In recent years, much progress has been made on the development of aerospace materials for structural and engine applications. Alloys, such as Al-based alloys, Mg-based alloys, Ti-based alloys, and Ni-based alloys, are developed for aerospace industry with outstanding advantages. Composite materials, the innovative materials, are taking more and more important roles in aircrafts. However, recent aerospace materials still face some major challenges, such as insufficient mechanical properties, fretting wear, stress corrosion cracking, and corrosion. Consequently, extensive studies have been conducted to develop the next generation aerospace materials with superior mechanical performance and corrosion resistance to achieve improvements in both performance and life cycle cost. This review focuses on the following topics: (1) materials requirements in design of aircraft structures and engines, (2) recent advances in the development of aerospace materials, (3) challenges faced by recent aerospace materials, and (4) future trends in aerospace materials.

In recent years, magnesium (Mg) and its alloy are being studied for their potential use in orthopedic implants with the novel ability to biodegrade after the implant serves its therapeutic function. Pure Mg, by itself, would not be suitable for use in a load-bearing implant application, due to its high corrosion rate and poor tribological properties. However, through proper alloying, this degradable metal is capable of achieving good mechanical properties reasonably similar to bone, a retarded rate of corrosion and enhanced biocompatibility. Previous studies have shown that alloying Mg with aluminum, lithium, rare earth (RE), zinc (Zn), and calcium (Ca) result in lower corrosion rates and enhanced mechanical properties. Despite the growing popularity of Mg and it alloys, there is relatively little information in the literature on their wear performance. In this paper, we report on an investigation of the directional tribological properties of Mg and Mg-Zn-Ca-RE alloy fabricated via two different manufacturing processing routes: as-cast and hot-extruded after casting, with extrusion ratios of 10 and 50. Pure Mg was cast 350°C. After casting, Mg-Zn-Ca-RE alloy was heat-treated at 510°C. Another Mg-Zn-Ca-RE alloy was hot-extruded at 400°C. Dry sliding wear tests were performed on as-cast and hot-extruded pure Mg and Mg-Zn-Ca-RE alloys using a reciprocating test configuration. Wear rate, coefficient of friction and wear coefficient were measured under applied loads ranging from 0.5–2.5N at sliding frequency of 0.2 Hz for 120 cycles, using microtribometery. Wear properties of the extruded specimen were measured in cross-section and longitudinal section. In the longitudinal section studies, wear properties were investigated along the extrusion direction and the transverse direction. Hardness properties were evaluated using microindentation. Cross-section and longitudinal section were indented with a Vickers indenter under applied load of 2.94 N. Alloying and extrusion enhanced the mechanical properties significantly, increased hardness by 80% and wear resistance by 50% compared to pure Mg. Despite the low hardness in both Mg and the Mg alloy cross-sections, the cross-sections for both displayed higher wear resistance compared to the longitudinal section. In the longitudinal section, wear resistance was higher along the transverse direction of the longitudinal section for both Mg and the Mg alloy. The wear coefficient was used to evaluate how the wear behavior of the material varied with respect to alloying, fabrication process, and direction of wear. The wear coefficient of pure Mg decreased as the extrusion ratio increased, thus, increasing the specific wear rate. The opposite behavior was found in the Mg alloy: as the wear coefficient increases, the specific wear rate decreases. The active wear mechanisms observed on the worn surface of Mg were fatigue, abrasive, adhesive and delamination wear. The same wear mechanisms were observed in the Mg alloy except for fatigue wear. Surface microstructure and topographical characterization were conducted using optical microscopy, scanning electron microscopy mechanical stylus profilometry, and optical profilometry.

The influence of Nb contents between 0.20 and 1.20wt% on the grain size and mechanical properties of 18wt% Cr ferritic stainless steel produced by investment casting was investigated. The average grain sizes of the three steels decreased apparently with increasing Nb content mainly due to the increasing number of pre-existing oxides formed at higher temperature, which were more likely to be the nuclei of heterogeneous nucleation. The thermodynamic analysis of Nb(C,N) formation was in conformity to the experimental result that the Nb(C,N) precipitates became larger with increasing Nb content. The as-cast specimen with the smallest grain size of steel C had the worse tensile strength and elongation in comparison with the as-cast specimens of steels A and B, mostly owing to the catenarian and dendritic Nb(C,N) particles distributed densely at the grain boundaries. The mechanical properties of specimens were not improved remarkably through high temperature solid-solution, whereas the mechanical properties of normalized specimens in the three steels were improved to different degrees. The coalescence and sparse distribution of smaller precipitates at grain boundaries after normalizing effectively weakened the local stress concentration arising from the reticular distribution of particles. The normalized specimen of steel A with 0.24wt% Nb still showed good mechanical properties. Normalizing at 850°C for 2h is the appropriate heat treatment for the 18wt% Cr ferritic stainless steel. The comparatively rational Nb content of the ferritic stainless steel is between 0.20 and 0.40wt% for investment casting production.

The effect of Ti contents between 0.10 and 0.50wt% on the grain size and mechanical properties of UNS S44100 ferritic stainless steel produced by investment casting was investigated. The mechanical properties were related to tensile strength and elongation. The average grain sizes of the as-cast specimens decreased obviously with increasing Ti content due to the increasing number of (Ti,Nb)(C,N) precipitates, with sizes of 2.0–4.0µm, acting as the nuclei for heterogeneous nucleation. The average sizes of TiN clusters in steels 2 and 3 were 3.6 and 7.0µm, respectively, whereas no TiN clusters were discovered in steel 1 with 0.13wt% Ti. The experimental results were in good agreement with the thermodynamic analysis of TiN formation. The precipitation temperature of TiN showed a rising trend with increasing Ti content, which implies that larger TiN clusters are more likely to be induced with Ti contents greater than 0.30wt%. Some as-cast specimens were normalized at 850°C for 2h in order to improve the mechanical properties. In addition, the morphology of the TiN clusters, which caused a sharply decline in the mechanical properties of the as-cast specimens with increasing Ti content, showed no change after normalizing. The tensile strengths of the normalized specimens in the three steels increased to different degrees and the improvement of elongation in steel 1 was remarkable. The comparatively rational Ti content of UNS S44100 ferritic stainless steel for meeting the requirements of investment casting production is between 0.10 and 0.20wt%.

Magnesium and its alloys have been found to potential candidates for biodegradable implant applications. However, magnesium and its alloys are broadly known to have poor tribological properties, but detailed specifics on wear performance are scarce. This research investigates the tribological characteristics on Mg-Zn-Ca-RE alloys and pure magnesium under as-cast and extruded conditions. Pure magnesium and Mg-Zn-Ca-RE alloys were hot extruded at 350°C and 400°C. Magnesium and Mg-Zn-Ca-Re alloy were also cast at 350°C and heat treated at 510°C. Directional wear properties were investigated using a CETR-UMT 2 microtribometer under unlubricated conditions in a reciprocating configuration for 120 cycles, with normal loads ranging from 0.5N–2.5N. Wear tests were conducted in directions: cross-sectional, longitudinal (along the extrusion direction) and transverse direction (perpendicular to the extrusion direction). Wear properties and friction properties were analyzed using a microtribometer, a mechanical stylus profiler, and microindentation. Surface morphology and microstructure were characterized using optical microscopy, scanning electron microscopy, and optical profilometry. The results show a lower wear rate in the transverse and cross-sectional direction compared to the longitudinal direction.

Grain refinement of AZ31 Mg alloy during cyclic extrusion compression (CEC) at 225–400 °C was investigated quantitatively by electron backscattering diffraction (EBSD). Results show that an ultrafine grained microstructure of AZ31 alloy is obtained only after 3 passes of CEC at 225 °C. The mean misorientation and the fraction of high angle grain boundaries (HAGBs) increase gradually by lowering extrusion temperature. Only a small fraction of twinning is observed by EBSD in AZ31 Mg alloys after 3 passes of CEC. Schmid factors calculation shows that the most active slip system is pyramidal slip and basal slip {0001} at 225–350 °C and 400 °C, respectively. Direct evidences at subgrain boundaries support the occurrence of continuous dynamic recrystallization (CDRX) mechanism in grain refinement of AZ31 Mg alloy processed by CEC.

Magnesium (Mg) based alloys are the most advanced cardiovascular stent materials. This new generation of stent scaffold is currently under clinical evaluation with encouraging outcomes. All these Mg alloys contain a certain amount of rare earth (RE) elements though the exact composition is not yet disclosed. RE alloying can usually enhance the mechanical strength of different metal alloys but their toxicity might be an issue for medical applications. It is still unclear how RE elements will affect the magnesium (Mg) alloys intended for stent materials as a whole. In this study, we evaluated MgZnCaY-1RE, MgZnCaY-2RE, MgYZr-1RE, and MgZnYZr-1RE alloys for cardiovascular stents applications regarding their mechanical strength, corrosion resistance, hemolysis, platelet adhesion/activation, and endothelial biocompatibility. The mechanical properties of all alloys were significantly improved. Potentiodynamic polarization showed that the corrosion resistance of four alloys was at least 3-10 times higher than that of pure Mg control. Hemolysis test revealed that all the materials were non-hemolytic while little to moderate platelet adhesion was found on all materials surface. No significant cytotoxicity was observed in human aorta endothelial cells cultured with magnesium alloy extract solution for up to seven days. Direct endothelialization test showed that all the alloys possess significantly better capability to sustain endothelial cell attachment and growth. The results demonstrated the promising potential of these alloys for stent material applications in the future.

Severe plastic deformation (SPD) has been widely employed to refine the grain size of Mg alloys, with the main objective to improve the strength and ductility of Mg alloys, since the well-known Hall-Petch equation suggests that a decreased grain size leads to an increased yield strength. However, the yield strength of Mg alloys processed by SPD is often decreased even though the grain size is effectively reduced. The abnormal flow behavior in Mg alloys processed by SPD has attracted great attention although this mechanism is still unclear, due to its complex and extensive influence factors. In this paper, the relationships between the processing conditions, grain refinement, and mechanical properties of the SPD treated Mg alloys are reviewed, with the emphasis on the effects of grain size and texture on the yield strength.

In recent years, much progress has been made on the development of biodegradable magnesium alloys as “smart” implants in cardiovascular and orthopedic applications. Mg-based alloys as biodegradable implants have outstanding advantages over Fe-based and Zn-based ones. However, the extensive applications of Mg-based alloys are still inhibited mainly by their high degradation rates and consequent loss in mechanical integrity. Consequently, extensive studies have been conducted to develop Mg-based alloys with superior mechanical and corrosion performance. This review focuses on the following topics: (i) the design criteria of biodegradable materials; (ii) alloy development strategy; (iii) in vitro performances of currently developed Mg-based alloys; and (iv) in vivo performances of currently developed Mg-based implants, especially Mg-based alloys under clinical trials.

The compression behaviour of a solid solution of ThO2 and hyperstoichiometric UO2 was examined up to 11 GPa hydrostatic pressure using x-ray diffraction. We observed a distortive phase transition above 3 GPa from fluorite-type to a structure isotypic with t-zirconia. The transition is tentatively explained by merging of local tetragonal UO2+δ clusters into an itinerant structural distortion. The bulk modulus increases from 162 ± 1 to 199 ± 4 GPa in the tetragonal phase by stiffening of the c-axis compression.

Lanthanum-III-pernitride is unique amongst pernitrides because of the seemingly misfit charge balance and stoichiometry. La+3N2 has been predicted to be stabilized by resonance between two mesomeric states of bonding in the pernitride group, which include a remarkable N‒N single bond limiting state. However, successful synthesis of this compound remained extant. Here, we report synthesis of LaN2 through shock-driven decomposition of lanthanum nitrate. The result is noteworthy as a first quantitative measure of the stability of N‒N single bonds in solids formed at GPa-level pressure and with respect to a possible cross-over of redox potentials in the nitrogen- and oxygen-systems at high pressure and temperature.

Novel deformation structures of commercial pure (CP) Ti induced by equal channel angular pressing (ECAP) at room temperature have been studied by electron backscattering diffraction (EBSD). All the deformation twins occurring in CP Ti, {101¯1}, {112¯1}, {101¯2} and {112¯2} have been revealed surprisingly in one original grain as first, secondary or third generation twins. 3 variants of {101¯2} twins have been identified. The deformation mechanism of CP-Ti during ECAP at room temperature in comparison to the ECAP at elevated temperatures is discussed.

Microstructural evolution was studied quantitatively by electron backscattering diffraction in commercial purity Ti processed by high-pressure torsion (HPT) at room temperature. The results show that a heterogeneous microstructure develops during HPT processing with regions of both nanocrystalline grains (<100 nm) and coarse grains (~1–30 μm). Tensile {10\( \overline{1} \)2} twins were observed in the center of the disk after the first turn of HPT. The microhardness near the disk center increases with increasing HPT turns and the hardness after 5 turns is reasonably homogeneous at radial positions >1 mm. The mechanism of grain refinement is characterized by dynamic recrystallization and the continuous formation of a necklace-like array of fine grains gradually consumes the larger grains in subsequent passes.

With the help of FESEM, high resolution electron backscatter diffraction can investigate the grains/subgrains as small as a few tens of nanometers with a good angular resolution (∼0.5°). Fast development of EBSD speed (up to 1100 patterns per second) contributes that the number of published articles related to EBSD has been increasing sharply year by year. This paper reviews the sample preparation, parameters optimization and analysis of EBSD technique, emphasizing on the investigation of ultrafine grained and nanostructured materials processed by severe plastic deformation (SPD). Detailed and practical parameters of the electropolishing, silica polishing and ion milling have been summarized. It is shown that ion milling is a real universal and promising polishing method for EBSD preparation of almost all materials. There exists a maximum value of indexed points as a function of step size. The optimum step size depends on the magnification and the board resolution/electronic step size. Grains/subgrains and texture, and grain boundary structure are readily obtained by EBSD. Strain and stored energy may be analyzed by EBSD.

Microstructure development and mechanical properties of Al-xMg alloys (x = 0, 1, 5-10 wt%), processed by ECAP at room temperature, have been investigated. The results show that the microstructures of Al-xMg alloys are refined by the interaction of shear bands and their increase in number during ECAP. The addition of magnesium to aluminum promotes the grain refinement. Misorientation increase induced by particles along grain boundaries is observed by using high resolution EBSD. As ECAP strain increases up to 4, the strength of Al-6 wt% Mg alloy increases progressively while the elongation decreases from 31.7% to 5.5%. A good combination of both strength and ductility has been obtained by annealed ECAP. The change in softening mechanism of the Al-6 wt% Mg alloy, processed by 6 passes of annealed ECAP, occurs in the range of 523-573 K. (C) 2012 Elsevier B.V. All rights reserved.

The potential for significant grain refinement in pure aluminium (99.9%) processed by means of equal channel angular pressing at 523 K (T/T m ∼ 0.6) has been demonstrated. The mean grain size was significantly reduced from 1.2 mm to 4 µm after eight passes. The stored energy and high-angle grain boundary fraction increase until 6 passes and then remain almost constant until 8 passes. The grain refinement occurred in two stages, initially driven by the evolution of deformation inhomogeneties (deformation bands, microbands) and then through recovery and the interaction of deformation inhomogeneties into fine grains.

Finite element method was used to study the strain distribution in ZK60 Mg alloy during multi-pass cyclic extrusion and compression (CEC). In order to optimize the CEC processing, the effects of friction condition and die geometry on the distribution of total equivalent plastic strain were investigated. The results show that the strain distributions in the workpieces are inhomogeneous after CEC deformation. The strains of the both ends of the workpieces are lower than that of the center region. The process parameters have significant effects on the strain distribution. The friction between die and workpiece is detrimental to strain homogeneity, thus the friction should be decreased. In order to improve the strain homogeneity, a large corner radius and a low extrusion angle should be used.

Strain-driven grain subdivision of commercial pure titanium during 1-8 passes of equal channel angular pressing (ECAP) at 723 K is investigated quantitatively by electron backscattering diffraction. The grain refinement mainly takes place in the first pass and the efficiency gradually decreases from 1 to 4 passes, reaching saturation with further increasing strain (6-8 passes). 11 0 1 21 tensile twins are firstly observed in the first pass of ECAP. Misorientation gradients and the driving force of dynamic recrystallization are discussed. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The production of primary aluminum is an energy costly process. With the global warming being of concern, the secondary aluminum stream is becoming an even more important component of aluminum production and is attractive due to its economic and environmental benefits. Recycling of aluminum by new solid state recycling techniques instead of conventional remelting and subsequent refining processing can result in significant energy savings. Severe Plastic Deformation (SPD) techniques have been applied for consolidating nano particles into fully dense materials with good mechanical properties. However, solid state recycling of scraps by SPD is only in the beginning. In the present study, degreasing of aluminum chips from the machine workshop was investigated by a thermal method and chemical treatment. Thereafter, the decoated chips were recycled by Cyclic Extrusion Compression (CEC) at deformation temperatures between 400 and 500 °C. The microstructure and mechanical properties of the recycled aluminum scrap processed by SPD were subsequently investigated. The results show that SPD technology provides a promising alternative for recycling of aluminum scrap. Thermal degreasing of aluminum scrap resulted in more oxidization of aluminum scrap particles. Visible interfaces between chips were observed even at a low magnification.

SummaryUltra‐fast pattern acquisition of electron backscatter diffraction and offline indexing could become a dominant technique over online electron backscatter diffraction to investigate the microstructures of a wide range of materials, especially for in situ experiments or very large scans. However, less attention has been paid to optimize the parameters related to ultra‐fast electron backscatter diffraction. The present results show that contamination on a clean and unmounted specimen is not a problem even at step sizes as small as 1 nm at a vacuum degree of 6.1 × 10−5 Pa. There exists an optimum step size at about 50 data acquisition board units. A new and easy method to calculate the effective spatial resolution is proposed. Effective spatial resolution tends to increase slightly as the probe current increases from 10 to 100 nA. The fraction of indexed points decreases slightly as the frame rate increases from 128 patterns per second (pps) to 835 pps by compensating the probe current at the same ratio. The value 96 × 96 is found to be the optimum pattern resolution to obtain optimum speed and image quality. For a fixed position of electron backscatter diffraction detector, the fraction of indexed points as a function of working distance has a maximum value and drops sharply by shortening the working distance and it decreases slowly with increasing the working distance.