Регистрация / Вход
Прислать материал

The microstructure and mechanical properties promising alloys Mg-Zn-Ca system

Name
Elena
Surname
Rzhevskaya
Scientific organization
Togliatti State University, Togliatti (Russia)
Academic degree
no
Position
junior researcher of the Research Institute of Progressive Technologies
Scientific discipline
New materials, Manufacturing technologies & Processes
Topic
The microstructure and mechanical properties promising alloys Mg-Zn-Ca system
Abstract
At present work have shown that a coarse intermetallic particles in the microstructure of magnesium alloys (Mg-Zn-Ca system) has no significant effect on fatigue crack initiation in low cycle fatigue test mode. The fatigue life of biodegradable magnesium alloys ZX50 and WZ21 after extrusion have shown comparable high-strength results in high cycle fatigue tests conducted in air atmosphere, like extruded magnesium ZK60 alloy.
Keywords
magnesium alloys; biocompatibility; mechanical properties; microstructure, corrosion fatigue
Summary

Now there is active development of new magnesium alloys for medical purposes, around the world. Magnesium has the basic qualities necessary for biomedical applications such as biocompatibility, non-toxicity and the non-carcinogenic. An important advantage is that magnesium can fully dissolve in the body without harm. As a result, the natural corrosion of the implant in a physiological environment of the need for re-operation was not require. However, biomedical magnesium alloys, which are used as bone implants should have a sufficient level of mechanical properties, which cannot provide the pure magnesium.

Using deformation hardening methods can improve mechanical properties of magnesium alloys.  Applying of severe plastic deformation (SPD) contributes high effective deformation in the workpiece and provides a mechanism of dynamic recrystallization. All this allows managing the microstructure by creating the crushed grain and the necessary distribution of secondary phases in the alloy.

Determination of fatigue properties plays an important role at the stage of detail’s designing, because it affects the prediction of detail’s working capacity.

The objects of study in this work are the alloys Mg-Zn-Ca system, because zinc and calcium are completely biocompatible with the human body. Furthermore, these elements in the alloy composition improve its properties. Thus, zinc is efficiently improves the mechanical properties of magnesium alloys, calcium contributes to the corrosion resistance.

All investigated alloys in this work are presented in Table 1. It should be noted that the zinc content at the level of 4-5% chosen because the higher concentration of this element in the alloy, lowers the resistance to corrosion.

At first the microstructure of the alloys with the base Mg-4Zn was investigated in the cast state. In the microstructure of these alloys in addition to the solid solution grains α-Mg, was discovered a network of intermetallic particles that are the Ca2Mg6Zn23 for alloys with calcium and Mg7Zn3 for non-calcium alloy.

After deformation hardening of alloy microstructure U3 (Mg-4Zn) has the most homogeneous structure with an average grain size (9 ÷ 10 microns). There is no inclusion in the alloy, so it means that zinc is in a solid solution. This homogeneity of structure in comparison with the alloys U1 (0,16Ca) and U2 (0,56Ca) is reached by multiple isothermal forging after just a single pass by ECAP.

The heterogeneity of microstructure after thermomechanical processing for all other alloys observed to a greater or lesser extent. Therefore, the microstructure of alloys is bimodal, where presented both relatively small grains and large non-recrystallized grains.

 

Tables 1. Chemical composition and thermomechanical treatment.

Alloy

Chemical composition

Thermomechanical treatment

U1

Mg-4Zn-0.16Ca

Homogenizing annealing (330°С, 10 h.) + ECAP, 1Вс, 120°, 320°С)

U2

Mg-4Zn-0.56Ca

U3

Mg-4Zn

Homogenizing annealing (330°С, 10 h.+460 °С, 7 h.) + ECAP ,1 Вс, 120°, 320°С+ MIF 2 pass 300 °С

ZX50

Mg-5Zn-0.25Ca

Homogenizing annealing (350°С, 12 h.) + extrusion (325 °С, 25:1)

WZ21

Mg-1.65Y-0.85Zn-0.25Ca

Homogenizing annealing (350°С, 12 h.) + extrusion (325 °С, 30:1)

 

Analysis of experiments on high-cycle fatigue results, in the air atmosphere, revealed that the extruded alloys ZX50 and WZ21 show fatigue life's properties like extruded ZK60 alloy, which is used as a reference material (table. 2). ZK60 is a bioalloy and high-strength structural magnesium alloy at the same time. It is important to note that ZX50 and WZ21 alloys after thermomechanical treatment can be attributed in the category of high-strength alloys, under fatigue characteristic

Were also performed cyclic testing of these alloys in a corrosive medical saline – 0.9 NaCl. A solution of 0.9NaCl simulates the human body solution. All samples have shown low rates of corrosion fatigue, even despite the fact that the corrosion rates of alloys samples was considerably different. Of course, the deterioration of fatigue properties was expected initially, but not to that extent.

 

Tables 2. Fatigue limit at cycle fatigue

Alloy

Chemical composition

Fatigue limit, MPa

U2

Mg-4Zn-0.56Ca

55

ZX50

Mg-5Zn-0.25Ca

90

WZ21

Mg-1.65Y-0.85Zn-0.25Ca

90

ZK60 [1]

extruded

Mg-6Zn-0,5Zr

89,5

 

Fractographic research of magnesium alloys were carried out on samples after low-cycle fatigue.

By studying the fatigue failure morphology near fracture nucleus, we can conclude that the crack initiation not connected with any superficial or micro structured defect's. These intermetallic particles are present predominantly in the unstable crack growth zone and of rupture zone. So, it's do not main affect the initiation of fatigue crack.

The initiation of fatigue crack and zone of stable crack growth are only quantitative, but not a qualitative difference. The surface of fracture is formed the mechanisms of brittle fracture to form a fine structure "serrated river line", in these zones.

The unstable crack growth zone has much more developed relief, as compared with the previous stages of fracture. The presence in this zone of dimples relief, formed by the merger of micropores indicates the ductile fracture behavior of the sample in the final stages of destruction.

After high-cycle fatigue testing in 0.9% NaCl environment the fracture surface of samples heavily covered with corrosion products. In this case uniquely identify characteristic regions of fatigue fracture is not possible.

Great influence on the geometry of the sample and the fracture has a load at which the test takes place. Thus, the section geometry of alloy samples tested under high loads (50 MPa), varies significantly under the influence of a corrosive medium. The cross section of samples is rounded only slightly (Fig. 1a), but he is still rectangular. Whereas a samples tested at low loads (25 MPa) in a corrosive environment is acquire virtually circular cross section (Fig. 1b).

Most probably, this is due to with the fact that the sample is longer in corrosion environment at low loads. Thus, the role of corrosion in cyclic tests is reduce the cross section of the sample during the experiment, which leads to a working stress increase. The high corrosion rate is a decisive factor in the degradation of mechanical properties during cyclic tests.

 

a

b

Figure 1 - A typical view of the fracture surface of the samples after fatigue testing in corrosive environment 0.9NaCl by the example alloy ZX50, tests under load: a - 50 MPa, b - 25 MPa

 

Conclusions

  1. Microstructure is heterogeneity after thermomechanical processing observed to a greater or lesser extent. The most homogeneous microstructure achieved after multiple isothermal forging (alloy U3).
  2. The best characteristics of fatigue in the air have extruded alloys ZX50 and WZ21.
  3. The particles of secondary phases in the alloys U1 and U2 do not substantially affect the mechanical properties, including fatigue failure.

 

Special thanks go to M. Linderov for his careful LCF and HCF experiments.

 

Reference

1. Constantinescu D.M., Moldovan P., Sillekens W.H., Sandu M., Bojin D., Baciu F., Apostol D.A., Miron M.C. Static and fatigue properties of magnesium alloys used in automotive industry. Sci. Bull. Automot. Ser. 2009. 19(B). p.33–39.