Modification of Al–7Si–0·3Mg alloy by sodium and strontium in presence of antimony

Abstract

Antimony exerts a degrading effect on the modification of Al–Si alloys with additions of either sodium or strontium, particularly at usual levels of either of the last two elements. The deleterious effect can be minimised if significantly higher levels of modifier are used. In each instance, antimony reacts with the modifier in the melt to form a dense solid compound which sinks to the bottom of the melt. Combinations of sodium and strontium used together are shown to provide beneficial long term modification when antimony is not present. In the presence of antimony these combinations can overcome its negative effect and once again provide long term modification. A modification treatment with 0·03%Sr and 0·01%Na is suggested for use with antimony levels of ≤ 0·01%. At 0·04%Sb, a treatment with 0·06%Sr and 0·02%Na is equally effective.

MST/925     

Subgrain growth in presence of nanosized dispersoids in Al–4·5Zn–1Mg alloy

Abstract

In this paper, an analytical model for subgrain growth in the presence of nanosized dispersoids is presented. The growth rate of subgrains is correlated to the mobility of low angle grain boundaries (LAGBs) and the net driving force for growth. The driving force is considered as the difference between stored energy, being inversely proportional to the average subgrain size, and the Zener drag pressure. A material dependent constant necessary for the determination of the mobility of LAGBs is estimated by fitting the model predictions into the experimental results. Model predictions of the evolution of subgrain sizes with annealing time at different temperatures show that subgrain growth intensifies with increasing annealing temperature. The magnitude of the Zener drag pressure has a predefined effect on the subgrain growth rate. The model predicts that when the P_Z/γ_s ratio is smaller than 1 μm^?1, the Zener drag pressure has an effect on subgrain size and the subgrain growth rate tends to decrease. However, when the P_Z/γ_s ratio is larger than 1 μm^?1, there is a limit beyond which the subgrain size does not increase with increasing annealing time. The limiting subgrain size is a function of the surface boundary energy and Zener drag pressure.     

Strength characteristics of Mg–Li alloys

We study the mechanical properties of Mg–Li alloys obtained by high-pressure die casting in a cold pressing chamber. A procedure of evaluation of the principal strength characteristics obtained under the conditions of high plastic strains is presented. The fracture toughness of materials under quasistatic and dynamic concentrated loads is investigated. Translated from Problemy Prochnosti, No. 3, pp. 78–88, May–June, 2009.     

Microstructure and mechanical properties of as aged Mg–3Sn–1Al and Mg–3Sn–2Zn–1Al alloy

Effect of Zn on the microstructure, age hardening response and mechanical properties of Mg–3Sn–1Al alloy which is immediately aged at 180°C after extrusion process (T5) was investigated. It was found that the Zn can refine the microstructure, remarkably improve the aging response with the peak hardness increases to 75 HV and the time to peak hardness reduces from ～110 to ～60 h, which is attributed to the solid solution hardening of Al, Zn and an amount of finer Mg₂Sn precipitates. The as aged Mg–3Sn–2Zn–1Al alloy exhibits better mechanical property at room temperature or 150°C than that of Mg–3Sn–1Al alloy, which is ascribed to the fine grained microstructure and thermally stable Mg₂Sn particles dispersed at grain boundaries and in the matrix.     

Continuation of twins across grain boundary in extruded Mg–3Al–1Zn alloy

Abstract

Paired twins in an extruded Mg–3Al–1Zn alloy are investigated by using electron backscattered diffraction in the current paper. The results show that these paired twins are discovered at low misorientation grain boundaries. The twin variant (1–102)[?1101] is operated in the paired twins. Additionally, a macroscopic angle exists in the paired twins and is determined by the c axis misorientation of the grains.     

Microstructure and tensile properties of as extruded and as aged Mg–Al–Zn–Mn–Sn alloy

The effect of 0–4 wt-% Sn addition on the microstructure and tensile properties of AZ80 alloys was investigated. The results indicated that Mg₂Sn particles were barely formed during the extrusion process until the content of Sn is >2 wt-%. The dislocation density in alloys after extrusion declined with the addition of Sn due to the promotion of dynamic recrystallisation after adding Sn. In aging treatment, Mg₁₇Al₁₂ precipitates were promoted by Sn and the phases distributed uniformly at low density level of dislocation. The AZ80-2 wt-% Sn alloy possessed the excellent tensile properties in as extruded and as aged state.     

Investigation of serrated flow behaviour in Al–10Mg alloy

Abstract

In view of reported anomalies in the serrated flow behaviour of aged Al–8.6Mg alloy, characteristics of serrated flow were investigated in an Al–10Mg alloy after solution treatment as well as after aging. The material was prepared by melting and casting, and then it was extruded, solution treated, and aged at either 150 or 200°C. Strain rate sensitivity, types of serration, onset strain of serrated flow, magnitude of serrations, and frequency of serrations were studied as a function of aging and strain rate. It was found that the alloy exhibited all the usual features of serrated flow except one, i.e. the magnitude of serration increased in the overaged condition after decreasing up to peak aging.     

Effects of minor additions of Ce and Y on as cast microstructure of Mg–3Sn–2Ca magnesium alloy

Abstract

The effects of minor additions of Ce and Y on the as cast microstructure of Mg–3Sn–2Ca (wt-%) magnesium alloy are investigated and compared. Results indicate that adding minor Ce or Y to Mg–3Sn–2Ca alloy does not cause formation of any new phases in the alloy. The as cast Mg–3Sn–2Ca alloy with addition of 0·5 wt-%Ce or Y is still composed of α-Mg, CaMgSn and Mg₂Ca phases. However, after adding 0·5 wt-%Ce or Y to Mg–3Sn–2Ca alloy, not only the formation of CaMgSn phase in the alloy is suppressed but also the CaMgSn phases in the alloy are effectively refined. In addition, adding 0·5 wt-%Ce to Mg–3Sn–2Ca alloy exhibits higher refinement efficiency to the CaMgSn phase in the alloy than adding 0·5 wt-%Y. Further investigations need to be considered in order to understand the difference of minor Ce and Y with regard to the refinement of CaMgSn phase in the Mg–3Sn–2Ca alloy.     

Roles of detwinning in strain hardening and ultimate elongation in extruded Mg–3Al–1Zn alloy

Abstract

For extruded magnesium alloy, prior compression along extrusion direction has great influences in the flow stress during subsequent tension. Detwinning plays an important role for these influences. In the present study, the effects of different prestrains on strain hardening behaviour during subsequent tension were examined in an extruded magnesium alloy AZ31. The results showed that the existence of detwinning decreased the tensile yield stress. Samples with different prestrains exhibited different strain hardening behaviour during subsequent tension. The reorientation due to detwinning had a great effect on strain hardening during tension. In addition, the effect of detwinning on ultimate elongation was investigated. The results showed that the sample with higher prestrain always has higher ultimate elongation due to the contribution of detwinning on macroscopic strain.     

Effect of phosphorus on modification of eutectic silicon in Al–7Si–0·3Mg alloy

Abstract

The influence of the impurity element phosphorus on the modification of eutectic silicon by the addition of strontium was examined by the observation of microstructures, differential scanning calorimetric thermal analysis, a tension test, and fractographic observation. When the phosphorus content was 1·3 ppm, the eutectic morphology was the fine needle phase. Where the phosphorus content was 17·5 ppm, the eutectic silicon appeared as a coarse flake phase. Increasing the phosphorus content of the melt deteriorated the modification of the eutectic silicon and increased the eutectic temperature. Tensile strength and elongation decreased with increasing phosphorus content.     

Effect of Texture on Biodegradable Behavior of an As-Extruded Mg–3%Al–1%Zn Alloy in Phosphate Buffer Saline Medium

In this work, the corrosion behavior of two differently oriented surfaces of an as-extruded Mg–3%Al–1%Zn(AZ31) bar in a simulated body ?uid of phosphate buffer saline(PBS) medium was investigated and compared, and the effect of crystallographic texture on corrosion resistance of the alloy was deeply described. The results showed that at the early stage of immersion, a layer of compact and ?at ?lm formed easily on surfaces of both oriented samples. With prolonged immersion time, the degradation of formed corrosive ?lms started and their severity was quite sensitive to the composed crystallographic planes of sample surfaces. For the surface containing highly concentrated orientation of {10–10} and {11–20} prism planes, the degradation of formed corrosive ?lm was quite slight and only occurred at some particular sites even after immersion for 48 h. Thus, the ?lm could keep good corrosive protection to the underneath substrate. However, for the surface containing {0002} basal planes, {10–10} and {11–20} prism planes, the degradation of corrosive production ?lm occurred widely, resulting in further decrease in the corrosion resistance of immersed samples.     

Effects of germanium on quench sensitivity in Al–Zn–Mg–Zr alloy

Effects of the trace element germanium (Ge) on the quench sensitivity of an Al–Zn–Mg–Zr alloy were investigated in the present work. The results showed that the Ge-bearing alloy exhibited lower quench sensitivity as compared to the Ge-free alloy. This phenomenon could be reasonably interpreted in terms of the stability of supersaturated solid solution of alloys after quenching from an elevated temperature. The apparent vacancy formation energies for the Ge-free and Ge-bearing alloys were determined to be 0.49 and 0.58 eV respectively. This suggested that the addition of a small amount of Ge was able to trap excess vacancies, leading to a decrease in the amount of coarse dispersoids and resultant low quench sensitivity in Ge-bearing alloys. Therefore, Ge could be used in alloy productions, which require a slow cooling rate to reduce the residual stresses and distortions.     

Compressive creep behaviour of Mg–Li–Al alloy

Abstract

The compressive creep behaviour of as cast Mg–14Li–1·3Al (wt-%) alloy was investigated in the temperature range of 20?85°C and under different compressive stress in the range of 37·3–74·6 MPa with special apparatus. Primary creep deformation and steady creep rate increase with temperature and applied stress. The compressive creep behaviour obeys an empirical equation ln t=C?nln σ + Q/RT, where t is the time to a selected creep strain, σ is the applied stress, T is the absolute temperature, R is the gas constant, and C, n, and Q are constants for the experimental alloy. The average values of the exponent n and the creep activation energy Q are 4·33 and 101·13 kJ mol^?1 respectively. The creep rate controlling mechanism is the dislocation climb and the lattice diffusion of Li in the experimental alloy under the testing conditions.     

Extrusion processing of Al–Li–Mg–Zr alloy

Abstract

The hot deformation behaviour of an Al–Li–Mg–Zr alloy was characterised in hot torsion and extrusion. The alloy was found to have similar hot ductility to existing high strength aluminium alloys, but this could be maintained at higher temperatures. Billets were extruded over a range of process conditions and a limit diagram was constructed for surface cracking. All the extrusions were found to be partially recrystallised after deformation, but the volume fraction of recrystallisation was a strong function of billet temperature and extrusion ratio. In addition, the unrecrystallised areas contained a recovered substructure where the subgrain size was inversely proportional to the temperature compensated strain rate. The as extruded structure was retained during solution treatment and as a result final mechanical properties were strongly dependent on the extrusion conditions. The use of high billet temperatures and low extrusion ratios gave the best combination of strength and toughness.

MST/839     

Precipitation hardening of Zr-modified Mg–Ca–Zn alloy

The microstructure and mechanical properties of Mg–Ca–Zn alloys with 1 wt.% Zr were investigated in as-cast and heat-treated conditions. A substantial decrease in grain size (from 65 μm for the Mg–Ca–Zn base alloy to 22 μm) was observed. The alloy was solution treated at 410 °C for up to 96 h followed by aging at 175 °C for up to 24 h. Conventional techniques, X-ray diffraction, EM + EDS, and TEM were used to characterize the microstructure of the alloy. The microstructure obtained after heat treatment had equiaxed grains with evenly distributed binary phase Zn₂Zr. The binary Mg₂Ca and ternary Mg₂Ca₆Zn₃ phases were identified in the matrix and at grain boundaries surrounded by precipitate-depleted zones (PDZs). The thermal stability of the Zr-modified alloys was examined by microhardness measurements conducted after prolonged exposures of the alloys to elevated temperatures. It was found that Zr is a structure-stabilizing factor. Its influence was associated with the formation of Zn₂Zr phase that does not undergo coarsening at the elevated temperatures used (due to the low diffusivity of Zr). The nanoscale mechanical properties of grain boundary PDZs were analyzed using combined nanoindentation and atomic force microscopy. These mechanical properties were then correlated to the composition and precipitate distribution in PDZs. An increase in the solution treatment duration from 10 to 96 h at 410 °C resulted in expansion of PDZs from ~0.75 to ~3 μm, while the following aging at 175 °C for up to 24 h did not lead to a detectable change in PDZs. The analysis indicates that the lowest hardness was found in the region where Zn₂Zr precipitates density was low, regardless of the solute concentration.     

Microstructure and mechanical properties of directionally solidified Mg–Zn alloy as a biomaterial

Directionally solidified Mg-4wt-% Zn alloy was prepared and the effect of growth rate on its microstructure evolution and mechanical properties was investigated. A typical cellular structure was observed when the growth rate was lower than 60?µm?s^?1. The microstructure evolved from cell to columnar dendrite as the growth rate increased. The ultimate tensile strength of the directionally solidified alloy was found to be higher than that of the alloy ingot with the same cooling rate. The ultimate tensile strength of the directionally solidified alloy increased with increasing growth rate but it decreased during the cell–dendrite transition. The results indicate that the mechanical properties of the directionally solidified alloy with fine cellular and columnar dendritic structures meet the requirements of biomaterials.     

Peculiarities of fracture in submicrocrystalline Al–Mg–Mn alloy under impact compression

The method of nondestructive X-ray computed tomography (CT) has been used to study the structure of A5083 (magnesium- and manganese-doped aluminum) alloy samples upon impact compression. The initial samples had an average grain size of 600 nm and submicrocrystalline (SMC) structure formed by dynamic equal-channel angular pressing. Three-dimensional CT images of local fracture regions were obtained and the degree of material damage was estimated by calculating the average and maximum size of discontinuities (pores and microcracks) in various cross sections. The techniques of transmission and scanning electron microscopy were used to trace evolution of the SMC structure of impact-compressed alloy and determine the morphological characteristics of spallation surfaces and other defects.     

Characterisation of interfaces in Al–Zn–Mg alloy reinforced with Sicabo fibres

Abstract

The microstructure of a metal matrix composite consisting of an Al–Zn–Mg alloy reinforced with SiC coated boron fibres has been examined by electron microscopy, electron probe microanalysis, and by optical microscopy. Considerable amounts of Mg₂Si phase were found to be segregated at the fibre/matrix interface. This intermetallic was not formed by a reaction between the fibre and matrix during the fabrication process, a liquid infiltration technique, but as a result of silicon impurities present as contaminants in the melt. It was concluded that the interface phase was precipitated from the metal matrix in the later stages of solidification without any nucleation role being played by the fibre. The Mg₂Si phase appears to be brittle and was present in amounts likely to have a deleterious effect on the strength of the composite.

MST/871     

STEM-HAADF tomography investigation of grain boundary precipitates in Al–Cu–Mg alloy

The high angle annular dark field scanning transmission electron microscopy (STEM-HAADF) tomography technique was applied to understand the three dimensional (3D) morphology and distribution characteristic of grain boundary precipitate (GBP) in peak-aged Al–Cu–Mg alloy. The results indicate that GBPs show both spherical and lenticular shapes and triangularly distribute along grain boundary (GB). Based on 3D observations from various directions, the values of GBP relevant parameters such as GBP size, center to center distance, number of GBPs per unit GB area and area fraction of GB covered by GBPs are further determined. The 3D method for GBP relevant parameter determination seems an effective way to avoid misunderstandings in the conventional two dimensional (2D) methods induced by GBP overlapping and projection effect as well as curved GB surface.     

Microstructure and mechanical properties of variable-plane-rolled Mg–3Al–1Zn alloy

A flawless bulk AZ31 magnesium alloy with extensive mechanical twins was produced by variable-plane rolling, in which the sample was rotated 90° around its longitudinal axis between passes. The unique orientation relationship between the parent grains and the twin grains favours twinning during variable-plane rolling, which leads to the formation of extensive twins. Tensile testing revealed an excellent balance of mechanical properties, with a yield strength of 280 MPa and 15.5% elongation to failure. The significant strengthening originates from the effective blockage of glide dislocations by numerous conventional grain boundaries and twin boundaries. A weak double-peak (slightly off-basal) texture is formed during variable-plane rolling, which helps in achieving the desired level of ductility.