K.M. Lin

Wear characteristics of ion-nitrided Ti50Ni50 shape memory alloys

Abstract The wear characteristics of Ti 50 Ni 50 shape memory alloy can be improved by ion nitriding. Both TiN and Ti 2 Ni compound layers are produced by 700–800°C ion nitriding. The thickness of TiN/Ti 2 Ni compound layers and the specimens surface hardness increase with increasing nitriding temperature. Main adhesive and minor abrasive wear mechanisms are observed for the TiN/Ti 2 Ni compound layers against a Cr-steel ball. The friction coefficient and weight loss of 700–800°C nitrided Ti 50 Ni 50 alloy are quite low before the TiN/Ti 2 Ni compound layers have been worn out. The thicker TiN/Ti 2 Ni compound layers can sustain higher wear loads and hence have better wear resistance. The mass transfer from TiNi martensite to Cr-steel ball and from Cr-steel ball to TiN/Ti 2 Ni compound layers occurs because of their significant difference in hardness.

The corrosion behavior of Fe-based shape memory alloys

Fe–30Mn–6Si, Fe–30Mn–6Si–5Cr and Fe–13Mn–5Si–12Cr–5Ni shape memory alloys were prepared by a VIM technique. The various corrosion tests were conducted to investigate the corrosion behaviors of these alloys. Experimental results show that in 3.5% NaCl solution, the Fe–13Mn–5Si–12Cr–5Ni alloy had the best chemical corrosion resistance, whereas the Fe–30Mn–6Si–5Cr alloy was locally attacked, forming many corrosion pits after immersion test. In addition, the detachment of the corrosion product covering the Fe–30Mn–6Si alloy caused an abrupt increase in the weight loss. After 2 h of heat treatment at 1000 °C, the corrosion potential of the Fe–30Mn–6Si alloy increased due to the formation of α-ferrite, while the Fe–30Mn–6Si–5Cr alloy became more active. In the stress-corrosion cracking test, the Fe–13Mn–5Si–12Cr–5Ni alloy, having the highest fracture stress in the atmosphere among these alloys, exhibited the largest decrease in fracture stress in the saturated H2S solution due to the existence of α-martensite.

A study of TiNiV ternary shape memory alloys

Abstract The TiNiV ternary shape memory alloys, obtained by equal substitution of V for both Ti and Ni, are investigated focusing on their basic transformation behavior, shape memory effect, pseudoelasticity and wear characteristic. Experimental results reveal that the Ti 49.25− x /2 Ni 50.75− x /2 V x ( x =0–4 at%) alloys exhibit a B2↔B19′ one stage martensitic transformation. The transformation temperatures will drop down about 10°C by adding 1–2 at% V due to the effect of solid-solution strengthening. There appear many (Ti,V) 2 Ni second-phase particles within the matrix of Ti 47.75 Ni 49.25 V 3 and Ti 47.25 Ni 48.75 V 4 alloys. The oxygen atoms in the matrix will be easily absorbed by the (Ti,V) 2 Ni second-phase particles to form the (Ti,V) 4 Ni 2 O oxide. This decreased oxygen content in the matrix will contribute to raise the transformation temperatures for both Ti 47.75 Ni 49.25 V 3 and Ti 47.25 Ni 48.75 V 4 alloys. The shape memory effect and pseudoelasticity of Ti 49.25 Ni 50.75 alloys can be improved by the addition of 1–2 at% V due to solid-solution strengthening. The energy storage efficiency ( E 2 / E 1 + E 2 ) can also be increased by the addition of V. However, the Ti 47.75 Ni 49.25 V 3 and Ti 47.25 Ni 48.75 V 4 alloys exhibit a worse shape memory effect due to the formation of second-phase particles. The wear mechanisms of Ti 49.25− x /2 Ni 50.75− x /2 V x alloys are found to be similar to those of TiNi binary alloys. The Ti 49.25 Ni 50.75 , Ti 48.75 Ni 50.25 V 1 , Ti 48.25 Ni 49.75 V 2 alloys have a better wear resistance than the Ti 47.75 Ni 49.25 V 3 and Ti 47.25 Ni 48.75 V 4 alloys, due to their higher hardness and pseudoelastic behaviors of the B2 structure.

The welding characteristics of Fe–30Mn–6Si and Fe–30Mn–6Si–5Cr shape memory alloys

Abstract After tungsten-inert gas (TIG) and laser welding, the microstructure, shape memory effect and chemical corrosion resistance of Fe–30Mn–6Si and Fe–30Mn–6Si–5Cr shape memory alloys have been investigated. Experimental results show that the dendrite structures in the laser-welded zones are finer than in the TIG-welded zones for both Fe–30Mn–6Si and Fe–30Mn–6Si–5Cr specimens. The Mn content in the TIG-welded zone of Fe–30Mn–6Si specimen is obviously reduced due to its vaporization during welding. The as-welded alloys still exhibit an excellent shape memory effect, with a variation of ±10% due to welding. The corrosion resistance of welded zones is worse than that of base material for both Fe–30Mn–6Si and Fe–30Mn–6Si–5Cr specimens. The degradation of corrosion resistance is ascribed to the micro-segregation and welding stress occurring within the welded zones. Meanwhile, the Cl − is more corrosive than the NO 3 − for Fe–Mn–Si shape memory alloys. After annealing treatment of 1100°C×2 h for these welded alloys, the dendrite structures in the welded zones disappear. The welded-then-annealed Fe–Mn–Si specimens still maintain the excellent shape memory effect, and their corrosion resistance is improved due to the annihilation of micro-segregation and welding stress.

A study of Fe-30Mn-6Si shape memory alloys prepared from different melting techniques

Newly developed shape memory alloys Ge-Mn-Si, governed by the stress-induced {gamma} {yields} {epsilon} martensitic transformation, are attracting attention due to their low cost and excellent workability. The shape memory effect (SME), although only a one way type, can be nearly perfect for alloys containing suitable amounts of manganese and silicon. However, there are few papers reporting the effect of the melting process, although it is an important factor affecting the material properties. Therefore, in the present study, the authors aim to investigate the casting morphology, shape memory effect, tensile property and fracture behavior for the Fe-30Mn-6Si alloys which are prepared by two melting techniques.

An investigation of grain-boundary phase in Fe–30Mn–6Si–5Cr shape memory alloy

Abstract The formation of the grain-boundary phase in the Fe–30Mn–6Si–5Cr shape memory alloy and how it affected the shape memory effect were investigated. The grain-boundary phase in the Fe–30Mn–6Si–5Cr shape memory alloy formed during annealing at temperatures ranging from 600 to 750°C, especially at 700°C. Conversely, the grain-boundary phase was not observed at annealing temperatures exceeding 800°C or lower than 550°C. The grain-boundary phase exhibited an ordered BCC structure with a lattice parameter of about 8.798 A, and had a similar chemical composition as that of the matrix. The shape memory effect and tensile property of the Fe–30Mn–6Si–5Cr alloy monotonously degraded with increasing density of the grain-boundary phase.

An investigation of martensitic transformation in an Fe-30Mn-6Si shape memory alloy

Fe-Mn-Si compositions represent newly developed shape memory alloys which are attracting attention due to their low cost and excellent workability. To obtain a shape memory effect, one must precisely control the Ms temperature and the Neel temperature (T{sub N}). In this study, the authors examine differential scanning calorimetry (DSC), magnetic susceptibility, internal friction, and electrical resistivity measurements as ways of determining Ms and T{sub N} in an Fe-30Mn-6Si alloy. The efficiency of these experiments are also discussed and compared.

Effects of annealing and NaOH pretreatment on an LmNi3.65Al0.34Mn0.27Co0.74 hydrogen storage alloy

Hydrogen storage alloys based on mischmetal (Lm=La-rich) are commercially used as negative electrode materials for Ni–MH batteries. In this paper, the effects of annealing and NaOH treatment on the LmNi3.65Al0.34Mn0.27Co0.74 alloys are investigated. Experimental results reveal that the annealing of the LmNi3.65Al0.34Mn0.27Co0.74 alloy will reduce the pulverization and raise the resistance of volume expansion during hydrogen absorption, and hence improve the alloy’s cycle stability and electrochemical properties. The NaOH treatment of the LmNi3.65Al0.34Mn0.27Co0.74 alloy produces a porous surface with enriched Ni, Co and Na atoms, which will enhance the electrocatalytic activity and inhibit the oxidation and pulverization of the alloy electrode. Hence, the NaOH treated electrode can exhibit an improved charge/discharge efficiency in Ni–MH batteries.

Characterization of internal friction of Fe-30Mn-6Si-5Cr

Fe-Mn-Si iron-based shape memory alloys have attracted much attention due to their low cost and excellent workability. Owing to its high sensitivity on constitutional structure, internal friction measurement has been used to investigate the martensitic transformations for thermoelastic alloys and ferrous alloys. In their previous paper, the internal friction characteristics due to {gamma}{yields}{epsilon} martensitic transformation have been examined by simple heating and cooling cycles. In the present study, the characterization of internal friction with the change in deformation states on an Fe-30Mn-6Si-5Cr shape memory alloy is investigated. In addition, the effect of thermo-mechanical training on the SME of this allow is also discussed.

SCREW DISLOCATIONS AROUND A RADIAL SURFACE CRACK IN A CIRCULAR BAR

Using the conformal mapping technique, the behavior of a screw dislocation near a radial surface crack in a circular bar was analyzed in detail. It was found that there exists a zero-force point ahead of the crack tip, and its location is closely related to the crack length. The region containing this point can form a plastic zone by accumulating dislocations during plastic deformation. For a certain radius R of the circular bar, both the strain energy of the dislocation and the stress intensity factor at the crack tip increase rapidly in magnitude with increasing crack length (R − a) as (R − a)/R ⪡ 1. When the crack gets longer, they become nearly independent of (R − a). The shielding effect on the crack tip induced by the dislocation is much smaller for a short crack than for a long crack, and it vanishes as the crack length approaches zero. Consequently, during crack propagation, brittle fracture will occur for a surface microcrack, but it becomes relatively ductile for a macrocrack. In addition, the strain energy and the shielding effect are significantly affected by R if (R − a) remains constant. They increase rapidly with increasing R for (R − a) < R < 5(R − a) and approach an upper limit respectively as R becomes infinity.