Magnesium alloys, while boasting light weight, suffer from a major drawback in their relatively low strength. Identifying the microstructural features that are most effective in strengthening is therefore a pressing challenge. Deformation twinning often mediates plastic yielding in magnesium alloys. Unfortunately, due to the complexity involved in the twinning mechanism and twin-precipitate interactions, the optimal precipitate morphology that can best impede twinning has yet to be singled out. Based on the understanding of twinning mechanism in magnesium alloys, here we propose that the lamellar precipitates or the network of plate-shaped precipitates are most effective in suppressing deformation twinning. This has been verified through quantitative in situ tests inside a transmission electron microscope on a series of magnesium alloys containing precipitates with different morphology. The insight gained is expected to have general implications for strengthening strategies and alloy design.
Friction stir welding (FSW) has been widely adopted in aerospace industry for fabricating high-strength aluminum alloy structures, such as large volume fuel tanks, due to its exceptional advantages including low distortion, less defects and high mechanical properties of the joint. This article systematically reviews the key technical issues in producing large capacity aluminum alloy fuel tanks by using FSW, including tool design, FSW process optimization, nondestructive testing (NDT) techniques and defect repairing techniques, etc. To fulfill the requirements of Chinese aerospace industry, constant-force FSW, retractable tool FSW, lock joint FSW, on-line NDT and solid-state equal-strength FSW techniques, as well as a complete set of aerospace aluminum FSW equipment, have been successfully developed. All these techniques have been engineered and validated in rocket tanks, which enormously improved the fabrication ability of Chinese aerospace industry.
Considerable progress has been achieved in friction stir welding (FSW) of steels in every aspect of tool fabrication, microstructure control and properties evaluation in the past two decades. With the development of reliable welding tools and precise control systems, FSW of steels has reached a new level of technical maturity. High-quality, long welds can be produced in many engineering steels. Compared to traditional fusion welding, FSW exhibits unique advantages producing joints with better properties. As a result of active control of the welding temperature and/or cooling rate, FSW has the capability of fabricating steel joints with excellent toughness and strength. For example, unfavorable phase transformations that usually occur during traditional welding can be avoided and favorable phase fractions in advanced steels can be maintained in the weld zone thus avoiding the typical property degradations associated with fusion welding. If phase transformations do occur during FSW of thick steels, optimization of microstructure and properties can be attained by controlling the heat input and post-weld cooling rate.
Grain refinement could effectively enhance yield strength of Mg alloys according to the well-known Hall-Petch theory. For decades, many studies have been devoted to the factors influencing the Hall-Petch slope (k) in Mg alloys. Understanding the factors influencing k and their mechanisms could offer guidance to design and produce high-strength Mg alloys through effective grain refinement hardening. A review and comments of the past work on the factors influencing k in Mg alloys are presented. Results of these previous investigations demonstrate that the value of k in Mg alloys varies with texture, grain size, temperature and stain. The influence of texture and grain size on k is found to be an essential result of the variation of deformation mode on k value. Without the variation of deformation modes, it is revealed that texture could also impose a significant effect on k and this is also summarized and discussed in this paper. The reason for texture effect on k is analyzed based on the mechanism of Hall-Petch relationship. In addition, it is found in face-centered cubic (fcc) or body-centered cubic (bcc) metals that boundary parameters (boundary coherence, boundary energy and boundary diffusivity) could strengthen twinning or slips to a different extent. Therefore, the role of boundary parameters is also extended into the k values in Mg alloys and discussed in this paper. In the end, we discuss the future research perspective of Hall-Petch relationship in Mg alloys.
A high strength Mg-5.1Zn-3.2Y-0.4Zr-0.4Ca (wt%) alloy containing W phase (Mg3Y2Zn3) prepared by permanent mold direct-chill casting is indirectly extruded at 350 °C and 400 °C, respectively. The extruded alloys show bimodal grain structure consisting of fine dynamic recrystallized (DRXed) grains and unrecrystallized coarse regions containing fine W phase and β2? precipitates. The fragmented W phase particles induced by extrusion stimulate nucleation of DRXed grains, leading to the formation of fine DRXed grains, which are mainly distributed near the W particle bands along the extrusion direction. The alloy extruded at 350 °C exhibits yield strength of 373 MPa, ultimate tensile strength of 403 MPa and elongation to failure of 5.1%. While the alloy extruded at 400 °C shows lower yield strength of 332 MPa, ultimate tensile strength of 352 MPa and higher elongation to failure of 12%. The mechanical properties of the as-extruded alloys vary with the distribution and size of W phase. A higher fraction of DRXed grains is obtained due to the homogeneous distribution of micron-scale broken W phase particles in the alloy extruded at 400 °C, which can lead to higher ductility. In addition, the nano-scale dynamic W phase precipitates distributed in the unDRXed regions are refined at lower extrusion temperature. The smaller size of nano-scale W phase precipitates leads to a higher fraction of unDRXed regions which contributes to higher strength of the alloy extruded at 350 °C.
Cold processing of magnesium (Mg) alloys is a challenge because Mg has a hexagonal close-packed (HCP) lattice with limited slip systems, which makes it difficult to plastically deform at low temperature. To address this challenge, a combination of annealing of as-cast alloy and multi-axial forging was adopted to obtain isotropic ultrafine-grained (UFG) structure in a lean Mg-2Zn-2Gd alloy with high strength (yield strength: ~227 MPa)-high ductility (% elongation: ~30%) combination. This combination of strength and ductility is excellent for the lean alloy, enabling an understanding of deformation processes in a formable high strength Mg-rare earth alloy. The nanoscale deformation behavior was studied via nanoindentation and electron microscopy, and the behavior was compared with its low strength (yield strength: ~46 MPa) - low ductility (% elongation: ~7%) coarse-grained (CG) counterpart. In the UFG alloy, extensive dislocation slip was an active deformation mechanism, while in the CG alloy, mechanical twinning occurred. The differences in the deformation mechanisms of UFG and CG alloys were reflected in the discrete burst in the load-displacement plots. The deformation of Mg-2Zn-2Gd alloys was significantly influenced by the grain structure, such that there was change in the deformation mechanism from dislocation slip (non-basal slip) to nanoscale twins in the CG structure. The high plasticity of UFG Mg alloy involved high dislocation activity and change in activation volume.
High strength-to-weight ratio, commendable biocompatibility and excellent corrosion resistance make Ti alloys widely applicable in aerospace, medical and marine industries. However, these alloys suffer from serious biofouling, and may become vulnerable to corrosion attack under some extreme marine conditions. The passivating and biofouling performance of Ti alloys can be attributed to their compact, stable and protective films. This paper comprehensively reviews the passivating and biofouling behavior, as well as their mechanisms, for typical Ti alloys in various marine environments. This review aims to help extend applications of Ti alloys in extremely harsh marine conditions.
This work aims to elucidate the impact of aluminum-content on microstructure and deformation mechanisms of transformation-induced plasticity (TRIP) steels through macroscale and nanoscale deformation experiments combined with post-mortem electron microscopy of the deformed region. The solid-state transformation-induced mechanical deformation varied with the Al contents, and influenced tensile strength-ductility combination. Steels with 2-4 wt% Al were characterized by TRIP effect. In contrast to 2Al-TRIP and 4Al-TRIP steels, twinning-induced plasticity (TWIP) was also observed in conjunction with strain-induced martensite in 6Al-TRIP steel. This behavior is attributed to the increase in stacking fault energy with the increase of Al content and stability of austenite, which depends on the local chemical variation. The study addresses the knowledge gap with regard to the effect of Al content on austenite stability in medium-Mn TRIP steels. This combination is expected to potentially enable cost-effective alloy design with high strength-high ductility condition.
High strength low alloy (HSLA) steels have been widely used in pipelines, power plant components, civil structures and so on, due to their outstanding mechanical properties as high strength and toughness, and excellent weldability. Multi-phase microstructures containing acicular ferrite or acicular ferrite dominated phase have been proved to possess good comprehensive properties in HSLA steels. This paper mainly focuses on the formation mechanisms and control methods of acicular ferrite in HSLA steels. Effect of austenitizing conditions, continuous cooling rate, and isothermal quenching time and temperature on acicular ferrite transformation was reviewed. Furthermore, the modified process to control the formation of multi-phase microstructures containing acicular ferrite, as intercritical heat treatments, step quenching treatments and thermo-mechanical controlled processing, was summarized. The favorable combination of mechanical properties can be achieved by these modified treatments.
A new cast Mg-2Gd-2Nd-2Y-1Ho-1Er-0.5Zn-0.4Zr (wt%) alloy was prepared by direct-chill semi-continuous casting technology. The microstructure, mechanical properties and thermal conductivity of the alloy in as-cast, solid-solution treated and especially peak-aged conditions were investigated. The as-cast alloy mainly consists of α-Mg matrix, (Mg, Zn)3 RE phase and basal plane stacking faults. After proper solid-solution treatment, the microstructure becomes almost Mg-based single phase solid solution except just very few RE-riched particles. The as-cast and solid-solution treated alloys exhibit moderate tensile properties and thermal conductivity. It is noteworthy that the Mg alloy with 8 wt% multiple RE exhibits remarkable age-hardening response (ΔHV = 35.7), which demonstrates that the multiple RE (RE = Gd, Nd, Y, Ho, Er) alloying instead of single Gd can effectively improve the age-hardening response. The peak-aged alloy has a relatively good combination of high strength/hardness (UTS (ultimate tensile strength) > 300 MPa; TYS (tensile yield strength) > 210 MPa; 115.3 HV), proper ductility (ε ≈ 6%) and moderate thermal conductivity (52.5 W/(m K)). The relative mechanisms mainly involving aging precipitation of β￠ and β′′ phases were discussed. The results provide a basis for development of high performance cast Mg alloys.