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Abstract:
Composite materials based on MgLi and MgY matrices reinforced with unidirectional T300 carbon fibres (45 vol.%) have been prepared by gas pressure infiltration technique. Nominal Li and Y alloying of Mg matrix were 0.5, 2, and 4 wt.% Li and 7, 13, and 20 wt.% Y, respectively. Back-scattered electron (BSE) observations and electron dispersive X-ray (EDX) analysis have revealed that penetration of lithium and formation of lithium carbide Li2C2 inside carbon fibres took place. Bending tests have shown that low Li alloying of Mg matrix (0.5 and 2 wt.% Li) has a favourable effect on the strength of MgLi/T300 composites. Higher Li content (4 wt.% Li) brings a much lower composite strengthening presumably due to degradation of carbon fibres with Li2C2. A negative effect of high yttrium alloying on the strength of MgY/T300 composites is explained by poor adherence of YC2 interfacial layer to carbon fibres.

Keywords:
magnesium matrix, magnesium composites, magnesium-lithium matrix, magnesium-lithium composites, magnesium-yttrium matrix, magnesium-yttrium composites, lithium carbide, yttrium carbide, reactive wetting

Abstract:
The gas pressure infiltration technique was used to prepare Saffil alumina fibers reinforced Mg-Li and Mg-Li-Zn matrix composites with a dual-phase matrix structure. There was investigated the effect of variable Li content (6.2–10.3 wt.% Li) and Zn alloying (∼ 1.5 wt.% Zn) on the proof stress Rp0.2 of prepared composites. Rp0.2 values increased monotonously with rising fraction of Saffil fibers (5, 10 and 15 vol.%) reaching the maximum of about 250 MPa for Mg-Li matrix composites. Rp0.2 values of Mg-Li-Zn matrix composites were lower. Strengthening effect of Saffil fibers was promoted by the displacement redox reaction with Mg-Li and Mg-Li-Zn melts in which only Li significantly participated. Zn alloying retarded the displacement redox reaction. Too extensive reaction, however, resulted in the fiber damage and the drop in composite strength.

Keywords:
Mg-Li alloys, Saffil fibers, metal matrix composites, short-fiber strengthening, reactive wetting, displacement reactions

Abstract:
Proof stress Rp0.2 of dual-phase α + β structured Mg-Li and Mg-Li-Zn alloys has been inspected in terms of the strengthening contributions of α- and β-phases. The alloys studied with a variable fraction of α- and β-phases have been subjected to compression straining tests, microhardness measurements and structural analysis by EDX and XRD. Alloying with 1.5 wt.% Zn results in the hardening of both α- and β-phases which however exhibit different hardening responses due to different Zn enrichment. The rule of the mixture has been used to interpret Rp0.2 values by taking into account the fraction of α- and β-phases and their strength level represented by their microhardness. Compression stress-strain curves indicate that work hardening of alloys studied depends considerably on the fraction of α-phase and is higher for Zn-containing alloys.

Keywords:
Mg-Li, Mg-Li-Zn, dual-phase alloy, solution hardening, ageing, work hardening