Age Hardening Response Of Mg Research Articles

A magnesium alloy with ultrahigh Gd content manufactured by laser directed energy deposition: Heat treatment and microstructure evolution

High RE content is able to improve the age-hardening response of Mg–RE alloys, thus enhancing mechanical properties. Here, we successfully produce a Mg–22Gd–2Zn–0.4Zr (wt%) alloy with ultrahigh Gd content using laser directed energy deposition (LDED) and investigate the microstructure evolution during heat treatment. Results show that for the studied alloy, the yield strength is 333 MPa after solution at 520 °C for 2 hours and aging at 200 °C for 64 hours, superior to most existing Mg–Gd alloys, due to the high number density of the strengthening β' phase resulting from high supersaturation. During solution treatment, increasing the temperature improves the solution efficiency by reducing the holding time, while preventing the overgrowth of grains. The peak hardness is around 139 Hv for both 175 °C and 200 °C aging, indicating that the peak hardness of Mg alloys with high Gd content is insensitive to aging temperature. This research can serve as guidance for the heat treatment and microstructure control of high-Gd-content magnesium alloys.

Remarkable age-hardening response and microstructural evolution of Mg–7Sn alloy

The poor age-hardening response of Mg alloys limited their development and applications. In this study, the age-hardening response of Mg–7Sn alloy has been improved significantly by a 40 MPa compressive stress aging at 220 °C. Phase transition from metastable βˈ (Mg3Sn) phase to β (Mg2Sn) phase occurs during the stress aging. The βˈ with L12 structure has an orientation relationship with the Mg matrix of (11¯0)β＇//(112¯0)α and [1¯1¯0]β＇//⌈0001¯⌉α. The precipitated sequence in the stress-aged alloy is S.S.S. (supersaturated solid solution)→GP→βˈ(Mg3Sn)→β(Mg2Sn). The stress-aged alloy exhibits a high peak-aged hardness (∼71 HV) and short time to reach the peak hardness (12 h) as compared with the aged alloy without the stress. Interestingly, a long-time aging platform with 71 HV for 96 h is observed during stress aging. The formation of the platform is related to a balance of hardening and softening arisen from microstructure evolution during stress aging. The stress aging provides a promising heat treatment way to improve aging hardening response of Mg alloys.

Improvement in the age-hardening response of Mg–7Sn alloy by compressive stress-assisted aging

Stress aging is an effective method to improve age-hardening behavior of Mg alloys. In this paper, the stress aging was applied to improve poor age-hardening response of Mg–7Sn alloy. The age-hardening behavior, microstructure evolution and mechanical properties of Mg–7Sn alloy with an external compressive stress were investigated. The stress aging significantly altered the size, morphology, amounts and distribution of Mg2Sn phase. During the stress aging, micro/nano scale Mg2Sn particles precipitated on the basal plane of the stress aged alloy and their volume fraction remarkably increased as compared with the isothermal aging alloy. Interestingly, small amounts of Mg2Sn phases precipitated on the non-basal planes. The stress aging significantly reduced the peak aging time (96 h) and increased peak aging hardness (68.4 HV). The tensile tests showed that the stress peak-aged alloy at 80 MPa exhibited the highest tensile strengths (σ0.2 = 140 MPa and σb = 217 MPa), which were mainly attributed to the precipitation strengthening of Mg2Sn phase.

Enhanced age hardening response and precipitation evolution of elastic stress aged Mg–Zn alloys

How to improve the age hardening response of Mg alloys in economical and effective ways is an important and interesting issue. The age hardening response and precipitation evolution of Mg–Zn alloys with and without external elastic stress are investigated. Microstructural observations of stress-free aged samples show that the low hardness is mainly due to the low number density and relatively small aspect ratio of β1′ precipitates, while a large number of nano-scale strengthening β1′ precipitates appear in the α-Mg matrix at the early stage of stress aging. In addition, the aspect ratio of β1′ precipitates in the stress aged samples is evidently higher than that of the stress-free aged samples at the same peak aged stage. Abundant dislocations introduced by the applied stress act as nucleation sites of precipitates and rapid diffusion path of solutes, which are mainly responsible for the remarkable increment in hardness and promoted kinetics. Precipitate-free zones ( PFZs) are observed in both samples with and without stress. In the stress aged samples, the formation of PFZs is mainly controlled by the motion of dislocations, which is different from the solute-vacancy depletion mechanism in the stress-free samples. Results imply that the external stress plays a similar role to that of commonly used methods in reducing the size, increasing the density, and promoting the precipitation kinetics, but has a negligible stress-orienting effect on the precipitate evolution.

Effect of Gd addition on the age hardening response of Mg–6Zn-1Mn alloy

The effect of Gd addition on the microstructure and mechanical properties of as-aged Mg–6Zn-1Mn (ZM61) alloy were investigated. The results showed that coarse second phase particles containing both Zn and Gd elements were detected after the addition of Gd element, leading to reduce the amount of Zn available as solutes in the α-Mg matrix. Compared with ZM61, the peak aging hardness had a little difference and the peak aging time was shortened with micro amount of Gd addition, while the hardness decreased significantly for the alloys with high Gd addition. The age hardening response was mainly dictated to the precipitation of fine rod-like β1′ phase, and the number density of β1′ phase was related to the Zn content in the α-Mg matrix. Aging treatment could effectively improve the strength of alloys with minor Gd addition and it was of little significance for alloys with high Gd content. The as-aged ZMG61-0.2 exhibited optimal mechanical properties, and the yield tensile strength, ultimate tensile strength, elongation were 360MPa, 390MPa, 8.3%, respectively.

Effects of Ce additions on the age hardening response of Mg–Zn alloys

The effects of Ce additions on the precipitation hardening behaviour of Mg–Zn are examined for a series of alloys, with Ce additions at both alloying and microalloying levels. The alloys are artificially aged, and studied using hardness measurement and X-ray diffraction, as well as optical and transmission electron microscopy. It is found that the age-hardening effect is driven by the formation of fine precipitates, the number density of which is related to the Zn content of the alloy. Conversely, the Ce content is found to slightly reduce hardening. When the alloy content of Ce is high, large secondary phase particles containing both Ce and Zn are present, and remain stable during solutionizing. These particles effectively reduce the amount of Zn available as solute for precipitation, and thereby reduce hardening. Combining hardness results with thermodynamic analysis of alloy solute levels also suggests that Ce can have a negative effect on hardening when present as solutes at the onset of ageing. This effect is confirmed by designing a pre-ageing heat treatment to preferentially remove Ce solutes, which is found to restore the hardening capability of an Mg–Zn–Ce alloy to the level of the Ce-free alloy.

Effects of Pb/Sn additions on the age-hardening behaviour of Mg–4Zn alloys

The effects of Pb/Sn additions on the age-hardening responses and corresponding microstructures of a Mg–4Zn alloy (wt%) were investigated by hardness test and transmission electron microscopy (TEM). Sn additions significantly refine the precipitates and lead to enhanced age-hardening response of Mg–4Zn alloys, while the addition of Pb slows down the ageing kinetics. The yield stress of peak-aged Mg–4Zn significantly improved from 87MPa to 129MPa through the addition of Sn, but slightly decreased to 84MPa with the addition of Pb. TEM showed that Mg–4Zn with Pb-added alloys consist of two main precipitates, rod/lath-like β1' phase and plate-like β2' phase. The precipitates in the peak-aged Mg–4Zn–1Pb alloy have much lower number density and larger size than that of the peak-aged Mg-4Zn alloy. In contrast, Sn-containing alloys show much finer precipitates, and high melting temperature Mg2Sn precipitates formed when the Sn content increased to 3wt%. Cliff–Lorimer plots of Mg–4Zn–1Pb and Mg–4Zn–3Sn alloys indicate that no Pb atoms preferentially exist in the precipitates, while Zn and Sn atoms are incorporated in the rod/lath-like β1' precipitates and Mg2Sn precipitates.

In situ neutron diffraction and polycrystal plasticity modeling of a Mg–Y–Nd–Zr alloy: Effects of precipitation on individual deformation mechanisms

In situ neutron diffraction compression tests were performed on Mg–Y–Nd–Zr alloy WE43, in the solution heat-treated, peak- and over-aged conditions. The flow curves and internal strain evolutions were modeled using polycrystal plasticity simulation, with the inclusion of an elastic phase to account for the presence of precipitates. The results reveal that prismatic plate-shaped precipitates strongly impede basal slip; the critical resolved shear strength (CRSS) of basal slip increases from 12 to 37MPa, an increase of over 200%. However, hard deformation modes such as non-basal slip of 〈a〉 dislocations are required for macroscopic yielding. These hard modes are not as strongly affected by aging, with CRSS values which increase from 78 to 92MPa, an increase of only 18%. The results of the study are consistent with recent modeling of the relative Orowan strengthening of individual deformation modes and the superposition of various strengthening effects (solid solution and precipitation). This finding helps to explain why the age-hardening response of Mg–Y–Nd–Zr alloys is not exceptional. It is concluded that future precipitation-strengthened alloy and process design strategies should focus on promoting high number densities of particles. The effect of aging upon twinning is surprising. The most age-hardened material exhibits more twinning than the solutionized material. To model this behavior using polycrystal plasticity, the critical stress to activate twinning (especially the strain hardening thereof) must be decreased.

The effect of Ag and Ca additions on the age hardening response of Mg–Zn alloys

The effect of sole and combined additions of Ag and Ca in enhancing the age hardening response in a Mg–2.4Zn (at%) alloy have been studied by systematic microstructure investigations using transmission electron microscopy (TEM) and three dimensional atom probe (3DAP). In the early aging stage of a Mg–2.4Zn–0.1Ag–0.1Ca (at%) alloy at 160°C, Zn-rich Guinier Preston (G.P.) zones form with Ag and Ca enrichment. Further aging lead to the formation of fine β′1 precipitates with Ag and Ca enrichment. We confirmed that the G.P. zones do not form in the Mg–2.4Zn (at%) binary alloy at 160°C, but form after a prolonged aging at 70°C. This suggests that the combined addition of Ag and Ca shifts the metastable solvus for the G.P. zones to a higher temperature, thereby making it possible to form G.P. zones even at the artificial aging temperature of 160°C. Since G.P. zones act as nucleation sites for the β′1 precipitates, the peak-aged microstructure is refined substantially by the addition of Ag and Ca.

Compositional optimization of Mg–Sn–Al alloys for higher age hardening response

The effect of Al additions and microalloying on the age hardening response of Mg–Sn alloys has been investigated to optimize the composition for the highest age hardening response. The addition of 3at%Al lead to the best enhancement in peak hardness being 72 HV at 200°C. The age hardening response was further enhanced from 73 to 81 HV by microalloying with 0.5 Zn, leading to the optimized alloy composition of Mg–2.2Sn–3Al–0.5Zn (at%) or Mg–9.8Sn–3.0Al–1.2Zn (wt%), i.e., TAZ1031. The enhancement in the peak strength and aging behavior is attributed to the solid solution hardening due to Al, the refinement of the Mg2Sn precipitates, the occurrence of Mg17Al12 precipitates, and the increase in the number density of non-basal Mg2Sn precipitates.

Enhanced age-hardening response of Mg–Zn alloys via Co additions

The addition of Co to a Mg–8 wt.% Zn alloy allows the use of a much higher solution treatment temperature that can lead to higher concentrations of Zn atoms and vacancies in Mg grains of the solution-treated samples, which upon subsequent ageing can significantly increase precipitate number density and age-hardening response.

Influence of solution treatments on the age-hardening response of Mg–Gd(–Mn–Sc) alloys

The microstructures and age-hardening response of a Mg–5 wt.%Gd alloy and a Mg–5 wt.%Gd–1.1 wt.%Mn–0.8 wt.%Sc alloy have been examined under different solution treatment conditions. Both alloys exhibit little or no age-hardening response during isothermal ageing at temperatures above 200 °C when the alloys are solution treated at 520 °C for 24 h. Solution treatment at 610 °C for 24 h does not improve the age-hardening response of the binary alloy. However, it leads to a significant increase in hardness during ageing for the quaternary alloy. This improved age-hardening response in the quaternary alloy is ascribed to the dense precipitation of two fine-scale precipitate phases during ageing.

Age-hardening response of Mg–0.3 at.%Ca alloys with different Zn contents

We have investigated the age-hardening responses and corresponding microstructures of Mg–0.3Ca–xZn (x=0.0, 0.1, 0.3, 0.6, 1.0, 1.6at.%) alloys by hardness test, transmission electron microscopy (TEM), and high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM). Zn additions up to x=0.6 lead to enhanced age-hardening responses with the highest peak hardness of HV=69 for x=0.6. Further addition of Zn degraded the age-hardening responses. HAADF-STEM images revealed that the finely dispersed monolayer G.P. zones with internal ordered structure are the major contributor to the age-hardening. Excess addition of Zn resulted in the formation of Ca2Mg6Zn3 precipitates suppressing the formation of the ordered G.P. zones.

The effect of micro-alloying addition of Cr on age hardening of an Mg–Zn alloy

Abstract A trace amount of Cr is soluble in the magnesium lattice in the presence of Zn. The presence of Cr does not notably affect the constituent particles, but it significantly improves the age hardening response of Mg–Zn alloy by increasing the number density of the precipitates. As a result of Cr addition, the hardness increment produced by artificial ageing at 160 °C is almost doubled compared to binary Mg–2.8 at.% Zn alloy. In particular, Cr noticeably accelerates the kinetics of precipitation and the near-peak hardness can be reached after only 4 h of ageing. Cr also moderately accelerates the natural ageing. The highest level of hardening in Mg–Zn–Cr alloy is produced by ageing at intermediate temperatures (e.g. 70 °C). Cr represents a suitable novel micro-alloying addition for the development of alloys with improved age hardening response that is particularly effective in enhancing the nucleation of precipitates.

Improvement in the age-hardening response of Mg–Y–Zn alloys by Ag additions

While being creep resistant, Mg–Y–Zn alloys exhibit a poor age-hardening response when they are processed by conventional ingot metallurgy. In this work, the effects of Ag on precipitation in an Mg–6Y–1Zn–0.6Zr alloy are examined. It shows that the Ag additions can remarkably enhance the age-hardening response and that this significant improvement in hardening is associated with the formation of densely distributed basal precipitate plates that are not observed in the Ag-free alloy.

Enhanced age hardening response by the addition of Zn in Mg–Sn alloys

The effect of Zn addition on the age hardening response of Mg–Sn alloys has been investigated. The peak hardness increased substantially from 60 HV to 80 HV with the addition of 0.5 at.% Zn. The increase in hardness is mainly attributed to the homogeneous distribution, the refinement of Mg2Sn precipitates and the increase in the number of precipitates lying on non-basal planes.