1,721,162 research outputs found
Estimation of dislocation densities in cold rolled Al-Mg-Cu-Mn alloys by combination of yield strength data, EBSD and strength models
No full textAl-Mg-Cu-Mn alloys have been developed for the packaging industry, in which large cold-working deformations are normally applied that can produce high dislocation densities. In this study, we present a simplified model for the yield strength contributions and apply that to obtain the dislocation densities by determining the orientation factors, which can be obtained via the crystal information of electron backscatter diffraction (EBSD). One alloy subjected to three cold-rolling reductions (10%, 40% and 90%) has been analysed by EBSD, and the density of dislocations are estimated using the strengthening model. This assessment suggests that dislocation densities by the Taylor model are roughly consistent but slightly lower than those determined by transmission electron microscopy
The assessment of GPB2/S" structures in Al-Cu-Mg alloys
No full textBased on experimental data presented in the literature, we propose a new structure for GPB2/S?? with the composition of Al10Cu3Mg3, which is aluminium-rich compared to S phase (Al2CuMg). The proposed structure is coherent with the fcc. Al matrix, is formed by the replacement of some Al atoms with Cu/Mg, and has orthorhombic structure (space group Imm2) with lattice parameters a = 0.405 nm, b = 1.62 nm and c = 0.405 nm. Simulated high resolution electron microscopy images and simulated diffraction patterns are compared with experimental data on a range of Al–Cu–Mg alloys. A good correspondence is found
A new structure of Nd1+?Fe4B4 phase in NdFeB magnet
No full textA new structure for Nd1+eFe4B4 phase has been observed, which has the same structure as Gd1+eFe4B4. The compound has Pccn structure with a = 0.71 nm and c = 2.74 nm, and its composition was found to be Nd2Fe7B7
The thermodynamics of and strengthening due to co-clusters: general theory and application to the case of Al-Cu-Mg alloys
Full text linkCo-clusters in ternary or higher order metallic alloys are metastable structures involving two or more distinct alloying atoms that retain the structure of the host lattice. A thermodynamic model based on a single interaction energy of dissimilar nearest neighbour alloying elements is presented, and a model for the strengthening due to these co-cluster dimers is derived. The model includes a new treatment of (short-) order strengthening relevant to these co-clusters and further encompasses modulus hardening and chemical hardening. The models are tested against data on a wide range of Al-Cu-Mg alloys treated at temperatures between 20 and 220ºC. Both quantitative calorimetry data on the enthalpy change due to co-cluster formation and strengthening due to co-clusters is predicted well. It is shown that in general (short-range) order strengthening will be the main strengthening mechanism
A model for the yield strength of Al-Zn-Mg-Cu alloys
No full textA model for the yield strength of multi-component alloys is presented and applied to overaged Al–Zn–Mg–Cu alloys (7xxx series). The model is based on an approximation of the strengthening due to precipitate bypassing during precipitate coarsening and takes account of ternary and higher order systems. It takes account of the influence of supersaturation on precipitation rates and of volume fraction on coarsening rates, as well crystallographic texture and recrystallisation. The model has been successfully used to fit and predict the yield strength data of 21 Al–Zn–Mg–Cu alloys, with compositions spread over the whole range of commercial alloying compositions, and which were aged for a range of times and temperatures to produce yield strengths ranging from 400 to 600 MPa. All but one of the microstructural and reaction rate parameters in the model are determined on the basis of microstructural data, with one parameter fitted to yield strength data. The resulting accuracy in predicting unseen proof strength data is 14 MPa. In support of the model, microstructures and phase transformations of 7xxx alloys were studied by a range of techniques, including differential scanning calorimetry (DSC), electron backscatter diffraction (EBSD) in an SEM with a field emission gun (FEG-SEM)
A simple approach of determination of the crystallographic orientation: Applications and accuracy
No full textA simple analytical solution for the crystallographic orientation is described. This method is based on one indexed Kikuchi pair in a known zone rather than the corresponding diffraction spots. The accuracy of this method is shown to be better than 0.1° even for cases in which a zone axis deviates by a large angle (e.g. 10°) from the centre of the beam direction. This approach simplifies experiments beacuse only one pair of Kikuchi lines and a zone axis are needed, and is especially suited when it is difficult or cumbersome to resolve a second pair of Kikuchi lines with sufficient accuracy
Two types of S phase precipitates in Al-Cu-Mg alloys
No full textTransmission electron microscopy (TEM) and differential scanning calorimetry (DSC) have been used to study S phase precipitation in an Al-4.2Cu-1.5Mg-0.6Mn-0.5Si (AA2024) and an Al-4.2Cu-1.5Mg-0.6Mn-0.08Si (AA2324) (wt-%) alloy. In DSC experiments on as solution treated samples two distinct exothermic peaks are observed in the range 250 to 350°C, whereas only one peak is observed in solution treated and subsequently stretched or cold worked samples. Samples heated to 270°C and 400°C at a rate of 10°C/min in the DSC have been studied by TEM. The selected area diffraction patterns show that S phase precipitates with the classic orientation relationship form during the lower temperature peak, and for the solution treated samples, the higher temperature peak is caused by the formation of a second type of S phase precipitates which have an orientation relationship that is rotated by ~4 degrees to the classic one. The effects of Si and cold work on the formation of second type of S precipitates have been discussed
A proposed new structure for GPB2/ S" in Al-Cu-Mg alloys
No full textBased on previously published HREM work, we propose a new structure for the ordered GPB2/ S" phase with a composition of Al10Cu3Mg3, which is aluminium-rich compared to S phase (Al2CuMg). The proposed structure is coherent with the f.c.c. Al matrix and is formed by the replacement of some Al atoms with Cu/Mg, and has a orthorhombic structure (space group Imm2) and the lattice parameters of a = 0.405 nm, b = 1.62 nm and c = 0.405 nm. The HREM simulation and reflection intensities based on this structure match the experimental image and diffraction patterns
Precipitates and intermetallic phases in precipitation hardening Al–Cu–Mg–(Li) based alloys
Full text linkThe present study contains a critical review of work on the formation of precipitates and intermetallic phases in dilute precipitation hardening Al–Cu–Mg based alloys with and without Li additions. Although many suggestions for the existence of pre-precipitates in Al–Cu–Mg alloys with a Cu/Mg atomic ratio close to 1 have been made, a critical review reveals that evidence exists for only two truly distinct ones. The precipitation sequence is best represented as:
supersaturated solid solution->co-clusters->GPB2/S"->S
where clusters are predominantly Cu–Mg co-clusters (also termed GPB or GPB I zones), GPB2/S" is an orthorhombic phase that is coherent with the matrix (probable composition Al10Cu3Mg3) for which both the term GPB2 and S" have been used, and S phase is the equilibrium Al2CuMg phase. GPB2/S" can co-exist with S phase before the completion of the formation of S phase. It is further mostly accepted that the crystal structure of S’ (Al2CuMg) is identical to the equilibrium S phase (Al2CuMg). The Perlitz and Westgren model for S phase is viewed to be the most accepted structure. 3DAP analysis showed that Cu–Mg clusters form within a short time of natural and artificial aging. Cu–Mg clusters and S phase contribute to the first and second stage hardening during aging. In Al–Cu alloys, the theta phase (Al2Cu) has I4/mcm structure with a=0.607 nm and c=0.487 nm, and theta’ phase with tetragonal structure and a=0.404 nm, c=0.58 nm, the space group is I4¯m2. Gerold’s model for theta" (or GPII) appears to be favourable in terms of free energy, and is consistent with most experimental data. The transformation from GPI to GPII (or theta") seems continuous, and as Cu atoms will not tend to cluster together or cluster with vacancies, the precipitation sequence can thus be captured as:
supersaturated solid solution->theta" (Al3Cu)->
theta’ (Al2Cu)->theta (Al2Cu).
The Omega phase (Al2Cu) has been variously proposed as monoclinic, orthorhombic, hexagonal and tetragonal distorted theta phase structures. It has been shown that Omega phase forms initially on {111}Al with c=0.935 nm and on further aging, the c lattice parameter changes continuously to 0.848 nm, to become identical to the orthorhombic structure proposed by Knowles and Stobbs (a=0.496 nm, b=0.858 nm and c=0.848 nm). Other models are either wrong (for example, monoclinic and hexagonal) or refer to a transition phase (for example, the Garg and Howe model with c=0.858 in a converted
orthorhombic structure). For Al–Li–Cu–Mg alloys, the L12 ordered metastable delta’ (Al3Li) phase has been observed by many researchers. The Huang and Ardell model for T1 phase (space group P6/mmm, a=0.496 nm and c=0.935 nm), appears more likely than other proposed structures. Other proposed structures are perhaps due to the T1 phase forming by the dissociation of 1/2<110> dislocations into 1/6<211> Shockley partials bounding a region of intrinsic stacking fault, in which
copper and lithium enrichment of the fault produces a thin layer of the T1 phase
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