined effect of the nanocrystalline matrix and the well-dispersed submicron TiB2 particles, the AZ31/TiB2 composites exhibited excellent micro-hardness. For the AZ31–10 wt % TiB2 composite, the micro-hardness was enhanced to 153 HV0.5. [ABSTRACT FROM AUTHOR]

Nanocrystalline AZ61 powders, dispersed with submicron/nanometer Ti particulates, were mechanically milled and a comparative study on thermal stability and mechanical properties was performed. After mechanical milling, Ti dispersions were cracked to submicron/nanometer particulates and Mg grains were refined to nanocrystalline. As illustrated by high-resolution transmission electron microscopy, coherent lattice and semi-coherent lattice interfaces between magnesium and titanium were achieved. Based on high angle annular dark field and corresponding energy dispersive spectrometer maps, Al dissolved into Ti particulates after mechanical milling. Moreover, the emergence of Ti 3 Al phase was verified by X-ray diffraction during mechanical milling and heat treatment. According to heat treatment results, this nanocrystalline magnesium composite exhibits a preeminent thermal stability among nano-structured magnesium alloys. After annealed at 673 K for 600 min, the composite still remained nanocrystalline with average grain size being 68.6 nm. Due to such microstructure, the composite revealed prominent mechanical property with the hardness being 146.6 HV. Simultaneously, the hardness of annealed composite remained high, with the value being 134.7 HV, which could ascribe to the excellent thermal stability. [ABSTRACT FROM AUTHOR]

[Display omitted] • Corrax stainless steel based SLM were prepared under different process parameters. • Many lath martensite (above 99%) and LAGBs (above 50%) were formed in SLMed samples. • The best process parameter was determined as 190 W-1.1 m·s−1. • The sample under the optimal parameter, exhibited superior mechanical properties. • Dislocation strengthening dominated the strengthening mechanism of the SLMed sample. In this paper, selective laser melting (SLM) was used to manufacture corrax stainless steel samples under different parameters. It was found that the SLMed samples were mainly composed by lots of fine martensite (including cellular structure, cellular dendritic grains and blocky grains), and trace austenite. During SLM forming process, a large number of low-angle grain boundaries (LAGBs) and high-density dislocations were formed in the matrix. Meanwhile, the samples showed weak texture and no obvious preference orientation. Moreover, the relative density of all SLMed samples reached above 90%, and the relative density was above 97% when the laser energy density was 54.13–78.19 J·mm−3. Under the optimal process parameters of P = 190 W, V = 1.1 m·s−1, the relative density of sample reached above 99.52 ± 0.09%, while the sample exhibited the best mechanical properties, including the highest microhardness (374.2 ± 6.5HV), yield strength (YS = 946 ± 7.3 MPa), ultimate tensile strength (UTS = 1084 ± 3 MPa) and elongation (EL = 17.64 ± 0.18%). Moreover, the strengthening mechanisms of SLMed samples mainly included grain boundary strengthening, dislocation strengthening and precipitation strengthening, while dislocation strengthening played a dominant role. Besides that, the fracture mechanisms of SLMed samples belonged to ductile fracture, except for the samples prepared with laser energy density below 54.13 J·mm−3. [ABSTRACT FROM AUTHOR]