Strengthening and toughening mechanism of Al-Li alloy
1. Strengthening mechanism
The strengthening effect of Al-Li alloy mainly comes from precipitation phase strengthening and solid solution strengthening. The main precipitation phase δ 'is a metastable phase coherent with the α-Al matrix, and has an ordered superlattice (LI2) structure. The degree of interface distortion of α / δ 'is very small, only about 0.08%, and the δ' phase is uniformly precipitated in the form of dispersed particles in the alloy. The strengthening of the metal comes from the hindering effect of its internal structure on slip dislocations. The main factor hindering the movement of dislocations in Al-Li alloys is the presence of δ 'precipitates in the alloy, and the factors that affect the dislocation cutting δ' particles are:
① Internal friction stress τ0 of the matrix
② lattice distortion distortion stress τg at the interface between δ 'phase and matrix
③ The difference between the shear modulus of the δ 'phase and the matrix τ △ G
④ The interfacial energy γ0 produced by the formation of antiphase chips in the δ 'phase
⑤ The surface energy of the new surface formed by the cut δ 'phase and the matrix γ0
⑥ Internal friction stress τp of δ 'phase
Experiments and calculations show that the main effect on the strength of the alloy is the reverse phase interface energy generated when the dislocation cuts the δ 'phase, and its contribution to the alloy strength is about 50%, followed by the internal friction stress of the δ' phase and the matrix τp and τ0, the other three items are about 5%. In addition, the change in the order of δ 'phase will also significantly change the strength of the alloy.
2. Toughening mechanism
①Coplanar slip
In Al-Li alloys, because the δ 'phase is completely coherent with the α matrix and its α / δ' phase interface strain is small, slip dislocation is easier to cut δ 'phase particles. The cut δ 'phase particles can provide a channel where slippage is easier, so a large amount of slippage displacement often slips on the same crystal plane without intersecting slip, forming a so-called coplanar slip zone. This coplanar slipping phenomenon leads to the accumulation of dislocations at the grain boundaries, resulting in local stress concentration and yielding, and finally to the initiation of grain boundary cracks. This coplanar slipping makes the toughness of the alloy improved.
② No precipitation zone at the grain boundary
The δ 'phase is uniform within the crystal, but the so-called δ' phase no precipitation zone (PFZ) appears near the grain boundary. Since PFZ is softer than the intragranular structure, the grain boundary dislocation accumulation and stress concentration caused by slip can cause early yield and plastic deformation, resulting in micropores near coarse grain boundaries and three-phase intersections. Nucleation and expansion along the PFZ to form micro-cracks, as a result of which the intergranular fracture will occur during stretching of the alloy and deteriorate the alloy's properties.
③Texture and recrystallization
The rolled Al-Li alloy sheet has deformed texture, and its main texture type is (110) [112] texture. Because of the existence of the texture, the orientation difference between the grains becomes small, only about 3 °, so the intragranular slip zone can spread across the grain boundary at this time. This is because the small-angle grain boundary has a small blocking effect on the dislocation, so once the dislocation passes through the grain boundary, a transcrystalline shear plane sliding along the {111} plane occurs until the material is destroyed.
Texture and recrystallization are closely related. After complete recrystallization, the deformation texture of the Al-Li alloy is also eliminated. After recrystallization, the strength of the Al-Li alloy decreases, and it is accompanied by grain growth, sub-grain boundary disappearance, and even a series of structural changes such as recrystallization texture.
④The influence and function of other precipitation phases
In the Al-Cu-Li-Mg-Zr series alloy, in addition to the δ 'phase, there are other binary or ternary precipitation phases. The basic precipitation process is:
The δ phase is the late product of the Al-Li binary precipitation process, and its precipitation will cause the volume fraction of the δ 'phase to decrease and the strength of the alloy to decrease, that is, the phenomenon of over-aging. In addition, the coarse δ-phase particles on the grain boundary not only promote the formation of PFZ, but also tend to cause micro-cracks around it. The Al2MgLi phase is also the equilibrium phase that appears at the later stage of aging, and precipitation at the grain boundary can also lead to the formation of PFZ and microcracks. The S phase preferentially precipitates unevenly near the dislocations and defects, which can effectively prevent the coplanar slip of the dislocations, thereby improving the strength and toughness of the alloy. The T1 phase is easily cut by dislocations and has a smaller effect on preventing coplanar slip than the S phase. The Al3Zr spherical precipitates have a pinning effect on the grain boundaries, and the addition of Zr also makes the aging speed of the alloy faster. Al-Cu binary precipitates can further strengthen the alloy without harming its fracture toughness.
3. Thin strip reinforcement and layered reinforcement
Taking an Al-Li alloy plate with a non-recrystallized flat grain structure as an example, its tensile fracture is layered, with a large number of intergranular secondary cracks perpendicular to the main fracture surface, which is called short lateral delamination. The short lateral delamination occurs because of the combined effects of flat grains, weak grain boundaries, plane slip grain boundary equilibrium phases and corresponding non-precipitated zones, which are evenly distributed, parallel to the rolling surface and extending perpendicular to the main fracture surface . Its generation and development will not only cause the specimen to break, but will divide it into many thin strips parallel to the tensile axis. The subsequent plastic deformation is restricted to independent thin strips, and the deformation transmission between each other is difficult to perform, and the deformation resistance With the increase, this effect is called thin strip strengthening. Secondly, due to the short transverse layered vertical main crack, the main crack will temporarily stagnate when it encounters 90 ° deflection. This effect is called layering strengthening.
4. Layer toughening
Still taking the non-recrystallized planar grain structure of Al-Li alloy sheet as an example, due to the triaxial tensile stress in front of the crack tip induced short lateral delamination, a series of vertical main crack thin strips were formed in front of the crack tip The crack tip changes from a whole plane strain state to a series of parallel plane stress states. Macroscopically, it shows an increase in fracture toughness. This toughening effect is called delamination toughening.
2. Strengthening and toughening of aluminum-lithium alloy
Generally speaking, the strength and toughness of aluminum-lithium alloy is relatively low, the main reasons are as follows:
① The superlattice structure of the δ '(Al3Li) phase is completely coherent with the matrix, which is easy to cause coplanar slip and cause local strain concentration
② The grain boundaries of δ and T2 phases precipitate, causing intergranular fracture
③ Alkali metal impurities such as Na, K, Ga are prone to segregate at the grain boundaries to form sodium brittleness
④ The presence of Li makes the aluminum-lithium alloy contain more hydrogen than the general aluminum alloy, seriously damaging the strength and toughness of the aluminum-lithium alloy
In view of the structural characteristics, strengthening mechanism and specific problems of improving the toughness and toughness of the aluminum-lithium alloy, in general, the following toughening measures can be taken.
1. Layered tissue
The unrecrystallized grain structure of the aluminum-lithium alloy is also called a layered structure. Under tensile conditions, when the grains are flat and along the crystal cracks in the form of short lateral layers, it can hinder the propagation of the main crack Helps improve plasticity. The flat unrecrystallized grain structure excludes the geometric conditions of crack propagation along the crystal under certain conditions. The main fracture surface is transgranular cracking, which improves the crack propagation work. More importantly, the triaxial tensile stress in front of the crack tip induces short lateral delamination, so that a series of thin strips of vertical main cracks are formed in front of the crack tip. The crack tip changes from a plane strain state with an overall millimeter thickness to hundreds. The plane stress state up to thousands of micrometers in thickness parallel shows the increase of fracture toughness value as a whole, showing obvious toughening effect.
2. Deformation heat treatment
Appropriate cold deformation of the Al-Li alloy after solution treatment before aging can form dense dislocations or dislocation entanglement in the alloy, which becomes the position of non-uniform nucleation of S ', T1 and other phases, thereby increasing The volume fraction of the precipitated phase that cannot be cut by dislocations reduces the coplanar slip of the alloy and the stress concentration at the grain boundaries. At the same time, the cold deformation before aging can accelerate the precipitation, make the precipitation phase more fine and uniformly distributed, and inhibit the formation of the grain boundary equilibrium phase. The effect of cold deformation before aging on room temperature tensile properties of 8090 alloy is shown in Table 1.
3. Microalloying
Relying on the introduction of T1 and S 'equal to the aluminum-lithium alloy contributes to dispersion slip, its effect depends largely on the dispersion of the precipitated phase itself, the use of pre-deformation helps to improve the T1 phase, S' equal dispersion However, it is difficult to achieve a completely uniform distribution of microscopic deformation. Therefore, the potential for improving the dispersion of the precipitated phase in Al-Li alloys is as limited as possible. Microalloying may change the thermodynamic and kinetic behavior of the components of the precipitated phase, improve the ageing characteristics of the precipitated phase from the level of component behavior, and then optimize the fine structure. A small amount of Zr and Sc are added to the aluminum-lithium alloy to form Al3Zr and Al3Sc dispersed particles, respectively, which play the role of dispersion strengthening and fine grain strengthening on the matrix. In addition, the addition of a small amount of Be can suppress the segregation of metal Na mixed into the alloy at the grain boundaries; the addition of Co, Ti, Ce and other elements form more non-coherent phases or δ 'symbiotic phases, thereby improving the strength and toughness. Adding Cu, Mg, Ag and other elements separately or simultaneously can effectively improve the strength and toughness of the aluminum-lithium alloy: first, Cu, Mg, Ag has a solid solution strengthening effect; second, after adding Cu, it promotes the precipitation of q '(Al2Cu) dispersion during alloy aging Phase, dislocations are difficult to cut and can only be bypassed, thereby reducing the tendency of aluminum-lithium alloy to coplanar slip, and exciting it to produce cross-slip, promoting uniform deformation of the alloy; finally, adding in Al-Cu-Li alloy A small amount of Mg and Ag, together forming Mg-Ag clusters, can more effectively promote the precipitation of T1 phase.
4. Grading aging
Studies have shown that aging treatment at low temperature and then high temperature can promote the dispersion, fine and uniform topography of a large number of S 'phases, and prevent the precipitation of coarse equilibrium phases along the grain boundaries and the formation of PFZ at the grain boundaries. In addition, the graded aging makes more Al3Li / Al3Zr composite particles appear in the alloy, so as to effectively improve the strength and toughness of the aluminum-lithium alloy. The effect of different deformation aging on the mechanical properties of 2091 alloy is shown in Table 2. Some people find that using multi-stage aging is better. This aging method mainly adopts slow heating at a certain speed (10 ℃ / h), so that the strengthening phase becomes fine and precipitates in a dispersed manner, and then aging at higher temperature Method to make the strengthening phase grow to a certain size.
5. Low Li
Low Li reduction reduces the co-planar slip caused by δ 'phase precipitation and hydrogen embrittlement caused by a large amount of hydrogen precipitation, but this is at the expense of some of the low-density advantages. The main reason is that the lithium content is not very large.
6. Purification
Strictly speaking, the purity of raw materials often has a great impact on the performance of materials. There is still a big gap between domestic and foreign countries in the handling and purification of raw materials. Therefore, the emphasis on the basic characteristics and purity of raw materials is the key to improving the performance of materials. Similarly, the purity of raw materials has a great influence on the strength and toughness of aluminum-lithium alloys. Purity issues include the effects of gas pollution, elemental pollution, inclusions, and dispersed particles. For aluminum alloys, the contaminated gas is mainly hydrogen, and a small amount of hydrogen will greatly reduce the toughness of the alloy, so the hydrogen content in the alloy should generally be less than 1 × 10-6. Other polluting elements mainly include Na, K, S, etc. They cannot be dissolved in the matrix, but are easily segregated at the grain boundary, causing embrittlement of the grain boundary. Inclusions refer to particles larger than 1μm such as Fe7, Si, Al7Cu2Fe and Al12 (FeMn) 3Si. Dispersed particles refer to particles of 0.1μm formed during solidification or high temperature homogenization, such as Mn-containing dispersed particles Al6Mn, Al20Cu2Mn5 It will have some harmful effects on the properties of the alloy. Therefore, the content of impurities such as Na, K, and S in the aluminum-lithium alloy should be less than (5 ~ 10) × 10-6, the content of Fe should be less than 0.06%, and the content of Si should be less than 0.02%. When preparing aluminum-lithium alloys, it is best to use high-purity aluminum with a purity of more than 99.9%. When smelting, it is covered with 20% LiF + 80% LiCl mixed flux or protected with argon. At the same time, the degassing process of the alloy must be strictly controlled, especially In the smelting process and before and after pouring, the process of degassing is very important, otherwise it will be difficult to obtain qualified products required by the industry.