Chang Qi Chen

Structure and properties of ultrafine-grained Al-Zn-Mg-Cu and Al-Cu-Mg-Mn alloys fabricated by ECA pressing combined with thermal treatment

Abstract An investigation was conducted on the structures and properties of aluminum-based alloys 7050 and 2224 with submicrocrystalline grains produced by severe plastic deformation through equal-channel angular pressing (ECA pressing). Special attention was paid to how the heat treatments before or after ECA pressing affected the structures and properties of alloys 7050 and 2224. The results showed that the ultimate tensile strength (UTS) of alloy 7050 reached 677 MPa, with elongation to failure of 15%, while the UTS of alloy 2224 reached 618 MPa, with elongation to failure of 12%. The forming of finer grains after ECA pressing resulted from cutting off shearing bands directly and from recrystallization at certain strain and annealing regimes. In this paper, three strengthening mechanisms were considered: grain refinement strengthening, high dislocation strengthening, and more homogenous and finer precipitations than the conventional T6-treated alloy.

Further verification of 1/3 type faulted dipoles in TiAl

The TiAl based alloy is one of the most promising candidate materials for high temperature applications in the next decade. However, its low-temperature brittleness hinders its wide usage. In the last twenty years the deformation mechanisms of TiAl alloys have been intensively investigated. It is well known that the extremely common feature of deformation microstructures at room temperature is the existence of numerous stacking faults surrounded by 1/6 partials. Further investigations by Hug et al showed that these planar faults were extrinsic faulted dislocation dipoles which were produced by the rearrangement of the core structure of a pinned 1/2 superdislocation. In their previous paper, the authors reported an experimental observation of a new type of faulted dipoles. The surrounding partials of these faulted dipoles have a Burgers vector of 1/3 rather than 1/6 . This is striking. One could argue that it seems impossible to identify unambiguously the Burgers vector of partial dislocations bounding a stacking fault by extinction rules only because the contrast of partial dislocations is often affected significantly by that of the bounded stacking faults. In order to check the previously reported result, image simulation has been performed and is presented in this note.

Effect of Zinc Content on the Ageing Behavior of Al-Zn-Mg-Cu-Li Alloys

In this paper, Al-5.6Zn-2.8Mg-1.6Cu-0.24Cr-1.1Li, Al-8.0Zn-2.4Mg-2.4Cu-1.1Li-0.18Zr and Al-11.8Zn-2.9Mg-2.8Cu-1.1Li-0.24Zr (in wt%) alloys were artificially aged at 80,100,120,140 and 160°C for different time, respectively. The ageing process, microstructure and hardness of different alloys are investigated by Vickers hardness and transmission electron microscopy (TEM). With the increment of zinc content, addition of Li to Al-Zn-Mg-Cu alloy containing high alloying elements has little effect on the nucleation of precipitates, but the growth of precipitates has been inhibited. Very fine precipitates with high density are formed during ageing due to the existence of high alloying elements, which result in highly strengthening alloy. As the alloying content is too high to exceed the solution saturation of matrix, coarse second phases are formed in grain boundary.

Stability of lamellar structure in a Ti-47.8at%Al alloy

The stability of the lamellar structure is known to be highly structure sensitive. Although some reports have investigated the stability of lamellae in lower aluminum content alloys, few reports have studied the stability of lamellae in near gamma titanium aluminide alloys. The purpose of this paper is to investigate the stability of lamellar structure with different features in Ti-47.8at%Al alloy. The following conclusions were reached: (1) A lamellar structure was obtained in furnace cooled (FC), air cooled (AC) and oil quenched (OQ) samples. As the cooling rate increased, the interlamellar spacing decreased. Thus, the stability of lamellae decreased. The coarsening rate increased with increasing temperature. (2) An {alpha}{sup 2} single phase was obtained in the water quenched (WQ) samples. The lamellae transformed from the {alpha}{sub 2} phase are less stable, and hence have a faster coarsening rate and finer coarse structure compared with the lamellar formed during cooling. (3) Since coarse lamellae nucleate in the {alpha}{sub 2} and {gamma} matrices, more than one habit plane exists, so that the alignment of coarse lamellae is complex. Parallel coarse lamellae with same close-packed plane index form a packet.

Texture Evolution in Rolled Mg-13wt%Li-X Alloy

Texture evolution in Mg-13wt%Li-X alloy cold-rolled from 1.35 mm to 0.34 mm thickness was investigated, by obtaining pole figures and orientation distribution functions (ODFs). Punching tests were conducted to reveal the effect of texture nature on formability. It was found that: (1) the textures of the as-received sheet are characterized by α fiber texture, a γ fiber texture and a cubic texture in both cold-rolled and annealed conditions; (2) with thickness reduction though rolling, the intensity of the γ fiber texture continuously increases and finally the γ fiber texture connects into {111} tube texture, the texture of <11 0> orientation flows towards {223}<11 0> along α fiber, the cubic texture of {001}<100> turns into {035}<100>, while some grains concentrate at {011}<41 1> orientation; (3) good punching behavior of the cold-rolled sheet corresponds to the appearance of a well-developed γ fiber texture.

Ageing Processes of Li-Containing Al-Zn-Mg-Cu Alloys

1.1%Li was added to 7075 alloys to obtain the Li-containing Al-Zn-Mg-Cu alloys. The microstructure and hardness of the alloys are investigated by transmission electron microscopy (TEM) and Vickers hardness. The hardness of the single-aged alloys is low. When the alloys were double-aged or multi-aged, the hardness is comparable to that of Al-Zn-Mg-Cu alloys at peak ageing. Two peaks were present in the hardness curves of the multi-aged Li-containing Al-Zn-Mg-Cu alloys. With the last-step temperature increases, two-peak phenomenon becomes prominent. The density and size of precipitates are influenced remarkedly by the ageing processes. Coarse grain boundary precipitates and PFZ (precipitate free zone)can be observed when the Li-containing Al-Zn-Mg-Cu alloys were multi-aged, and the higher the last-step ageing temperature, the wider the PFZ is.

Types of serrations in Al-Li single crystals

Abstracts are not published in this journal

Effect of Erbium on Precipitation of Aged Al-Zn-Mg-Cu-Li Alloy

0.3%Er is added to Al-5.6Zn-1.9Mg-1.6Cu-1.0Li alloy (in wt.%) to investigate the effect of erbium on microstructure of the alloy. Most of erbium exists in the primary Al3Er phases which are incoherent with the alloy matrix unevenly. A few of fine Al3Er phases, precipitated from supersaturated solid solution, are coherent with the matrix. η′-MgZn2 precipitates in the single-aged alloy are sparser than that in the double-aged alloy. A few of fine Al3Er phases act as the heterogeneous sites of η phases. There exist some precipitation free zones in the matrix around fine Al3Er precipitates. Erbium is segregated to grain boundary seriously. A few of Al3Er phases are present in grain boundary. The grain boundary precipitates are not continuous about the Al3Er phases.

Effect of Natural Ageing on Precipitation of Al-Zn-Mg-Cu-Li Alloys

Effect of natural ageing on the precipitation of Al-5.6Zn-1.9Mg-1.6Cu- 1.1Li-0.24Cr (in wt.%) alloys were investigated. The ageing processes, microstructure and hardness of the alloys were investigated by Vickers hardness and transmission electron microscopy (TEM). The results show that natural ageing makes the exothermic temperature range of the Al-Zn-Mg-Cu-Li alloys to shift to lower temperature. Natural ageing for 48h and followed by ageing at 120 , precipitates in the matrix of Al-Zn-Mg-Cu-Li alloys are sparse, which lowers the age hardness of the Al-Zn-Mg-Cu-Li alloys.

Small Angle X-Ray Scattering Study in Equal-Channel Angular Pressed 7055 Al Alloys

A study was conducted on 7055 Al alloys processed by Equal-channel Angular Pressing (ECAP). The quantitative information about precipitate size, distribution and volume fraction in ECAPed samples was investigated by small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). It is found that with the number of ECAP increasing, the quantitative information about volume fraction (fv), precipitate size (R), specific inner surface of particle-matrix (Sp), and integrated intensity (Q0) all increased, indicating that ECAP enhanced the nucleation and growth of precipitates. The microhardness of samples after two passes of ECAP is higher than that of T6 treated samples even after 96 h at 120°C. The formation of fine grains, higher volume fraction of precipitates, together with strain hardening induced by severe deformation are the main factors leading to a high microhardness in ECAPed samples.