Shinichiro Komatsu

Fabrication of TiNi Powder by Mechanical Alloying and Its Shape Memory Characteristics of Sintered Alloy

This paper presents the fabrication condition of the TiNi alloy powder by mechanical alloying and its shape memory characteristics of the sintered alloy. The effect of mechanical alloying condition on the characteristics of mechanically alloyed powder (MA powder) was investigated. Also, the difference in sintering behavior between the MA powder and the mixed elemental powders by V-blender and the microstructure, shape memory characteristics of the sintered alloys were also examined. The MA powder was fabricated by milling using a planetary ball mill in rotational speed from 200 rpm to 500 rpm for various milling times in an atmosphere of Ar gas. These two types of powders prepared in different processes were sintered by a pulse-current pressure sintering equipment at various sintering temperatures. The powder agglomerated and its particle size became larger with an increase in milling time. The mixture of Ti and Ni powders changed into an amorphous state by processing for 3.6 ks over 300 rpm. The alloy sintered by the MA powder showed more uniform phase of TiNi than that by the mixed elemental powders sintered in a same manner, however, the former showed a lower density than the later due to a larger particle size of its MA powder of before-sintering.

Shape Memory Characteristics of Ti-Ni Alloy Using Mechanically-Alloyed Powder

In this research, we investigated the fabrication conditions of Ti-Ni alloy powder by MA process and the shape memory characteristics of the sintered alloy by a pulse-current sintering equipment using mechanically alloyed powder (MA powder). The microstructure of the sintered alloy of the MA powder was more homogeneous than that of the alloy of the elemental powders. The application of the MA powder makes the width between transformation temperatures of the shape memory treated alloy of the MA powder became narrower, that is, it improves the temperature response of the compact. The tensile strength and elongation of the shape memory compact of the MA powder were approximately 780 MPa and 7.5 %, respectively. This is one of the superior tensile properties of SMA compact fabricated by powder metallurgy process. The superelastic behavior took place in the alloy of the MA powder. Thus, the MA process in short process time resulted in lower contamination of the MA powder and the application of the MA powder resulted in superior shape memory characteristics of the sintered alloy.

Influence of water embrittlement effect on mechanical properties of ADI

Water embrittlement in ADI has been studied experimentally for several loading conditions. The phenomenon has been explained on the basis of a hypothesis of hydrogen embrittlement. In addition to conventional tensile specimens, hole-edged CT specimens that can be used to measure the bending load were used. It was found that the latter were more susceptible to water embrittlement than the former, at the maximum tensile load. Some specimens were pre-strained in dry conditions before tensile testing in wet conditions. Specimens that had been pre-strained to values beyond the non-prestrained water rupture point exhibited very similar values in their rupture strain and rupture stress, regardless of the pre-strain value. A hydrogen embrittlement hypothesis is proposed to explain the experimental fact that the water embrittlement effect is observed for ADI but not in the case of a ferrite matrix.

Influence of Cyclic Deformation on Shape Memory Characteristics of Ti-Ni-Cu Alloys Fabricated by Pulse-Current Pressure Sintering Method

Ti-Ni-Cu shape memory alloy by elemental powders was fabricated by using a pulse-current pressure sintering method, and it was subjected to solid-solution heat treatment after sintering to homogenize the microstructure of the alloy. The influence of the thermo-mechanical cyclic deformation on the shape memory and thermo-mechanical characteristics of Ti-Ni-Cu alloy was investigated. The thermo-mechanical cyclic deformation improved the deformation behavior of the sintered alloy and made the thermo-mechanical behavior of the alloy stable caused by the introduction of dislocations. As a result, the recovery stress of the alloy after the cyclic deformation fairly increased. The transformation temperatures of the alloy after the cyclic deformation became lower than those of the alloy before the cyclic deformation. Therefore, the superelastic-like behavior of the alloy after the cyclic deformation appeared in an isothermal tensile test at lower holding temperature.

Low thermal-expansion gray cast iron strengthened by crystallizing dispersed carbide particles

Low thermal-expansion gray cast iron with high strength and hardness was produced by dispersing carbide particles by means of alloying carbide-stabilizing elements such as Nb, Cr, W, V, Ti and Mo up to 5 at.%. Increasing the alloying content of each element results in increasing the area fraction of carbide and decreasing in that of the graphite fraction, and it results in remarkable increasing in hardness. Tensile strength increases remarkably with increasing in alloying content of Cr or V, but it becomes maximum at some 1 at.% in the case of W or Nb. The increasing in tensile strength and hardness was discussed from such viewpoints as increasing in area fraction of carbide and increasing in effective sectional area ratio resulted from decreasing in area fraction of graphite. The coefficient of mean linear thermal-expansion at 373K increases largely with increasing alloying contents in the case of Cr, W or Mo, but it increases only slightly in the case of Ti, V or Nb.

Accuracy Improving Methods in Estimation of Graphite Nodularity of Ductile Cast Iron by Measurement of Ultrasonic Velocity

Ductile cast iron is one of the very useful and economical engineering materials and it is of‐ ten used as the material for the members that are required good mechanical properties such as high tensile strength and high elongation. The microstructure of ductile cast iron basically consists of two kinds of basic components: the metallic matrix and many spheroidal graph‐ ite nodules dispersed among the matrix. As the bonding strength at the boundaries between the matrix and the graphite nodules is considered to be very little or nothing at all and also as the mechanical properties of the graphite nodules themselves are considered to be much less than those of the matrix, the tensile properties of ductile cast iron are considered to de‐ pend mostly upon the two kinds of conditions of the matrix. One of the conditions of the matrix is its microstructure (such as ferrite, pearlite and others), and the other one is the graphite nodularity which determines the continuity condition of the matrix. The latter is usually expressed by a kind of comparison number called “graphite nodularity” or “graph‐ ite spheroidizing ratio” which indicates how much the outer shapes of the graphite nodules are close to those of perfect spheres.

Thermo-Mechanical Characteristics of Ti-Ni-Cu Shape Memory Alloy Fabricated by Pulse-Current Pressure Sintering Method

The Ti-Ni-Cu shape memory alloy by elemental powders was fabricated by means of a pulse-current pressure sintering method that can produce high-density sintered compacts in a very short sintering time. The fabrication conditions of Ti-Ni-Cu alloy and the influence of Cu content in Ti-Ni-Cu alloy on the tensile properties and thermo-mechanical characteristics were investigated. The relative density of the as-sintered compacts was around 97% at any Cu content. The microstructure, tensile properties and thermo-mechanical characteristics of the as-sintered compacts were improved greatly by performing a solid solution heat-treatment. The yielding behavior due to the stress-induced martensite in stress-strain curves of the alloy took place after elastic deformation at any Cu content. The deformation resistance of the alloys changed with Cu content, and it was lowest around 9at% in Cu content. The tensile strength and elongation of the alloy with Cu content around 9at% were more than 400 MPa and 6%, respectively. It was found that the thermo-mechanical behavior of the alloy becomes stable by performing cyclic deformation.