Study On Mechanochemical Interaction Of Newly Developed High-performance Mg Alloys

Due to the low density and high specific strength,Mg alloys have recently received considerable attention for the applications in aerospace and automobile industries.It has become the focus in the new materials field.Compared with other Mg alloys,I-phase strengthened Mg-Zn-Y-Zr alloys and high strength Mg-Gd-Y-Nd-Zr alloys have attracted great interest because they have higher mechanical properties at both ambient and elevated temperatures.However,in real service conditions,the structural components often suffer from stress corrosion cracking(SCC)and corrosion fatigue failure under the interaction between the mechanics and chemistry.It dramatically restricts the wide application and development of Mg alloys.In this dissertation,we have systematically investigated the mechanochemical interaction of Mg-Zn-Y-Zr and Mg-Gd-Y-Nd-Zr alloys and their failure mechanisms by using thermal analysis,microstructural analysis,corrosion and mechanical testing methods,etc.Moreover,the underlying mechanism of the influence of heat treatment on the mechanochemical interaction of Mg-Zn-Y-Zr alloys was deeply discussed.It can provide important guidance and reference for improving the mechanochemical interaction resistance of Mg alloys and improving their service safety.The effect of heat treatment on the corrosion resistance and mechanical properties of an as-forged Mg-Zn-Y-Zr alloy has been investigated.It revealed that T4 treatment(i.e.300 ℃/1 h+400 ℃/2 h)can basically eliminate the inhomogeneous distribution associated with the MgZn2 precipitates.Meanwhile,nano-scale I-phase particles can precipitate during the T4 treatment.Due to the dissolution of MgZn2 precipitates,the corrosion resistance of the alloy was greatly improved.Moreover,nano-scale I-phase precipitates can counteract the strength decrease of the alloy due to the MgZn2 dissolution and grain growth.Through investigating the influence of heat treatment on the stress corrosion cracking(SCC)of an as-forged Mg-Zn-Y-Zr alloy,it revealed that T4 treatment can improve the SCC resistance of the alloy.The SCC susceptibility index(Iscc)of as-forged samples was 0.95 and its elongation-to-failure(εf)was only 1.1%.After T4 treatment,the ISCC and εf values of T4 samples were 0.86 and 3.4%,respectively.Fractography and surface observation indicated that the SCC mode for as-forged samples was dominated by transgranular and partially intergranular morphology,whereas the cracking mode for T4 samples was transgranular.In both cases,the main cracking mechanism was associated with hydrogen embrittlement(HE).Through alleviating the corrosion attack of Mg matrix,the influence of HE on the SCC resistance of T4 samples can be greatly suppressed.Through investigating the influence of heat treatment on the fatigue behaviour of an as-forged Mg-Zn-Y-Zr alloy in air,it disclosed the mechanism of the influence of T4 treatment on the fatigue crack initiation.S-N curves showed that the fatigue strength of as-forged samples was 110 MPa,whereas the fatigue strength of T4 samples was only 80 MPa.Observations to fracture surfaces demonstrated that for as-forged samples,fatigue crack initiation sites were covered with a layer of oxide film.However,due to the coarse grain structure and the dissolution of MgZn2 precipitates,the activation and accumulation of twins in T4 samples were much easier,resulting in the preferential fatigue crack initiation at cracked twin boundaries(TBs).Surface characterization demonstrated that TB cracking was mainly ascribed to the incompatible plastic deformation in the twinned area and nearby a-Mg matrix.Through investigating influence of heat treatment on the fatigue behaviour of an as-forged Mg-Zn-Y-Zr alloy in 3.5 wt.%NaCl solution,it demonstrated that the fatigue strength of as-forged samples corresponding to 5 × 106 cycles was 30 MPa,whereas the fatigue strength of T4 samples increased to 50 MPa.Fracture observation showed that for as-forged samples,the crack initiated at localized corrosion areas from sample surface.However,for T4 samples,the crack initiation was related with localized corrosion and cracked twin boundaries(TBs)at low and high stress amplitudes,respectively.In all cases,the main cracking mechanism was associated with hydrogen embrittlement(HE).Through alleviating the corrosion attack of Mg matrix,the influence of HE on the corrosion fatigue resistance of T4 samples can be greatly suppressed.However,due to the easier activation of twins and their subsequently brittle cracking at high stress amplitudes(e.g.90 MPa),the corrosion fatigue lives of T4 sample was greatly degraded.The influence of phase dissolution and hydrogen on the corrosion and stress corrosion cracking(SCC)behavior of an as-cast Mg-Gd-Y-Nd-Zr alloy has been investigated.It revealed that the proposed T4 treatment can improve corrosion resistance of the alloy,which is ascribed to the complete dissolution of coarse eutectic pockets by T4 treatment.After cathodic charging,the corrosion resistance of as-cast samples was significantly decreased due to the formation of vast defects in the surface film.However,the corrosion resistance of cathodically charged T4 samples was only slightly decreased when compared with uncharged samples.The results showed that the SCC susceptibility index(ISCC)of as-cast samples was 0.82 and its elongation-to-failure(εf)was only 0.10%.After T4 treatment,the SCC resistance was improved.The ISCC and εf values of T4 samples were 0.55 and 0.27%,respectively.Observations to the surfaces and fracture surfaces of failed SCC samples indicated that the cracking mode for as-cast samples was dominated by intergranular associated with the coarse eutectic pockets and partially transgranular morphology,whereas the cracking mode for T4 samples was transgranular.In both cases,the main SCC mechanism was associated with hydrogen embrittlement(HE).Through dissolving coarse secondary phases and alleviating the corrosion attack of Mg matrix,the influence of HE on the SCC resistance of T4 samples can be greatly reduced.The fatigue behavior of an as-cast Mg-Gd-Y-Nd-Zr alloy in both laboratory air and 3.5 wt.%NaCl solution has been investigated.It demonstrated that the fatigue strength of the alloy corresponding to 106 cycles is 120 MPa both at 20 Hz and 5 Hz in laboratory air.However,in NaCl solution,the fatigue strength is sensitive to the loading frequency and decreases from 80 to 60 MPa with the frequency decreasing from 20 Hz to 5 Hz.In air,MgO inclusions at the surface of samples preferentially act as the fatigue crack initiation sites.When localized corrosion area occurs even with a size smaller than that of oxide inclusions,the fatigue cracks will tend to nucleate at localized corrosion areas.Moreover,fatigue cracks can initiate at multiple localized corrosion areas when severe corrosion attack occurred at the fatigue sample surfaces.Based on the measured "defect area" of oxide inclusions,the fatigue strength in air can be well predicted and fit well with the experimental data.However,due to the occurrence of hydrogen embrittlement and crack initiation at multiple sites,the fatigue strength of samples tested in NaCl solution cannot be predicted.