Syed Ansar Tofail

Directly created electrostatic micro-domains on hydroxyapatite: probing with a Kelvin Force probe and a protein

Micro-domains of modified surface potential (SP) were created on hydroxyapatite films by direct patterning by mid-energy focused electron beam, typically available as a microprobe of Scanning Electron Microscopes. The SP distribution of these patterns has been studied on sub-micrometer scale by the Kelvin Probe Force Microscopy method as well as lysozyme adsorption. Since the lysozyme is positively charged at physiological pH, it allows us to track positively and negatively charged areas of the SP patterns. Distribution of the adsorbed proteins over the domains was in good agreement with the observed SP patterns.

Probing Martensitic Transition in Nitinol Wire: A Comparison of X‐ray Diffraction and Other Techniques

Martensitic to austenite transformation in Nitinol wire can be measured by a number of techniques such as XRD (X‐Ray Diffraction), DSC (Differential Scanning Calorimetry), BFR (Bend and Free Recovery) and Vickers indentation recovery. A comparison of results from these varied characterisation techniques is reported here to obtain a greater understanding of the thermal‐elastic‐structural changes associated with martensitic transformation. The transformation temperatures measured by DSC were found to correspond well with the structural and mechanical information obtained from XRD, BFR and Vickers indent recovery methods. Indent recovery is a relatively new and accurate method of monitoring stress induced martensitic transformations in NiTi and is one of only a few methods of stress inducing martensitic transformation in large scale samples. It is especially useful for NiTi in the as‐cast billet form, where tensile testing is impossible. BFR is uniquely popular in the NiTi wire manufacturing sector and is recognised as the most accurate method of measuring the transformation temperature. Here the material is stressed to a representative in‐service stress level during the test. No other test uses the shape memory effect for measuring the transformation temperature of NiTi. The results show that the DSC thermogram and XRD diffractogram have a peak overlap which is a common occurrence in NiTi that has been extensively processed. The XRD method further explains the observations in the DSC thermogram and in combination they confirm the transformation temperature.

3.21 Powder Metallurgical Processing of NiTi Using Spark Plasma Sintering

Shape memory alloys were generally produced by casting but powder metallurgy has fast become an alternative method for producing shape memory alloys, specifically NiTi. Powder metallurgy became a popular method due to its ability to avoid the problems of segregation and excessive grain growth associated with casting and it allows for better control over the composition of NiTi alloy and can therefore control the transformation temperature and in turn the alloys unique shape memory and superelastic properties. Spark plasma sintering has emerged as the most efficient powder metallurgical method of melting binary and ternary NiTi.