Biomedical Engineering Reference
In-Depth Information
8.2
Prospecting for the Best
Better performance for lower weight was an industrial incentive to develop Mg
alloys: with a density around 1,800 kg.m
3
and a YS of some 300 MPa the ratio
of YS or UTS to
is positioned in between those for CoCr and Ti alloys (see
Tab le
8.3
). However, a 'best' alloy is not identified yet. In Table
8.2
, we collected a
set of data on the mechanical performance of potential biomedical candidate alloys.
The table shows the highly diverging data, a demonstration of the high sensitivity
to thermal and mechanical history and composition. Compare the data for YS and
UTS of AZ91 (as cast) at the top of the table with those for the same type of alloy
but reported by another author. Alternatives are Mg-Al-RE containing rare earths
(Ce, Nd,...with refined microstructure and improved strength) and Mg-Li-Al,RE
alloys (LAE, WEL, LE).
The crystallographic structure of the pure metal is h.c.p. as the other elements in
the same column of the periodic table. The matrix is mainly dendritic
˛
-Mg with an
eutectic
-phase consisting of the intermetallic compound Mg
17
Al
12
,plusafinely
grained (1-5
ˇ
m) Mn-rich phase. UTS and grain size are strongly related, following
Tabl e 8. 2
Mechanical properties of Mg alloys: collation of data from [
258
,
261
-
263
]
Alloy
E
YS
UTS
Elongation
Grain size
GPa
MPa
MPa
%
m
AZ91(as cast)
69
210
7
AZ91(heat treated)
108
240
3
AZ91(473)
a
364
400
1.9
1.2
AZ91(623)
a
277
325
7.1
15.6
AZ91(ECAE)
b
277
318
2.5
1.0
Bending
Mg cp
14
9.5
Mg0.6Ca
15
143
Mg1.2Ca
18
132
Mg2.0
18
108
Compression
Mg cp
45
90
198
Mg0.6Ca
402
Mg1.2Ca
50
97
254
Mg2.0Ca
59
73
233
MgCa5
85
AZ91Fe0.03
95
145
AZ91Fe0.05
105
200
AZ91/50
c
218
315
AZ91/5
215
315
AZ91/2.5
130
235
a
Extruded at 473 or 623
ı
C
b
ECAE: Equal Channel Angular Extrusion, a large shear strain extrusion process. For more detail
and the effect on grain size, see [
264
]
c
AZ91 but in bars of 50, 5 or 2.5 mm diameter
