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

\n Grade 91 steel is widely used as steam pipes in ultrasupercritical (USC) steam boilers. In residual creep life assessment of the pipes by calculation, one needs creep rupture life of the steel as a function of stress and temperature in a time range longer than 105 h. Four regions with different creep rupture characteristics appear in a stress versus creep rupture life diagram of the steel. Main steam pipes made of the steel are used in a long-term region with low values of stress exponent and activation energy for creep rupture life (referred to as region G in this paper). Creep rupture lives of the steel in this region vary from heat to heat depending on their prior austenite grain size. This paper proposes a grain size-dependent equation representing creep rupture life of the steel in region G. The equation is verified with creep rupture data up to 232,833 h at 600 °C. Region G is absent in some heats with a large grain size. The equation can rationalize the absence in the heats. In a stress versus creep rupture life diagram of grade 92 steel, there is the same long-term region G. In the region, a creep rupture life of each heat is dependent on its grain size as is the case in grade 91 steel. The proposed equation accords well with the creep rupture lives of the grade 92 steel in region G.

A grain size-dependent equation for creep rupture life of grade 91 steel verified up to 233,000 hours

Oxide dispersion strengthened (ODS) steels are the most promising candidate materials for high temperature nuclear applications. Mechanical alloying and subsequent thermomechanical treatments are applied to manufacture the ODS steels. Recently improved chemical composition and manufacturing processes have been developed to produce ultrafine grain size with high number-density of nanoscale oxide particles and high dislocation density in the microstructure. Usually, fine grains degrade creep resistance at elevated temperatures. However, the fine-grained ODS steels exhibit not only good radiation resistance, but also superior creep properties. The present paper reviews the chemical compositions, manufacturing processing, microstructural features, thermal creep properties and radiation resistance of recently developed ODS steels. Special attention is paid to the effects of the fine-scale microstructural features on thermal creep and radiation resistance.

Development of Oxide Dispersion Strengthened Steels for High Temperature Nuclear Structural Applications

Synopsis : Temperature and stress dependence of creep rupture life was examined on 24 heats of type 304, 316, 321 and 347 stainless steels. Collective evaluation of the results indicated the existence of three regions, H, M and L, with different activation energy, Q, and stress exponent, n, for rupture life : region H at a shorter rupture life, region L at a longer rupture life, and region M in between the two regions. The values of Q and n decrease with increasing rupture life as expected from creep fracture theories, suggesting that a longer rupture life than the actual value would be predicted erroneously if it is estimated from the short term data taking the high values of Q. Transgranular and intergranular fracture take place respectively in the regions H and M, and the fracture process in the two regions is controlled by creep deformation. The intergranular fracture in the region L starts from cavities nucleated at sigma-phase/matrix interfaces, and this fracture process is controlled by grain boundary diffusion. The changes in Q and n coincide fairly well with the changes in creep mechanism. This fact points out that the changes in Q and n are inherent to the austenitic stainless steels and the same thing will happen in any materials similar to the four types of stainless steels.

Collective Evaluation of Temperature and Stress Dependence of Creep Rupture Life in Austenitic Stainless Steels

A Postassessment Test of 100,000 h Creep Rupture Strength of Grade 91 Steel at 600 °C

Tensile tests were performed in Ti—(7 ∼ 13)mol.%Al alloys at 1100 K. Stress—strain curves showed a large region of constant stress deformation. The results were analyzed by a Dorn type creep equation. The strain rate and the steady-state stress showed anomalous strength of the examined alloys in comparison with the previously reported Ti—(1 ∼ 5)mol.%Al alloys (H. Oikawa and T. Oomori, Mater. Sci. Eng., A104 (1988) 125). Both X-ray diffraction and transmission electron microscopy indicated the appearance of additional diffraction intensities that may owing to the formation of short range ordered (SRO) clusters. The anomalous creep strength observed in the present work seems to be caused by the retardation of dislocation glide by the SRO clusters.

Kouichi Maruyama

Papers

A grain size-dependent equation for creep rupture life of grade 91 steel verified up to 233,000 hours

Development of Oxide Dispersion Strengthened Steels for High Temperature Nuclear Structural Applications

Collective Evaluation of Temperature and Stress Dependence of Creep Rupture Life in Austenitic Stainless Steels

A Postassessment Test of 100,000 h Creep Rupture Strength of Grade 91 Steel at 600 °C

High temperature strength of α TiAl alloys with a locally ordered structure