Metals and Materials International最新文献

In this work, the influence of equal channel angular pressing (ECAP) on the microstructure, texture, and mechanical properties of ZK60 Mg alloy was investigated. The deformation process by equal channel angular pressing has been performed at the three temperatures of 588, 628, and 668 K and different ECAP pass numbers up to 4. The microstructural evolution was investigated using an optical microscope (OM), scanning electron microscope (SEM) equipped with an EDS detector, and X-ray diffraction (XRD) analyses. After the ECAP process, the microstructure of the cast alloy with an average grain size of about 208 μm converted to the bimodal grain structure. The fractions of fine grains increase and their size decreases with the increasing ECAP pass number and the decreasing deformation processing temperature. The results show that the ECAP process improves the mechanical properties at room temperature and weakens them at high temperatures. In addition, enhancement of the deformation severity through increasing the number of ECAP passes and decreasing the ECAP temperature led to an increase in the hardness of the alloy at room temperature and a decrease in its creep resistance at high temperatures.

Graphical Abstract

Abstract

The wire arc additive manufacturing (WAAM) deposition of Ti–6Al–4V (Ti64) and Ti–6Al–2Sn–2Zr–2Mo–2Cr (Ti62222) alloys produce long columnar grains, indicating tensile anisotropy in various directions. Therefore, this study applied interlayer peening (ILP) during WAAM to modify the solidification morphology. The index of plane anisotropy (IPA) of the as-built Ti64 deposit was 30%, whereas that of the ILP Ti64 alloy was 1%, indicating the significant effect on modifying the solidification morphology and reducing the IPA of the Ti64 deposit. However, the IPA of the as-built Ti62222 alloy was 79%, whereas that of the ILP Ti62222 alloy was 72%, indicating anisotropy in the mechanical properties despite the ILP process. In Ti64 alloy, ILP resulted in larger D_ILP (3.2 mm) than D_remelt (1.87 mm), which impeded the growth of columnar β grains and nucleated the equiaxed β grains during subsequent layer deposition. However, in Ti62222 alloy, owing to its higher hardness and Yield Strength, D_ILP (2.2 mm) and D_remelt (1.97 mm) were approximately the same, thus allowing the growth of columnar β grains to continue during subsequent layer deposition.

Graphic Abstract

Large numbers of long strip-shaped pure MnS inclusions in steel will result in anisotropy of mechanical properties. To obtain more duplex (Ca,Mn)S inclusions in steel through adding Ca can decrease the proportion of long strip-shaped pure MnS inclusions, and anisotropy of mechanical properties can be reduced. In this paper, based on three heats of Ca–S free cutting steel with Al content under 0.01%, the characteristics and formation of Ca–Mg–Al–Si–O + (Ca,Mn)S duplex inclusions in bars were analyzed. The results indicate these duplex inclusions can be classified as four types, named as “Type-C”, “TypeMC-H”, “TypeMC-L”, and “Type-M”, respectively. For Type-C, although they behave spherical, CaS is enriched in (Ca,Mn)S and CaO is enriched in core oxides. For TypeMC-H, CaS content in (Ca,Mn)S is appropriate, they behave spindle-shaped, but CaO content in core oxides closes to Type-C. The formation of Type-C or TypeMC-H consumes lots of Ca element, which makes the overall number of duplex inclusions decrease. For TypeMC-L, their compositions and shapes are both appropriate. For Type-M, although they have higher aspect ratios, their formation can reduce the formation of pure MnS inclusions and improve the distribution of sulfides. Under the condition with specific Ca/S ratio in steel, to obtain more duplex (Ca,Mn)S inclusions for reducing anisotropy of mechanical properties, numbers of Type-C and TypeMC-H should be decreased, and numbers of TypeMC-L and Type-M should be increased. The key is to make SiO₂ content in RH-end oxides as lower as possible, and Al content in steel should be controlled not less than 0.007%.

Graphical abstract

The yield-point phenomenon in recrystallized ferritic steels is often associated with the dislocation multiplication mechanism, wherein the yield drop can be attributed to the lack of mobile dislocations in materials. However, the yield-point phenomenon is not consistently observed in all recrystallized ferritic steels, implying that the dislocation multiplication mechanism has constraints in delineating the yielding behavior of these materials. Therefore, in this study, we introduced grain boundary strength as a critical parameter for elucidating the yielding behavior of recrystallized ferritic steels. Three types of steels—interstitial-free (IF) steel, precipitation-hardened (PH) steel, and Mn-added interstitial-free (IF-2Mn) steel—were analyzed for grain boundary strength using nanoindentation, and the reliability of this methodology was verified by Hall–Petch analysis. The IF steel, which lacked the yield-point phenomenon, demonstrated a much lower grain boundary strength than the PH and IF-2Mn steels, where the phenomenon occurred. Microstructural analysis confirmed that the enhanced grain boundary strengths of the PH and IF-2Mn steels were due to carbon and manganese segregation at the grain boundaries, respectively. Further, the grain boundary strength significantly influenced the tensile properties and yielding behavior. In PH steels, the enhanced grain boundary strength increased the yield strength owing to Hall–Petch hardening; however, it also increased the resistance to plastic deformation propagation, resulting in reduced ductility. In the IF-2Mn steels, the two specimens with different grain sizes exhibited similar yield strengths, which could be attributed to differences in the grain boundary strength. Our findings have significant implications for the design and optimization of ferritic steels.

Graphical Abstract

This study investigates the severe plastic deformation of Ti–6Al–7Nb alloy synthesized through mechanical alloying (MA, 0–120 h) and subsequently processed via spark plasma sintering (SPS, 50 MPa, 1050 °C, 6 min). Advanced characterization techniques such as XRD, optical microscopy, HRSEM, HRTEM, EDAX, and EBSD analysis were employed to analyze the powder and consolidated specimens, focusing on the grain microstructure and formation mechanisms during MA and SPS. Mechanical properties were evaluated through micro-hardness, nano-indentation, and compression tests. The SPS 120 h sample exhibited a fine-grained microstructure dominated by the α-Ti phase, with needle-shaped minor β-Ti phases, while the SPS blended sample (0 h) displayed coarse-grained phases. Processing via MA and SPS significantly influenced the material, rendering it suitable for medical and dental applications. It is confirmed that the 120 h milled nanocrystallite sample demonstrated higher strength, with a micro-hardness of 760 VHN and compressive strength of 905 MPa, compared to the initial blended sample (0 h) with a coarse grain, which exhibited a micro-hardness of 120 VHN and compressive strength of 874 MPa. The influence of various strengthening mechanisms, such as grain boundary strengthening, solid solution strengthening, and dislocation strengthening, were elucidated and correlated with the total strength of the material.

Graphical Abstract

This research aimed to explore the influence of the rare-earth element La on the interface microstructure and mechanical properties of Al/steel bimetallic composites produced through liquid–solid casting. The addition of the rare earth element La refined the morphology of eutectic silicon and ensured its uniform and continuous distribution. The interface structure of the Al/steel bimetallic composite exhibited distinct layering, primarily comprising two layers. The first layer, termed reaction layer I, comprised Al₅Fe₂ and τ₁-Al₂Fe₃Si₃ phases. While the second layer, termed reaction layer II, consisted of Al₁₃Fe₄, τ₅-Al₇Fe₂Si, and τ₆-Al₉Fe₂Si₂ phases. The addition of La did not alter the types of intermetallic compounds present in the Al/steel reaction layer. As the La content increased to 0.3%, there was a notable reduction in the average thickness of both reaction layers I and II, reaching a minimum. The presence of La effectively restrained the growth of intermetallic compounds within the reaction layer. Consequently, the shear strength of the Al/steel bimetallic sample exhibited an initial increase followed by a subsequent decrease with increasing La content. With the addition of 0.3% La, the shear strength of the sample peaked at 30.1 MPa, representing a 66% increase.

Graphical Abstract

Reticular crystal phases and abnormally coarse grains are key problems that restrict the improvement of the mechanical properties and uniformity of Al-Li alloys. The effects of the multidirectional forging (MDF) process on the microstructure at the edge and center and the three-directional mechanical properties of the 2195 Al-Li alloy were investigated. The results show that the strong deformation resistance produced by one heat forging at 400 ℃ with seven upsetting and six stretching (400-7U6S-1) fully broke the reticular crystal phases at the grain boundaries and obtained the dispersed phase structure. The high density of dislocations accumulated by strong deformation promoted the dissolution of the dispersed secondary phases, and the area fraction of the secondary phase particles at the edge and center decreased from 3.88% and 1.97–0.75% and 0.61%, respectively, which prevented the occurrence of intergranular fractures and dramatically improved the ductility. Meanwhile, the dissolution of the second phases enhanced the precipitation force of the T1 phases and inhibited the precipitation of δ’ phases. Furthermore, the higher density of dislocations significantly increased the nucleation rate of dynamic recrystallization and eliminated the abnormally coarse grains, and thus acquired a uniform ultra-fined grain structure and the average grain diameter was reduced from 159 μm to 17 μm. The tensile strength, yield strength and elongation in the width direction increased to 592 MPa, 545 MPa and 8.0%, respectively, and increased by 7.2%, 7.2% and 90.5%, respectively. In particular, the maximum difference in the elongation of the forgings in the width direction decreased from 83.3 to 11.1%.

Graphical Abstract

In this study, a Ni–Fe–Si–B amorphous composite coating is coated on H13 steel by laser cladding. Coatings are systematically investigated for their microstructure, phase composition, tribological behavior, and mechanical characteristics. X-ray diffraction results demonstrate that the cladding layer can be divided into the interface, transition, and compositionally stable zones, where the coating has both crystalline and amorphous phases, with up to 57% of the coating being amorphous. According to scanning electron microscopy and transmission electron microscopy analyses, the middle and surface regions of the coating mainly consist of (Fe_0.5Ni_0.5)₃Si, Fe₂B, Fe₂NiB, Ni₃₁Si₁₂, and amorphous phases. The in-situ generated Fe₂B phase is uniformly distributed within the coating, leading to a significant enhancement in microhardness. The greatest hardness of the coating is approximately 927.04 HV_0.2. The composite coating exhibits excellent wear resistance, which is approximately 1.71 times greater than that of the substrate. Minor abrasive wear constitutes the primary wear mechanism for the coatings.