Plastic anisotropy and the role of non-basal slip in magnesium alloy AZ31B

The activity of non-basal slip systems and dynamic recovery at room temperature in fine-grained AZ31B magnesium alloys

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

Magnesium and its alloys do not in general undergo the same extended range of plasticity as their competitor structural metals. The present work is part I of a study that examines some of the roles deformation twinning might play in the phenomenon. A series of tensile test results are reported for the common wrought alloy AZ31. These data are employed in conjunction with a simple constitutive model to argue that View the MathML source twinning (which gives extension along the c-axis) can increase the uniform elongation in tensile tests. This effect appears to be similar to that seen in Ti, Zr and Cu–Si and in the so called TWIP phenomenon in steel.

Twinning and the ductility of magnesium alloys Part I: “Tension” twins

Friction stir processing (FSP), developed based on the basic principles of friction stir welding (FSW), a solid-state joining process originally developed for aluminum alloys, is an emerging metalworking technique that can provide localized modification and control of microstructures in near-surface layers of processed metallic components. The FSP causes intense plastic deformation, material mixing, and thermal exposure, resulting in significant microstructural refinement, densification, and homogeneity of the processed zone. The FSP technique has been successfully used for producing the fine-grained structure and surface composite, modifying the microstructure of materials, and synthesizing the composite and intermetallic compound in situ. In this review article, the current state of the understanding and development of FSP is addressed.

Friction stir processing technology: A review

Theoretical Consideration of Measurement of Work-Hardening and Recovery Rates during High Temperature Creep

This paper aims at predicting long term creep curves and rupture lives of 2.25Cr-1Mo steel on the basis of the following creep equation:e=e0+A{1–exp(–αt)}+B{exp(αt)–1}, where, e0, A, B, α: parameters to be determined by curve fitting to a measured creep curve.The parameters are expressed as simple functions of stress and temperature, and can easily be extrapolated to a lower stress or a lower temperature, i.e., to a long term creep condition. Inserting the extrapolated values into the creep equation, one can draw a long term creep curve up to the tertiary creep stage.Rupture life is linearly related to the following rupture parameter P derived from the creep equation:P=(1/α)ln{(er–e0–A)/B}, where, er: the strain to rupture.By the aid of this parameter long term rupture lives can be predicted.The predicted creep curves and rupture lives were confirmed by long term creep tests up to about 400 Ms (13 years).

Prediction of Long Term Creep Curve and Rupture Life of 2.25Cr-1Mo Steel

Synopsis : Long term creep rupture life has been estimated from short term data by the extrapolation based of the timetemperature parameter (TTP) methods. This work aims at diScussing causes of the extrapolation error. Temperature dependence of rupture life can change at longer lives due to the change in fracture mechanism. The present forms of TTP, however, do not take the change into account, and this disregard is responsible for the extrapolation error. When the change occurs, one cannot correctly extrapolate rupture life even less than three times the longest test duration. On the other hand, rupture life can be extrapolated more than ten times without introducing a serious error when the change is absent. The change in temperature dependence also influences the direction of deviation from true values. The extrapolation provides a conservative estimate when the temperature dependenceof rupture life increases at longer lives, whereas it gives a nonconservative estimate when the dependence decreases.

Errors of Creep Rupture Life Extrapolated by Time-Temperature Parameter Methods

Long-Term Creep Curve Prediction Based on the Modified θ Projection Concept

Heat-to-heat variation of creep strength is significant in grade 91 (Gr.91) steel, but the causes of the variation have not been well understood yet. In the present paper, creep rupture data of 14 heats of Gr.91 steel were analyzed paying attention to their chemical compositions and microstructures. The longest creep rupture lives analyzed are 2 × 105 h at 500 and 550 °C and 105 h at 600 °C. The causes of the heat-to-heat variation are different, depending on creep test conditions. At low temperature and high stress (creep rupture life of 104 h at 500 and 550 °C), creep rupture strength increases with increase of hardness after tempering. This suggests strengthening by a fine subgrain microstructure developed during normalizing and subsequent tempering. At higher temperature and intermediate time range (104 h at 600 °C), creep rupture strength depends on Cr concentration of the heats in addition to the hardness. This finding suggests an important contribution of recovery process of the subgrain microstructures to creep strength of the steel. In long-term creep (2×105 h at 550 °C and 105 h at 600 °C) creep rupture strength primarily increases with increasing grain size of the heats. This suggests that grain boundary sliding is an important deformation mode at low strain rate because of fine grain size usual with Gr.91 steel. Specifications on Ni concentration and N%/Al% ratio are newly introduced in the type II version of Gr.91 steel. They are not effective to eliminate a heat with low creep strength.

Kouichi Maruyama

Papers

Theoretical Consideration of Measurement of Work-Hardening and Recovery Rates during High Temperature Creep

Prediction of Long Term Creep Curve and Rupture Life of 2.25Cr-1Mo Steel

Errors of Creep Rupture Life Extrapolated by Time-Temperature Parameter Methods

Long-Term Creep Curve Prediction Based on the Modified θ Projection Concept

Causes of heat-to-heat variation of creep strength in grade 91 steel