Under-aged Alloy Research Articles

Addition rules and the contribution of δ' precipitates to strengthening of aged AlLiCu alloys

The room temperature yield strength of two alloys, Al-2.30 wt% Li-2.85 Cu-0.12 Zr (the 2–3 alloy) and Al-2.90 Li-0.99 Cu-0.12Zr (the 3-1 alloy), was investigated as a function of aging time at 160 and 190°C. Reversion experiments were used to separate the contributions of the δ' and T 1, precipitates to the overall strength of the aged samples. The reversion heat treatments nearly completely dissolved the δ' precipitates, leaving the dispersion of T 1 precipitates essentially undisturbed and enabling its contribution, Δτ T 1 , to be measured experimentally. Assuming linear additivity of the precipitation hardening contribution, Δτ p , and all the others (due to grain size strengthening, solid solution strengthening, etc.), the δ' contribution, Δτ δ' , was estimated using a generalized addition rule of the type Δτ p q = Δτ δ' q + Δτ T 1 q , where q is a constant (the shear stresses, Δτ, were obtained from the measured yield stresses using data on the textures of the alloys). These values of Δτ δ' , were then analysed according to a recent version of the theory of order strengthening which requires that the trailing dislocation be pulled through the sheared ordered precipitates immediately on encountering them. It was found that for the underaged alloys only q = 1.4 yielded values of the APB energy on {111}, γ apb , that were identical for both the 3-1 and 2–3 alloys (0.150 J/m 2). Using this value of γ abp , the theoretically predicted values of Δτ δ' at peak strength and the δ' particle size at peak strength were in very good agreement with the experimentally measured values, and the predicted spacings of the dislocation pairs were in excellent agreement with those measured. It is demonstrated that the value of q is not merely an empirical constant required to bring theory and experiment in agreement, but is justified by the results of computer simulation experiments on strengthening by point obstacles of two distinct strengths. The possible role of other strengthening mechanisms is discussed.

Microscopic and macroscopic aspects of fracture in lithium-containing aluminum alloys

The effects of alloy composition and aging treatment on plane strain fracture initiation toughness ( K lc , J lc ) and crack growth resistance (characterized by the tearing modulus, T R ) were investigated in high purity alloys belonging to the Al-Li-Cu-Zr system. The Li Cu ratio (expressed in terms of atomic fractions) was varied from 2.2 to 25.2 and in each of these alloys, microstructures spanning the very underaged to the severely overaged conditions were studied. In peak-aged and over-aged alloys of higher lithium content, intergranular fracture arising from voids initiated around grain boundary particles dictates fracture initiation toughness and crack growth resistance. In such cases, both K lc and T R are insensitive to aging beyond the peak strength. When the amount of grain boundary precipitates is limited, the existence of ordered δ′ (Al 3Li) particles in the matrix promotes intense slip planarity. While there are reports of experimental evidence indicating a detrimental effect of planar slip on fracture toughness upon artificial aging, there is also a concomitant beneficial (geometrical) effect on crack growth due to severe bifurcation of the crack during quasi-static fracture. We demonstrate with the aid of branched crack models that such bifurcation can account for an important part of the microstructural effects on fracture toughness in some underaged alloys. The magnitude of microstructurally-influenced crack deflection is systematically correlated with the volume fraction of the precipitates using quantitative transmission electron microscopy. It is shown that microstructural characteristics, which are generally considered undesirable for fracture initiation toughness, can in fact lead to marked improvements in the tearing modulus. We rationalize such results based on the observed variations in crack path. In order to separate out the individual effects of Li and Cu and to address some key issues pertinent to the micromechanisms of failure, fracture experiments have also been performed in a high purity Al-3 wt% Li alloy containing only Mn (used for grain size control). Furthermore, results of heat treatments designed to produce microstructural reversion are discussed to identify the individual contributions to fracture from matrix deformation and grain boundary failure. An attempt is made to present, as quantitatively as possible, an overall mechanistic perspective on fracture initiation toughness and crack growth resistance in lithium-containing aluminum alloys.

Strengthening of nitrided FeTi alloys

The temperature dependence of the yield and flow stresses of nitrided FeTi alloys have been investigated in the range 77–900 K for a number of particle dispersions. In as-nitrided or underaged alloys, from the magnitude of the strengthening, its dependence on solute concentration and temperature it is concluded that strengthening is due to chemical factors arising when new interfaces are created as titanium-nitrogen atom clusters (GP zones) are cut by glide dislocations. Such alloys have poor ductility due, in part, to their high strength and the coarse slip distribution which develops as a result of particle shear. When overaged, the titanium-nitrogen atom clusters transform to the equilibrium precipitate (TiN) and a change in deformation mechanism from cutting to looping occurs which is accompanied by a rapid increase in work hardening rate and an improvement in ductility. The strength of overaged alloys can be adequately accounted for by Orowan hardening.

Effect of aging and dispersoid content on tensile properties of Al–0·6Mg–1 Si alloys

Abstract Slip distribution was varied in a series of Al–Mg–Si alloys by changing the amount of manganese-bearing dispersoid present and by under- and overaging, which also altered the grain–boundary precipitate-free zones and hence the grain–boundary strength. Dispersoids are shown to increase ductility by slip homogenization. Slip is more heterogeneous in underaged alloys, but these show greater ductility than overaged alloys because the grain boundaries are stronger. Work-hardening rates increase with dispersoid content, although for a given dispersoid content, the underaged alloys have higher work-hardening rates. This effect is interpreted in terms of the effect of aging upon the properties of the grain-boundary regions.MST/340

δ′-L1<SUB>2</SUB>型規則相を含むAl-Li 2元合金の降伏応力の歪速度依存性

The effect of strain rate on the yield stress of an Al-11.1 mol%Li alloy containing δ′-precipitates has been investigated at temperatures between 77 and 523 K and over the strain rate range from 1.77×10−4 s−1 to 1.77×10−2 s−1. At testing temperatures below 373 K, the yield stress is almost independent of strain rate at any aging stage. At testing temperatures above 373 K, the yield stress increases linearly with the logarithm of strain rate, and the strain rate dependence increases with increasing testing temperature. The yield stresses of under-aged alloy at temperatures between 373 and 473 K at high strain rates are greater than the yield stress at 77 K. For the alloy under-aged or aged nearly to its peak strength, the temperature range within which the positive temperature dependence of yield stress appears expands to the higher temperature side with increasing strain rate. The strain rate dependence of the yield stress is slightly negative at this aging stage. Within this temperature range, dislocations move in pairs cutting the δ′-L12 ordered particles. The yield stress of the over-aged alloy decreases monotonically with decreasing strain rate and with increasing testing temperature above 373 K. The modulus normalized yield stress is nearly constant at testing temperatures below 373 K at any strain rate investigated. In the over-aged alloy, dislocations by-pass the precipitates at any temperature and strain rate.

Ａｌ‐Ｚｎ‐Ｍｇ‐Ｃｕ系合金の破壊じん性値と機械的性質の関係

Effect of alloy purity and aging on the fracture toughness of three type alloys has been examined to clear the relationship between the fracture toughness and tensile properties. The fracture toughness were evaluated by the critical crack tip opening displacement (CTODc) derived from stretched zone analysis. The general magnitud of CTODc increases with reduction in iron and silicon contents. The value of CTODc depends on the aging condition. It lowers as the yield strength rises, or the value of the strain-hardening exponent decreases. All underaged alloys show a superior to toughness to the overaged of ones having the same yield strength. The total impurity content is, accordingly, a dominant fractor governing the fracture toughness of these alloys. The final values of fracture toughness were also determined by the tensile parameters, yield strength and strain-hardening exponent.

Some aspects of dislocation-precipitate interaction in under-aged aluminium-germanium alloy

The glide dislocation-precipitate interaction in under-aged alloy is investigated by microscopy and low-temperature deformation techniques. Slip-line features of room temperature deformed samples were observed by optical microscopy. From tensile tests at various temperatures the variation of flow stress withT 2/3 was established. Comparing the present results with those obtained on the peak-hardened condition of the same alloy it is concluded that moving dislocation shear through the small rod-shaped germanium precipitates.

The effect of microstructure and environment on fatigue crack closure of 7475 aluminum alloy

The effects of slip character and grain size on the intrinsic material and extrinsic closure contributions to fatigue crack growth resistance have been studied for a 7475 aluminum alloy. The alloy was tested in the underaged and overaged conditions with grain sizes of 18 μm and 80 μm. The fracture surface exhibited increased irregularity and planar facet formation with increased grain size, underaging, and tests in vacuum. These changes were accompanied by an increased resistance to fatigue crack growth. In air the 18 μm grain size overaged material exhibited relatively poor resistance to fatigue crack growth compared with other microstructural variants, and this was associated with a lower stress intensity for closure. All materials exhibited a marked improvement in fatigue crack growth resistance when tested in vacuum, with the most significant difference being ˜1000× at a ΔK of 10 MPa m1/2 for the 80 μm grain size underaged alloy. This improvement could not be accounted for by either an increase in closure or increased crack deflection and is most likely due to increased slip reversibility in the vacuum environment. The intrinsic resistance of the alloy to fatigue crack growth was microstructurally dependent in vacuum, with large grains and planar slip providing the better fatigue performance.

等しい強さの不完全時効状態と過時効状態の延性の比較

In order to determine the optimum aging temperature to give superior toughness to an age-hardening alloy, a comparison of the experimental values of ductility in the under- and over-aged states having similar strength was made using various kinds of iron and aluminum alloys, for example, Fe-Cu, Fe-Cr, Fe-Ni-Cu, Fe-Ni-Mo-C, Fe-Co-Cr-W, Al-Si and Al-Ag alloys. The following conclusions have been drawn from the data by the authors and other investigators: (1) In general, the under-aged alloy has a higher ductility, a lower yield point and a larger strain hardening exponent than those in the over-aged alloy having similar tensile strength. (2) In the alloys strengthened by precipitation of G.P. zone or by decomposition into two phases, in other words, having a coherent second phase, premature fracture often takes place in the under-aged state. (3) In the heavily cold worked alloys and secondary-hardened high alloy steels, the superior toughness may be obtained after slight over-aging.