Hot Isostatic Pressing (HIP) of Elemental Powder Mixtures and Prealloyed Powder for NiTi Shape Memory Parts

Abstract

we used. If one phase would stay or become disconnected, one would either obtain a loose powder or the etching process would stop after the particles connected to the specimen surface have been leeched out.A section through the nanoporous metal is also shown in Figure 2d. It demonstrates that the porosity is not confined to the surface but penetrates the entire specimen thickness. This result suggests, but does not prove gas permeability. Thus, gas permeability tests using hydrogen were also performed. The set up consists of a vacuum tight holder with inlet and outlet, a vacuum pump, a gas supply and a gas chromatograph. The nanoporous sheets, approximately 6 mm 20 mm wide and 0.1 mm to 0.25 mm thick, were placed in the holder. The whole system was evacuated prior to testing to a pressure of less than 1 Pa. Constant hydrogen pressure was then applied on one side of the porous membrane for 300 seconds and the permeated gas was collected at the other side. The pressure on the inlet side was varied between 20 Pa and 102 kPa. The volume of the recipient was selected such, that the pressure increase at the outlet side was always less than a tenth of the inlet pressure. In other words, the pressure difference was essentially equal to the applied pressure at the inlet. The permeated gas content was then measured by the gas chromatograph.The test results are shown in Figure 3, whereby the flow rate is related to standard conditions. They confirm gas permeability of the nanoporous material and a linear relation between pressure drop and flow rate. At Dp = 102 kPa a column of hydrogen gas, approximately 1 mm high, permeates through a given filter area per second. It should be noted that no attempts to degas the filters prior to testing were made so far. Probably this will further improve permeability as some of the aqueous electrolyte is likely to remain in the porous structure due to capillary forces. Furthermore, it is anticipated that the permeability can be widely varied, as volume fraction and size of the open channels can be influenced by chemical composition of the alloy and processing conditions. We anticipate use of the new nanoporous Ni-alloys in such diverse fields of application as medicine, mechanical engineering and chemical engineering. Filtration of bacteria is one example, where the ability to sterilize the microorganisms by resistance heating of the metallic filter is an added advantage not amenable to ceramic filters or polymer membranes. Solid oxide fuel cells are another example, where nanoporous metals exhibiting gas permeability would be highly beneficial as they would enable thin film deposition of the electrode and membrane materials, thus reducing the electrical resistance of the cell. Nanoporous metals could also aid the development of miniaturized low temperature fuel cells, believed to power laptops and cellular phones in the future, [6] as further miniaturization of the fuel processing system, requiring micro heat exchangers and reactors, becomes possible. This is just a small selection of po...

The influence of Ni powder size, compact composition and sintering profile on the shape memory transformation and tensile behaviour of NiTi

“…A double transformation peak has been reported by a number of researchers to be due to a two-stage transformation involving a metastable R phase [26,35]. However, in those cases the two transform peaks are more separate than observed in Fig.…”

Porous Metal Sheets of NiTi made by Wet Powder Spraying

“…But the formation of second phase such as stable Ni 3 Ti or NiTi 2 which can hardly be suppressed affect the mechanical properties and the phase transformation behavior of the shape memory alloys (SMA's). Starting with pre-alloyed NiTi powders, pressureless sintering will lead to 5 -10 % porosity [5]. In principal, the building of pores by using place holder additives (e. g. salts) as described for titanium powder can also be envisaged for NiTi [6].…”

“…Metal Injection Molding (MIM), a technique related to polymer injection, is commonly used for several metals such as stainless steel or bronze because of high economic efficiency and automation potential [10]. For NiTi, this method has only been described a few times [5,9,11,12]. Only pre-alloyed powder can be used for MIM of NiTi.…”

Laser Processing of Net-Shape NiTi Shape Memory Alloy

“…[7] Considerable research work has been devoted to powder metallurgical (PM) processing of NiTi shapememory alloys during the last two decades with the aim of fabricating near net-shape components. These techniques include solid-state sintering, [8,9,10] hot extrusion, [8] hot pressing, [8,11] hot isostatic pressing, [12,13,14] metal injection molding, [13] field-activated pressureassisted synthesis, [15] mechanical alloying, [16] and selfpropagating high-temperature/combustion synthesis. [17,18] However, there are several important problems associated with those routes.…”