Mechanical alloying of an equiatomic mixture of crystalline Ti and Pt elemental powders in a high-energy ball mill formed an amorphous alloy by solid-state reactions. At an intermediate stage of mechanical alloying, scanning electron microscopy showed the formation of characteristic layered structures of inhomogeneous composition within the powder particles. X-ray diffraction analysis showed the gradual disappearance of crystalline Bragg peaks and the emergence of broad amorphous maxima as milling progressed.
The amorphization process was completed after hours. The amorphous state of the product indicated that temperatures during processing did not exceed the crystallization temperature of the alloy. Keywords: TiPt alloy, mechanical alloying, amorphization, solid-state reactions. Shape memory alloys SMAs undergo a reversible martensite transformation, which drives the thermoelastic properties of shape memory and pseudoelasticity Otsuka and Ren, The Ti at.
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The pseudoelasticity and shape memory properties of the Ti at. While the addition of certain alloying elements can slightly raise the M S of Ti-Ni alloys Firstov et al. Alloying elements can then be used to reduce the MS according to specific applications and to improve ductility. Reducing the MS also lowers the effect of diffusion-controlled processes such as recrystallization, recovery, and phase separation, all of which are known to be detrimental to the reversible martensite transformation. Conventional methods of forming the TiPt alloy usually involve plasma arc, electron beam, or vacuum induction melting, followed by a suitable thermomechanical treatment to homogenize and age the alloy.
The current work explores mechanical alloying MA as an alternative means of producing the alloy. MA is a process of alloy formation from a mixture of powder particles in a high-energy ball mill Soni, Chemical homogenization proceeds through a series of ball-powder-ball collisions Figure 1 , resulting in the repeated flattening, fracturing, and mutual cold welding of the powder particles coupled with short-range diffusion.
This leads to the formation of a particulate alloy material suitable for consolidation and further processing. MA of crystalline powder mixtures of two transition metals often results in the formation of amorphous alloys Koch et al. It is generally accepted that this is due to solid-state amorphization reactions SSARs , driven by a a large negative heat of mixing in the amorphous state, and b one element in the powder mixture having anomalously fast diffusivity in the other Johnson, These conditions ensure the availability of a sufficient thermodynamic driving force for the amorphization reaction, and that the amorphous phase forms at a reasonable rate, faster than the competing crystalline equilibrium phase s.
The free energy state of such a process can be generally illustrated by the schematic free energy diagram Figure 2 , where it is assumed that a fully homogenous amorphous alloy with composition X is formed from a starting crystalline powder mixture of A and B. The amorphous MA product formed by SSAR is a metastable phase that requires devitrification to form an equilibrium crystalline phase suitable for the envisaged shape memory applications.
As Figure 2 suggests, such alloys typically have no thermodynamic barrier against crystallization of the amorphous phase, only a kinetic barrier that suppresses the long-range atomic diffusion required for crystallization Johnson, The amorphous phase will therefore readily undergo crystallization when heated above the crystallization temperature T x , where enough activation energy becomes available to overcome the kinetic barrier against crystallization.
Compared to conventional melting methods, MA offers the advantages of solid-state processing and better composition control, with the possibility of directly forming near net shape parts that require minimum machining and metal loss to produce the final part dimensions. The disadvantages of MA include easy oxidation of powder particles due to the large activated surface area involved, and the formation of a nanocrystalline parent austenite phase.
Grain refinement is known to suppress the martensite transformation Waitz et al. This is due to the increasing difficulty of accommodating the martensite transformation shape strain in the nanocrystalline austenite parent phase. In the Ti-Pt system, however, Ti at. For such alloys, the amorphous phase can be directly crystallized into the martensite phase with no prior austenite phase, as demonstrated for the Zr-Cu-Ni system Firstov et al. The possibility of directly crystallizing BTiPt martensite from an amorphous phase removes the limitations that would otherwise be associated with martensite formation from a nanocrystalline B2-austenite parent phase.
Experimental methods. Mechanical alloying. Elemental powders of commercially pure Ti and Pt were mixed in a atomic ratio. The Ti particles were spherical and the Pt particles were spongy and irregular Figure 3.
The charge, composed of powder and milling balls, was loaded into the jar and the jar was sealed in an Ar-filled glove box. The MA parameters are shown in Table I. The jar was periodically discharged to minimize the cold welding of powder to the milling equipment. The steady state external temperature of the jar did not exceed Cooling the jar increases the transfer of heat away from the contents, promoting brittle particle fracture over ductile coating of the jar walls, balls, and rotor.
The major contaminants were Fe, Cr, and Mo, corresponding to the hardened steel composition of the mill lining and rotor. Hence the contamination was attributed to wear of milling equipment. Structural characterization. Where applicable, the resulting diffraction patterns were analysed for average domain size and internal strain using the Scherrer method Suryanarayana and Norton, Morphology of powder. During the first few hours of MA, the powder displayed a strong tendency to cold weld to the balls and walls of the milling jar due to the ductility of the powder.
While the formation of a thin coating on milling equipment can help prevent excessive wear and minimize powder contamination, the authors have previously observed that not dislodging the welded particles regularly to reintroduce the powder back into the MA process as free-flowing powder will result in a heterogeneous final product. As MA progressed, plastic deformation strongly reduced the ductility of the powder particles, until cold welding to the milling equipment was no longer observed.
SEM micrographs of sectioned particles at different stages of the alloying process after MA for 4 hours are shown in Figure 5. EDX analysis showed that the dark grey regions are pure Ti and the lighter grey regions have a composition of at. The repeated impact experienced by the powder particles when trapped between colliding balls causes plastic deformations and flattening.
Mutual cold welding of these flattened particles result in the lamellar structure of Figure 5 a. A similar layered structure at an intermediate stage of alloying of ductile metal powders was also reported by Benjamin and Volin The repeated fragmentation and cold welding of the powder particles refined the lamellar structure, resulting in a random orientation of the lamellae within the particles as seen in Figure 5 b. Continued structural refinement resulted in homogenization and disappearance of the layered structure, with the Pt-rich layers in Figure 5 c being the last to disappear.
Compositional Analysis of Ni-Zr Powder during Amorphization by Mechanical Alloying - IOPscience
Beyond 4 hours of MA, all particles displayed a homogenous structure similar to Figure 5 d with a near equiatomic composition. Crystallinity of powder. Figure 6 shows a series of selected XRD patterns of Ti at. The pattern at zero hours is a superposition of hcp Ti and fcc Pt reflection peaks, showing that the starting material was a mixture of elemental crystalline powders of Ti and Pt. After 4 hours, all Ti reflection peaks have disappeared and only broad low- intensity Pt reflections can be identified. At the same time, there is an emergence of the broad maxima characteristic of amorphous alloys.
Hence, no Ti reflections appear on the 4-hour scans. The average crystallite size is reduced and the lattice strain is increased from 13 nm 0. For MA of 12 hours and beyond, all Bragg reflections had disappeared, indicating completion of the amorphization process. The MA process was further studied by TEM for the time intervals corresponding to partial amorphization 4 hours' MA , fully amorphous alloy 12 hours , and heavily contaminated amorphous alloy 32 hours.
This is reflected by the fine lattice fringes on the high-magnification image arrow and diffraction spots on the SAD. The lack of diffraction spots and lattice fringes on the powder after 12 hours Figure 8a and b shows that the powder is now fully amorphous, in agreement with the XRD results.
After 32 hours Figure 9a and b , SAD patterns show traces of crystalline diffraction spots in the amorphous matrix. The crystalline spots can be due to either: a crystallization of the previously fully amorphous powder when the powder particles trapped between colliding balls experience a momentary temperature rise exceeding the T x of the alloy as Figure 2 suggests , or b the introduction of crystalline contaminant phases into the amorphous alloy.
The temperature rise from ball impacts during MA has been estimated to be only a few hundred degrees Joarder et al.
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The crystallites can therefore be attributed to contam- inants from wear of milling equipment, as detected by the low-resolution analytical techniques. This indicates that to avoid contamination, MA should not be carried out for unnecessarily long time intervals, such as beyond the point of complete amorphization. The formation of amorphous Ti at.
Similar results of amorphous phase formation by MA from elemental powder mixtures have been reported for the closely related TiNi Schwarz et al. The deformation, fracturing, and cold welding of powder particles is illustrated by Figure 4 a , b , where the formerly spherical and spongy particles had formed agglomerates, and in some cases had become flattened and fissured.
These structural changes also resulted in a reduction in crystallite size and an increase in lattice strain. The characteristic layered structure shown in Figure 5 , which precedes the appearance of the homogenous structure, indicates that the alloying process proceeded by SSAR. The layered structure was due to the preferential cold welding of newly created clean Ti and Pt boundaries formed by particle fracture.
It has been suggested that conditions at the layer interfaces during an intermediate stage of MA of two metals closely resemble those within conventional thin-film diffusion couples Johnson, , leading to alloy formation by solid-state interfacial interdiffusion reactions. According to the classification of Weeber and Bakker , SSAR of two metals can proceed along three different paths, depending on the alloy system and experimental conditions.
The SSAR alloying mechanism observed in the current work is different from the observation made by Maweja et. It is suggested that the different MA product phases were due to differences in milling parameters, such as the effect of using a process control agent PCA to control the cold welding of powder particles to milling equipment. A PCA was not used in the current work to avoid contamination and to optimize alloy formation. Besides improving the amount of free-flowing powder in the milling jar, PCAs have been shown to have a generally negative effect on the alloying process Machio et al.
Similar inconsistencies in the crystallinity of the milled product have been observed in the MA of other alloy systems such as Ti-Ni, where formation of solid solutions Mousavi et al. Determination of the relative thermodynamic stability of the fcc Pt Ti solid solution reported by Maweja et. While it is currently possible to measure the process temperature during operation of the high-energy ball mill only with a thermocouple attached externally to the wall of the milling jar, some general deductions can be made about thermal conditions inside the jar during operation.