The high chemical reactivity and low thermal conductivity of titanium-alloys are the main reasons for not using PCD tools in machining of Ti-alloys. The aim of this recent research study on three different PCD grades involving tool life and wear development is to understand the role of characteristic PCD properties (i.e. grain size, its distribution and cobalt content) in machining of β-titanium alloy (Ti5Al5V5Mo3Cr). The results show a clear relationship between the characteristic PCD properties and the development of crater wear while abrasive wear does not play a dominant role in the evaluation of tool performance. Within a specific cutting data window PCD tools can contribute to improved machinability of Ti5Al5V5Mo3Cr compared to commonly used carbide tools. Different advanced analysis methods such as scanning electron microscopy (SEM) and (scanning) transmission electron microscopy ((S)TEM) in combination with energy dispersive X-ray spectroscopy (EDXS) and electron energy loss spectroscopy (EELS) have been applied to understand the basic wear mechanisms on the atomic scale. The performance results clearly show that the design of the PCD grade is one of the main factors for improving performance of the diamond tool in terms of wear development and tool life. Based on the presented results the crater wear development can be explained by atomistic processes.

Cemented carbides with sub-micron grain size have increased the need to restrict grain growth during sintering. Commonly used inhibitors like V, Ti, and Cr have been observed to form interface layers in the interfaces between WC grains and the Co binder. Atomistic modeling has predicted the composition and thickness of the interface layers. Earlier, the interface layers have been characterized qualitatively using high resolution transmission electron microscopy (TEM). To get more information about the structure and composition of the interface layers in a Ti containing cemented carbide in this work, Z contrast imaging and spectroscopy using scanning transmission electron microscopy (STEM) have been combined. Elemental maps revealing the structure of the interface layers will be presented.

In order to gain a better understanding of the WC-Co sintering process, the change in Sigma = 2 grain boundaries was studied in two series of alloys with different carbon contents and increasing Co content in the 10-50 vol% range. The samples were sintered at 1410 degrees C for I h and 5 h. The frequency of Sigma = 2 grain boundaries and the rotation angle distribution were determined using electron backscatter diffraction (EBSD) in order to make a statistical analysis of the effect of composition and sintering time on the special boundary characteristics. Complementary TEM observations were conducted for a more thorough determination of the rotation aspects (angle/axis). TThe rotation angle is scattered in the WC powder with a maximum peak close to 90, as expected for Sigma = 2 grain boundaries. However, the average rotation angle tends towards 88.7 degrees in sintered alloys. This deviation is explained by a dislocation array compensating for the misfit and angular deviation in the grain boundary. Its characteristics are calculated using the 0-lattice approach and the associated elastic energy is evaluated from these data. The observed angular deviation is consistent with a minimum elastic energy of the boundary. Particle rearrangement, which can occur during liquid phase sintering, might explain the slight rotation of adjacent crystals in the sintered materials, in order to find a lower energy configuration. A mechanism involving stacking faults in successive prismatic planes is also proposed to explain the formation of Sigma = 2 boundaries and their high frequency in thepowder. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

A systematic characterisation of the boundary structure in WC-Co cemented carbides was made as a function of the cobalt content and carbon potential using electron backscattered diffraction technique, with the aim to analyse the mechanisms of WC/Co phase boundary and WC/WC grain boundary development. The fraction of grain boundaries with basal or prismatic habit planes was estimated using a stereological method. Special grain boundaries were classified as a function of the rotation axis between adjacent grains. Although a few special grain boundaries originate from the powder, most grain boundaries form during heat treatment. The development of grain boundaries and phase boundaries during sintering is discussed from the results. The surprising lack of variation in the distribution of boundary types over large changes in volume fraction and composition is particularly commented. Finally, a mechanism of cooperative migration of grain boundaries and phase boundaries is proposed to explain microstructural observations associated with grain growth.

In this work, the effects of cobalt content and C/W ratio on grain growth are studied, in order to better understand the mechanisms. Powder compacts were sintered at 1410°C for 1 and 5 h. The relative contribution of carbide dissolution in the binder and grain growth for the evolution of the size distribution is discussed. Grain size measurements were made with the Electron BackScattered Diffraction technique. The distribution of 2D equivalent disk diameters was converted in a distribution of 3D equivalent sphere diameters, with the Saltykov method. High cobalt content favors grain growth in W‐rich samples whereas it has a slight influence on C‐rich samples. But still grain growth is significantly faster in C‐rich samples. Variation in the D90/D10 ratio indicates a tendency to abnormal growth, with a broadening of the size distribution between 1 and 5 h, favored by high binder content.

One of the most challenging issues in the characterization of magnetic materials is to obtain a quantitative analysis on the nanometer scale. Here we describe how electron magnetic circular dichroism (EMCD) measurements using the transmission electron microscope can be used for that purpose, utilizing reciprocal space maps. Applying the EMCD sum rules, an orbital to spin moment ratio of mL/mS=0.08+/-0.01 is obtained for Fe, which is consistent with the commonly accepted value. Hence, we establish EMCD as a quantitative element-specific technique for magnetic studies, using a widely available instrument with superior spatial resolution.

Physical vapor deposition coatings for cutting tools may be deposited by, e.g. reactive magnetron sputtering. Alumina growth in Ar/O2 gas mixtures gives rise to problems due to insulating layers on targets, and hysteresis effects with respect to oxygen gas flow. In this paper is described a technology for the deposition of crystalline alumina: reactive high power impulse magnetron sputtering. Pure Al was used as target material, and the cemented carbide (WC/Co) substrates were kept at 500–650°C. Hysteresis effects with respect to oxygen gas flow were alleviated, which enabled stable growth at a high deposition rate. The high power impulses were helpful in obtaining a crystalline oxide coating. X-ray diffraction and cross-section transmission electron microscopy showed that α-alumina films were formed. Technological testing of these PVD alumina coatings, with state-of-the-art AlTiN as benchmark, showed significantly improved crater wear resistance in steel turning.

The transfer of work material to the tool surface limits the tool life in many forming operations. Using a dedicated load-scanning test equipment with crossed-cylinder geometry, dry forming of austenitic stainless steel was simulated by provoking adhesion to the TiN-coated tool specimen. High-resolution electron microscopy combined with analytical techniques was used to examine the interface between tool and work material. The decisive mechanism for adhesion and transfer of steel to the TiN surface is suggested. The oxide layer on the steel surface, especially the Fe-oxide, initiates the metal transfer. The interfacial oxide acts as a glue between stainless steel and TiN and increases the adhesive forces. Obviously, the adhesion and internal strength of the oxide layer is far stronger than anticipated. It may even be stronger than the bonding to the austenitic steel itself. A consequence of these findings is that the development of galling resistance-forming tool materials and coatings for austenitic stainless steels should not only aim to improve the bulk tool material, but also to reduce the adhesion strength between the tool surface and the oxide layer on the work material.

Electron magnetic circular dichroism -Optimization of signal acquisition and data evaluation

This paper is focused to the metallurgical consequences of severe adhesive wear of metallicmaterials. Early examples from the late 70:ies from sliding wear tests of different steels areshown together with some high-resolution TEM micrographs of a cemented carbide cutting tooledge, prepared by using a Focused Ion Beam.Irrespective of sliding conditions, severe metallic wear of the adhesive type results in a surfacelayer, the structure of which is totally different from that of the original bulk material. Theoutermost surface layer displays a nano-crystalline structure followed by a textured layer inwhich the original grains are heavily deformed. For carbon steels, the nano-crystalline layeroften represents untempered martensite.During the wear process, oxide fragments and wear particles from the counter-material may alsobe mixed into the surface layer.The consequence for all metallic materials is that severe wear generates a hard superficial layer.For carbon steels, the hardness of the outermost layer may well exceed 1000 HV. The hardeningmechanisms are well known to a metallurgist and consist of grain refinement, deformationhardening through dislocation generation and tangling, solute hardening (martensite in carbonsteels) and second phase or particle strengthening through intermixing.Consequently, the wear process generates a surface layer on metallic materials that has a muchhigher wear resistance than the original material. This was also demonstrated in one of theexperiments.

Austenitic stainless steels are known to be very difficult to machine and form. This is due to their tendency to adhere to the tool material, which then initiates galling. In this paper, austenitic stainless steel has been tested against TiN in un-lubricated sliding contact. The interface between TiN and adhered stainless steel was investigated by transmission electron microscopy. An oxide layer, emanating from the stainless steel, was revealed to be present between the tool surface and the adhered stainless steel. It is concluded that the stainless steel oxide plays a major role for the sticky behavior of this material. To further investigate this phenomenon, austenitic stainless steel samples were also pre-oxidised before testing to examine if the oxide thickness and composition are important for the adhesion tendency. The test showed that the sample oxidised at 800°C had less tendency to adhere than those oxidised at lower temperatures. The explanation is either its higher thickness, or its high content of Cr-oxide in its surface. The surface of the oxides formed at lower temperatures consists mainly of iron oxide.