Magnesium alloys have received an increasing interest in the past 12 years for potential applications in the automotive, aircraft, aerospace, and electronic industries. Many of these alloys are strong because of solid-state precipitates that are produced by an age-hardening process. Although some strength improvements of existing magnesium alloys have been made and some novel alloys with improved strength have been developed, the strength level that has been achieved so far is still substantially lower than that obtained in counterpart aluminum alloys. Further improvements in the alloy strength require a better understanding of the structure, morphology, orientation of precipitates, effects of precipitate morphology, and orientation on the strengthening and microstructural factors that are important in controlling the nucleation and growth of these precipitates. In this review, precipitation in most precipitation-hardenable magnesium alloys is reviewed, and its relationship with strengthening is examined. It is demonstrated that the precipitation phenomena in these alloys, especially in the very early stage of the precipitation process, are still far from being well understood, and many fundamental issues remain unsolved even after some extensive and concerted efforts made in the past 12 years. The challenges associated with precipitation hardening and age hardening are identified and discussed, and guidelines are outlined for the rational design and development of higher strength, and ultimately ultrahigh strength, magnesium alloys via precipitation hardening.

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Precipitation and Hardening in Magnesium Alloys

The 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). Th...

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Precipitates and intermetallic phases in precipitation hardening Al–Cu–Mg–(Li) based alloys

Recovery, recrystallization, grain growth and phase stability of a family of FCC-structured multi-component equiatomic solid solution alloys

An equation expressing the volume fraction,x, of crystals precipitating in a glass heated at a constant rate, α, was derived. When crystal particles grow m-dimensionally,x is expressed as In [- ln(1 -x)] = -n (nα - 1.052mE/RT + Constant whereE is the activation energy for crystal growth andn is a numerical factor depending on the nucleation process. When the nuclei formed during the heating at the constant rate,α, are dominant,n is equal tom + 1, and when the nuclei formed in the previous heat-treatment before thermal analysis run are dominant,n is equal tom. The validity and usefulness of this equation was ascertained by applying it to a Li2O·2SiO2 glass. A method for determining the values ofn andm from DSC curves was proposed and it was concluded that the modified Ozawa-type plot is very useful and convenient to obtain the activation energy for crystal growth.

Kinetics of non-isothermal crystallization process and activation energy for crystal growth in amorphous materials

The fracture of bulk metallic glasses

The structure evolution of the WE43 Mg alloy during different thermal treatments is studied by differential scanning calorimetry (DSC), transmission electron microscopy (TEM) and positron annihilation spectroscopy (PAS). Isothermal annealings at different temperatures are made, with the aim to correlate the alloy's structure with its hardness response and to compare these treatments to the standard industrial T6 one (16 h at 250 °C). The highest hardening effect is obtained with a treatment at 210 °C with the formation of both β″ and β′ phases. A residual amount of solute atoms into the matrix after each thermal treatment promotes the renucleation of β″ and β′. The standard T6 treatment mainly leads to the precipitation of the β′ phase, while the thermal treatment at 280 °C essentially promotes the formation of the equilibrium β phase.

Structure evolution of a WE43 Mg alloy submitted to different thermal treatments

Formation and evolution of the hardening precipitates in a Mg–Y–Nd alloy

Early ageing mechanisms in a high-copper AlCuMg alloy

Electrical resistivity and structural relaxation in Fe75TM5B20 amorphous alloys (TM = Ti, V, Cr, Mn, Co, Ni)

The effect of impurities and of slight deformations as factors modifying the kinetics of normal grain growth after primary recrystallization in pure iron is investigated. Samples of pure iron and Armco iron, each with or without a content of precipitated oxides, were examined. Inclusions inhibit grain growth up to high temperatures, whilst the influence of strain depends on its amount: very low deformations (∼2%) generally block grain growth; for deformations around 5%, secondary recrystallization takes place; higher deformations (∼10%) accelerate the early stages of growth, leading to not very large final grain dimensions. 2.0% elongation has been found to be, in our conditions, the critical strain limit for secondary recrystallization.

Giuseppe Riontino

Papers

Structure evolution of a WE43 Mg alloy submitted to different thermal treatments

Formation and evolution of the hardening precipitates in a Mg–Y–Nd alloy

Early ageing mechanisms in a high-copper AlCuMg alloy

Electrical resistivity and structural relaxation in Fe75TM5B20 amorphous alloys (TM = Ti, V, Cr, Mn, Co, Ni)

Grain growth and secondary recrystallization in iron