Effect of Nitrogen on Shape Memory Behaviour of FE-MN-SI-CR-NI Alloys
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ABSTRACT The composition of an Fe-Mn-Si-Cr-Ni stainless steel with shape memory effect was altered in this work in order to increase the strength of the alloy. The alloy possessed a low yield strength which is a major draw back for structural applications. Nitrogen alloying, using nitrogen pressurized melting (P=1-10 atm), was employed to introduce a nitrogen concentration of up to 0.36 wt%. The effect of nitrogen alloying on shape memory effect was studied through mechanical testing. It was found that nitrogen alloying increased the hardness; however, nitrogen as an interstitial alloying element suppressed the 7=> transformation and therefore decreased the shape memory effect. Introducing small amount of Nb (e. g., 0.36 wt%) to the nitrogen containing alloys caused formation of NbN. The NbN compound was in the form of globular dispersed particles (200 nm) which increased the strength of the alloy without significantly changing the shape memory effect. INTRODUCTION A weak shape memory effect in an Fe-Mn alloy was reported for the first time in 1975 (1). Upon adding Si to an Fe-Mn binary alloy Sato et. al., (2) obtained a perfect shape memory effect (SME) in single crystalline material. An optimum range of Mn and Si concentrations were determined by Murakami et. al., (3) in order to have a good shape memory effect in Fe-Mn-Si alloys. This range was 28-34% of Mn and 4-6.5% of Si. The maximum shape memory effect which has been reported for polycrystalline Fe-Mn-Si alloys is only 2-4% (4). However, the lower price of these alloys compared to Ni-Ti and better corrosion resistance compared to Cu-based alloys has made them worthwhile for further investigation. Since certain application requires high corrosion resistance (such as fasteners and couplings) Otsuka et. al., (5) studied the effect of Cr and Ni addition on Fe-Mn-Si- based alloys. They found that alloys with the composition of Fe-28%Mn-6%Si-5%Cr, Fe20%Mn-5%Si-8%Cr, and Fe-16%Mn-5%Si-12%Cr-5%Ni (wt%) have good corrosion resistance without showing any degradation in SME. Improvement of corrosion resistant Fe-Mn-Si-based alloys has provided a great potential for employing these alloys in structural applications such as tendon rods in prestressed concrete and armour materials. The main disadvantage of the existing alloys is their low strength. The objective of this work involves altering the composition of
357 Mat. Res. Soc. Symp. Proc. Vol. 459 01997 Materials Research Society
existing Fe-16%Mn-6%Si-9%Cr-6%Ni (wt%) shape memory alloy in order to improve the strength of this alloys without degrading the shape memory effect. Nitrogen as an interstitially dissolved element is known to improve the strength of steels. The strengthening effect of nitrogen is explained by Pickering's equation (6): a'yield=l
5.4(4.4+23C+1.3Si+0.24Cr+0.94Mo+1.2V+0.29W 2
+2.6Nb+1.7Ti+0.82A1+32N+0.166 ferrite+0.46/d"/ where d is the average grain size. Corrosion studies on nitrogen containing steels also imply the benefits of presence of nitrogen (7-8). These studies show that nitrogen co
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