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Mechanism for corrosion protection of β-TCP reinforced ZK60 via laser rapid solidification

Figure 9. CCK-8 assay for MG-63 cells cultured in the extracts of ZK60/xβ-TCP composites for 1, 3
and 5 days. (n = 3, *p < 0.05, **p < 0.01).

composites by conventional techniques. Unfortunately, believed to be far faster than the movement of β-TCP
agglomeration phenomenon of β-TCP occurred even at particles. In this condition, the β-TCP particles would be
[11]
a low content of 1.5 wt. % in casting process . Huang captured by the solid/liquid interface and remained the
[22]
et al. reported that β-TCP aggregated in the matrix of original uniform distribution. On the other hand, laser
casted Mg–2Zn–0.5Ca/1β-TCP at a lower content of 1 rapid solidification could also cause a grain refinement
[23]
wt. %. Yan et al. fabricated a kind of Mg-Zn/β-TCP with a high density of grains boundaries . More grain
[6]
composite by powder metallurgy, and also observed boundaries would provide more distribution space for
the aggregation of β-TCP in Mg matrix. The physical β-TCP particles, thus avoiding the aggregation of β-TCP
differences between β-TCP and α-Mg would be used to particles.
explain the agglomeration of β-TCP in the Mg matrix.
β-TCP possessed a rhombohedral structure (lattice 4.2 The Effect of β-TCP on Mechanical
parameters a, b = 1.04352 nm, c = 3.7403 nm, a, β = 90° Properties
and γ = 120°), while α-Mg had a hexagonal structure Mechanical tests revealed that the incorporation of
(lattice parameters a, b = 0.32092 nm and c = 0.52105
nm) [24] . According to the heterogeneous nucleation β-TCP significantly improved the compressive strength
theory, such a difference in crystal structure made it and hardness of ZK60. The increased compressive
extremely difficult for α-Mg to nucleate on the surfaces strength of ZK60/xβ-TCP composites was due to: (I)
of β-TCP particles. Thus, most of the β-TCP particles a good interface bonding between the α-Mg grains
would be pushed by the growing front of α-Mg grains and β-TCP particles gave rise to effective load transfer
during the solidification. In equilibrium solidification from α-Mg matrix to β-TCP particles, which possessed
[25]
with a low cooling rate, the β-TCP particles would be better load-bearing capacity ; (II) the homogeneously
squeezed out continuously by slowly-advancing solid/ distributed β-TCP would serve as an obstacle to
liquid interfaces, finally gathered at the crystal interface the dislocation movement and then ended up with
and caused component segregation. dislocation pile ups; (III) the addition of β-TCP particles
Combined processes have been reported to overcome as second phase inhibited the growth of ɑ-Mg grains,
the agglomeration of β-TCP in Mg matrix. For example, resulting in fine grain strengthening. However, the
a melt shearing technology combined with high- compressive strength of the ZK60/xβ-TCP composites
pressure die casting was applied to fabricated β-TCP/Mg decreased with β-TCP further increasing to 12 wt. %. For
[12]
composite . Besides, powder metallurgy, hot extrusion ZK60/12β-TCP, excess β-TCP aggregated at the grain
and aging treatment were combined to fabricated boundaries and formed coarsened second phase, which
β-TCP/Mg-Zn composites [23] . In this study, laser rapid weakened the bonding interface between the ɑ-Mg
solidification, as one step process, was proposed to solve grains and adjacent β-TCP particles. A large number of
the problem. SEM images clearly showed that β-TCP pores and defects formed in the matrix, thus reducing
homogeneously distributed along grains boundaries in the compressive strengths of the composite. Besides, the
Mg matrix, with β-TCP contents up to 8 wt. % (Figure enhanced hardness was primarily attributed to that hard
2). In laser rapid solidification, the velocity of the so- β-TCP particles acted as reinforcement phases, which
lid/li quid interface was extremely high, which was impeded the dislocation movement.

8 International Journal of Bioprinting (2018)–Volume 4, Issue 1