Li Jia Chen

Low-Cycle Fatigue Behavior of Magnesium Alloy AZ91

Fully-reversed total-strain-amplitude-controlled fatigue tests were performed in laboratory air at room temperature for the magnesium alloy AZ91. Experimental results showed that during the experiment significant cyclic strain hardening occurred throughout the imposed strain amplitudes. It was found that the relationship between plastic strain amplitude, elastic strain amplitude and reversals to failure can be well described by Basquin and Coffin-Manson equations. In addition, the strain fatigue parameters of the AZ91 alloy were determined through analyzing the corresponding strain fatigue life data.

Microstructure and indentation toughness of Cr/CrN multilayer coatings by arc ion plating

Abstract Cr/CrN multilayer coatings with bilayer periods in the range from 1351 to 260 nm were prepared on 304 stainless steel substrates by arc ion plating to study the microstructure and properties of multilayer coatings and stimulate their application. SEM results confirm the clear periodicity of the Cr/CrN multilayer coatings and the clear interface between individual layers. XRD patterns reveal that these multilayer coatings contain Cr, CrN and Cr2N phases. Because Cr layer is softer than its nitride layer, the hardness decreases with the shortening of the bilayer period (or increasing volume fraction of Cr layer). The Cr/CrN multilayer coating with 862 nm period possesses the highest indentation toughness due to a proper individual Cr and nitride layer thickness. However, for the Cr/CrN multilayer with the bilayer period of 1351 nm, it possesses the lowest toughness due to more nitride phase. The indentation toughness of Cr/CrN multilayer coatings is related with their bilayer period. A coating with a proper individual Cr and nitride layer thickness possesses the highest indentation toughness.

Microstructural evolution of rolled Mg-5Zn-3Nd(-Zr) alloy

Abstract A mini-type rolling machine was employed for multi-passes rolling of a Mg-5Zn-3Nd(-Zr)(mass fraction, %) wrought alloy. For the sake of providing experimental basis for magnesium alloy rolling process, optical microscopy, SEM and TEM observations were used to study the microstructure evolution of magnesium alloys subjected to different rolling reductions before and after annealing. Investigations show that multi-passes can be achieved for this alloy at ambient temperature, but 330 °C, 15 min annealing was needed for next pass rolling, and total deformation degree was 66%. With the increase in total deformation degree, rolling streams form and the average grain size decreases gradually. The microstructure after rolling is mainly composed of twining, and multiple twining in parallel distribution is also observed. The average grain size in the as-cast condition is about 50 μm and decreases to about 10–20 μm after rolling, whereas the twinning spacing is limited to 1–2 μm. SAED analysis in the twinning area indicates that twinning takes place at {10 1} plane. Complete recrystallization can occur in Mg-5Zn-3Nd(-Zr) alloys with various rolling reductions and after 200 °C, 120 min or 300 °C, 10 min annealing. Meanwhile, grain growth is apparent under heat treatment at 300 °C.

Solute Pre-Precipitation and Phase Transformation in a Mg-Zn-Gd Alloy

A novel precipitation sequence, super-saturated-solution → stacking faults → Mg3Gd, i.e. S.S.S. → SF → β1, was established for a Mg-3Zn-2Gd (wt%) alloy. Solute pre-precipitation can occur during the solidification of as-cast alloy and subsequent solution-treatment, resulting in the formation of lamella structures with …ABCABC… stacking sequence. The lamella structure will transform to the Mg3Gd phase, even that Mg3Gd is not the most stable phase in this serial alloys.

Cyclic Deformation Behaviour and Potential Automobile Application of Magnesium Die Casting Alloys AZ91 and AM50

Low-cycle fatigue life, cyclic strain hysteresis loops, cyclic strain hardening, cyclic mean stress relaxation of magnesium die casting alloys AZ91 and AM5 are investigated in this paper. Depending on the material characters and components working load, the potential automobile application has been discussed. It is considered that the magnesium alloy die castings with a big cyclic strain hysteresis loop and cyclic strain hardening tendency is beneficial for increasing NVH of a car, but the mean stress relaxation phenomenon remind that the tightness of bolt connection is easy to loos under a cyclic stress for a moving car. Compared with AZ91HP-F, AZ91HP-T6 and AM50HP -F, AZ91HP-T4 is more favourable to make automobile components.

An Investigation of Low Temperature Superplasticity of ZK40 Magnesium Alloy Subjected to Equal Channel Angular Pressing

Microstructure evolution and superplastic behaviors of ZK40 magnesium alloy were investigated in the temperature range of 473 ~ 623 K. Twinning occurred significantly after being processed by equal channel angular pressing (ECAP) for one pass through the die, the mean grain size was 5.6 µm. Finer grains can be obtained after further processing through ECAP, the average grain size of the alloy processed by ECAP for three passes was as low as 0.8 µm; this alloy exhibited low temperature superplasticity at 473 ~ 523 K, elongations at an initial strain rate of 1×10-3 s-1 were 260 % at 473 K and 612 % at 523 K, respectively. The incompatibility between fine and coarse grains was thought to be unfavorable to the improvement of superplasticity. Low temperature superplasticity of the as-ECAP ZK40 alloy can be attributed to the relatively finer grain size and the homogeneity of microstructure.

Dynamic Strain Aging during Tensile Deformation of Extruded AZ81 Magnesium Alloy

Serrated flow has been observed in AZ81 alloy during tensile deformation. The observed static strain ageing effect and negative strain rate sensitivity suggest that the serrated flow is due to interaction between dislocations and solute atoms, know as dynamic strain ageing (DSA). The Portevin-Le Chatelier effect is observed at temperatures between 150oC~200oC and 125oC~200oC. In the microstructure of deformed samples dislocations and twins is observed. It is suggested that the occurrence of the dynamic strain aging is associated with interactions between solute atoms and dislocations.

Low-Cycle Fatigue Behavior of Three Die Cast Magnesium Alloys

Totally strain-controlled low cycle fatigue tests with a strain ratio Rε= -1 were carried out on die-cast AZ91HP, AM50HP and AE 42 alloys at a constant strain rate of 2.5×10-3 s-1 and room temperature. The cyclic deformation behavior of the three alloys was investigated through the characteristics of representative hysteresis loops at various total strain amplitudes. Cyclic deformation hardening was observed and the low-cycle fatigue life data were analyzed using the well known Basquin and Coffin-Manson equations. The transition life of the three alloys is considerably low, which can be attributed to the low ductility of these die cast Mg alloys.

Superplasticity of Magnesium Alloy ZK40 Processed by Equal Channel Angular Pressing

The superplasticity and microstructure of magnesium alloy ZK40 processed by ECAP for 1 and 4 passes were investigated. Tensile experiments at 523 K showed that the elongation of ZK40 was improved with the increasing passes. The elongation-to-failure of ZK40 processed by ECAP for 4 passes tested at 523 K and strain rate of 1×10-4s-1 was 660%. The microstructural analyses explained these results.

Strain rate-dependent high temperature compressive deformation characteristics of ultrafine-grained pure aluminum produced by ECAP

Abstract To explore the effect of strain rate on the high temperature deformation characteristics of ultrafine-grained materials, the deformation and damage features as well as microstructures of ECAP-treated pure Al at different temperatures T and strain rates were systematically studied through compression tests and microscopic observations. The increase in eliminates strain softening at T ≤473 K, and largely enhances the yield strength and flow stress at 473–573 K. The shear deformation dominates the plastic deformation of ECAP-treated Al. Many cracks along shear bands (SBs) are formed at T ≥473 K and secondary SBs basically disappear at 1×10 −3 s −1 ; however, at 1×10 −2 s −1 , cracks are only observed at temperature below 473 K, and secondary SBs become clearer at T ≥473 K. The microstructures of ECAP-treated Al mainly consist of sub-grains (SGs). The increase in inhibits the SG growth, thus leading to the increases both in yield strength and flow stress at high temperatures.