Role of Grain Boundary Segregation and Nanoprecipitation on the Tensile Properties and Thermal Stability of Dilute Mg–0.7Al–0.3Ca (wt%) Alloy

The effect of dose rate on the response of austenitic stainless steels to neutron radiation

In the present study, the influence of the Zn content (0.0, 0.5, 1.0, and 2.0 wt.%) on the microstructure, texture, tensile mechanical properties and formability of Mg-1.0Sn-0.5Ca-based alloys after extrusion were investigated via extrusion of Mg sheets and analysis of the extruded materials. Adding Zn improved the microstructure uniformity and accelerated the formation of more CaMgSn and the Ca2Mg6Zn3 phase. In addition, after the addition of Zn, the texture evolved from the initial extrusion direction tilted bimodal texture to a weakened and symmetrical texture. Compared with the Mg-1.0Sn-0.5Ca alloy, the average tensile yield strength and elongation values were simultaneously improved after 0.5 wt.% Zn addition due to the synergistic effect of the solid solution of elemental Zn, the decreased amount of coarse unrecrystallized grains, and the existence of an additional tilted direction texture component. The formability first increased and then decreased with increasing Zn content. In contrast, the deterioration of formability after the further addition of Zn was closely associated with the increased volume fractions of the coarse CaMgSn and Ca2Mg6Zn3 phases.

The effects of Zr addition on the as cast microstructure, tensile and creep properties of Mg–3·8Zn–2·2Ca (wt-%) magnesium alloy were investigated. The results indicate that adding 0·19–0·81 wt-%Zr to the Mg–3·8Zn–2·2Ca does not cause the formation of any new phases but causes the morphology of the Ca2Mg6Zn3 phase to change from the initial continuous and/or quasi-continuous net to the quasi-continuous and/or disconnected shapes. Furthermore, adding 0·19–0·81 wt-%Zr can effectively refine the grains of the Mg–3·8Zn–2·2Ca alloy, and an increase in Zr amount from 0·19 to 0·81 wt-% causes the grain size to gradually decrease. In addition, adding 0·19–0·81 wt-%Zr to the Mg–3·8Zn–2·2Ca alloy can improve the tensile properties but decreases the creep properties. Among these Mg–3·8Zn–2·2Ca alloys with the additions of 0·19, 0·52 and 0·81 wt-%Zr, the alloy with the addition of 0·81 wt-%Zr exhibits the relatively optimal tensile and creep properties.

As-cast microstructures and mechanical properties of Mg–4Zn–xY–1Ca (x=1.0, 1.5, 2.0, 3.0) magnesium alloys

A semi-solid press-forming process was applied to Mg–4~8%Al, Mg–4~8%Al–2%Ca and Mg–4~8%Al–l%Si– 0.5%Ca alloys for a purpose of toughness improvement. Effects of heat treatment were investigated on tensile properties and microstructure of the semi-solid formings. Fine globular particles of the α-phace are distributed uniformely together with the fine eutectic constituent in the semi-solid formings made from 15% strained materials. The grain size of the α-phase particles in as-formed specimens decreases with increasing aluminum and calcium contents. A network of intermetallic compound, Mg17Al12 crystallized along grain boundaries in the Mg–Al binary and Mg–Al–1%Si–0.5% Ca alloys, breaks up with decreasing aluminum content, resulting in increases both of tensile strength and of elongation. The Mg17Al12 compounds disappear after the T4–treatment in the all investigated alloys, while the shape of Al2Ca compounds becomes globular after the T4–treatment of Mg–Al–2%Ca alloys alone. The microstructural changes clearly improve tensile strength and elongation. The age hardning by precipitation of Mg17Al12 phase occurs by T6–treatment in the Mg–Al binary and Mg–Al–1%Si–0.5%Ca alloys with aluminum content more than 6%, and in the Mg– Al–2%Ca alloys with aluminum larger than 8%. The aged specimens exhibit higher tensile strength and 0.2% proof stress than as-formed spesimens.

Tensile properties were investigated from room temperature to 673 K of a binary Mg–0.9 mass%Ca alloy where Mg2Ca phase was dispersed as lamella. The 0.2% proof stress significantly increased by the Mg2Ca phase in the temperature range investigated. It was suggested that strengthening by stable lamella Mg2Ca in the grains and limitation of deformation related to grain boundaries by Mg2Ca phase at grain boundaries contribute to improvement of high temperature strength for the Mg–Ca alloy.

Microstructure and Mechanical Properties of Extruded Mg-Sm-Ca Alloys

The present study carries out systematic thermodynamics analysis of Grain Boundary (GB) segregation and relaxation in Nano-Grained (NG) polycrystalline alloys. GB segregation and relaxation is an internal process towards thermodynamic equilibrium, which occurs naturally in NG alloys without any applied loads, causes deformation and generates internal stresses. The analysis comprehensively investigates the multiple coupling effects among chemical concentrations and mechanical stresses in GBs and grains. A hybrid approach of eigenstress and eigenstrain is developed herein to solve the multiple coupling problem. The analysis results indicate that the GB stress and grain stress induced by GB segregation and relaxation can be extremely high in NG alloys, reaching the GPa level, which play an important role in the thermal stability of NG alloys, especially via the coupling terms between stress and concentration. The present theoretic analysis proposes a novel criterion of thermal stability for NG alloys, which is determined by the difference in molar free energy between a NG alloy and its reference single crystal with the same nominal chemical composition. If the difference at a temperature is negative or zero, the NG alloy is thermal stable at that temperature, otherwise unstable.

Correlation between stabilizing and strengthening effects due to grain boundary segregation in iron-based alloys: Theoretical models and first-principles calculations

Microstructure, tensile properties, and corrosion resistance of extruded Mg-1Bi-1Zn alloy: The influence of minor Ca addition

In this paper, microstructure and mechanical properties of AZ61 magnesium alloys with the Y and Ca combined addition were investigated. The results exhibit that the addition of Y and Ca into AZ61 alloy leads to effective refinement of the microstructure and additional block-like Al2Y and long-rod Al2Ca phases found in AZ61–1.2Y alloy and AZ61–1.2Y–1.0Ca alloy, respectively. The tensile strength, elongation and creep properties of AZ61–Y–Ca alloy are significantly increased by the addition of Y and Ca. The tensile strength of AZ61–1.2Y–1.0Ca alloy at 20 and 175 °C is up to 225 and 175 MPa, and compared with AZ61 alloy, the strengths of the alloy are enhanced by 37 and 80%, respectively. AZ61–1.2Y–1.0Ca alloy achieves the lowest steady creep rate 2.5027 × 10−7 s−1 that is about 4 times lower than that of AZ61 alloy. The poor mechanical properties of AZ61 magnesium alloy is attributed to the softening of β-phase Mg17Al12, especially at elevated temperature. The tensile strength and creep resistance of AZ61–1.2Y–1.0Ca alloy are effectively improved by the Al2Ca and Al2Y phases.

In this paper, the effect of adding 1.0 wt.% Ce on the as-cast microstructure and mechanical properties of the Mg-3.8Zn-2.2Ca (wt.%) magnesium alloy were investigated. The results indicate that, after adding 1.0 wt.%Ce to the Mg-3.8Zn-2.2Ca alloy, small amounts of Mg12Ce phase are formed and an obvious equiaxed trendance is observed. At the same time, the average grain size decreases from 234mm to 71mm and the morphology of some Ca2Mg6Zn3 phases changes from initial coarse blocks to fine particles. In addition, adding 1.0 wt.%Ce to the Mg-3.8Zn-2.2Ca alloy also improve the tensile and creep properties of the alloy. Further investigations need to be considered in order to optimize the amounts of Ce additions and understand its effects on the tensile and creep properties and age-hardening behaviour.

Effects of solution and aging treatment on microstructure and mechanical properties of rolled AM50+xCa alloys(x=0, 1, 2 wt. %) were studied. The results indicated that, with increasing solution time i, the secondary phase Mg17Al12 was dissolved into the Mg matrix and Al2Ca became thinner and shorter, then gradually broken and spheroidized.With an increase of aging time, Mg17Al12 precipitated from the Mg matrix in the form of particles and Al2Ca changed a little. After solution treatment, hardness and tensile properties of the alloy’s decreased. After the aging treatment, the alloy’s hardness increased first and decreased later while the tensile properties increased little. The solution and aging treatment can increase the ductility of AM50 and AM50+1Ca alloys. For AM50+2Ca alloy, the ductility increased after solid solution treatment and decreased after aging treatment.

The effects of Zn addition on the microstructure and tensile properties of as-extruded Mg-2Al-0.5Ca-xZn (x = 0, 0.3, 0.6, 0.9 wt.%) alloys were investigated in this work. The results showed that the extruded sheets exhibited a completely dynamically recrystallized microstructure, the grain size was refined, and texture weakening was achieved with Zn addition because of the segregation of Zn atoms on grain boundaries, which suppresses the growth of dynamic recrystallized grains. The addition of 0.6 wt.% Zn improved both the tensile strength and ductility of the as-extruded Mg-2Al-0.5Ca alloy. The as-extruded Mg-2Al-0.5Ca-0.6Zn alloy showed a 0.2% proof stress of 145 MPa, an ultimate tensile strength of 317 MPa, and an elongation of 30.0% along the extruded direction. The simultaneous improvement of strength and ductility was mainly due to the fine and homogeneous grain microstructure and the weakened extrude direction (ED)-tilted texture. The as-extruded Mg-2Al-0.5Ca-0.6Zn alloy showed little in-plane anisotropic tensile properties, with a 0.2% proof stress, ultimate tensile strength, and elongation in the 45° direction of 148 MPa, 299 MPa, and 25.0%, and those in the transverse direction of 148 MPa, 269 MPa, and 16.8%, respectively.

Insights into the effect of different thermomechanical processing on the microstructure, phases, texture and tensile properties in Mg-0.9Al-0.6Mn-0.2Si-0.1Ca alloy