Pre-precipitate clusters and precipitation processes in Al–Mg–Si alloys

The solute clusters and the metastable precipitates in aged Al–Mg–Si alloys have been characterized by a three-dimensional atom probe (3DAP) and transmission electron microscopy (TEM). After long-term natural aging, Mg–Si co-clusters have been detected in addition to separate Si and Mg atom clusters. The particle density of β″ after 10 h artificial aging at 175°C varies depending on pre-aging conditions, i.e. pre-aging at 70°C increases the number density of the β″ precipitates, whereas natural aging reduces it. This suggests that the spherical GP zones formed at 70°C serve as nucleation sites for the β″ in the subsequent artificial aging, whereas co-clusters formed at room temperature do not. Atom probe analysis results have revealed that the Mg:Si ratios of the GP zones and the β″ precipitates in the alloy with excess amount of Si are 1:1, whereas those in the Al–Mg₂Si quasi-binary alloy are 2:1. Based on these results, the characteristic two-step age-hardening behavior in Al–Mg–Si alloys is discussed.     

Morphology and formation of Fe–Al intermetallic layers on iron hot-dipped in Al–Mg–Si alloy melt

We have examined the morphology and the growth of Fe–Al intermetallic layers of η-Fe₂Al₅ and θ-FeAl₃ phases formed on pure Fe sheets dipped in an Al-8.2Mg-4.8Si (wt.%) alloy melt and pure Al melt at 750 °C. The η phase layer grows one order of magnitude slower in the Al–Mg–Si alloy melt than in the pure Al melt. The change in thickness of Fe sheets with dipping time is less pronounced in the Al–Mg–Si alloy melt than in a pure Al melt. Microstructure observations suggest that the retarded interfacial reaction between solid Fe and liquid Al–Mg–Si alloy is associated with a continuous θ phase layer formed in the Al–Mg–Si alloy melt, which acts as the diffusion barrier.     

Phase separation kinetics in a Fe–Cr–Al alloy

The α–α′ phase separation kinetics in a commercial Fe–20 wt.% Cr–6 wt.% Al oxide dispersion-strengthened PM 2000? steel have been characterized with the complementary techniques atom probe tomography and thermoelectric power measurements during isothermal aging at 673, 708, and 748 K for times up to 3600 h. A progressive decrease in the Al content of the Cr-rich α′ phase was observed at 708 and 748 K with increasing time, but no partitioning was observed at 673 K. The variation in the volume fraction of the α′ phase well inside the coarsening regime, along with the Avrami exponent 1.2 and activation energy 264 kJ mol^?1, obtained after fitting the experimental results to an Austin–Rickett type equation, indicates that phase separation in PM 2000? is a transient coarsening process with overlapping nucleation, growth, and coarsening stages.     

Oxide film characteristics of Al–7Si–Mg alloy in dynamic conditions in casting

Abstract

'New' oxide film which forms in a very short time in the casting process was studied. Samples for the study were prepared based on a technique in which an oxide–metal 'sandwich' could be made. Alloy A356 (Al–7Si–0.4Mg) was selected for the study. Features such as thickness of the oxide film, its morphology, rigidity and presence of eutectic phase have been examined and shown by SEM study. Possible consequences of the morphology of the oxide film are discussed.     

High strain rate superplasticity in a commercial Al–Mg–Sc alloy

It was shown that an Al–5.7%Mg–0.32%Sc–0.3%Mn alloy subjected to severe plastic deformation through equal-channel angular extrusion exhibits superior superplastic properties in the temperature range of 250–500 °C at strain rates ranging from 1.4 × 10⁻⁵ to 1.4 s⁻¹ with a maximum elongation-to-failure of 2000% recorded at 450 °C and an initial strain rate of 5.6 × 10⁻² s⁻¹.     

The effect of microstructure on the shape recovery of a Fe–Mn–Si–Cr–Ni stainless steel shape memory alloy

In this work the effect of varying the microstructure on the shape memory properties of a Fe-15Mn-7Si-9Cr-5Ni (wt.%) stainless steel shape memory alloy was evaluated using a simple bend test. The best shape recovery was obtained for a single-phase austenite microstructure and for a two-phase microstructure composed of an austenite matrix and Fe₅Ni₃Si₂ type intermetallic grain boundary phase. The maximum shape recovery was achieved at the reversion temperature of 600 °C and when the pre-stain was less than 2%.     

Cavity formation and early growth in a superplastic Al–Mg alloy

Knowledge of the exact physical mechanism of cavity formation and early growth is important for the prediction of the extent of internal damage following superplastic deformation. To this end, the early stages of cavitation in a superplastic Al–Mg–Mn–Cu alloy have been experimentally studied and reported here. Small cavities (<0.5 μm) were detected by scanning electron microscopy and the number of cavities per unit volume was monitored by image analysis through optical microscopy. Before deformation, some cavities were seen at the particle–matrix interfaces. However, during tensile deformation in the temperature range of 450–550°C (and strain rates ∼10⁻⁴ to 10⁻² s⁻¹), additional cavities emerge and grow. Most cavities are observed at the interface between particles and the matrix from submicrometer size range, and grow initially along the interface. This suggests that early cavity growth is by matrix/particle decohesion, possibly starting from interfacial defects, and this growth has rapid kinetics. The density of observable cavities increases with strain, i.e. “nucleation” is continuous. The number of cavities increases at higher strain rates and at lower test temperatures. This is due to the higher flow stresses, reduced strain-rate sensitivity and poorer diffusional accommodation process, which assist in the initial growth of the submicrometer and nanoscale interface defects. But the evidence for diffusional cavity growth in the initial stages was not found.     

Strength properties and structure of a submicrocrystalline Al–Mg–Mn alloy under shock compression

The results of studying the strength of a submicrocrystalline aluminum A5083 alloy (chemical composition was 4.4Mg–0.6Mn–0.11Si–0.23Fe–0.03Cr–0.02Cu–0.06Ti wt % and Al base) under shockwave compression are presented. The submicrocrystalline structure of the alloy was produced in the process of dynamic channel-angular pressing at a strain rate of 10⁴ s^–1. The average size of crystallites in the alloy was 180–460 nm. Hugoniot elastic limit σ_HEL, dynamic yield stress σ_y, and the spall strength σ_SP of the submicrocrystalline alloy were determined based on the free-surface velocity profiles of samples during shock compression. It has been established that upon shock compression, the σ_HEL and σ_y of the submicrocrystalline alloy are higher than those of the coarse-grained alloy and σ_sp does not depend on the grain size. The maximum value of σ_HEL reached for the submicrocrystalline alloy is 0.66 GPa, which is greater than that in the coarse-crystalline alloy by 78%. The dynamic yield stress is σ_y = 0.31 GPa, which is higher than that of the coarse-crystalline alloy by 63%. The spall strength is σ_sp = 1.49 GPa. The evolution of the submicrocrystalline structure of the alloy during shock compression was studied. It has been established that a mixed nonequilibrium grain-subgrain structure with a fragment size of about 400 nm is retained after shock compression, and the dislocation density and the hardness of the alloy are increased.     

Atomic model for GP-zones in a 6082 Al–Mg–Si system

Heat treatments of 6082 Al-alloy with 0.6 wt% Mg, 0.9 wt% Si, 0.5 wt% Mn and 0.2 wt% Fe can lead to a considerable increase in hardness. This increase is due to the presence of several metastable phases (in particular β″). To determine the structure of the phase formed before the β″ phase, a detailed high resolution electron microscopy (HREM) study was performed. The pre-β″ phase is needle like, as is the β″ phase. Based on reconstructed exit waves, two models were possible, one of which could be rejected because of the interatomic distances. The model resembles that of the β″ but with different positions for some of the Mg atoms along the needle direction. The structure is more similar to the Al matrix than that of the β″ phase. The Mg sites, and to a lesser extent also the Si sites are partly occupied by Al atoms. The composition is therefore less Mg-rich than β″ (Mg₅Si₆). The content of Al in the structure of the precipitates increases with the degree of coherency in the Al matrix. The space group of the new phase is C2/m, as for β″.     

Controlled semisolid microstructures and mechanical properties of Al–Mg–Si–Mn wrought alloy produced by D-SSF

Abstract

The semisolid microstructures and the mechanical properties of Al–1˙35Mg–1˙04Si–0˙67Mn alloy produced by deformation semisolid forming (D-SSF) process were studied. Fine α-Al₁₅Mn₃Si₂ compounds precipitate homogeneously during the homogenisation treatment. These compounds effectively inhibit the coarsening of recrystallised grains during heating to the semisolid temperature. When the liquid fraction is controlled to be ～23%, the complete die filling is not achieved. Therefore, in order to achieve good fluidity, it is necessary to control the liquid fraction to be more than 30%. The average grain size and the liquid fraction at the semisolid temperature influence directly mechanical properties. Therefore, the relationship among the average grain size, the liquid fraction at the semisolid temperature and mechanical properties was evaluated. Furthermore, the optimum semisolid microstructure was determined and the condition for the D-SSF process was established.     

Friction welding of Al–Mg–Si alloy to Ni–Cr–Mo low alloy steel

Abstract

It is difficult to weld the dissimilar material combination of aluminium alloys and low alloy steels using fusion welding processes, on account of the formation of a brittle interlayer composed of intermetallic compound phases and the significant difference in physical and mechanical properties. In the present work an attempt has been made to join these materials via the friction welding method, i.e. one of the solid phase joining processes. In particular, the present paper describes the optimisation of friction welding parameters so that the intermetallic layer is narrow and joints of acceptable quality can be produced for a dissimilar joint between Al-Mg-Si alloy (AA6061) and Ni-Cr-Mo low alloy steel, using a design of experiment method. The effect of post-weld heat treatment on the tensile strength of the joints was then clarified. It was concluded that the friction time strongly affected the joint tensile strength, the latter decreasing rapidly with increasing friction time. The highest strength was achieved using the shortest friction time. The highest joint strength was greater than that of the AA6061 substrate in the as welded condition. This is due to the narrow width of the brittle intermetallic layer generated, which progressed from the peripheral (outer surface) region to the centreline region of the joint with increasing friction time. The joints in the as welded condition could be bent without cracking in a bend test. The joint tensile strength in the as welded condition was increased by heat treatment at 423 K (150° C), and then it decreased when the heat treatment temperature exceeded 423 K. All joints fractured in the AA6061 substrate adjacent to the interface except for the joints heated at 773 K (500° C). The joints fractured at the interface because of the occurrence of a brittle intermetallic compound phase.     

Low temperature superplasticity in a friction-stir-processed ultrafine grained Al–Zn–Mg–Sc alloy

Friction stir processing (FSP) was used to create a microstructure with ultrafine grains (0.68 μm grain size) in an as-cast Al–8.9Zn–2.6Mg–0.09Sc (wt.%) alloy. The ultrafine grained alloy exhibited superplasticity at relatively low temperatures and higher strain rates. Optimum ductility of 1165% at a strain rate of 3 × 10⁻² s⁻¹ and 310 °C was obtained. Enhanced superplasticity was also achieved at a temperature as low as 220 °C. Experimentally observed parametric dependencies and microstructural examinations indicated that the operating deformation mechanism might be the Rachinger grain boundary sliding accommodated by intragranular slip. The FSP microstructure became highly unstable at 390 °C onwards, thus, affecting ductility adversely. In situ transmission electron microscopy heating was used to understand the instability phenomenon, which has been attributed to the drop in particle pinning forces due to the dissolution of metastable precipitates and microstructural heterogeneity.     

Microstructure characterization of Cu-rich nanoprecipitates in a Fe–2.5 Cu–1.5 Mn–4.0 Ni–1.0 Al multicomponent ferritic alloy

The evolution of precipitates in a Fe–2.5 Cu–1.5 Mn–4.0 Ni–1.0 Al multicomponent ferritic alloy during annealing at 500 °C was systematically investigated by aberration-corrected scanning transmission electron microscopy. The atomic-scale structure and chemistry characterization reveal that primary precipitates with enriched Cu, Ni, Mn and Al originate from continuous growth of B2 ordered domains in the as-quenched alloy. The formation of a Cu-rich body-centered cubic (bcc) phase takes place by the decomposition of the B2 ordered primary phase, which forms a Cu-rich bcc core and ordered B2-Ni(Al,Mn) shell. The B2 shells serve as a buffer layer to moderate the coherent strain and to prohibit the inter-diffusion between the Cu-rich precipitates and bcc-Fe matrix, giving rise to a low coarsening rate of the precipitates. The Cu-rich precipitates experience a structural transformation from bcc to 9R at a critical size of ～6 nm during long time annealing, corresponding to obvious coarsening of the precipitates and dramatic loss in hardness of the alloy.     

The role carbon plays in the martensitic phase transformation of an Fe–Mn–Al alloy

We observed direct evidence that 18R martensite is induced by carbon atoms in the BCC grains of an Fe–27.0wt.%Mn–5.3wt.%Al–0.1wt.%C alloy via high-temperature quenching. A single BCC phase structure formed 18R martensite in the present study. The lowest carbon content found for the formation of 18R martensite is 0.035 wt.% in Fe–Mn–Al alloys.     

Prediction of the morphology of Mg32(Al,Zn)49 precipitates in a Mg–Zn–Al alloy

A geometrical model has been applied to predict the morphology of faceted Mg₃₂(Al, Zn)₄₉ precipitates in a Mg–Zn–Al alloy using the observed orientation relationship (OR) and the lattice parameters of the precipitates and the matrix as inputs. Planes in rational or in irrational orientations with higher densities of good matching sites are more likely to be preferred, which agrees well with experimental observations.     

Effect of pre-straining on structure and formation mechanism of precipitates in Al–Mg–Si–Cu alloy

The effect of pre-straining on the structure and formation mechanism of precipitates in an Al–Mg–Si–Cu alloy was systematically investigated by atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Elongated and string-like precipitates are formed along the dislocations in the pre-strained Al–Mg–Si–Cu alloy. The precipitates formed along the dislocations exhibit three features: non-periodic atomic arrangement within the precipitate; Cu segregation occurring at the precipitate/α(Al) interface; different orientations presented in one individual precipitate. Four different formation mechanisms of these heterogeneous precipitates were proposed as follows: elongated precipitates are formed independently in the dislocation; string-like precipitates are formed directly along the dislocations; different precipitates encounter to form string-like precipitates; precipitates are connected by other phases or solute enrichment regions. These different formation mechanisms are responsible for forming different atomic structures and morphologies of precipitates.