Processing and mechanical properties of pure mg and in situ aln reinforced mg 5al composite 7

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155
Chapter 7
Conclusions and recommendations

7.1 Conclusions
Physical, thermal and mechanical properties of nanostructured Mg-5Al-1AlN
composite are studied in comparison with nanostructured pure Mg which is
synthesized and tested with the same parameters as the composite. The Mg composite
and pure Mg consolidated from MMed powders inherently contain processing defects.
Combined effect of grain refinement, solid solution, reinforcement and in-situ formed
second phase particles result in very complex microstructure and consequently very
different mechanical properties at different milling durations. Based on the
experimental results, some conclusions can be drawn.

1. Similar behavior of overall tensile deformation is observed in both material
systems despite additional alloying and reinforcement in the composite. The
only difference is that the composite samples possess higher strength due to
additional solid solution strengthening and particle dispersion strengthening.
2. Short milling durations such as 10h and 20h produce microstructures with
higher yield stress compared to those of the as-blended samples. Grain
refinement, solid solution and particle dispersion strengthening resulted from
the interactions between: (i) dislocations and grain boundaries and (ii)
dislocations and particles are responsible for the strength enhancement in these
composite samples.
3. Decline in yield stress but enhanced ductility with strain softening behaviors
are found from those samples after 20h-MM. Extremely high ductility of 34%
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with yield stress of 205 MPa is observed from the composite sample with grain
size of about 33 nm after 40h-MM. Similar results of 33% ductility and 210
MPa yield stress are produced for 40h-MMed pure Mg samples. This finding
implies that the grain refinement to a certain length scale diminishes the
significance of solid solution strengthening and dispersion hardening. 30h-
MMed composite exhibited the best combination of strength and ductility.
4. Time dependence of mechanical properties of both material systems is verified
by tensile tests at three different strain rates, 3.33x10
-5
s
-1
, 3.33x10
-4
s
-1
,
3.33x10
-3
s
-1
. At room temperature, strain rate sensitivity factors of 0.087 and
0.106, and unusually small apparent activation volume of 41b
3
and 34b
3
are
observed from the 40h-MMed composite and the 40h-MMed pure Mg samples
respectively.
5. Apparent activation energy of 50 KJ mol
-1

which is close to the activation
energy for grain boundary diffusion in Mg has been estimated from creep
curves of 120 MPa constant applied stress on the 40h-MMed composite
samples at different temperatures. It implies that grain boundary deformation
process is dominant in the nanoscaled grain size region.
6. Investigation of grain boundary deformation process in the nanostructured
composite through constant stress creep tests at various temperatures less than
0.35 T
m
show stress exponent n of 2 and activation energy Q of 61 KJ mol
-1
.
7. Coble creep alone is not sufficient to describe the creep behavior of the present
nano-composite samples. In addition to the AlN reinforcement, processing
defects such as impurities and excess free volume render the grain boundaries
imperfect making the experimental results deviate from theoretical prediction.
Although creep deformation parameters (n and Q) favor grain boundary sliding
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as dominant deformation mode, the co-existence of Coble creep is evident with
two sets of competitive results which are quite close to each other especially at
higher stress level.

7.2 Recommendations for future investigation
In general, in order to understand the nature and origin of deformation mechanisms at
high temperatures, information related to the dependence of strain rate on stress,
temperature, and grain size is essential. In the case of nc materials, this type of
information may be obtained by
(a) conducting creep tests over a wide range of strain rates (at least five orders of

magnitude) and at several temperatures to identify unambiguously the values of
the stress exponent, n, and the activation energy for creep, Q,
(b) adopting an effective method that would produce nc materials with different
grain sizes and then performing systematic tests under identical conditions of
temperature and stress for the purpose of determining the values of the grain size
sensitivity, and
(c) performing experiments that involve large strains to ensure the presence of
steady-state creep. In addition, in order to avoid complications that may be
caused by the presence of different alloying elements and impurities, it is
desirable to perform such tests on pure nc metals. However, caution must be
exercised in adopting such an approach due to the occurrence of significant grain
growth in pure nc metals at elevated temperatures.

From the experimental results in the present research, various milling durations
produced different mechanical behaviors. It can be observed that quite extreme
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combinations of tensile properties such as high strength and low ductility to low
strength and high ductility have been obtained by varying the milling time, in other
words, varying grain sizes. Due to technological restrictions and the pyrophoric nature
of Mg, it is a tedious task to carry out microstructural study using TEM. The following
tasks are recommended for the analysis on the influence of grain boundary,
reinforcement and second phase particles, dislocation, processing history, structural
flaws to comprehend the deformation behaviors of Mg composite.

1. Both pure Mg and composite samples showed similar deformation behaviors.
Further creep tests and TEM microstructural examination can be performed on
the pure Mg samples to confirm the diminishing effect of reinforcements (in-
situ formed second phase particles and/or ex-situ reinforcement particles) when

the grain size becomes very small in nanometer range.
2. Grain boundary activities played an important role in the deformation process
of nanostructured Mg composite. In-depth analysis of the nature of grain
boundary, types of impurities, excess free volume should be carried out to
study their influence on the grain boundary diffusivity.
3. For convincing microstructural evidence, an attempt should be made to
facilitate the in-situ observation of grain boundary activities and its interaction
with other structural defects and particles during deformation process.
4. Combined positron-lifetime and Fe-tracer diffusion measurement could provide
the correlation between free volumes and atomic diffusion. The variation of the
free volume pattern and the diffusion behavior with different milling durations
can verify the role of excess volume at the grain boundaries in grain boundary
diffusion and deformation process.