Fe–Al–Mn–C lightweight structural alloys: a review on the microstructures and mechanical properties

Abstract

Adding a large amount of light elements such as aluminum to steels is not a new concept recalling that several Fe–Al–Mn–C alloys were patented in 1950s for replacement of nickel or chromium in corrosion resistance steels. However, the so-called lightweight steels or low-density steels were revisited recently, which is driven by demands from the industry where steel has served as a major structural material. Strengthening without loss of ductility has been a triumph in steel research, but lowering the density of steel by mixing with light elements will be another prospect that may support the competitiveness against emerging alternatives such as magnesium alloys. In this paper, we review recent studies on lightweight steels, emphasizing the concept of alloy design for microstructures and mechanical properties. The influence of alloying elements on the phase constituents, mechanical properties and the change of density is critically reviewed. Deformation mechanisms of various lightweight steels are discussed as well. This paper provides a reason why the success of lightweight steels is strongly dependent on scientific achievements even though alloy development is closely related to industrial applications. Finally, we summarize some of the main directions for future investigations necessary for vitalizing this field of interest.     

Effect of the rolling speed on microstructural and mechanical properties of aluminum–magnesium alloys prepared by twin roll casting

In the present investigation the microstructures and mechanical properties of Al–Mg alloys prepared by twin roll casting (TRC), varying rolling speed, (i.e., 3, 4, and 5 rpm) were studied. Optical microstructures of the all the samples show dendritic morphology. Scanning electron microscopy (SEM) and energy dispersive X-ray spectrometer (EDS) studies show segregation of alloying elements at the interdendritic regions. Transmission electron microscopy (TEM) observation confirms the presence of intermetallic phase (Mg₅Al₈) at the grain boundaries. It was observed that the secondary dendritic arm spacing (SDAS) increases (marginally) with rolling speed. Also the microhardness and tensile strength decreases with increase in rolling speed.     

Effects of Ti addition on the microstructures and mechanical properties of the Al–Mn–Mg–RE alloy

The effects of higher Ti addition (near peritectic point) on microstructures and mechanical properties of a designed Al–Mn–Mg–RE alloy were investigated by optical microscopy, scanning electron microscopy, energy dispersive spectroscopy and tensile tests, respectively. The results show that the addition of Ti refined grains evidently, meliorated the morphology and distribution of iron-rich phase, and hence improved the mechanical properties of the Al–Mn–Mg–RE alloy. The fracture mechanisms changed from brittle fracture to ductile fracture after extruding. The addition of Ti refined the constituent particles and resulted in deeper and more homogenized dimples of the tensile fracture surfaces.     

Amorphous Ti–Cu–Ni–Al alloys prepared by mechanical alloying

Ti _x (CuNi)_90–x Al₁₀ (x = 50, 55, 60) amorphous powder alloys were synthesized by mechanical alloying technique. The evolution of amorphization during milling and subsequent heat treatment was investigated by scanning electron microscopy, X-ray diffraction, differential scanning calorimetry and transmission electron microscopy. The fully amorphous powders were obtained in the Ti₅₀Cu₂₀Ni₂₀Al₁₀, Ti₅₅Cu_17.5Ni_17.5Al₁₀ and Ti₆₀Cu₁₅Ni₁₅Al₁₀ alloys after milling for 30, 20 and 15 h, respectively. Differential scanning calorimetry revealed that thermal stability increased with the increasing (CuNi) content: Ti₆₀Cu₁₅Ni₁₅Al₁₀, Ti₅₅Cu_17.5Ni_17.5Al₁₀ and Ti₅₀Cu₂₀Ni₂₀Al₁₀. Heating of the three amorphous alloys at 800 K for 10 min results in the formation of the NiTi, NiTi₂ and CuTi₂ intermetallic phases.     

Effect of heat treatment and Fe content on the microstructure and mechanical properties of die-cast Al–Si–Cu alloys

The effect of solution and ageing heat treatment on the microstructure and mechanical properties of the die-cast Al–9 wt.%Si–3.5 wt.%Cu alloys containing 0.1–1.0 wt.% Fe was investigated. The results showed that the dendritic primary α-Al phase was varied from 20 to 100 μm in size and the globular α-Al grains were smaller than 10 μm in size. The Fe-rich intermetallics exhibited coarse compact or star-like shapes with the sizes from 10 to 20 μm and the fine compact particles at an average size of 0.75 μm. The solution treatment of the alloys could be achieved in a short period of time, typically 30 min at 510 °C, which dissolved the Cu-rich intermetallics into the primary α-Al phase and spheroidised the eutectic Si phase. During the subsequent ageing treatment, numerous fine precipitates of θ′ and Q′ phases were formed to provide effective strengthening to the α-Al phase, significantly improving the mechanical properties. Therefore, Fe content in the die-cast Al–Si–Cu alloys needs to be controlled at a low level in order to obtain the improved ductility and strength under solution and aged condition.     

Effect of Al and C on structure and mechanical properties of Fe–Al–C alloys

Abstract

Effect of aluminium and carbon content on the microstructure and mechanical properties of Fe–Al–C alloys has been investigated. Alloys were prepared by combination of air induction melting with flux cover (AIMFC) and electroslag remelting (ESR). The ESR ingots were hot forged and hot rolled at 1373 K. As rolled alloys were examined using optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to understand the microstructure of these alloys. The ternary Fe–Al–C alloys containing 10·5 and 13 wt-%Al showed the presence of three phases: FeAl with disordered bcc structure, Fe₃Al with ordered DO₃ structure and Fe₃AlC_0·5 precipitates with L′1₂ structure. Addition of high concentration of carbon to these alloys resulted in excellent hot workability and superior tensile at room temperature as well as tensile and creep properties at 873 K. An increase in Al content from 9 to 13 wt-% in Fe–Al–C alloys containing the same levels of carbon has no significant influence on strength and creep properties at 873 K, however resulted in significant improvement in room temperature strength accompanied by a reduction in room temperature ductility.     

Thermal stability and mechanical properties of nanocrystalline Fe–Ni–Zr alloys prepared by mechanical alloying

The thermal stability of nanostructured Fe_100?x?y Ni _x Zr _y alloys with Zr additions up to 4 at.% was investigated. This expands upon our previous results for Fe–Ni base alloys that were limited to 1 at.% Zr addition. Emphasis was placed on understanding the effects of composition and microstructural evolution on grain growth and mechanical properties after annealing at temperatures near and above the bcc-to-fcc transformation. Results reveal that microstructural stability can be lost due to the bcc-to-fcc transformation (occurring at 700 °C) by the sudden appearance of abnormally grown fcc grains. However, it was determined that grain growth can be suppressed kinetically at higher temperatures for high Zr content alloys due to the precipitation of intermetallic compounds. Eventually, at higher temperatures and regardless of composition, the retention of nanocrystallinity was lost, leaving behind fine micron grains filled with nanoscale intermetallic precipitates. Despite the increase in grain size, the in situ formed precipitates were found to induce an Orowan hardening effect rivaling that predicted by Hall–Petch hardening for the smallest grain sizes. The transition from grain size strengthening to precipitation strengthening is reported for these alloys. The large grain size and high precipitation hardening result in a material that exhibits high strength and significant plastic straining capacity.     

Effect of CeLa addition on the mechanical properties of Al–Cu–Mn–Mg–Fe alloy

The rapid development of new energy automobiles leads to an increasing demand for high-strength lithium battery shell alloy. The microstructures, electrical conductivity and mechanical properties of CeLa-containing Al–Cu–Mn–Mg–Fe alloys were investigated with scanning electron microscopy (SEM), X-ray diffraction, Eddy Current conductivity tester, tensile testing and Erichsen cup testing. Experiment results indicate that Al₆(Mn, Fe) particles could be refined by CeLa alloying and AlCuCeLa phase nucleates and grew up at the surface of Al₆(Mn, Fe) particle. Major texture of the CeLa-containing alloys was different from that of the CeLa-free alloy. The electrical conductivity decreased with increase of the CeLa content. CeLa addition could greatly enhance the tensile strength of the alloy at temperatures ranging from –40°C to 300°C.     

Investigation of corrosion and mechanical properties of Al–Cu–SiC–xNi composite alloys

In this study, dry sliding wear behavior and corrosion resistance of Al–Cu–SiC–xNi (x: 0, 0.5, 1, 1.5 wt.%) composites were investigated. Effect of nickel content on the microstructure and hardness of the alloys was also studied. Wear tests were conducted using a ball on disc wear test device. Corrosion behavior of Al–Cu–SiC–xNi composite alloys in 3.5% NaCl solution was investigated by using potentiodynamic polarization, impedance spectroscopy and cronoamperometric methods. The results showed that the hardness of the composite alloy increases with increasing nickel content. Maximum wear resistance is reported with the addition of 1 wt.%Ni. It was determined that corrosion resistance of Al–Cu–SiC composite alloys improved with increasing nickel content in the alloy.     

Thermal and mechanical properties of La–Al–Sb alloys

We fabricated La–Al–Sb alloy by arc melter system and the fabrication conditions were described in detail. Microstructural analyses were performed and it was found that LaSb, Al₁₁La₃ Al₄La, AlLa₃ and Sb phases formed for different heat treatment conditions. The resistivity results showed the metallic and semiconducting type behavior depending on heat treatment temperatures. The thermal-conductivity measurement was performed in the range of 2–300 K and the data were analyzed by the sum of lattice and carrier components. The linear temperature dependence of thermo power indicates metallic type characteristic of the samples. The micro-hardness values of the phases in the samples were analyzed and it was found that there are two different hardness regions in the samples.     

Influence of atomized powder size on microstructures and mechanical properties of rapidly solidified Al–Cr–Y–Zr aluminum alloys

Rapidly solidified powders of Al–5.0Cr–4.0Y–1.5Zr (wt%) were prepared by using a multi-stage atomization-rapid solidification powder-making device. The atomized powders were sieved into four shares with various nominal diameter level and were fabricated into hot-extruded bars after cold-isostatically pressing and vaccum degassing process. Influence of atomized powder size on microstructures and mechanical properties of the hot-extruded bars was investigated by optical microscopy, X-ray diffraction, transmission electronic microscopy with EPSX and scanning electron microscopy. The results show that the fine atomized powders of rapidly solidified Al–5.0Cr–4.0Y–1.5Zr aluminum alloy attains supersaturated solid solution state under the exist condition of multi-stage rapid solidification. With the powder size increasing, there are Al₂₀Cr₂Y (cubic, a = 1.437 nm) and Ll₂ Al₃Zr (FCC, a = 0.407 nm) phase forming in the powders, and even lumpish particles of Al₂₀Cr₂Y appearing in the coarse atomized powders, as can be found in the as-cast master alloy. Typical microstructures of the extruded bars of rapidly solidified Al–5.0Cr–4.0Y–1.5Zr aluminum alloy can be characterized by fine grain FCC α-Al matrix with ultra-fine spherical particles of Al₂₀Cr₂Y and Al₃Zr. But a small quantity of Al₂₀Cr₂Y coarse lumpish particles with micro-twin structures can be found, originating from lumpish particles of the coarse powders. The extruded bars of rapidly solidified Al–5.0Cr–4.0Y–1.5Zr aluminum alloy by using the fine powders eliminated out too coarse powders have good tensile properties of σ_0.2 = 403 MPa, σ_b = 442 MPa and δ = 9.4% at room temperature, and σ_0.2 = 153 MPa, σ_b = 164 MPa and δ = 8.1% at high temperature of 350 °C.     

The effects of Mg amount on the microstructure and mechanical properties of Al–Si–Mg alloys

In this study, the effects of magnesium (Mg) addition to A356 aluminum alloy at different amounts on the microstructure and mechanical properties of this alloy were examined. For the experimental studies, three different alloys (0.43, 0.67 and 0.86 wt%) having various amounts of Mg were prepared through casting process in the form of plates. The plates were homogenized and cooled in the furnace. All the samples were treated with aging process (T6) and then tensile samples were prepared from the homogenized samples. The samples treated with T6 process were characterized by optical microscopy, laser confocal microscopy, Scanning Electron Microscope (SEM), Energy Dispersive Spectrometer (EDS) and X-Ray Diffraction (XRD) examinations as well as hardness measurements and tensile tests. The phases which were formed in the microstructures for different amounts of Mg were examined. It was observed that iron-rich intermetallic compounds were also formed in addition to the phases resulting from the aging process. Fe-rich intermetallic compounds, observed from the fracture surfaces, were found to reduce the tensile strength the alloy. The results also indicate that the tensile strength and hardness of the alloy increase with increasing Mg amount.     

Correlation between microstructures and mechanical properties in Ni-rich Ni–Mn–Ga high-temperature shape-memory alloys

Microstructures and mechanical properties of polycrystalline and single-crystalline Ni_55+xMn₂₅Ga_20?x (x?=?1, 2, 3) alloys are investigated. The alloys exhibit a single martensite phase for x?=?1, and two phases containing tetragonal martensite phase and face-centered γ phase for x?=?2 and 3. The appearance of γ phase can apparently raise the strength and plasticity of the alloys. Moreover, single crystals show better mechanical properties compared with the polycrystals with the same composition. Furthermore, a kind of net-like γ phase is observed in directionally solidified Ni₅₈Mn₂₅Ga₁₇ single crystal. This alloy exhibits excellent plasticity due to that the crack propagation is seriously hindered by the plastic net-like γ phase.     

Toughness and fatigue behaviour of eutectic and hypereutectic Al–Si–Cu–Mg alloys produced through lost foam and squeeze casting

Abstract

The strength and toughness of four high silicon content Al–Si–Mg–Cu alloys have been studied at room temperature (RT), 200°C and 300°C. The fatigue behaviour has also been investigated. The alloys were produced using two very different processing routes: lost foam and squeeze casting. In the tensile tests, the ductility was low for alloys produced via both routes irrespective of the testing temperature. The strength was similar at RT and 200°C, but at 300°C it fell abruptly. The toughness followed the same trend with testing temperature. Direct observation of fatigue cracks revealed that the brittle silicon and intermetallic particles broke ahead of the crack tip; the fatigue crack advanced by linking the main crack with cracks formed ahead of it. The T6 thermal treatment improved fatigue resistance in the squeeze cast material, especially at high D K values.     

Effect of Si content on microstructure and mechanical properties of Al–Si–Mg alloys

Microstructure and mechanical properties of as-cast and as-extruded Al–Si–Mg alloys with different Si content are investigated by tensile test, microstructure observation. High density of Si particles in the Al alloys can induce dynamic recrystallization during hot extrusion and it becomes more matured with an increase in the density of Si particles. The tensile strength of as-cast and as-extruded alloys can be improved with the increase of Si content and hot extrusion make the elongation of alloys increase dramatically. Considerable grain refining effect caused by recrystallization occurred during hot extrusion of S2 (equivalently commercial A356 alloy) and S3 (near eutectic alloy) alloys plays an important role in the improvement of elongation. A good combination of strength and elongation for the as-extruded S3 alloy indicates that near eutectic Al–Si alloys can be hot-extruded to produce aluminum profiles with high performance.     

Microstructures and mechanical properties of spark plasma sintered Cu–Cr composites prepared by mechanical milling and alloying

Nanograined Cu–8 at.% Cr composite was produced by a combination of mechanical milling (MM), mechanical alloying (MA) and spark plasma sintering (SPS). Commercial Cu and Cr powders were pre-milled separately by MM. The milled Cu and Cr powders were then mechanically alloyed with as-received Cr and Cu powders respectively. After milling, the powder mixtures were separately subjected to SPS. It was found that pre-milling Cr can efficiently decrease the size of grain and reinforcement, resulting in remarkable strengthening. The grain size of Cu matrix was about 82 nm after SPS. The Vickers hardness, compressive yield strength and compression ratio of the composite were 327 HV, 1049 MPa and 10.4%, respectively. The excellent mechanical properties were primarily attributed to dispersion strengthening of the Cr particles and fine grain strengthening of the Cu matrix. The strong Cu/Cr interface and dissolved Cr atoms can also contribute to strengthening of the composite.     

Effect of microstructure on the mechanical properties of as-cast Ti–Nb–Al–Cu–Ni alloys for biomedical application

The correlation between the microstructure and mechanical behavior during tensile loading of Ti_68.8Nb_13.6Al_6.5Cu₆Ni_5.1 and Ti_71.8Nb_14.1Al_6.7Cu₄Ni_3.4 alloys was investigated. The present alloys were prepared by the non-equilibrium processing applying relatively high cooling rates. The microstructure consists of a dendritic bcc β-Ti solid solution and fine intermetallic precipitates in the interdendritic region. The volume fraction of the intermetallic phases decreases significantly with slightly decreasing the Cu and Ni content. Consequently, the fracture mechanism in tension changes from cleavage to shear. This in turn strongly enhances the ductility of the alloy and as a result Ti_71.8Nb_14.1Al_6.7Cu₄Ni_3.4 demonstrates a significant tensile ductility of about 14% combined with the high yield strength of above 820 MPa already in the as-cast state. The results demonstrate that the control of precipitates can significantly enhance the ductility and yet maintaining the high strength and the low Young's modulus of these alloys. The achieved high bio performance (ratio of strength to Young's modulus) is comparable (or even superior) with that of the recently developed Ti-based biomedical alloys.     

Effects of solution treatment on the microstructure and mechanical properties of Al–Cu–Mg–Ag alloy

The effects of solution treatment on the microstructure and mechanical properties of Al–Cu–Mg–Ag alloy were studied by optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray (EDX), differential scanning calorimeter (DSC), transmission electron microscopy (TEM) and tensile test, respectively. The results show that the mechanical property increases and then decreases with increasing the solution temperature. And the residual phases are dissolved into the matrix gradually, the number fraction of the precipitation and the size of recrystallized grains increase. Compared to the solution temperature, the solution holding time has less effect on the microstructure and the mechanical properties of Al–Cu–Mg–Ag alloy. The overburnt temperature of Al–Cu–Mg–Ag alloy is 525 °C. The yield strength and the elongation get the best when the alloy is solution treated at 515 °C for 1.5 h, is 504 MPa and 12.2% respectively. The fracture mechanism of the samples is ductile fracture.