**4. Thixotropic compression deformation behavior of composites**

The characteristics of semi-solid composites deformed mechanism can be understood well only when the relationships between stress and strain are described. So the semi-solid compression tests for SiCp/AZ61 composites were conducted, whose mechanical properties and destruction model were investigated.

The experiments were conducted in a Thermecmastor-Z dynamic material testing machine, whose set-up was shown in Fig.11. The specimen was heated by electromagnetic wave, whose temperature was monitored by thermocouples. The graphite slices were placed between the specimen and the compression heads for reducing the influence of friction on experiment. In order to study and master the characteristic mechanics of semi-solid magnesium matrix composites at high solid volume fractions, the deformation temperatures were taken as 530℃, 545℃, 560℃ and 570℃ respectively. According to the heating procedure shown in Fig.12, the initial heating rate was 10℃/s; when the specimen temperature reached 500℃, the temperature rate was down to 1℃/s. Then the semi-solid compression experiments were done under the strain rates of 0.1s-1, 0.5 s-1, 5.0 s-1and 10 s-1 respectively, in which the total strain was 0.6.

Fig. 11. Schematic diagram for the compressive tests

The stress-strain curves of semi-solid SiCp/AZ61 composites with various volume fractions of SiC particles are shown in Fig.13. The tendency of curves implies that the deformation temperature has a significant effect on the flow stress. It is observed that for a constant strain

Study on Thixotropic Plastic Forming of Magnesium Matrix Composites 265

Relationships between peak stress and temperature of composites with strain rate of 10s-1 are shown in Fig.14. It can also be seen that the variation of volume fractions of SiC has a significant effect on the peak stress of the composites. When the deformation temperature raises slightly higher than the solid phase line in semi-solid zone, the volume fractions of liquid phase is low in the matrix, and the peak stress decreases rapidly almost as linear form with the increasing of temperature initially, and then slows as the temperature increases further. The results are thought to be of the lower volume fractions of liquid phase. When the specimens has high volume fractions of solid phase , the solid grains contact with each other to form a net, the sliding and rotation of grains become hard. The plastic deformation of solid particles is the main mechanisms. Besides SiC particles increase the resistance to grain boundaries sliding and impose barriers to dislocation motion, which leads to higher resistance for plastic deformation. With the increasing of temperature, the volume fractions of liquid phase increase, and the solid grains are surrounded by liquid phase, which requires smaller force for the solid grains to slid and rotate. Thus the peak flow stress

decrease rapidly, and then slowed as the temperature increases further.

Fig. 14. Relationships between the peak stress and temperature of three

The relations of stress-strain rate at various strain rates are shown in Fig. 15. It can be seen that the peak stress increases as the strain rate increases at the constant temperature. When the composites are compressed at high strain rate (for example 10 s-1), the liquid phase can not be squeezed out timely, in which the flow stress is very high during the initial deformation stage, and then decreases as the result of high shearing rate. With the further deformation, the liquid is squeezed out and pushed together, in which the solid particles are smashed. During this stage, the grain boundaries sliding and flow become easy and the flow stress decreases rapidly. At lower strain rate the solid grains are surrounded by liquid phase, which requires smaller force for sliding and rotation. So the flow stress is lower.

Fig. 16 presents stress-strain curves of the SiCp/AZ61 composite with different SiC fractions at 545℃and 560℃ and constant strain rate of 0.1s-1 and 10s-1. The fractions of SiC have a significant effect on the flow stress. The compression stress increases with the increasing of volume fractions of SiC particles. The reason is that SiC particles are mainly located in the inter-granular and boundary regions in the composites. The SiC particles impose barriers to dislocation motion and resistance to the solid grains sliding during the steady-state compressive deformation. So it increases the resistance for dislocation and grain boundaries

SiCp/AZ61composites in semi-solid thixotropic compression

sliding with the increasing of volume fractions of SiC particles.

rate and constant volume fraction of SiC in the composites, the flow stresses and peak stresses decrease with the increasing of deformation temperature, which presents that the thixotropic plastic deformation of the composites is highly sensitive to temperature. The tendency is thought to be the result of variation of volume fractions of solid α phase. When the specimens has high solid volume fractions, the solid grains contact with each other and form a net, sliding and rotation of grains become hard. The plastic deformation of solid particles is the main mechanisms. With the increasing of temperature, the solid volume fractions decrease, and the solid grains are surrounded by liquid phase, which makes the solid grains to slid and rotate easily. Thus the sliding and rotation of grains plays a more significant role in the thixotropic plastic deformation.

Fig. 12. Heating processing

Fig. 13. Curves of stress-strain relation at various temperatures for SiCp/AZ61 composites

rate and constant volume fraction of SiC in the composites, the flow stresses and peak stresses decrease with the increasing of deformation temperature, which presents that the thixotropic plastic deformation of the composites is highly sensitive to temperature. The tendency is thought to be the result of variation of volume fractions of solid α phase. When the specimens has high solid volume fractions, the solid grains contact with each other and form a net, sliding and rotation of grains become hard. The plastic deformation of solid particles is the main mechanisms. With the increasing of temperature, the solid volume fractions decrease, and the solid grains are surrounded by liquid phase, which makes the solid grains to slid and rotate easily. Thus the sliding and rotation of grains plays a more

=0.1 s-1 (b) fp=6%,

(c) fp=9%,

=10s-1

Fig. 13. Curves of stress-strain relation at various temperatures for SiCp/AZ61 composites

=5s-1

significant role in the thixotropic plastic deformation.

Fig. 12. Heating processing

(a) fp=3%,

Relationships between peak stress and temperature of composites with strain rate of 10s-1 are shown in Fig.14. It can also be seen that the variation of volume fractions of SiC has a significant effect on the peak stress of the composites. When the deformation temperature raises slightly higher than the solid phase line in semi-solid zone, the volume fractions of liquid phase is low in the matrix, and the peak stress decreases rapidly almost as linear form with the increasing of temperature initially, and then slows as the temperature increases further. The results are thought to be of the lower volume fractions of liquid phase. When the specimens has high volume fractions of solid phase , the solid grains contact with each other to form a net, the sliding and rotation of grains become hard. The plastic deformation of solid particles is the main mechanisms. Besides SiC particles increase the resistance to grain boundaries sliding and impose barriers to dislocation motion, which leads to higher resistance for plastic deformation. With the increasing of temperature, the volume fractions of liquid phase increase, and the solid grains are surrounded by liquid phase, which requires smaller force for the solid grains to slid and rotate. Thus the peak flow stress decrease rapidly, and then slowed as the temperature increases further.

Fig. 14. Relationships between the peak stress and temperature of three SiCp/AZ61composites in semi-solid thixotropic compression

The relations of stress-strain rate at various strain rates are shown in Fig. 15. It can be seen that the peak stress increases as the strain rate increases at the constant temperature. When the composites are compressed at high strain rate (for example 10 s-1), the liquid phase can not be squeezed out timely, in which the flow stress is very high during the initial deformation stage, and then decreases as the result of high shearing rate. With the further deformation, the liquid is squeezed out and pushed together, in which the solid particles are smashed. During this stage, the grain boundaries sliding and flow become easy and the flow stress decreases rapidly. At lower strain rate the solid grains are surrounded by liquid phase, which requires smaller force for sliding and rotation. So the flow stress is lower.

Fig. 16 presents stress-strain curves of the SiCp/AZ61 composite with different SiC fractions at 545℃and 560℃ and constant strain rate of 0.1s-1 and 10s-1. The fractions of SiC have a significant effect on the flow stress. The compression stress increases with the increasing of volume fractions of SiC particles. The reason is that SiC particles are mainly located in the inter-granular and boundary regions in the composites. The SiC particles impose barriers to dislocation motion and resistance to the solid grains sliding during the steady-state compressive deformation. So it increases the resistance for dislocation and grain boundaries sliding with the increasing of volume fractions of SiC particles.

Study on Thixotropic Plastic Forming of Magnesium Matrix Composites 267

The appearances of specimens compressed at semi-solid state are shown in Fig. 17. In can be seen that the surface longitudinal cracks of specimen happen at compression ratio of 20% and the liquid phase is squeezed out along the cracks. When the compression ratio reaches up 30%, the volume fractions of liquid phase squeezed out of the surface increases, which generates the mixed liquid-solid outer surfaces. The surface strength is very low and generates easily cracks. When all of liquid is squeezed out, the flow stress starts to ascend.

(a) 0% (b) 20% (c) 30% (d) 60%

On the basis of analysis of behavior of thixotropic plastic deformation of composites in compression process, its constitutive model is established. Then the model parameters are

Based on the experimental analysis of axial compression of composites in semi-solid state,

temperature T, liquid phase rate *Lf* , as well as the volume fraction of reinforcement *pf* [Yan & Wang, 2011]. At the same time Hong Yan present the constitutive relationship of semi-

1 2 <sup>3</sup> exp( / ) ( ) 1 1 *a a <sup>a</sup>*

 

*T fL* (3)

– strain rate, T – temperature, β – geometric

 

In the study of deformation behavior of composites under high strain rate [Bao & Lin, 1996] and [Li & Ramesh, 2000] found that the influence of volume fraction of reinforcement on the

> ( ) ( , ) ( )[ ( ) ] 1 *<sup>m</sup> p p p*

*f gf f*

(,) 

So the constitutive model of thixotropic plastic deformation of composites reinforced with

(4)


 , strain *<sup>z</sup>* ,

**5. Constitutive model for thixotropic plastic forming of composites** 

there is a certain non-liner relationship among stress σ and strain rate *<sup>z</sup>*

At last the compressed specimen looks like popcorn.

Fig. 17. Appearances of specimens compressed at semi-solid state

determined using the multiple nonlinear regression method.

mechanical behavior of the material was present as following:

 


solid magnesium alloy as follow [Yan & Zhou, 2006]:

Where σ – stress, ε – strain,

Where σ- stress, ε- strain,

particles is proposed.

volume fraction of reinforcement.

parameters(β=1.5), fL – liquid phase rate.

Fig. 15. Curves of stress-strain relations at various strain rates for SiCp/AZ61 composites

Fig. 16. Curves of stress-strain relation at 545℃and 560℃ for SiCp/AZ61 composites with different SiC fractions

(a) fp=3%,T=530℃ (b) fp=6%,T=545℃

(c) fp=9%,T=570℃ Fig. 15. Curves of stress-strain relations at various strain rates for SiCp/AZ61 composites

=0.1s-1 (b) T=560℃,

Fig. 16. Curves of stress-strain relation at 545℃and 560℃ for SiCp/AZ61 composites with

=10s-1

(a) T=545℃,

different SiC fractions

The appearances of specimens compressed at semi-solid state are shown in Fig. 17. In can be seen that the surface longitudinal cracks of specimen happen at compression ratio of 20% and the liquid phase is squeezed out along the cracks. When the compression ratio reaches up 30%, the volume fractions of liquid phase squeezed out of the surface increases, which generates the mixed liquid-solid outer surfaces. The surface strength is very low and generates easily cracks. When all of liquid is squeezed out, the flow stress starts to ascend. At last the compressed specimen looks like popcorn.

Fig. 17. Appearances of specimens compressed at semi-solid state
