Processing and mechanical properties of pure mg and in situ aln reinforced mg 5al composite 1

Introduction



1
Chapter 1
Introduction

1.1 The challenges and motivation of this project
Magnesium, one of the lightest structural metals, has a density of 1.74 g cm
-3
which is
respectively 35.6% and 61.3% lower than that of Al and Ti [1]. Weight saving
possibilities from the application of Mg in structural parts has prompted intensive
research, especially in the automotive and aerospace industries. Due to its low density,
high strength-to-weight ratio and high specific stiffness at both ambient and elevated
temperatures, Mg and Mg alloys are attractive choices among these three light weight
metals. Good processing, machining and recycling possibilities are the additional
advantages of Mg alloys [2].

In some applications, Mg shows poor corrosion resistance and low strength and poor
creep resistance at elevated temperature. Sand cast unalloyed 13 mm diameter Mg at
20˚C shows yield strength of 90 MPa and 2-6% of elongation. [3]. This limitation
shrinks the range of applications of Mg. Therefore, the enhancement in mechanical
properties through proper control and manipulation of the structures is critical to
broaden its applications. Mg is usually alloyed with other metals such as Al, Zn, Mn,
Li, Y and rare earth elements or used as composites [4]. It was found that Mg alloy
with 3 wt% Al provides the optimal ductility and that with 6 wt% Al produces the
optimum combination of strength and ductility [3]. However, the solubility of alloying
elements in Mg is limited, restricting the possibility of improving mechanical
properties and chemical behavior. Enhanced mechanical properties have been reported

in Mg composite reinforced with nanoscaled ceramic particles such as SiC [5], Al
2
O
3

Introduction



2
[6] and AlN [7]. The demand for greater performance of Mg alloys or composites leads
to the development of Mg-based metal matrix composite (MMC) with nanostructure
reinforced by in-situ ceramics [8]. Composites not only have the combined properties
of their constituents, they also can be tailored to offer improved properties to meet
different engineering requirements.

Ductility is determined by the number of operating slip systems. The main deformation
mode in magnesium and magnesium alloys is basal slip, i.e. slip on the (0001) plane
with a
 0211 Burgers vector and secondary slip which is the prismatic slip on
}0110{ in the  0211 direction. This limits the inherent ductility of Mg at low
temperatures. At elevated temperatures, the pyramidal slips
}0110{  0211 have also
been observed, but their critical resolved shear stress at room temperature is roughly a
100-fold greater than for basal slip [9]. The limited number of operating slip systems
makes Mg extremely orientation dependent and low ductility.

As the production of Mg die-castings for automotive applications increases and
environmentally approved disposal costs rise [10], the recycling of in-house scrap and
post consumer scrap plays an increasingly important role in the supply of magnesium

in the future. Mg chips and scraps have been recycled by consolidation followed by hot
extrusion [11,12]. It was found that MM can provide an alternative cheaper way of
recycling the Mg scraps as nc powders can be produced directly from the solid state
without going through the melting process which is very expensive and
environmentally harmful. MM is one of the most impressive methods for reducing
grain size and producing nc powders [13,14]. It is also well documented that the
Introduction



3
mechanical properties of Mg alloys, especially yield strength and ductility, strongly
depend on the grain size due to the large Taylor factor of Mg [15].

Bulk nc materials are normally produced from MMed fine grained powders through
secondary processes, for example, conventional PM techniques consisting of
mechanical consolidation, sintering and extrusion. Mechanical testing however often
exhibits controversial results and the understanding on deformation behavior of fine
grained/nc materials consolidated from MMed powders has yet to be fully
comprehended.

An increase in strength of the MMed materials is usually and inherently accompanied
with the sacrifice of ductility. Although mechanical alloying (MA)/MM processes are
effective in grain refinement, defects are introduced inevitably during processing such
as contaminants, oxide particles and residual porosity which are detrimental to the
mechanical properties. The major sources of contamination are (i) the milling media
(balls and vial), (ii) process control agent (PCA) and (iii) atmosphere during
processing. Residual porosity may result from poor interparticle bonding during
consolidation and sintering. Mechanical properties are influenced by microstructural
features including grain size, shape, pores and their distribution, flaws, surface

condition, impurity level, second phase/dopants, stress, duration of its application and
temperature, and crystal defects. To minimize and control the level of defects and
contaminants without sacrificing material properties is the major challenge in this MM
process.

Introduction



4
Unique properties of the nc materials with grain size less than 100 nm have attracted
intensive research in recent years. Both types of the nc materials which comply with
Hall-Patch relationship and those deviate from this relationship (“inverse Hall Petch”)
have been reported in the literatures. A number of investigators have suggested that the
inverse Hall–Petch relation can be attributed to the increased grain-boundary activity
due to grain-boundary sliding and/or diffusional mass transfer via grain-boundary
diffusion. As such, it is essential to understand and investigate the mechanical
behaviors in fine grained materials consolidated from MMed or alloyed powders.
Unfortunately, only limited fundamental information is available in the literature
regarding the microstructural evolution and deformation behaviors of Mg composite
consolidated from MMed powders. A fundamental understanding on bulk Mg
composites via MM at different milling durations will provide the opportunity to
understand how these materials are sensitive to processing parameters and grain size
effect on their deformation behaviors.

Contamination during milling and grain growth during secondary process of
conventional pressureless sintering are the main concerns in this synthesis process.
Normally contamination is more severe with longer milling duration but it can be
controlled to maintain at an acceptable level by shortening the milling duration. It is
important to optimize the processing temperature to avoid excessive grain growth

without sacrificing the good bonding of the particles which in turn affects the
mechanical properties. It is one of the objectives of the present project to find out the
optimum primary and secondary processing parameters which produce the optimum
combination of strength and ductility of the nc Mg composite.

Introduction



5
Grain refinement, reinforcement particles, dislocation and solid solution contribute to
the strengthening of Mg composite consolidated from MMed powders. In nanoscale
grain size region, different deformation behaviors have been reported when the grain
size is reduced below a critical size. Experimental investigation will be carried out to
validate the intrinsic relation between structural evolution after various milling
duration and mechanical properties. Mechanical properties are expected to be
enhanced in this nc Mg composite due to high density of grain boundary with different
atomic structures. Microstructural evolution will be examined using different types of
microscopes such as optical microscope, field emission scanning electron microscope
(FESEM) and transmission electron microscope (TEM). Physical properties such as
thermal properties, electrical resistivity, coefficient of thermal expansion and
mechanical properties such as tensile tests at different strain rates, creep tests at
various temperatures will be included in this study.

1.2 Objectives
The main objectives of the present study are:
1.
Synthesis of AlN composite powder by mechanochemical milling.
2.
Synthesis of Mg-5wt%Al reinforced with 1wt% of in-situ formed AlN via

mechanical milling and power metallurgy techniques.
3.
Study of microstructures and mechanical properties of Mg MMCs at different
ball milling durations.
4.
Investigation of the deformation behavior of Mg-5wt%Al-1wt%AlN systems.



Introduction



6
1.3 Scope of the thesis
This thesis is composed of seven chapters including this chapter of introduction and
the remaining chapters are organized as follows:

Chapter 2 reviews the unique mechanical properties of nc materials relative to their
coarse grained counterparts. Nc materials processed via consolidation of MMed
powders, their mechanical properties and deformation behaviors will be the focus in
the literature review.

Chapter 3 presents the synthesis of AlN powder by mechanochemical process using Al
and pyrazine as the starting materials. The mass structure, morphologies and thermal
properties of the as-milled powder samples were examined after several milling
durations ranging from 20 to 100 hours. Details of AlN morphologies for as-milled and
annealed powders are discussed with the general explanation of growth mechanism.

Chapter 4 reports the processing of the pure nc Mg and nc Mg composite. Comparative

studies on tensile, thermal, electrical properties between the nc pure Mg and the nc Mg
composites at room temperature are presented in this chapter.

Chapter 5 presents the experimental investigation of strain rate effect on nc pure Mg
and nc Mg composites in tension. True stress versus true stain rate curves and TEM
micrographs were used to support the detailed discussion of strain rate sensitivity,
activation volume and strain hardening behaviors.
Introduction



7
Chapter 6 presents the experimental investigation on creep behaviors of the bulk nc
Mg composites at various temperatures to further examine the possible deformation
mechanism.

In the last chapter, Chapter 7, conclusions from present study are presented and
recommendations for further study of deformation mechanism for Mg MMCs are
proposed.

1.4 References
1. FH Froes, YW Kim, S Krishnamurthy, Mat Sci Eng A 117 (1989) 19-32.
2. B Wolf, C Fleck, D Eifler, Int J Fatigue 26 (2004) 1357-1363.
3. M Avedesian, H Baker (ed.), Magnesium and magnesium alloys, ASM
International, Materials Park, OH, USA, 1999, pp. 14.
4. BL Mordike, KU Kainer, Magnesium alloys and their applications, Werkstoff-
Information-sgesellschaft, Frankfurt, Germany, 1998.
5. H Ferkel, BL Mordike, Mater Sci Eng A 298 (2001) 193-199.
6. SF Hassan, M Gupta, Mater Sci Eng A 392 (2005) 163-168.
7. AT Maung, L Lu, MO Lai, Compos Struct 75 (2006) 206-212.

8. SC Tjong, ZY Ma, Mater Sci Eng R 29 (2000) 49-113.
9. EW Kelley, WF Hosford, Trans AIME 242 (1968) 5-13.
10.
ART2003b.pdf.
11. ML Hu, ZS Ji, XY Chen, ZK Zhang, Mater Charact 59 (2008) 385-389.
12. H Watanabe, K Moriwaki, T Mukai, K Ishikawa, M Kohzu, K Higashi, J Mater
Sci 36 (2001) 5007-5011.
13. JS Benjamin, T Volin, Metall Mater Trans B 5 (1974) 1929-1934.
14. C Suryanarayana, Prog Mater Sci 46 (2001) 1-184.
15. K. Kubota, M. Mabuchi, K. Higashi, J Mater Sci 34 (1999) 2255-2262.