Room temperature deformation modes in DO3-structured Fe-34AI and Fe-28AI-6Cr

The dislocation substructures in the DO₃-structured alloys, Fe-34AI and Fe-28AI-6Cr, were examined by transmission electron microscopy following room temperature compression. In both alloys a small number of a<001> dislocations, produced through the energetically favourable interaction of two pairs of antiphase boundary-coupled a/2<1 1 1> dislocations, was observed. It was also found that, relative to the binary alloy, one effect of the chromium addition was to increase the B2 character of the alloy, but conversely to decrease the DO₃ character. These results are rationalized on the basis of the atom site location of the chromium atoms in the DO₃ structure.     

TEM observation of DO3 structure in Fe3 Al alloy with Mn addition

The structures of DO₃ Fe-28Al-1.5Mn alloy, including ordering degree, superdislocation, APD and APB, were investigated by TEM. The results showed that addition of manganese into DO₃ Fe₃Al could not change the ordered type of the alloy, but could reduce APD size and then reduce ordering degree of the alloy. The fourfold superdislocations were retarded in DO₃ Fe₃Al alloy after Mn addition. Undeformed alloy with Mn has mainly twofold superdislocations. As deformation increases, the twofold superdislocations slip and decompose into unit dislocations, and unit dislocations slip and slip cross, leading to better ductility. The deformation mechanism of DO₃ Fe-28Al-1.5Mn alloy was controlled at first by twofold superdislocation and at last by unit superdislocation.     

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.     

DO22 to L12 transition in intermetallic systems

Ternary additions of metals such as chromium, manganese, iron, cobalt, nickel, copper and zinc to tetragonal (DO₂₂) Al₃Ti are known to lead to stabilization of the cubic (L1₂) structure. This DO₂₂ to L1₂ transition has been studied in the Al-Ti-Ni system using X-ray diffraction and scanning electron microscopy. The results show that nickel substitution has no significant effect on the lattice parameter (and therefore on the tetragonality) of the DO₂₂ phase and that the solid solubility of nickel in the DO₂₂ phase is very limited. The L1₂ phase precipitates out on addition of nickel to Al₃Ti, its amount increasing with increasing nickel content of the alloy. The compositions of the DO₂₂ and L1₂ phases do not change significantly with the alloy composition. These results are discussed in terms of theories of structural transitions in ordered alloys. Similar transitions have been reported in transition metal-based systems. An analysis of the transition in intermetallic systems is presented.     

Ultrafine-grained Zn-0.45Li alloy with enhanced mechanical property,degradation behavior and cytocompatibility prepared by hot extrusion and multi-pass drawing

Biodegradable Zn-0.45Li alloys with high strength and ductility were successfully fabricated by hot extrusion and multi-pass drawing to obtain ultrafine-grained microstructures with a secondary phase of fine LiZn₄ participates. The mechanical properties, degradation behavior and cytocompatibility of the alloys were subsequently investigated. Results showed that grain refinement could be achieved in the alloys after hot extrusion and multi-pass drawing. The yield strength, ultimate tensile strength, and elongation to failure of the ultrafine-grained Zn-0.45Li alloys reached 416 MPa, 567 MPa and 55.4 %, respectively. Enhancements in both strength and elongation could be attributed to interactions between LiZn₄ and matrix dislocations, the pinning effect of LiZn₄ on grain boundaries, and grain refinement. Immersion tests and MTT cytotoxicity assay indicated that the Zn-0.45Li alloys have a corrosion rate and cytocompatibility similar to the values reported for biomedical implants.     

Phase decompositions of Fe-Si-Al ordered alloys

Stable structures of Fe-Si-Al ternary alloys and Fe-Si and Fe-Al binary alloys containing up to about 40 at% solute atoms were investigated by means of transmission electron microscopy. The following results were obtained. Two types of phase separation, B2+DO₃ and + DO₃ were observed in the alloys whose compositions lie in a narrow band connecting Fe-10 to 14 at% Si with Fe-20 to 25 at % Al and also in the neighbourhood of a Fe-30 at%Al-3 at% Si alloy. Such compositions of the alloys are located in the phase boundary of B2 and DO₃ single phases or and DO₃ single phases. The phase separation in the Fe-Si-Al and Fe-Si alloys produce the 100 modulated structure which differs from the morphology formed by the phase separation of the Fe-Al system.     

Microstructural evolution and mechanical properties of CoCrFeNiMnTix high-entropy alloys

This study was conducted to investigate the effect of titanium addition on the microstructure and properties of an equitaomic CoCrFeNiMn high-entropy alloy. Homogenized microstructures of CoCrFeNiMnTi_x (x = 0.1 and 0.3) alloys consist of face-centered cubic phase; however, addition of more titanium led to formation of a (chromium, titanium)-rich σ phase in CoCrFeNiMnTi_0.4 alloy. The average electron hole number calculations indicate the higher possibility of σ phase formation by adding more titanium. Furthermore, addition of an atom like titanium with a larger atomic radius in comparison with other elements can affect stability of face-centered cubic structure. Chromium and manganese has a destabilizing influence on the single face-centered cubic phase and manganese may reject chromium to facilitate the formation of a (chromium, titanium)-rich phase in alloys containing more than 5.5 at.% titanium (x>0.3). The mechanical properties revealed an improvement in strength without losing the ductility drastically by adding titanium up to 5.5 at.% (x = 0.3). Nevertheless, the strength remarkably increased and ductility significantly decreased in CoCrFeNiMnTi_0.4 alloy due to formation of brittle σ phase in the microstructure.     

Microstructure and Mechanical Properties of Al25 − xCr25 + 0.5xFe25Ni25 + 0.5x (x = 19, 17, 15 at%) Multi‐Component Alloys

A series of Al_{25 ? x}Cr_{25 + 0.5x}Fe₂₅Ni_{25 + 0.5x} (x = 19, 17, 15 at%) multi‐component alloys are prepared by arc‐melting and rapid solidification of copper molds. The technique of thermal‐mechanical processing is further applied to the master alloys to improve their mechanical properties. These alloys consist of face‐centered cubic (FCC) and body‐centered cubic (BCC) structure. The volume fraction of the BCC phase increases as Al content increase and Cr and Ni contents decrease, accompanied with a microstructural evolution from dendritic structure to lamella‐like structure. Due to the increase of volume fraction of BCC phase, the master alloys exhibit an increased strength and a declined ductility as Al content increases. The rapid solidified alloys have more BCC phase compared with the master alloys, which enhances the strength and decreases the ductility. After homogenization, hot‐rolling, and annealing at 1000 °C, the Al₈Cr_33.5Fe₂₅Ni_33.5 alloy displays excellent combination of strength (yield strength is ～635 MPa and fracture strength is ～1155 MPa) and ductility (tension strain is ～11%).

     

High strength and ductility Mg-8Gd-3Y-0.5Zr alloy with bimodal structure and nano-precipitates

To resolve the strength-ductility trade-off problem for high-strength Mg alloys, we prepared a high performance Mg-8 Gd-3 Y-0.5 Zr(wt%) alloy with yield strength of 371 MPa, ultimate tensile strength of419 MPa and elongation of 15.8%. The processing route involves extrusion, pre-deformation and aging,which leads to a bimodal structure and nano-precipitates. Back-stress originated from the deformationincompatibility in the bimodal-structure alloy can improve ductility. In addition, dislocation density in coarse grains increased during the pre-deformation strain of 2%, and the dislocations in coarse grains can promote the formation of chain-like nano-precipitates during aging treatment. The chain-like nanoprecipitates can act as barriers for dislocations slip and the existing mobile dislocations enable good ductility.     

Effect of iron additions on mechanical properties of Co3 Ti alloy

High temperature tensile tests were carried out on L1₂ type Co₃ Ti alloys, both undoped and doped with 1–4 at.-%Fe. There were anomalous increases of the 0·2% yield stress (yield strength) with increasing test temperature from 473 to 1073 K (or 1173 K, depending on the composition). The elongation and ultimate tensile stress (UTS) monotonically decreased with increasing temperature. The fracture surfaces of specimens showed a variety of fracture modes which were dependent on the test temperature and composition. There was a correlation between the ductility and the fracture mode: the more transgranular the fracture mode, the higher the ductility. It was found that Co₃ Ti with 2 at.-%Fe exhibited improved ductility and it exhibited the highest peak value of yield strength and peak temperature. The alloys were also hydrogen charged to investigate their hydrogen embrittlement behaviour. Room temperature tests indicated that the addition of 2 at.-%Fe decreased the hydrogen related embrittlement.

MST/3479     

Interfaces in ordered intermetallics

In this paper, we have examined different types of interfaces that occur between orientational/translational variants generated during the ordering process. This has been illustrated citing examples of ordering of the FCC structure into DO₂₂, Dl_a and Pt₂Mo type structures in some nickel base alloys. Microstructures consisting of more than one ordered structure have also been investigated. Superlattice domains of DO₂₂ and Pt₂Mo type structures have also been found to coexist in a microscopic scale of mixing in Ni-V alloys while mixed domains of Dl_a and Pt₂Mo type structures on a much finer scale have been observed in Ni-Mo alloys. The formation of different variants (rotational and translational) of ordered structure(s) from the disordered lattice has been explained on the basis of group theoretical and symmetry considerations.     

Ultrastrong High-Ductility Ni35Co35Fe10Al10Ti8B2 High-Entropy Alloy Strengthened with Super-High Concentration L12 Precipitates

L1₂ phase hardening alloys with excellent mechanical properties are of great significance for structural applications. However, low volume fractions of L1₂ precipitates in conventional alloys (nearly lower than 60%) tend to limit their practical usage, while the strengths of the alloys generally increase with L1₂ precipitation contents. Herein, a novel high-entropy alloy (HEA) Ni₃₅Co₃₅Fe₁₀Al₈Ti₁₀B₂ with ultrahigh concentration L1₂ precipitates is successfully designed aided by the calculation of phase diagrams (CALPHAD). The volume fraction of L1₂ precipitates in this HEA is up to 75% and outperforms that of most of traditional superalloys. The novel L1₂-strengthened Ni₃₅Co₃₅Fe₁₀Al₈Ti₁₀B₂ has an ultrahigh tensile yield strength of ≈1.45 GPa, ultimate tensile strength of ≈1.9 GPa, and great ductility of ≈23% at room temperature. The desirable strength–ductility combination is superior to most of conventional superalloys and reported HEAs, mainly due to the presence of ultrahigh concentration L1₂ precipitates that act as dislocation obstacles and the formation of numerous stacking faults and deformation twining. This work is expected to provide guidance for developing new high-performance HEAs with an excellent combination of strength and ductility.     

Transformation of BCC and B2 High Temperature Phases to HCP and Orthorhombic Structures in the Ti-Al-Nb System. Part II: Experimental TEM Study of Microstructures

Possible transformation paths that involve no long range diffusion and their corresponding microstructural details were predicted by Bendersky, Roytburd, and Boettinger [J. Res. Natl. Inst. Stand. Technol. 98, 561 (1993)] for Ti-Al-Nb alloys cooled from the high temperature BCC/B2 phase field into close-packed orthorhombic or hexagonal phase fields. These predictions were based on structural and symmetry relations between the known phases. In the present paper experimental TEM results show that two of the predicted transformation paths are indeed followed for different alloy compositions. For Ti-25Al-12.5Nb (at%), the path includes the formation of intermediate hexagonal phases, A3 and DO₁₉, and subsequent formation of a metastable domain structure of the low-temperature O phase. For alloys close to Ti-25Al-25Nb (at%), the path involves an intermediate B19 structure and subsequent formation of a translational domain structure of the O phase. The path selection depends on whether B2 order forms in the high temperature cubic phase prior to transformation to the close-packed structure. The paper also analyzes the formation of a two-phase modulated microstructure during long term annealing at 700 °C. The structure forms by congruent ordering of the DO₁₉ phase to the O phase, and then reprecipitation of the DO₁₉ phase, possibly by a spinodal mechanism. The thermodynamics underlying the path selection and the two-phase formation are also discussed.     

The deformation behavior of DyCu ductile intermetallic compound under compression

In this study polycrystalline specimens of as-cast DyCu with B2 crystal structure were compressed to different strains at room temperature to test if stress-induced phase transformation and twinning occur during deformation. In these tests DyCu exhibited high ductility with plastic strain as high as 35% and ultimate compressive strength of ∼790 MPa. X-ray diffraction and transmission electron microscopy (TEM) showed no indications that DyCu had undergone stress-induced phase transformation or twinning. 〈1 1 1〉-type dislocations have been observed to play an important role for imparting the ductile behavior. TEM analyses showed the presence of second phases, Dy₂O₃ and DyCu₂ within the DyCu. The mechanisms of ductility and impurity phase formation are discussed.     

Superior mechanical properties of a selective-laser-melted AlZnMgCuScZr alloy enabled by a tunable hierarchical microstructure and dual-nanoprecipitation

Achieving high mechanical strength and ductility in age-hardenable Al7000 series (Al–Zn–Mg) alloys fabricated by selective laser melting (SLM) remains challenging. Here, we show that crack-free AlZnMgCuScZr alloys with an unprecedented strength–ductility synergy can be fabricated via SLM and heat treatment. The as-built samples had an architectured microstructure consisting of a multimodal grain structure and a hierarchical phase morphology. It consisted of primary Al₃(Sc_x,Zr_1−x) particles which act as inoculants for ultrafine grains, preventing crack formation. The metastable Mg-, Zn-, and Cu-rich icosahedral quasicrystals (I-phase) ubiquitously dispersed inside the grains and aligned as a filigree skeleton along the grain boundaries. The heat treated SLM-produced AlZnMgCuScZr alloy exhibited tunable mechanical behaviors through trade-off among the hierarchical features, including the dual-nanoprecipitation, viz, η′ phase, and secondary (Al,Zn)₃(Sc₉Zr), and grain coarsening. Less coarsening of grains and (Al,Zn)₃(Sc₉Zr) particles, due to a reduced solution treatment temperature and time, could overwhelm the more complete dissolution of I-phase (triggering more η′ phase), resulting in higher yield strength. Optimal combination of the hierarchical features yields the highest yield strength (∼647 MPa) among all reported SLM-produced Al alloys to date with appreciable ductility (∼11.6%). The successful fabrication of high-strength Al7000 series alloys with an adjustable hierarchical microstructure paves the way for designing and fine-tuning SLM-produced aluminum engineering components exposed to high mechanical loads.     

Enhancement of high temperature strength and room temperature ductility of iron aluminides by alloying

Abstract

The chemical ordering in intermetallics results in reduced atomic mobility and therefore increased resistance to plastic deformation at elevated temperatures. This intrinsic source of high temperature strength leads to the inherent brittleness of polycrystalline ordered intermetallics at room temperature. The requirements for optimum high temperature strength and ductility at ambient temperature are often incompatible. Iron aluminides possess high strength up to 873 K. There is an anomalous (positive) temperature dependence of yield and flow strengths. Iron aluminides have yet to achieve satisfactory elevated load bearing capability. Alloy additions have the potential for improving elevated temperature strength and room temperature ductility; whichever is more critical for the application. Elements such as Cr, Ti, Mn, Co, and Mo produce higher flow stress due to solid solution strengthening. Elements such as Zr, Ta, Nb, Re, and Hf go into solution partly, reprecipitate, effectively pin dislocations and thereby cause strengthening. Mo, Zr, and Hf produce good tensile strength at elevated temperatures but ductility decreases. Element B strengthens by grain boundary cohesion. The improvement in room temperature ductility can be achieved through modification of the crystal structure by changes in stoichiometry, macroalloying, microalloying, and control of the environment. B, TiB₂, and Cr are notable for enhancing ductility. The paper is an overview of the present status of iron aluminides in this respect.     

Tensile behaviour of two DO3-ordered alloys: Fe3Si and Fe-20 at % Al-5 at % Si

The tensile behaviour, including fracture modes and deformation substructures, of two powder-produced DO₃-ordered alloys having compositions Fe-25 at % Si (Fe₃Si) and Fe-20 at % Al-5 at % Si, has been investigated from room temperature to 800° C. The brittle-to-ductile transition temperature for the Fe₃Si alloy occurred at a temperature between 500 and 550° C, while that of the Fe-20 at % Al-5 at % Si alloy was approximately room temperature. In both alloys fracture occurred by transgranular cleavage at room temperature, with the occurrence of an increasing proportion of intergranular cavitation with increasing temperature. At low strains plastic deformation occurred chiefly by movement of perfect superlattice dislocations which, with increasing strain, dissociated to produce next-nearest-neighbour antiphase boundary trails.     

Microstructure and mechanical properties of a novel refractory AlNbTiZr high-entropy alloy

The microstructure and mechanical properties of a novel refractory AlNbTiZr high-entropy alloy (HEA) with a low density of ～5.85?g?cm^?3 were investigated after arc melting and homogenisation at 1473?K for 5?h. The as-cast HEA exhibits a single-phase ordered body-centred cubic (B2) structure. A hexagonal Zr₅Al₃-type second phase is introduced into the HEA through homogenisation treatment, resulting in increase of the yield strength, ultimate compressive strength and fracture strain by 70?MPa, 308?MPa and 9.2%, respectively. These results indicate that the introduction of the hexagonal Zr₅Al₃-type second phase into the B2 matrix can simultaneously improve the HEA strength and ductility, showing a strength–ductility combination superior to those of most reported refractory HEAs.     

High-temperature creep and structure investigation of nearly stoichiometric Fe3Si

X-ray diffraction measurements were carried out on powdered single crystals of nearly stoichiometric Fe₃Si. The experimental data obtained in the temperature range from room temperature up to 750‡ C in terms of long-range order, thermal expansion, phase transition and Debye temperature (together with values of the Curie temperature) support the existence of two modifications of the DO₃ structure for Fe-26 at% Si alloys and a phase transition in the DO₃ structure field at 595‡ C. The high-temperature modification has a smaller thermal expansion coefficient, a higher Curie temperature and a higher Debye temperature.     

Why L12 intermetallics are brittle and how to make them ductile

Origins of the intrinsic and extrinsic brittleness of L1₂ intermetallics are analyzed. It is shown that the intrinsic behavior is determined by the dislocation process in the alloy, and that the extrinsic behavior is related to moisture-induced embrittlement. Converting DO₂₂ structure to L1₂ structure does not necessarily yield a ductile alloy. To improve the intrinsic ductility, the alloying element must be able to change the bonding nature so that the glissile complex stacking fault-coupled dissociation of superdislocations is promoted, while an improvement in the extrinsic ductility might be achieved by partially replacing the light element in the alloy with a heavier alloying element.