The effects of W content on the hard magnetic properties and microstructure of $Nd_{15}(Fe_{100-x}W_x)_{77}B_8$ alloys have been investigated by magnetic analyses, differential scanning calorimetry(DSC) and microstructural examinations. The alloys wth x up to 1.2 were prepared by employing conventional powder metallurgical processes.
Results of magnetic measurements have shown that the intrinsic coercive force[$_iH_c$] of $Nd_{15}(Fe_{100-x}W_x)_{77}B_8$ increased by about 15.2% from 6.6kOe for x=0 to 7.6kOe for x=1.2. On the other hand, the remanence[B$_r$] and maximum energy product[$(BH)_{max}$] decreased from 12.4kG and 35MGOe for x=0 to 10.4kG and 25MGOe for x=1.2, respectively. Electron probe microanalyses have shown that the fine precipitates of W-rich phase was formed in $Nd_2Fe_{14}B$ matrix and Nd-rich phase by the addition of W. The increase in $_iH_c$ was due to the suppression of grain growth by the W-rich precipitates. The DSC traces of $Nd_{15}(Fe_{100-x}W_x)_{77}B_8$ alloys revealed that the Curie temperature (T$_c$) was approximately constant independent of W content. This indicates that Nd-Fe-B alloys containing W do not form any other hard magnetic phases besides of $Nd_2Fe_{14}B$ phase. The rapid decrease in $B_r$ and $(BH)_{\max}$ with W content may be ascribed to the reduced amounts of ferromagnetic $Nd_2Fe_{14}B$ phase.
The variation of magnetic properties with milling time was also investigated. The best magnetic properties were obtained at a milling time of 15 min. Further milling deteriorated the magnetic properties due to the oxygen pick up during milling process.
