Shape memory alloys (SMAs) with high transformation temperatures can enable simplifications and improvements in operating efficiency of many mechanical components designed to operate at tem...

High temperature shape memory alloys

Through single‐crystal x‐ray diffraction methods, the crystal structure of TiNi has been determined in the temperature range −70° to 900°C. Contrary to what has been assumed from previous work based on the powder pattern methods, the TiNi crystal structure is not a simple CsCl type. Rather, it has an a0=9A superlattice and an a0=3A sublattice with 54 atoms per unit cell complex structure.The 9A superlattice undergoes, at about 166°C, a ``martensitic'' pseudo order‐disorder transition which is not accompanied by a crystallographic transformation. Through the understanding of this unique transition the apparent contradicting observations made on TiNi by various past investigators can now be reconciled and the unusual physical properties associated with the alloy are explained qualitatively.

Crystal Structure and a Unique ``Martensitic'' Transition of TiNi

Martensite and equilibrium phases in CuZn and CuZnAl alloys

The general mechanism of martensitic nucleation by faulting from groups of existing dislocations, as proposed in Part I, is applied to the fcc → bcc, bcc → fcc, bcc → hcp, and related transformations, including mechanical twinning. Where thermodynamic data are available, the conditions at the observedMs temperatures are consistent with nucleation from a defect composed of four or five properly spaced lattice dislocations. Examples of nucleation by faulting on the planes predicted are found in published electron microscopy. The faults are observed at the types of sites where the required dislocation groups are expected. These include grain boundaries, incoherent twin boundaries, and inclusion particle interfaces. Having defined the function of a nucleation site, mechanisms of strain induced nucleation and autocatalysis are then considered.

A general mechanism of martensitic nucleation: Part II. FCC → BCC and other martensitic transformations

The properties of linear defects in crystals (dislocation lines) and of planar defects (stacking faults) are important in almost every branch of solid state physics. This article is intended to give a comprehensive introduction to dislocation theory for the physicist who is not a specialist in crystal plasticity. It begins with a survey of the established theory relating to the geometrical and topological properties of dislocations, the elastic theory of dislocations in a continuum, the atomistic or core properties of dislocations, and the dynamics of moving dislocations. General methods for finding the elastic field of an arbitrary dislocation loop are outlined, and the results of some recent calculations using anisotropic elasticity are summarized. These include the prediction, partially confirmed by experiment, that dislocations in certain ranges of orientation may have negative line tension. The current importance of atomistic calculations of core structure and related problems is emphasized, and the methods available for these calculations are discussed. More detailed descriptions of dislocation and stacking fault configurations are given for some of the common crystal structures, and recent work on complex defects resulting from vacancy aggregation in close-packed structures is included. The experimental and theoretical evidence for the recent conclusion that screw dislocations in body-centred cubic metals have an asymmetric core is also reviewed. The rather controversial theory of thermally activated dislocation motion is described in a separate section, and examples are given of the application of this theory to various models of the obstacles encountered by moving dislocations. The final section is concerned with the theory of dislocations in grain boundaries and interphase boundaries, and includes the concept of the surface dislocation tensor.

https://iopscience.iop.org/article/10.1088/0034-4885/33/1/307/pdf

H. Warlimont

Papers

The electron-metallography and crystallography of copper-aluminum martensites

The isothermal ordering of a β copper-aluminum alloy