Abstract
High pressure torsion (HPT) processing technology, consisting in the obtainment of (ultra)fine bulk metallic structure during 2–3 complete rotations of the superior anvil at low speed (~10-1 rpm) under high applied pressure (~ GPa) applied on the lower anvil, has been modified as to allow the application of elevated number of rotation numbers (~102 rpm). By high-speed high pressure torsion (HS-HPT), coned-disk spring shape modules were processed from an as cast Fe-28Mn-6Si-5Cr (mass %) shape memory alloy (SMA). Scanning electron microscopy (SEM) and X-ray diffraction (XRD) studies revealed that the modules became nanostructured as an effect of HS-HPT processing. After processing, a hardness gradient was obtained along the truncated cone generator, increasing from inner to outer areas, due to different deformation degrees in these zones. After complete flattening, the measurements revealed that the hardness gradient maintained its value but reversed its variation sense.
Highlights
Amongst intelligent materials, shape memory alloys (SMAs) are characterized by the ability to recover from being deformed, when heated [1]
A special attention has been recently paid to the possibility to obtain nanostructured FeMnSi-based SMAs by means of High Pressure Torsion (HPT), a severe plastic deformation (SPD) procedure which has been modified as to allow the application of elevated number of rotation numbers (~102 rpm) concomitant with high applied pressure (~ GPa) [17]
The global amounts of γ-fcc austenite, ε-hcp martensite and α’-bcc martensite varied as follows: 1. in initial condition 74 % γ, 23 % ε and 3 % α’; 2. in compressed state 77 % % γ, 19 % ε and 4 % α’ 3. in heated condition 72 % γ, 23 % ε and 5 % α’. These results suggest that, at the end of a compression-heating cycle, a small amount of austenite irreversibly transformed to α’-bcc martensite
Summary
Shape memory alloys (SMAs) are characterized by the ability to recover from being deformed, when heated [1]. FeMnSi base Shape Memory Alloys (SMAs) have been developed since the years 1980s [3], as cheap substitutes of Ti-Ni base SMAs [4] and became commercially available under the form of Fe-28 Mn-6 Si5 Cr [5] and Fe-14 Mn-5 Si-9 Cr-5 Ni [6] The present paper aims to further investigate the response of HS-HPT modules during compression cycles, between flat surfaces, both without and with lubrication, and to corroborate the changes in hardness variation along cone generator with accompanying microstructural changes.
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